Breathing and exchange of gases

Respiratory organs

Mechanisms of breathing vary among different groups of animals depending mainly on their habitats and levels of organisation. Lower invertebrates like sponges, coelenterates, flatworms, etc., exchange O₂ with CO₂) by simple diffusion over their entire body surface. Earthworms use their moist cuticle and insects have a network of tubes (tracheal tubes) to transport atmospheric air within the body. Special vascularised structures called gills (branchial respiration) are used by most of the aquatic arthropods and molluscs whereas vascularised bags called lungs (pulmonary respiration) are used by the terrestrial forms for the exchange of gases. Among vertebrates, fishes use gills whereas amphibians, reptiles, birds and mammals respire through lungs. Amphibians like frogs can respire through their moist skin (cutaneous respiration) also.

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Human Respiratory System

We have a pair of external nostrils opening out above the upper lips. It leads to a nasal chamber through the nasal passage. The nasal chamber opens into the pharynx, a portion of which is the common passage for food and air. The pharynx opens through the larynx region into the trachea. Larynx is a cartilaginous box which helps in sound production and hence called the sound box.

During swallowing glottis can be covered by a thin elastic cartilaginous flap called epiglottis to prevent the entry of food into the larynx.
Trachea is a straight tube extending up to the mid-thoracic cavity, which divides at the level of 5th thoracic vertebra into a right and left primary bronchi.
Each bronchi undergoes repeated divisions to form the secondary and tertiary bronchi and bronchioles ending up in very thin terminal bronchioles.
The tracheae, primary, secondary and tertiary bronchi, and initial bronchioles are supported by incomplete cartilaginous rings.
Each terminal bronchiole gives rise to a number of very thin, irregular-walled and vascularised bag-like structures called alveoli.
The branching network of bronchi, bronchioles and alveoli comprise the lungs (Figure).
We have two lungs which are covered by a double layered pleura, with pleural fluid between them.
It reduces friction on the lung-surface.
The outer pleural membrane is in close contact with the thoracic lining whereas the inner pleural membrane is in contact with the lung surface.
The part starting with the external nostrils up to the terminal bronchioles constitute the conducting part whereas the alveoli and their ducts form the respiratory or exchange part of the respiratory system.
The conducting part transports the atmospheric air to the alveoli, clears it from foreign particles, humidifies and also brings the air to body temperature. Exchange part is the site of actual diffusion of O₂ and CO₂ between blood and atmospheric air.
The lungs are situated in the thoracic chamber which is anatomically an air-tight chamber.
The thoracic chamber is formed dorsally by the vertebral column, ventrally by the sternum, laterally by the ribs and on the lower side by the dome-shaped diaphragm.
The anatomical setup of lungs in thorax is such that any change in the volume of the thoracic cavity will be reflected in the lung (pulmonary) cavity. Such an arrangement is essential for breathing, as we cannot directly alter the pulmonary volume.

Respiration involves the following steps:

(i) Breathing or pulmonary ventilation by which atmospheric air is drawn in and CO₂ rich alveolar air is released out.
(ii) Diffusion of gases O₂ and CO₂ across alveolar membrane.
(iii) Transport of gases by the blood.
(iv) Diffusion of O₂ and CO₂ between blood and tissues.
(v) Utilisation of O₂ by the cells for catabolic reactions and resultant
release of CO₂. 00

Mechanism of breathing

Breathing involves two stages : inspiration during which atmospheric air is drawn in and expiration by which the alveolar air is released out.
The movement of air into and out of the lungs is carried out by creating a pressure gradient between the lungs and the atmosphere.

Inspiration can occur if the pressure within the lungs (intra-pulmonary pressure) is less than the atmospheric pressure, i.e., there is a negative pressure in the lungs with respect to atmospheric pressure.

Similarly, expiration takes place when the intra-pulmonary pressure is higher than the atmospheric pressure. The diaphragm and a specialised set of muscles – external and internal intercostals between the ribs, help in generation of such gradients. Inspiration is initiated by the contraction of diaphragm which increases the volume of thoracic chamber in the antero-posterior axis. The contraction of external inter-costal muscles lifts up the ribs and the sternum causing an increase in the volume of the thoracic chamber in the dorso-ventral axis. The overall increase in the thoracic volume causes a similar increase in pulmonary volume.
An increase in pulmonary volume decreases the intra-pulmonary pressure to less than the atmospheric pressure which forces the air from outside to move into the lungs, i.e., inspiration (Figure).

Relaxation of the diaphragm and the inter-costal muscles returns the diaphragm and sternum to their normal positions and reduce the thoracic volume and thereby the pulmonary volume. This leads to an increase in intra-pulmonary pressure to slightly above the atmospheric pressure causing the expulsion of air from the lungs, i.e., expiration (Figure ). We have the ability to increase the strength of inspiration and expiration with the help of additional muscles in the abdomen.

On an average, a healthy human breathes 12-16 times/minute. The volume of air involved in breathing movements can be estimated by using a spirometer which helps in clinical assessment of pulmonary functions.

Respiratory Volumes and Capacities

Tidal Volume (TV): Volume of air inspired or expired during a normal respiration. It is approx. 500 mL., i.e., a healthy man can inspire or expire approximately 6000 to 8000 mL of air per minute.

Inspiratory Reserve Volume (IRV): Additional volume of air, a person can inspire by a forcible inspiration. This averages 2500 mL to 3000 mL.

Expiratory Reserve Volume (ERV): Additional volume of air, a person can expire by a forcible expiration. This averages 1000 mL to 1100 mL.

Residual Volume (RV): Volume of air remaining in the lungs even after a forcible expiration. This averages 1100 mL to 1200 mL. By adding up a few respiratory volumes described above, one can derive various pulmonary capacities, which can be used in clinical diagnosis.

Inspiratory Capacity (IC): Total volume of air a person can inspire after a normal expiration. This includes tidal volume and inspiratory reserve volume ( TV+IRV).

Expiratory Capacity (EC): Total volume of air a person can expire after a normal inspiration. This includes tidal volume and expiratory reserve volume (TV+ERV).

Functional Residual Capacity (FRC): Volume of air that will remain in the lungs after a normal expiration. This includes ERV+RV. Vital Capacity (VC): The maximum volume of air a person can breathe in after a forced expiration. This includes ERV, TV and IRV or the maximum volume of air a person can breathe out after a forced inspiration.

Total Lung Capacity (TLC): Total volume of air accommodated in the lungs at the end of a forced inspiration. This includes RV, ERV, TV and IRV or vital capacity + residual volume.
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EXCHANGE OF GASES

Alveoli are the primary sites of exchange of gases. Exchange of gases also occur between blood and tissues. O₂ and CO₂ are exchanged in these sites by simple diffusion mainly based on pressure/concentration gradient. Solubility of the gases as well as the thickness of the membranes involved in diffusion are also some important factors that can affect the rate of diffusion. Pressure contributed by an individual gas in a mixture of gases is called partial pressure and is represented as pO₂ for oxygen and pCO₂ for carbon dioxide. Partial pressures of these two gases in the atmospheric air and the two sites of diffusion are given in Table and in the Figure. The data given in the table clearly indicates a concentration gradient for oxygen from alveoli to blood and blood to tissues. Similarly, a gradient is present for CO₂ in the opposite direction, i.e., from tissues to blood and blood to alveoli. As the solubility of CO₂ is 20-25 times higher than that of O₂, the amount of CO₂ that can diffuse through the diffusion membrane per unit difference in partial pressure is much higher compared to that of O₂.

