Transport of gasses

Oxygen is transported in the blood by a respiratory pigment called haemoglobin, which is a conjugated protein. It  consists of a large protein molecules, the globin consisting of 4 polypeptide chains (two alpha and two beta chains) to each of which attached a prosthetic heme group. Heme is based on a structure known as piphyrin ring structure which includes four pyrol groups around a central Fe.

The transport of oxygen by the blood from the lungs to the tissues is mainly due to the ability of haemoglobin to combine reversibly with oxygen. One molecule of haemoglobin has four heme groups, and each of Hb molecule is capable of taking up four molecules of oxygen. Haemoglobin is dark red whereas oxyhaemoglobin is bright red in colour. Oxy-haemoglobin on reaching the tissue dissociates into free oxygen and haemoglobin. Again oxygen diffuses into tissue because of difference in partial pressure. -

Transport of carbon dioxide from tissues to lungs: Small amount of carbon dioxide is transported in dissolved state in plasma of blood. About 70% of carbon dioxide produced enters R.B.C. by diffusion. Each decilitre (100 ml) of blood receives 3.7 ml of carbon dioxide from tissue and rest of carbon dioxide diffuses into blood plasma to form carbonic acid with water. Carbon dioxide is also transported as carbamino- haemoglobin (reduced haemoglobin) inside R.B.C

Oxy-haemoglobin behaves as a strong acid and as more and more oxy-haemoglobin is formed, it releases more of Hydronium ions which decrease pH of blood. Role of haemoglobin in transport of oxygen from lungs tissue.

In a normal healthy person there is about 15 gm of Haemoglobin per 100 ml of blood. The capacity of 1 gm of Hb to combine with oxygen is 1.34 ml. Thus on an average 100 ml of blood transports 20 ml (19.4) ml of oxygen. When oxygen diffuses into the blood in external respiration most of it enters the red blood or erythrocytes and unites with the haemoglobin in these cells forming a compound called oxy-haemoglobin. Haemoglobin is a complex protein compound containing iron which has a great affinity for oxygen. This property of haemoglobin enables the blood to carry about 50 times more oxygen than the plasma alone can carry. As the blood passes through the alveolar capillaries, the haemoglobin becomes saturated with oxygen. This reaction may be represented as follows:

Oxy-haemoglobin is a very unstable compound and, when the blood reaches the capillaries in the tissue, throughout the body where the oxygen concentration is low, the compound breaks down into hemoglobin and oxygen. The oxygen diffuses into the cells.

Oxy-haemoglobin has a bright scarlet colour while reduced haemoglobin and has dull purplish colour. This accounts for the diference in colour between arterial and venous blood.

Role of carbonic anhydrase enzyme in the transport of gases during respiration.

Carbonic anhydrase is an enzyme found in. the erythrocytes. It takes part in joining of carbon dioxide with water to form Carbonic acid. It is completed in less than one second.