1. Energy Production:

All living beings require energy to carry out various activities of the body, its parts and cells. The energy required for daily metabolic activities is derived from the oxidation of food going on continuously in the body.

2. Excretion: Respiration excretes carbon dioxide, water, etc.

3. Maintenance of Acid-base Balance: Elimination of CO2 maintains the acid-base balance in the body.


4. Maintenance of Temperature: A large amount of heat is expelled out during expiration which maintains the body temperature.

5. Return of Blood and Lymph: During inspiration the intra-abdominal pressure increases and the intrathoracic pressure decreases. This results the return of blood and lymph from the abdomen to the thorax.

Exchange of Gases :

The air reaches the alveoli of the lungs during the inspiration. The atmospheric air contains. Oxygen 20.9 per cent Carbon dioxide 0.04 per cent Nitrogen and other inert gases 79 per cent Water Vapour Variable (0 Gases always diffuse from an area of higher concentration to the area of lower concentration. Gases always exert pressure upon all the walls of their container. Gases always fill their container completely. (iii) The molecules of gases are always in motion.


The respiratory gases such as oxygen and carbon dioxide move freely by the process of diffusion. 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 P02, PC02, PN2 for oxygen, carbon dioxide and nitrogen respectively.

Exchange of Gases of Lungs :

The pulmonary arteries carry deoxygenated (venous) blood to the lungs and the pulmonary veins carry oxygenated (arterial) blood from the lungs. In the lungs, exchange of gases takes place between alveoli and blood capillaries. This is also called external respiration.

The wall of the alveoli is very thin and has rich network of blood capillaries. Due to this, the alveolar wall seems to be a sheet of flowing blood and is called respiratory membrane alveolar- capillary membrane). The respiratory membrane consists mainly of- (a) alveolar epithelium; (b) epithelial basement membrance; (c) a thin interstitial space; (d) capillary basement membrane and (e) capillary endothelium. All these layers form a membrane of 0.2 mm thickness. The respiratory membrane has a limit of gaseous exchange between alveoli and pulmonary blood. It is called diffusing capacity. The diffusing capacity is defined as the volume of gas that diffuses through the membrane per minute for a pressure difference of 1mm 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. The partial pressure of oxygen (P02) in the alveoli is higher than that in the deoxygenated blood in the capillaries of the pulmonary arteries. As the gases diffuse from a higher to a lower concentration, the movement of oxygen is from the alveoli to the blood. The reverse is the case in relation to carbon dioxide. The partial pressure of carbon dioxide (PC02) is higher in deoxygenated blood (46 mm Hg) then in alveoli, (40 mm Hg), therefore, carbon dioxide passes from the blood to the alveoli. The partial pressure of nitrogen (PN2) is the same (573 mm Hg) in the alveoli as it is in the blood. This condition is maintained because nitrogen as a gas is not used up by the body.

Exchange of Gases in Tissues :

The oxygenated blood is sent from the blood capillaries to the heart. The heart distributes this oxygenated blood to various body parts through arteries. The arteries divide to form arterioles. The latter further divide to form capillaries. The exchange of oxygen and carbon dioxide between tissue blood capillaries and tissue cells takes place which is also called internal respiration. The partial pressure of oxygen is higher (95 mm Hg) than that of the body cells (20 mm Hg) and the partial pressure of carbon dioxide is lesser (46 mm Hg) than that of the body cells (52 mm Hg). Therefore, oxygen diffuses from the capillary blood to the body cells through tissues fluid and carbon dioxide diffuses from the body cells of the capillary via tissue fluid. Now the blood becomes deoxygenated. The latter is carries to the heart and hence to the lungs.

Transport of Gases in Blood :


Blood carries oxygen from the lungs to the heart and from the heart to various parts. The blood also brings carbon dioxide from the body parts to the heart and then to the lungs.

Transport of Oxygen :

(i) As dissolved gas: About 3 per cent of oxygen in the blood is dissolved in the plasma which carries oxygen to the body cells.

(ii) As oxyhaemoglobin: About 97% of oxygen is carried in combination with haemoglobin of the erythrocytes. Oxygen and haemoglobin combine in an easily reversible reaction to form oxyhaemoglobin.


Heamoglobin Oxygen Oxyhaemoglobin Under the high partial pressure, oxygen easily binds with haemoglobin in the pulmonary (lung) blood 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 blood capillaries.

A normal person has about 15 grams of haemoglobin per millilitres of blood. 1 gram of haemoglobin binds about 1.34 ml of oxygen. Thus on an average 100 ml of blood carries about 20 ml (19.4 ml exactly) of 02. Hence under normal conditions, about 5 ml of oxygen is transported buy 100 ml of blood.

During exercise or under strenuous conditions, the muscles cells consume more oxygen, The partial pressure of oxygen in the tissue falls, as a result of which, the blood at the tissue level has only 4.4 ml of oxygen/v100 ml of blood. Thus about 15 ml of oxygen is transported by haemoglobin during exercise.