The diffusion membrane is made up of three major layers (Figure 17.4) namely,
the thin squamous epithelium of alveoli,
the endothelium of alveolar capillaries and
the basement substance (composed of a thin basement membrane supporting the squamous epithelium and the basement membrane surrounding the single layer endothelial cells of capillaries) in between them.

However, its total thickness is much less than a millimetre. Therefore, all the factors in our body are favourable for diffusion of O₂ from alveoli to tissues and that of CO₂ from tissues to alveoli.

Transport of gases

Blood is the medium of transport for O₂ and CO₂. About 97 per cent of O₂ is transported by RBCs in the blood. The remaining 3 per cent of O₂ is carried in a dissolved state through the plasma. Nearly 20-25 per cent of CO₂ is transported by RBCs whereas 70 per cent of it is carried as bicarbonate. About 7 per cent of CO₂ is carried in a dissolved state through plasma.

Transport of Oxygen

Haemoglobin is a red coloured iron containing pigment present in the RBCs. O₂ can bind with haemoglobin in a reversible manner to form oxyhaemoglobin. Each haemoglobin molecule can carry a maximum of four molecules of O₂. Binding of oxygen with haemoglobin is primarily related to partial pressure of O₂. Partial pressure of CO₂, hydrogen ion concentration and temperature are the other factors which can interfere with this binding.
A sigmoid curve is obtained when percentage saturation of haemoglobin with O₂ is plotted against the pO₂. This curve is called the Oxygen dissociation curve (Figure ) and is highly useful in studying the effect of factors like pCO₂, H⁺ concentration, etc., on binding of O₂ with haemoglobin. In the alveoli, where there is high pO₂, low pCO₂, lesser H⁺concentration and lower temperature, the factors are all favourable for the formation of oxyhaemoglobin, whereas in the tissues, where low pO₂, high pCO₂, high H+ concentration and higher temperature exist, the conditions are favourable for dissociation of oxygen from the oxyhaemoglobin. This clearly indicates that O₂ gets bound to haemoglobin in the lung surface and gets dissociated at the tissues. Every 100 ml of oxygenated blood can deliver around 5 ml of O₂ to the tissues under normal physiological conditions.

Transport of Carbon dioxide
CO₂ is carried by haemoglobin as carbamino-haemoglobin (about 20-25 per cent). This binding is related to the partial pressure of CO₂. pO₂ is a major factor which could affect this binding. When pCO₂ is high and pO₂ is low as in the tissues, more binding of carbon dioxide occurs whereas, when the pCO₂ is low and pO₂ is high as in the alveoli, dissociation of CO₂ from carbamino-haemoglobin takes place, i.e., CO₂ which is bound to haemoglobin from the tissues is delivered at the alveoli. RBCs contain a very high concentration of the enzyme, carbonic anhydrase and minute quantities of the same is present in the plasma too. This enzyme facilitates the following reaction in both directions. At the tissue site where partial pressure of CO₂ is high due to catabolism, CO₂ diffuses into blood (RBCs and plasma) and forms HCO3^– and H+. At the alveolar site where pCO₂ is low, the reaction proceeds in the opposite direction leading to the formation of CO₂ and H₂ O. Thus, CO₂ trapped as bicarbonate at the tissue level and transported to the alveoli is released out as CO₂. Every 100 ml of deoxygenated blood delivers approximately 4 ml of CO₂ to the alveoli.

Regulation of respiration

Human beings have a significant ability to maintain and moderate the respiratory rhythm to suit the demands of the body tissues. This is done by the neural system. A specialised centre present in the medulla region of the brain called respiratory rhythm centre is primarily responsible for this regulation. Another centre present in the pons region of the brain called pneumotaxic centre can moderate the functions of the respiratory rhythm centre. Neural signal from this centre can reduce the duration of inspiration and thereby alter the respiratory rate. A chemosensitive area is situated adjacent to the rhythm centre which is highly sensitive to CO₂ and hydrogen ions. Increase in these substances can activate this centre, which in turn can signal the rhythm centre to make necessary adjustments in the respiratory process by which these substances can be eliminated. Receptors associated with aortic arch and carotid artery also can recognise changes in CO₂ and H⁺ concentration and send necessary signals to the rhythm centre for remedial actions. The role of oxygen in the regulation of respiratory rhythm is quite insignificant.

Disorders of respiratory system

Asthma is a difficulty in breathing causing wheezing due to inflammation of bronchi and bronchioles.
Emphysema is a chronic disorder in which alveolar walls are damaged due to which respiratory surface is decreased. One of the major causes of this is cigarette smoking.
Occupational Respiratory Disorders: In certain industries, especially those involving grinding or stone-breaking, so much dust is produced that the defense mechanism of the body cannot fully cope with the situation. Long exposure can give rise to inflammation leading to fibrosis (proliferation of fibrous tissues) and thus causing serious lung damage. Workers in such industries should wear protective masks. 00

1. Define vital capacity. What is its significance?
Vital capacity is the maximum volume of air that can be exhaled after a maximum inspiration. It is about 3.5 – 4.5 liters in the human body. It allows the intake of the maximum amount of fresh air along with getting rid of the foul air in a single stroke of respiration. Hence, it increases the gaseous exchange between the various tissues of the body, this leads to the increased amount of energy available for body functioning. 2. State the volume of air remaining in the lungs after a normal breathing.
Functional Residual Capacity The volume of air remaining in the lungs after a normal expiration is known as functional residual capacity (FRC). It includes expiratory reserve volume (ERV) and residual volume (RV). ERV is the maximum volume of air that can be exhaled after a normal expiration. It is about 1000 mL to 1500 mL. RV is the volume of air remaining in the lungs after maximum expiration. It is about 1100 mL to 1500 mL. ∴ FRC = ERV + RV ≅ 1500 + 1500 ≅ 3000 mL The functional residual capacity of the human lungs is about 2500 – 3000 mL. 3. Diffusion of gases occurs in the alveolar region only and not in the other parts of respiratory system. Why?
For efficient exchange of gases, the respiratory surface must have certain characteristics such as (I) it must be thin, moist and permeable to respiratory gases. (ii) it must be very large (iii) it must be highly vascular. Only the alveolar region has these characteristics. Thus, diffusion of gases occurs in this region only. 4. What are the major transport mechanisms for CO₂? Explain.
4. What are the major transport mechanisms for CO2 ?Explain.
Plasma and red blood cells transport carbon dioxide. This is because they are readily soluble in water. (1) Through plasma : About 7% of CO2 is carried in a dissolved state through the plasma. Carbon dioxide combines with water and forms carbonic acid. Since the process of forming carbonic acid is slow, only a small amount of carbon dioxide is carried this way. (2) Through RBCs : About 20 – 25% of CO2 is transported by the red blood cells as carbaminohaemoglobin. Carbon dioxide binds to the amino groups on the polypeptide chains of haemoglobin and forms a compound known as carbaminohaemoglobin. (3) Through sodium bicarbonate : About 70% of carbon dioxide is transported as sodium bicarbonate. As CO2 diffuses into the blood plasma, a large part of it combines with water to form carbonic acid in the presence of the enzyme carbonic anhydrase. Carbonic anhydrase is a zinc enzyme that speeds up the formation of carbonic acid. This carbonic acid dissociates into bicarbonate and hydrogen ions (H+). Carbonic anhydrase 5. What will be the pO2 and pCO2 in the atmospheric air compared to those in the alveolar air ? (i) pO2 lesser, pCO2 higher (ii) pO2 higher, pCO2 lesser (iii) pO2 higher, pCO2 higher (iv) pO2 lesser, pCO2 lesser (ii) The pO2 (partial pressure of oxygen) will be higher in the atmospheric air compared to that in the alveolar air. The pCO2 (partial pressure of carbon dioxide)will be lesser in the atmospheric air compared to that in the alveolar air. In atmospheric air, pO2 is about 159 mm Hg. In alveolar air, it is about 104 mm Hg. In atmospheric air, pCO2 is about 0.3 mm Hg. In alveolar air, it is about 40 mm Hg. 5. What will be the pO₂ and pCO₂ in the atmospheric air compared to those in the alveolar air ?
(i) pO₂ lesser, pCO₂ higher

