The respiration occurs in two phases namely 1. Inspiration and 2. Expiration. The oxygen is taken inside during inspiration while carbon dioxide is expelled out during expiration.
During inspiration the external intercostal muscles contract, so that the ribs are bent forward and outward and the sternum moves forwards. At the same time the diaphragm contracts and becomes flat. As a result the thoracic cavity increases in all dimensions. The volume of the lungs also increases as the air rushes into them through the respiratory passage. This occurs until the pressure of air in the lungs becomes equal to that of the atmosphere. The air passes through external nares, nasal chamber, internal nares, pharynx, glottis, larynx, trachea, bronchi, bronchioles, air sacs and ultimately into alveoli. While air passes through nasal chambers, it becomes warm, moist, sterilized and free from dust particles.
Exchange of gases takes place in the alveoli. The blood flowing in the walls of the alveoli is very nearer to the fresh air. Oxygen from the air diffuses into the blood and the carbon dioxide of the blood diffuses into the air through the thin walls of alveoli. Thus the air of alveoli becomes impure and the carbon dioxide is to be sent outside immediately.
During expiration the intercostal muscles relax. The ribs are bent inwards and backwards and the sternum comes to its normal position. The diaphragm also relaxes and becomes dome shaped. As a result the volume of the thoracic cavity decreases, producing a huge pressure on the lungs which are then compressed. Thus the air with carbondioxide is expelled out from the lungs into bronchi, trachea nasal passage and ultimately to outside through external nostrils.
During expiration, the lungs never become completely empty and some amount of residual air always remains in them. This air is called Residual air.
TRANSPORT OF RESPIRATORY GASES
The intake of oxygen and the expulsion of carbon dioxide by the blood of alveolar capillaries can be explained by diffusion. The gases pass from regions of high pressure to those of low pressure. The partial pressure of O2 in the alveolar air is about 100 mm Hg. While that of atmospnere is 159 mm Hg. The partial pressure of O2 in the blood of alveolar capillaries is 40 mm Hg. Accordingly the oxygen diffuses from alveoli into the blood of alveolar capillaries.
Similarly the partial pressure of carbon dioxide in the alveolar capillaries is 46 mm Hg while that of alveoli is 40 mm Hg. This results in the diffusion of CO2 out of the blood into alveoli. Thus the diffusion gradient in the diffusion of O2 and CO2 determines the direction of their flow.
Transport of oxygen:
Oxygen from the capillaries of alveoli is transported to the level of tissues through the blood. Haemoglobin present in R.B.C helps in transportation of O2 to different organs. Haemoglobin is made up of four subunits, each of which comprises a haeme and a polypeptide chain (globin). The haeme consists of a porphyrin ring with one atom of iron in the center. The iron of each haeme unit can combine with one molecule of oxygen. Thus each haemoglobin molecule can carry four molecules of oxygen. The oxygen molecules are added one at a time.
Hb4 + O2 <-------> Hb4O2
Hb4O2 + O2 <-------> Hb4O4
Hb4O4 + O2 <-------> Hb4O6
Hb4O6 + O2 <-------> Hb4O8
Combination of the first subunit of Haemoglobin with O2 increases the affinity of the second, and oxygenation of the second increases the affinity of the third and so on. The oxyhaemoglobin is transported, to the tissues along with blood. The oxygen pressure in the arterial blood is 100 mm Hg while in the cells it is 1 to 40 mm Hg . Accordingly O2 is dissociated from the oxyhaemoglobin and diffuses out from the blood through the capillary walls into the cells. This is made possible because the combination of O2 with haemoglobin is reversible. The reduced haemoglobin is further transported through the blood to the lungs to repeat the cycle once again. The O2 diffused into the tissues is involved in oxidation of carbohydrates to release CO2 along with energy.
At high O2 pressure, the haemoglobin combines with O2 to form oxyhaemoglobin. At low O2 pressure oxyhaemoglobin dissociates to liberate O2 that diffuses into tissues. At any given O2 concentration, there is definite proportion between the amount of haemoglobin and oxyhaemoglobin.
Transport of carbon dioxide:
Carbon dioxide is released from the tissues as a result of metabolic activities and diffuses into the blood. Carbon dioxide is transported in the blood by the following three ways.
i. Transport of CO2 in the form of carbonic acid:
As carbon dioxide enters the blood from the tissues, it combines with water of the plasma to form carbonic acid. About 5% of CO2 is carried in the plasma as carbonic acid.
CO2 + H2O <-------> H2CO3
ii. Transport of CO2 as carbaminocompounds:
About 10% of the CO2 is transported in the form of carbaminocompounds. It combines directly with the amino group of (-NH2) haemoglobin to form carbaminohaemoglobin.
CO2 + Hb.NH2 <-------> Hb.NH.COOH
iii. Transport of CO2 as bicarbonates:
About 85% of the total CO2 is carried in the form of bicarbonates in the plasma and red blood cells. As CO2 enters the blood cells from the tissues, it combines with water to form carbonic acid (H2CO3). It dissociates into hydrogen ions (H+) and bicarbonate ions (HCO3-). The latter diffuse into the plasma and forms sodium and potassium bicarbonates with sodium and potassium ions.
H2CO3 <-------> H+ + HCO3-
Na+ + HCO3 <-------> NaHCO3
K+ + HCO3e <-------> KHCO3
The red blood cells contain an enzyme called carbonic anhydrase. This increases the speed of reaction between CO2 and H2O resulting in the formation of H2CO3 that is further converted into bicarbonates. Carbonic anhydrase also catalyzes the reversible reaction that is spliting of carbonic acid into water and carbondioxide.
H2CO3 <-------> CO2 + H2O