Oxygen-haemoglobin Dissociation Curve: The percentage of haemoglobin that is bound with 02 is called percent saturation of haemoglobin. It depends upon the P02 in the blood. P02 in the oxygenated blood leaving the lungs is about 95 to 97 ml Hg, and at this P02, haemoglobin is about 97% saturated with 02. The P02 in deoxygenated blood returning from body tissues is only 40 mm Hg and at this P02, haemoglobin is only about 70% to 75% saturated with 02. This relationship between P02 and percentage saturation of haemoglobin with 02 is graphically represented by curve called oxygen-haemoglobin dissociation curve, which is a typical sigmoid due to this relationship. In other Words this curve shows how oxygen is loaded and unloaded due to partial pressure. This curve is dependent on P02, PC02, temperature and pH.


Bohr Effect: In an acid medium oxygen splits more readily from heamolobin. The effect of increased acidity on heamoglobin is called Bohr effect named after the Danish physiologist Christian Bohr. The Bohr effect facilitates the release of 02 from oxyhaemoglobin in the tissues where C02 production lowers pH. Thus the overall result of Bohr effect is to allow greater oxygen transport from lungs to the tissues because the amount of oxygen that binds with haemoglobin considerably increases.

Transport of Carbon Dioxide :

In the oxidation of food, carbon dioxide, water and energy are produced. Carbon dioxide in gaseous form diffuses out of the cells into the capillaries, where it is transported in three ways. (z) As dissolved gas: Because of its high solubility, about 7 percent carbon dioxide gets dissolved in the blood plasma ana is carried in solution to the lungs.

Deoxygenated (Venous) blood (PC02 is 40 mm Hg) and oxygenated (arterial) blood (PC02 is 40 mm Hg) carry about 2.7 ml and 2.4 ml of C02 per 100 ml of blood in dissolved state in plasma respectively. Hence about 0.3 (2.7 minus 400 ml, 2.4) ml of C02 is transported per 100 ml of blood in dissolved state in blood plasma. This is about 7% of all the C02 transported by blood from tissues to the lungs. (ii) As bicarbonate ions’. The dissolved carbon dioxide in the blood reacts with water to from carbonic acid. This reaction is very slow in blood plasma, but occurs very rapidly inside RBCs because a zinc-containing enzyme, the carbonic anhydrase, present in RBCs, accelerates its rate about 5000 times. Due RBCs where it reacts with water to form Carbonic acid (H2 C03).

Most of the hydrogen ions then combine with the haemoglobin in the RBCs because heamoglobin is a powerful acid-base buffer. In turn, many of the bicarbonate ions diffuse into the blood plasma while chloride ions diffuse into the RBCs. This is made possible by the presence of a special bicarbonate-chloride carrier protein in the RBC membrane that moves there two ions in opposite directions at rapid velocites. Thus, the chloride content of venous (deoxygenated) RBCs is greater than that of arterial (oxygenated) RBCs.

Chloride shift (=Hamburger’s phenomenon). Exit of bicarbonate ions, considerably change ionic balance between the plasma and the erythrocytes (RBCs). To restore this ionic balance, the chloride ions diffuse from the plasma into the erythrocytes. This movement of chloride ions is called chloride shift (=Hamburger’s phenomenon). The latter maintains an acid base equilibrium of pH 7.4 for the blood and electrochemical balance between erythrocytes and plasma. The chloride ions (Cl~) inside RBC combine with potassium ion (K+) to form potassium chloride (KC1), whereas hydrogen carbonate ions (HC03) in the blood plasma combine with Na+ to form sodium hydrogen carbonate (NaHC03). (iii) As carbamino heamoglobin: In addition to reacting with water, carbon dioxide also reacts directly with amino radicals (NH2) of haemoglobin to form an unstable compound carbamino heamoglobin(C02 HHb, also written HbCOz). This is reversible reaction. A small amount of carbon dioxide also reacts in the same way with the plasma proteins. About 23 percent C02 is transported in combination with haemoglobin and plasma proteins.

Release of carbon dioxide in the alveoli of lung: The pulmonary arteries carry deoxygenated blood to the lungs. This blood contains carbon dioxide as dissolved in blood plasma, as bicarbonate ions and as corbomino heamoglobin.

(i) C02 is less soluble in arterial blood than in venous blood. Therefore, some C02 diffuses from the blood plasma of the lung capillaries into the lung alveoli.

(ii) For the release of C02 from the biocarbonate a series of reverse reactions takes place. When the haemoglobin of the lung blood capillaries takes up 02, the H+ is released from it. Then, the Cl~ and HC03 ions are released from KC1 blood, and NaHC03 in the RBC respectively. After this HC03 reacts with H+ to form H2C03. As a result H2C03 splits into carbon dioxide and water in the presence of carbonic anhydrase enzyme and C02 is released into the alveoli of the lungs.

(iii) High P02 in the lung blood capillaries due to oxygenation of haemoglobin favours separation of C02 from carbomino haemoglobin.

Haldane Effect: Binding of oxygen with haemoglobin tends to displace carbon dioxide from the blood. This effect is called Haldane effect (J.S. Haldane, a Scotish physiologist). It is quantitatively far more important in promoting carbon transport than is the Bohr effect in promoting oxygen transport.