(ii) pO₂ higher, pCO₂ lesser
(iii) pO₂ higher, pCO₂ higher
(iv) pO₂ lesser, pCO₂ lesser
6. Explain the process of inspiration under normal conditions.
Inspiration or inhalation is the process of bringing air from outside the body into the lungs. It is carried out by creating a pressure gradient between the lungs and the atmosphere. When air enters the lungs, the diaphragm expands toward the abdominal cavity, thereby increasing the space in the thoracic cavity for accommodating the inhaled air. The volume of the thoracic chamber in the anteroposterior axis increases with the simultaneous contraction of the external intercostal muscles. This causes the ribs and the sternum to move out, thereby increasing the volume of the thoracic chamber in the dorsoventral axis. The overall increase in the thoracic volume leads to a similar increase in the pulmonary volume. Now, as a result of this increase, the intra-pulmonary pressure becomes lesser than the atmospheric pressure. This causes the air from outside the body to move into the lungs. 7. How is respiration regulated?
a. The respiratory rhythm centre present in the medulla region of the brain is primarily responsible for the regulation of respiration. b. The pneumotaxic centre can alter the function performed by the respiratory rhythm centre by signalling to reduce the inspiration rate. c. The chemosensitive region present near the respiratory centre is sensitive to carbon dioxide and hydrogen ions. This region then signals to change the rate of expiration for eliminating the compounds. d. The receptors present in the carotid artery and aorta detect the levels of carbon dioxide and hydrogen ions in the blood. e. As the level of carbon dioxide increases, the respiratory centre sends nerve impulses for necessary changes. 8. What is the effect of pCO₂ on oxygen transport?
pCO2 plays an important role in the transportation of oxygen. At the alveolus, the low pCO2 and high pO2 favours the formation of haemoglobin. At the tissues, the high pCO2 and low pO2 favours the dissociation of oxygen from oxyhaemoglobin. Hence, the affinity of haemoglobin for oxygen is enhanced by the decrease of pCO2 in blood. Therefore, oxygen is transported in blood as oxyhaemoglobin and oxygen dissociates from it at the tissues. 9. What happens to the respiratory process in a man going up a hill?
As altitude increases, the oxygen level in the atmosphere decreases. Therefore, as a man goes uphill, he gets less oxygen with each breath. This causes the amount of oxygen in the blood to decline. The respiratory rate increases in response to the decrease in the oxygen content of blood. Simultaneously, the rate of heartbeat increases to increase the supply of oxygen to the blood. 10. What is the site of gaseous exchange in an insect?
Insects have a complex system of whitish, shining, intercommunicating air tubes called tracheae to enable them to exchange gases between the environment and the body cells (tracheal respiration). Openings to these tubes are called spiracles. Oxygen rich air rushes through spiracles into the tracheal tubes, diffuses into the body tissue and reaches every cell of the body. Similarly carbon dioxide from the cells goes into the tracheal tubes and moves out through the spiracles. 11. Define oxygen dissociation curve. Can you suggest any reason for its sigmoidal pattern?
Oxygen dissociation curve: The oxygen dissociation curve is a graph which shows the relationship between the partial pressure of oxygen and oxygen saturation of haemoglobin. The oxygen dissociation curve is a graph describing the saturation percentage of oxyhaemoglobin at various partial pressures of oxygen. This curve also shows the equilibrium of haemoglobin and oxyhaemoglobin at various partial pressures. Reason for sigmoidal pattern of oxygen dissociation curve: The oxygen dissociation curve is in a sigmoid shape or S-shaped because of the co-operative binding of oxygen to haemoglobin. The oxygen dissociation curve is obtained by plotting the percentage saturation of haemoglobin with oxygen against the partial pressure of oxygen. 12. Have you heard about hypoxia? Try to gather information about it, and discuss with your friends.
Hypoxia is a condition characterised by an inadequate or decreased supply of oxygen to the lungs. It is caused by several extrinsic factors such as reduction in pO2, inadequate oxygen, etc. The different types of hypoxia are discussed below. Hypoxemic hypoxia: In this condition, there is a reduction in the oxygen content of blood as a result of the low partial pressure of oxygen in the arterial blood. Anaemic hypoxia: In this condition, there is a reduction in the concentration of haemoglobin. Stagnant or ischemic hypoxia: In this condition, there is a deficiency in the oxygen content of blood because of poor blood circulation. It occurs when a person is exposed to cold temperature for a prolonged period of time. Histotoxic hypoxia: In this condition, tissues are unable to use oxygen. This occurs during carbon monoxide or cyanide poisoning. 13. Distinguish between (a) IRV and ERV (b) Inspiratory capacity and Expiratory capacity. (c) Vital capacity and Total lung capacity. 13. Distinguish between
(a) IRV and ERV
(b) Inspiratory capacity and Expiratory capacity.
(c) Vital capacity and Total lung capacity.
Inspiratory Reserve Volume(IRV) Expiratory Reserve Volume (ERV) 1. The maximum volume of air that can be inhaled after normal inspiration is called inspiratory reserve volume. 1. The maximum volume of air that can be forcefully exhaled after normal expiration is called expiratory reserve volume. 2. The IRV of the human lungs is about 2500 - 3500 mL. 2. The ERV of the human lungs is about 1000-1100 mL. Part (b): Inspiratory Capacity vs Expiratory Capacity Inspiratory capacity (IC) Expiratory capacity (EC) 1. It is the volume air that can be inhaled after normal expiration. 1. It is the volume of air that can be exhaled after a normal inspiration. 2. It includes tidal volume and inspiratory reserve volume 2. It includes the tidal volume along with the expiratory reserve volume 3. IC = TV + IRV 3. EC = TV + ERV Part (c): Vital capacity vs Total lung capacity Vital capacity (VC) Total lung capacity (TLC) 1. It is the maximum volume of air that can be exhaled after maximum inspiration. 1. It is the maximum volume of air in the lungs after maximum inspiration. 2. IC + ERV = VC = 4000 mL in humans. 2. TLC = IC + ERV + RV = 5000-6000 mL in humans. 14. What is Tidal volume? Find out the Tidal volume (approximate value) for a healthy human in an hour.
Tidal volume is the volume of air inspired or expired during normal respiration. It is about 6000 to 8000 mL of air per minute. The hourly tidal volume for a healthy human can be calculated as: Tidal volume = 6000 to 8000 mL/minute Tidal volume in an hour = 6000 to 8000 mL × (60 min) = 3.6 × 105 mL to 4.8 × 105 mL Therefore, the hourly tidal volume for a healthy human is approximately 3.6 × 105 mL to 4.8 × 105 mL. 00

1117SV Comprehension
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1. Which gas is released during catabolic reactions?
2. Name the lower vertebrates that exchange oxygen with carbon dioxide by simple diffusion over their entire body surface.
3. How the earthworms undergo respiration?
4. What is the mechanism in insects to transport atmospheric air within the body?
5. Give names of two animals or group of animals where branchial respiration take place.
6. Frogs can respire through their moist skin, what do you this type of respiration?
7. What do you understand by nasal passage.
8. Fill in the blanks
a. During swallowing glottis can be covered by a thin elastic cartilaginous flap called ……….. to prevent the entry of food into the …….. Trachea is a straight tube extending up to the mid-thoracic cavity, which divides at the level of ……… thoracic vertebra into a right and left primary bronchi.
b. Each bronchi undergoes repeated divisions to form the secondary and tertiary bronchi and bronchioles ending up in very thin …………… bronchioles.
c. The tracheae, primary, secondary and tertiary bronchi, and initial bronchioles are supported by incomplete ………………. rings.
d. Each terminal bronchiole gives rise to a number of very thin, irregular-walled and vascularised bag-like structures called ………………..
e. We have two lungs which are covered by a double layered ………….., with …………….. fluid between them. The latter reduces friction on the lung-surface.
9. Which membrane is in contact with the lung surface?
10. Name the parts or structures which form the respiratory or exchange part of the respiratory system. Also write role of this part.
Fill in the blanks
11. The thoracic chamber is formed dorsally by the ………………., ventrally by the …………….., laterally by the ribs and on the lower side by the ………shaped diaphragm.
12. Write five steps of Respiration.
Also Explain the following
a. inspiration
b. expiration.
13. Which conditions lead to inspiration?
14. Which conditions lead to expiration?
15. Write the location of external and internal intercostals.
16. How Inspiration is initiated?
17. How ribs and the sternum are lifted up during inspiration. What does it lead to?
18. How the diaphragm and sternum come back to to their normal positions during expiration?
19. How many times a healthy human breathes in a minute?
20. What is a spirometer?
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21. Explain the following-
a. Tidal Volume (TV)
b. Inspiratory Reserve Volume (IRV)
c. Expiratory Reserve Volume (ERV)
d. Residual Volume (RV)
e. Inspiratory Capacity (IC)
f. Expiratory Capacity (EC)
g. Functional Residual Capacity (FRC)
h. Vital Capacity (VC)
i. Total Lung Capacity (TLC)
Also mention the volume of above in ml.
22. How many time solubility of CO2 is higher than that of O2?
23. Name three major layers which form diffusion membrane.
24. Fill in the blanks
About ………. per cent of O2 is transported by RBCs in the blood. The remaining …………. per cent of O2 is carried in a dissolved state through the plasma. Nearly ………… per cent of CO2 is transported by RBCs whereas …………….. per cent of it is carried as bicarbonate. About …………. per cent of CO2 is carried in a dissolved state through plasma.
25. How many molecules of O2 are carried by a haemoglobin molecule?
26. Name the factors responsible for binding of O2 with haemoglobin.
27. What is the shape of Oxygen dissociation curve?
28. How much oxygen is delivered by every 100 ml of oxygenated blood.
29. What is carbamino-haemoglobin?
30. Write function of enzyme carbonic anhydrase.
31. How much CO2 is delivered by every 100 ml of deoxygenated blood to the alveoli?
32. Where respiratory rhythm centre is present?
33. What is the role of respiratory rhythm centre?
34. To which gas and ions chemosensitive area situated close to the respiratory rhythm centre is sensitive?
35. What is the role of receptors associated with aortic arch and carotid artery in respiration?
36. Explain the following disorders of respiratory system
Asthma,
Emphysema and
Occupational Respiratory Disorders 00
Notes Mammalian Respiratory System The system brings about inspiration (breathing in), expiration (breathing out), exchange of gases in lungs and transport of gases between the lungs and tissues. Transport of gases occurs with the help of blood. The remaining respiratory system consists of a respiratory tract, a pair of lungs and structures involved in ventillation. Respiratory tract consists of external nares, nasal cavity, internal nares, nasopharynx, larynx, trachea, bronchi and bronchioles. A large leaf-like cartilaginous epiglottis guards the opening of the larynx called glottis. Trachea is a tubular structure of about 12 cm in length and 2.5 cm in diameter; it starts posterior to larynx and extends up to the middle of the thoracic cavity where it divides into right and left primary bronchi that enter into the lungs. The tracheal tubule is supported by incomplete (C-shaped) ring of cartilage at regular intervals to prevent collapsing of the tubule. In each lung. The bronchus divides and redivides to from secondary bronchi, tertiary bronchi, bronchiole and. ultimately the terminal bronchioles. that further subdivide into many alveolar ducts that lead into the alveoli or air sac (Fig. 6.6). There are 300 millions of alveoli in the two lungs. Lungs are paired structure present in the thoracic or pleural cavity. A double-layered pleural membrane encloses the lung for its protection. The outer layer of pleura Mechanism of Respiration The main purpose of respiration is to provide oxygen to the tissues and to remove carbon dioxide from them. This entire process is accomplished in three steps: breathing or pulmonary ventilation. exchange of oxygen and carbon dioxide. and transport of gases in blood. Breathing and pulmonary ventilation: It means the inflow (inspiration) and outflow (expiration) of air between atmosphere and the alveoli of the lung. Breathing is effected by the expansion and contraction of lungs. There are two processes by which the lungs are expanded or contracted: (1) The downward and upward movement of the diaphragm. which lengthens and shortens the chest cavity. (ii) The elevation and depression of the ribs which increase or decrease the diameter of the chest cavity. During expansion. the volume of lungs increases. As a result. the pressure of air inside the lung decreases. In order to bring the pressure at normal level. atmospheric air is inhaled. When the lungs contract, their volumes decrease, resulting in the increase of air pressure in lungs. Hence. the air is exhaled from the lungs. These two processes are called inspiration and expiration. respectively. During normal or quiet breathing, the downward and upward movement of the diaphragm takes place. When the diaphragm contracts. the lower surface of the lung is pulled downward. Consequently. the volume of the lungs increases. This causes inhalation of air or inspiration. During exhalation of air or expiration. the diaphragm relaxes and the lungs are compressed During exercise,. the rate of breathing increases due to the increased demand for oxygen. The elastic force, resulting from contraction and relaxation of diaphragm, is not sufficient for this purpose. The demand of extra oxygen is fulfilled by the expansion of rib cage. Either of the two movements, or both, create a partial vacuum or reduction of air pressure inside the thoracic cavity, including the lungs. This results into the rushing of air in the lungs to fill up the space and equalise the air pressure. When we breathe out, the capacity of thoracic cavity decreases due to the inward, as well as downward movement of the rib cage along with upward movement of the diaphragm. A high pressure is generated in the lungs and air moves out (expiration). The upward movement of rib cage is caused mainly by the external intercostal muscles present between the ribs, along with the assistance of few other adjacent muscles. Similarly, the downward movement of rib cage is facilitated by the internal intercostals, external oblique and internal oblique muscles (Fig. 6.8 The volume of air inspired and expired with every normal breath during effortless respiration is called the tidal volume (TV is about 500 ml of air). Sometimes. extra amount of air can be forcefully inspired. The extra volume of air that can be inspired beyond the normal tidal volume is called inspiratory reserve volume (IRV, is about 2500-3000 ml of air). Similarly, an extra amount of air can be expired forcefully even beyond the normal tidal expiration. The measure of this capacity of lung is called expiratory reserve volume (ERV, is about 1000 ml of air). Even after a forceful expiration to maximum capacity, some amount of air remains in the lung. It is called residual volume (RV, is about 1500 male of air). When any two or more of the above¬mentioned pulmonary volumes are considered together, such combinations are called pulmonary capacities. The total amount of air a person can take in distending the lungs to the maximum, beginning at normal expiratory level, is called inspiratory capacity (IC, is about 3000-3500 ml of air). It is equal to the sum of tidal volume and inspiratory reserve volume (IC = TV + IRV). When a person breathes normally, then, the amount which remains in the lung after normal expiration, is called functional residual capacity (FRC, is about 2500 ml of air). It can be measured as the total of expiratory reserve volume and the residual volume (FRC = ERV + RV). Vital capacity (VC) is an important measure of pulmonary capacity. It is the maximum amount of air a person can expel from the lungs after first filling the lungs to their maximum extent (VC varies from 3400 ml to 4800 mI, depending on age. sex and height of the individual). Vital capacity is the sum total of inspiratory reserve volume. tidal volume and expiratory reserve volume (VC = IRV + TV + ERV). Exchange of gas : The inspired air ultimately reaches the alveoli of the lung. which. in turn. receives the blood supply of the pulmonary circulation. At this place. the oxygen of the inspired air is taken in by the blood. and carbon dioxide is released into the alveoli for expiration. These respiratory gases move freely by the process of diffusion. The kinetic motion of the molecules provides the energy required for this diffusion of gaseous molecule itself. Diffusion of any molecule takes place from high to low concentration. The process of diffusion is directly proportional to the pressure caused by the gas alone. The pressure exerted by an individual gas is called partial pressure. It is represented as PO2, PCO2, PN2 for oxygen. carbon dioxide and nitrogen, respectively. Inside the alveoli, the inspired air remains in a very close contact with blood. It is because the alveolar wall is very thin and contains a rich network of interconnected capillaries. Due to this, the alveolar wall seems to be a sheet of flowing blood. and is called the respiratory membrane. It consists mainly of the alveolar epithelium, epithelial basement membrane, a thin interstitial space. capillary basement membrane and capillary endothelial membrane. All these layers cumulatively form a membrane of 0.2 mm thickness (Fig. 6.9). The respiratory membrane has a limit of gaseous' exchange between alveoli and pulmonary blood. It is called diffusing capacity, and is deemed as the volume of gas. that diffuses through the membrane per minute for a pressure difference of 1 mm Hg. It is further dependent on the solubility of the diffusing gases. In other words. at a particular pressure difference. the diffusion of carbon dioxide is 20 times faster than oxygen. and that of oxygen is two times faster than nitrogen. Due to the existing pressure difference of oxygen. and carbon dioxide between the alveoli and the blood capillaries. oxygen diffuses from alveolar air to the capillary blood. whereas carbon dioxide diffuses from capillary blood to the alveolar air Transport of gases in blood: Blood is the medium for the transport of oxygen from the respiratory organ to the different tissues. and carbon dioxide from tissues to the respiratory organ. The pulmonary artery supplies oxygenated blood to the left atrium. from where it is pumped into left ventricle. and ultimately. to the different tissues of the body. As much as 97 per cent of the oxygen is transported from :he lungs to the tissues in combination with haemoglobin (Hb+O2žHbO2 oxyhaemoglobin), and only 3 per cent is transported in dissolved condition by the plasma. Under the high partial pressure, oxygen easily binds with haemoglobin in the pulmonary capillaries. When this oxygenated blood reaches the different tissues, the partial pressure of oxygen declines and the bonds holding oxygen to haemoglobin become unstable. As a result, oxygen is released from the capillaries. Under strenuous conditions, or during exercise, the muscle cells consume oxygen at a comparatively faster rate.. The partial pressure of oxygen in the tissue falls, as a result of which, the blood at the tissue level has merely 4.4 ml of oxygen/ 100 ml of blood. Thus, approximately 15 ml of oxygen is transported by haemoglobin during exercise. In a normal and healthy person, the measurement of haemoglobin is approximately 15 g per 100 ml. The capacity of 1 g of haemoglobin to combine with oxygen is 1.34 ml. Thus, on an average, 100 ml of blood carries about 20 ml (19.4 ml exactly) of oxygen. When blood reaches the tissues, its oxygen concentration is reduced gradually to 14.4 ml, which is then collected, by the veinules and veins. Thus, under normal conditions, approximately 5 ml of oxygen is transported by 100 ml of blood. This can be verified by deducting the quantity of oxygen of venous blood from that of arterial blood. The amount of oxygen that can bind with haemoglobin, is determined by oxygen tension. This is expressed as a partial pressure (PO2). that is the fraction of atmospheric oxygen. Figure 6.10 shows the saturation level of haemoglobin in relation to the PO2 of blood. Haemoglobin cannot take up oxygen beyond a saturation level of 95 per cent. A 100 per cent saturation of haemoglobin is rare. At lower PO2 oxygen is released from haemoglobin. It is 50 per cent saturated at 30 mm of Hg. Haemoglobin would be completely free from oxygen at zero PO2. This relationship is expressed by plotting the oxygen saturation of blood against the PO2 of oxygen. An S¬ shaped curve, called oxygen dissociation curve, is obtained. It is dependent on PO2, PCO2 temperature and pH. The blood transports carbon dioxide comparatively easily because of its higher solubility. There are three ways of transport of carbon dioxide. (a) In dissolved state: Approximately 5-7 per cent of carbon dioxide is transported. being dissolved in the plasma of blood. The partial pressures (PCO2) of the venous blood and arterial blood is 45 mm of Hg (i.e. 2,7 ml of CO2/100 ml) and 40 mm of Hg (2.4 ml CO2/ 100ml). respectively. Hence. 0.3mlofcarbon dioxide is transported per 100 ml of blood, (b) In the form of bicarbonate: Carbon dioxide produced by the tissues. diffuses passively into the blood stream and passes into the red blood corpuscles. where it reacts with water to form carbonic acid (H2CO3, This reaction is catalysed by the enzyme. carbonic anhydrase. found in the erythrocytes, and takes less than one second to complete the process. Immediately after its formation, carbonic acid dissociates into Hydrogen (H+) and bicarbonate (HCO3 -) ions. The oxyhaemoglobin (HbO2) of the erythrocytes is weekly acidic and remains in association with K+ ions as KHbO2, The hydrogen ions (H +) released from carbonic acid combine with haemoglobin after its dissociation from the potassium ions. The majority of bicarbonate ions (HCO3-) formed within the erythrocytes diffuse out into the plasma along a concentration gradient. These combine with haemoglobin to form the haemogIobinic acid (H .Hb). In response, chloride ions (Cl- )diffuse from plasma into the erythrocytes to maintain the ionic balance. Thus. electrochemical neutrality is maintained. This is called chloride shift (Fig. 6.11). The chloride ions (Cl-) inside RBC combine with potassium ion (K+) to form potassium chloride (KC1), whereas hydrogen carbonate ions (HCO3-) in the plasma combine with Na+ to form sodium hydrogen carbonate (NaHCO3). Nearly 70 per cent of carbon dioxide is transported from tissues to the lungs in this form (c) In combination with amine group of protein: Besides the above two methods, carbon dioxide reacts directly with the amine radicals (NH2) of haemoglobin molecule and forms a carbaminohaemoglobin (HbCO2) molecule. This combination of carbon dioxide with haemoglobin is a reversible reaction. Nearly 23 per cent of carbon dioxide is transported through this mode. Release of carbon dioxide in the alveoli of lung: When the deoxygenated blood reaches the alveoli of the lung, it contains carbon dioxide as dissolved in plasma, as carbaminohaemoglobin, and as bicarbonate ions. In the pulmonary capillaries, the carbon dioxide dissolved in plasma diffuses into alveoli. Carbaminohaemoglobin also splits into carbon dioxide and haemoglobin. For the release of carbon dioxide from the bicarbonate, a small series of reverse reactions takes place. When the haemoglobin in the pulmonary blood takes up oxygen, the H+ is released from it. Then, the Cl- and HCO3- ions are released from KCI in blood, and NaHCO3 in the RBC, respectively. Then HCO3- reacts with H + to form H2CO3. This H2CO3 ultimately, then splits into carbon dioxide and water in the presence of carbonic anhydrase enzyme and carbon dioxide is released into lungs. REGULATION OF RESPIRATIOPN The respiratory rhythm is controlled by the nervous system. The rate of respiration can be enhanced as per demand of the body during strenuous physical exercises. A number of groups of neurons located bilaterally in the medulla oblongata control the respiration. These are called respiratory centres. Three groups of respiratory centres have been identified, namely: dorsal respiratory group, ventral respiratory group and pneumotaxic centre (Fig. 6.12). The dorsal respiratory group is present in the dorsal portion of medulla oblongata. The signals from these neurons generate the basic respiratory rhythm. The nervous signal released from this group is transmitted to the diaphragm, which is the primary inspiratory muscle. The ventral respiratory group of neurons are located anterolateral to the dorsal respiratory group. During normal respiration, this remains inactive and even does not play any role in the basic respiratory rhythm. But, under the enhanced respiratory drive, the respiratory signal of this group contributes to fulfill the demand by regulating both inspiration and expiration. Few of the neurons of this group control inspiration, while few other control expiration, thus regulating both. The pneumotaxic centre is located dorsally in the upper pons. It transmits signals to the inspiratory area. Primarily, it controls the switch off point of inspiration. When this signal is strong, the inspiration lasts only for 0.5 seconds, and lungs are filled partially. During weak pneumotaxic signal, inspiration lasts for 5 seconds, or more, resulting into complete filling of lungs. The strong signal causes increased rate of breathing because inspiration, as well as expiration, is shortened. The concentration of CO2 and H+ cause increased strength of inspiratory, as well as expiratory signal. However, oxygen has no such direct effect. RESPIRATORY DISORDERS (a) Bronchitis: It is the inflammation of the bronchi, which is characterised by hypertrophy and hyperplasia of sera-mucous gland and goblet cells lining the bronchi. The symptom is regular coughing, with thick greenish yellow sputum that indicates the underlying infection. resulting into excessive secretion of mucous. It may also be caused by cigarette smoking and exposure to air pollutants like carbon monoxide. Prevention and cure: Avoiding exposure to the cause, i.e.. smoke, chemicals and pollutants, can prevent Bronchitis. The underlying infection of the disease is treated with suitable antibiotics. Bronchodilator drugs (for widening the constriction of bronchial passage by relaxing the smooth muscles) provide symptomatic relief. (b) Bronchial Asthma: This is characterised by the spasm of the smooth muscles present in the walls of the bronchiole. It is generally caused due to the hypersensitivity of the bronchiole to the foreign substances present in the air passing through it. The symptoms of the disease may be coughing, or difficulty in breathing mainly during expiration. The mucous membranes on the wall of the air passage start secreting excess amount of mucous, which may clog the bronchi. as well as bronchiole. Prevention and cure: It is an allergic disease hence, avoiding exposure to the foreign substance or allergens is the best preventive measure. In case the patient is sensitive to a very few number of allergens, then hyposensitisation (by exposing small doses of !:be specific allergen) is the other preventive measure. Treatment of the disease includes antibiotic therapy for removing the infection. and use of bronchodilator drugs, as well as inhalers for symptomatic relief. (c) Emphysema: It is an inflation or abnormal distension of the bronchiole or alveolar sac, which results into the loss of elasticity of these parts. As a result, the alveolar sac remains filled with air even after expiration. and ultimately, the lung size increases. The reason for such a condition can be assigned to cigarette smoking and chronic bronchitis. Prevention and cure: Emphysema is a chronic obstructive disease oflung, causing irreversible iistension and loss of elasticity of alveoli. Hence, it can't be cured permanently. However, treatment may retard the progression of the disease. Its treatment is also symptomatic. Bronchodilators, antibiotics and oxygen therapy are used. This disease is preventable if chronic exposure to smoke (cigarette and others) and pollutant is avoided. (d) Pneumonia: It is an acute infection or inflammation of the alveoli of the lung. This disease is caused mainly due to infection of the bacteria (Streptococcus pneumoniae). Sometimes, other bacteria or fungi, protozoan, viruses and mycoplasma may also be responsible. Infants, elderly persons and immuno compromised individuals are susceptible to it. In this disease, the fluid with dead WBC occupies most of the air space of the alveolar sac. Uptake of oxygen is adversely affected in the inflamed alveoli, as a result of which, the oxygen level of the blood falls. Prevention and cure : Since infection is the main cause of pneumonia, use of antibiotics to remove the infection cures it. Patient may require symptomatic treatment like bronchodilator drugs. In case of immuno compromised ¬individuals, the disease can be prevented by proper and timely vaccination. (e) Occupational Lung Disease: It is caused because of the exposure of potentially harmful substances, such as gas, fumes or dusts, present in the environment where a person works. Silicosis and asbestosis are the common examples, which occur due to chronic exposure of silica and asbestos dust in the mining industry. It is characterised by fibrosis (proliferation of fibrous connective tissue) of upper part of lung, causing inflammation. Prevention and cure : Almost all the occupational lung diseases, express symptoms after chronic exposure, i.e. 10-15 years or even more. Not only this, diseases like silicosis and asbestosis are incurable. Hence, the person likely to be exposed to such irritants, should adopt all possible preventive measures, These measures include: (i) Minimising the exposure of harmful dust at the work place. (ii) Workers should be well informed about the harm of the exposure to such dusts. (iii) Use of protective gears and clothing by the workers at the work place. (iv) Regular health check up. (v) Holiday from duty at short intervals for the workers in such areas. The patient may be provided with symptomatic treatment, like bronchodilators and antibiotics, to remove underlying secondary infection. External Nares (Nostrils). They are a pair of slit-like openings present on the lower end of nose. Nasal Cavity. It occurs between palate and cranium. Nasal cavity is divisible into two nasal chambers by a nasal septum. Each nasal chamber has three parts. (a) Vestibule. It is lower small part just above the external naris which is lined by skin and bears hair as well as oil glands. Hair help in filtering out dust particles from incoming air. (b) Conditioner (Respiratory Region). It is middle part of nasal chamber. There are three bony projections called nasal conchae, or turbinates (superior, middle and inferior) and some sinuses (maxillary, frontal, sphenoid and ethmoid). The conditioner part is reddish pinkish in colour due to abundant blood capillaries. It is covered by ciliated pseudostratified columnar epithelium with mucous (mucus producing) and serous (secreting watery fluid) glands. The inhaled air is moistened, warmed and cleaned (trapping dust and germ particles). (c) Olfactory Region. Upper part of nasal chamber and superior nasal concha are yellowish brown. They are covered by olfactory epithelium which perceives sensation of smell. Internal Nares (Choanae). The two nasal chambers open into nasopharynx through internal nares . Nasopharynx occurs at the base of skull and has a lining of ciliated stratified squamous epithelium. Nasopharynx leads to oropharynx or common pathway of respiratory- and digestive systems. Oropharynx passes into laryngopharynx which contains epiglottis and passes into larynx. Larynx. Larynx or voice box opens into laryngo-pharynx through a slit-like glottis which can be widened by intrinsic muscles. Glottis can be closed by a large leaf-like cartilaginous flap called epiglottis. Larynx has C-shaped thyroid cartilage (on sides and in front where it can be felt as Adam's Apple), a pair of triangular arytenoid (arytaenoid) cartilages (on back), a ring-like cricoid cartilage and a pair of nodule-like cartilages of Santorini (upper end of arytenoid cartilages). Internally larynx has ciliated columnar mucous epithelium and a pair of vocal cords (attached to thyroid and arytenoid cartilages). Vocal cords become thickened in adult males. Vocal cords are shorter and thinner in women producing high pitched voice. Voice is produced by passage of air between vocal cords and modulations created by tongue, teeth, lips and nasal cavity. Trachea (Wind Pipe). It is 10-12 cm long tube with 2-3 cm diameter which arises from larynx and passes upto middle of thorax. Trachea is supported by 16 - 20 C-shaped incomplete cartilaginous rings and lined by ciliated pseudostratified mucous epithelium. Bronchi. Trachea divides into right and left primary bronchi. Left bronchus is about 5 cm long while right bronchus is only 2.5 cm long. Right bronchus almost directly enters the right lung. Infection of right lung is more common due to this. Inside the lung, the primary bronchus divides into secondary bronchi, secondary bronchi into segmental bronchi and the latter into bronchioles. All bronchi are lined by ciliated and mucus secreting pseudostratified epithelium and supported by incomplete cartilaginous rings. Bronchioles divide into terminal bronchioles, respiratory bronchioles, alveolar ducts, air sacs and alveoli. Mucus secreting cells are absent from terminal bronchioles and their branches. Epithelium is ciliated in bronchioles and terminal bronchioles. It is nonciliated in respiratory bronchioles and their branches. Lungs. A pair of conical spongy elastic lungs of pinkish to slate grey colour occur inside air tight thoracic cavity. A small space called mediastinum lies in between the two lungs, (especially due to concavity called cardiac notch of left lung). It encloses heart. Each lung is covered by a pleural sac made of an outer parietal pleuron in contact with wall of thoracic cavity and inner visceral pleuron in contact with surface of lung. A narrow pleural cavity (0.02 mm) occurs between them. It contains pleural fluid. It allows frictionless sliding of pleura during inspiration and expiration: Protection and moistening of lungs are also provided. Pleurisy is painful infection involving inflammation of pleura and over-production of pleural fluid. Normally pleural fluid is under negative pressure due to higher rate of its drainage as compared to its formation from the membranous covering. Left lung is slightly narrower and longer than the right one. Right lungs has three lobes - right superior, right middle and right inferior. Left lung has two lobes, left superior and left inferior. It contains a cardiac notch in antero-median region for accomodating heart. Each lobe is divided internally into segments and segments into lobules. A lobule receives a terminal bronchiole. Terminal bronchiole produces a few respiratory bronchioles. A respiratory bronchiole gives rise to 2-11 alveolar ducts, each of which ends in an alveolar sac infundibulum. The latter has a number of small pouches named alveoli or air sacs. Number of alveoli in human pulmonary system is 300-400 million with a surface area of 100 m2. Each alveolus is polyhedral in outline with a thin wall made of nonciliated squamous epithelium with a few cubical cells that secrete a lipoprotein surfactant to prevent collapse and sticking of alveolar walls during expiration. Blood capillaries occur on the surface of alveoli for gaseous exchange. Diaphragm. It is a membranous musculo-tendinous partition between thorax and abdomen. Normally it is convex with convexity towards thorax. Phrenic muscles attach diaphragm to ribs and vertebral column. Contraction of muscles straighten the diaphragm to increase thoracic cavity. Intercostal Muscles. There are (i) external intercostal (ii) internal intercostal (iii) external oblique and (iv) internal oblique muscles. Abdominal Muscles. Relaxation allows compression of abdominal organs. Then diaphragm straightens. Contraction presses the abdominal viscera against diaphragm to bulge a more upwardly (for expiration). Breathing and Pulmonary Ventilation Breathing or pulmonary ventilation consists of three steps- inspiration (=inhalation or inflow of air), expiration (= exhalation or outflow of air) and pause. Lungs are in contact with thoracic walls through pleural sacs. Expansion and contraction of thoracic cavity brings about similar expansion and contraction of lungs. Expansion of lungs decreases air pressure in them so that air rushes from outside in the process of inspiration. During inspiration air is filtered (nasal hair in vestibule), cleaned (precipitation of finer particles), sterilised (by antibacterial components of glandular secretions and macrophages), air conditioned and moistened before entering in alveoli. Contraction of lungs increases air pressure in them so that a part of air is pushed out in expiration. Expansion and contraction of thoracic cavity and hence lungs is performed by two methods (i) Downward and upward movement of diaphragm. It lengthens and shortens the thoracic (chest) cavity. (ii) Increase or decrease of diameter of thoracic cavity by elevation and depression of rib cage. Normal or quiet breathing is accomplished almost entirely by movement of diaphragm. It is also called abdominal breathing. Phrenic muscles contract and straighten the diaphragm. It lengthens chest cavity pulling down the lower surface of lungs as well. Volume of lungs increases resulting in inspiration or inhalation. Phrenic muscles relax and diaphragm becomes convex. It shortens the thoracic cavity, pushing upwards the lower surface of lungs. Volume of lungs decreases and expiration or exhalation occurs. A forceful expiration can be achieved by contraction of abdominal muscles. It pushes the abdominal contents upwards, increasing convexity of diaphragm and further shortening the thoracic cavity. During exercise or manual work, there is increased demand for oxygen. At this time the thoracic cavity is further expanded by increasing the dimensions of rib cage. During rest the ribs slant downwards and the sternum backwards towards vertebral column. External intercostal muscles contract. They elevate the rib cage projecting ribs in almost forward position and moving the sternum outwardly and upwardly. Some more muscles are involved in lifting the sternum and ribs. It expands the thoracic cavity by some 20% and increases the amount of inspiration. External intercostals muscles relax. The rib cage is brought downwardly by internal intercostal muscles, external oblique and internal oblique muscles. Tidal Volume (TV). Volume of air inspired or expired in relaxed or resting position-500 ml. It consists of 150 ml of dead space volume and 350 ml of alveolar volume. Dead Space (Anatomic Dead Space). Part of respiratory tract not involved in gaseous exchange (Nose to terminal bronchi, vol. 150 ml). Total Lung Capacity (T.LC). Maximum amount of air the lungs can hold after forceful inspiration. 4.5-6.0 litres. Residual Volume (RV).Air left in lungs and dead space after forceful expiration. 1.5 litres. The air left in lungs is useful in uninterrupted gaseous exchange. Pulmonary Capacities. Pulmonary volume is obtained through summation of two or more values. Vital Capacity (V.C). Maximum amount of air- which can be breathed out through forceful expiration after a forceful inspiration. It is sum total of tidal volume (TV), inspiratory reserve volume (IRV) and expiratory reserve volume (ERV). The value is 3.4-4.8 litres. Higher in athletes (than nonatheletes), mountain dwellers (than plain dwellers), nonsmokers (than smokers), Inspiratory Reserve Volume (I.R.V. = Complementary. Air). Volume of air in excess of tidal volume which can be inhaled due to forceful inspiration. 2.5-3.0 litres (Complemental air). Expiratory Reserve Volume (E.R.V. = Supplementary Air). Volume of air in excess of tidal volume which can be exhaled due to forceful expiration. 1.0-1.5 litres (Supplemental air). Inspiratory Capacity (I.C.). Total volume of air that can be inhaled due to forceful inspiration after a normal expiration. It is equal to inspiratory reserve volume plus tidal volume, (IC = IRV + TV) or 3.0-3.5 It. Functional Residual Capacity (F.R.c.). It is sum total of expiratory reserve volume (E.RV.) and residual volume (RV.), 2.0-3.0 lt Exchange Of Gases Alveolar air is separated from blood present in surrounding blood capillaries by a very thin partition of 0.2 p.m thickness (average 0.6 p.m). It is called respiratory membrane. The membrane consists of alveolar surfactant, alveolar epithelium, epithelial basement membrane, a thin interstitial space, capillary basement membrane and capillary endothelial membrane. Diffusing capacity of a gas across a membrane is the volume of gas that diffuses per minute for a pressure difference of 1 mm Hg. The rate of diffusion of CO2 is 20 times faster than that of oxygen while oxygen diffuses twice as fast as nitrogen. Partial pressure of O2 in alveolar air (Po) is about 100 -104 mm Hg while that of deoxygenated blood in alveolar capillary is 40 mm Hg. Therefore, oxygen diffuses into blood and combines with haemoglobin to form oxyhaemoglobin. Po of oxygenated blood is 95 mm Hg. Pco of alveolar capillary blood is 46 mm Hg while in fresh alveolar air it is 40 mm Hg. As the diffusing capacity of CO2 is 20 times higher than that of O2 , CO2 of blood rapidly passes out into alveolar air. Its partial pressure in oxygenated blood is 40 mm Hg (equal to that of fresh alveolar air). Gaseous exchange occurs again in the tissues between tissue cells and capillary blood through the interstitial fluid. Partial pressure of oxygen, Po in respiring cells is 20 mm Hg, tissue fluid 40 mm Hg while it is 95 mm Hg in capillary blood. Therefore, O2 diffuses from blood into tissue fluid and from there into cells. Blood leaving the tissue capillaries have a Po2 of about 40 mm Hg. Pco2 of blood capillaries is 40 mm Hg, tissue fluid 46 mm Hg and that of cells 52 mm Hg. Therefore, carbon dioxide diffuses out of cells into tissue fluid and from tissue fluid into blood. Blood leaving the tissue capillaries has a Pco2 of 46 mm Hg. Respiratory Centre It controls rate of respiration. Respiratory centre is located in medulla oblongata and pons. It has the following well dispersed components (i) Dorsal Respiratory Group. Located dorsally along length of medulla with neurons interconnected to sensory termination of glossopharyngeal (sensory signals from peripheral chemoreceptors) and vagus (sensory signals from lungs and stretch receptors of bronchi) nerves. The area is connected through nerves to phrenic muscles of diaphragm. Nervous signal from this group brings about normal resting inspiration. Expiration occurs through elastic recoil of thoracic wall and lungs. (ii) Pneumotaxic Area. It occurs in pons and is meant for switching off normal inspiration when the limit of lung filling is reached. The latter is, however, also dependent upon the strength of signal- 0.5 sec when signal is strong and 5.0 sec when signal is poor. (iii) Ventral Respiratory Group. It occurs ventrolaterally anterior to dorsal respiratory group. The group has two types of neurons, inspiratory and expiratory. They are normally inactive but when the respiratory drive is greater than normal, the group is activated. It results in deeper and quicker inspiration and expiration. (iv) Chemosensitive Area. It lies in the medulla near the place of entry of glossopharyngeal and vagus nerves. It is sensitive to blood carbon dioxide and hydrogen ion concentration. Chemosensitive area is connected to other areas of respiratory centre. Chemoreceptors located on carotid and aortic bodies are sensitive to oxygen deficiency in arterial blood. They send information to respiratory centre. Transport of Oxygen The oxygen carrying capacity of blood is about 20 ml/l00 ml. The value is 1.34 ml per gm of haemoglobin. At 15 gm/l00 ml of haemoglobin, the carrying capacity would be 1.34 X 15 = 19.4 ml. Blood leaving tissue blood capillaries carries about 14.4 ml of oxygen/100 mI. It means that 5 ml of oxygen/l00 ml of blood is passed to tissues for respiration. Same will be the rate of O2 transport. However, during strenuous exercise as much as 15 ml of oxygen can be passed into tissues and only 4.4 ml of oxygen remains in 100 ml of venous blood. Of the total oxygen, 3% dissolves in plasma while 97% combines with haemoglobin to form a loose reversible complex called oxyhaemoglobin. A molecule of haemoglobin has four iron ions (Fe 2+ ), each of which can pick up a molecule of oxygen through coordinate bond O = O. The degree of saturation of haemoglobin with oxygen depends upon oxygen tension (Po2). 100% saturation is rare. Normally the maximum is 95% at Po2 found in alveoli (100 mm Hg). The degree of saturation decreases with the fall of Po2. Dissociation of haemoglobin begins. At 30 mm Hg of Po2, only 50% saturation can be maintained. The relationship between oxyhaemoglobin saturation and oxygen tension is called oxygen dissociation Curve. It is sigmoid. For myoglobin of muscles, the oxygen dissociation curve is hyperbolic. Oxygen-haemoglobin dissociation curve shifts to right by increase in H+ concentration, CO2 tension, temperature and diphosphoglycerate concentration of erythrocytes. It decreases oxygen carrying capacity of haemoglobin. Bohr effect is shift of oxygen haemoglobin dissociation curve by changes in blood carbon dioxide. Haldane Effect. Oxyhaemglobin functions as an acid. It decreases pH of blood. Increased number of H+ ions convert HC03 into H2O and CO2. Transport of Carbon Dioxide (7% as dissolved carbonic acid, 70% as bicarbonate and 23% as carbamino-haemoglobin) Carbon dioxide is produced in tissues. Consequently, its partial pressure is the highest in tissues. Carbon dioxide diffuses into tissue fluid and from there into blood. At 45 mm Hg, venous blood carries 2.7 ml of C02/100 ml. Arterial blood at 40 mm Hg of Peoz carries 2.4 ml of C02/l00 ml. Therefore, O. 3 ml of CO2 is transported per 100 ml of blood. In blood carbon dioxide is transported in three forms. (i) In Dissolved State. About 5 - 7% C02 is transported in dissolved form in plasma. (ii) As Bicarbonate. CO2 diffuses into erythrocytes where it first reacts with water to' form carbonic acid (H2C03)' The reaction is catalysed by carbonic anhydrase found in erythrocytes. It takes less than I second. Carbonic acid dissociates into HC03' (bicarbonate) and H+ ions. Normally, oxyhaemoglobin is slightly acidic and is balanced by K+ ion. H+ released by dissociation of H2C03' replaces K+ ion. It forms haemoglobinic acid (HHb). Most of bicarbonate passes out into plasma along concentration gradient. On the other hand, chlO1:ide ion present in plasma diffuses into erythrocytes. It combines with K+ ion to form KCI. The phenomenon is called chloride shift or Hamburger phenomenon. Bicarbonate in plasma combines with Na + to form NaHC03' It carries 70% of CO2' CO2 + H2O carbonic . H2C03 '" ..'- H+ + HC03' anhydrase KHb02 + H+ _ H. Hb + K+ + 02 erythrocyte _ KCI erythrocyte Na + + HC03' _ NaHC03 Plasma (iii) As Carbaminohaemoglobin. A part of CO2 passing into erythrocytes combines with amino groups (- NH2) of protein part of haemoglobin. It is a reversible process. The complex is called carbaminohaemoglobin. It carries 23% of total CO2' K+ + CI' . Release of CO2 in Alveolar Blood CO2 dissolved in plasma diffuses into alveolar air. Carbaminohaemoglobin splits up to release CO2' The same passes out of erythrocytes and diffuses into alveolar air. Ha_moglobin releases H+ as it combines with oxygen. There is dissociation of KCI and NaHC. 00
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