Gaseous Exchange in Mammals

 Ventilation of the lungs

In mammals, the structure of the thorax is designed to bring about the ventilation of the lungs. The ribcage, intercostal muscles and diaphragm work together to draw air into and out of the lungs. How this ventilation mechanism works in humans is described.

 Gaseous exchange in the alveoli

Inhaler air in the alveoli is rich in oxygen and poor in carbon dioxide. Blood flowing to the capillaries in the alveoli, k. The other hand, is poor in oxygen and rich in carbon dioxide. These differences in the concentrations of the gases in the alveolar air and blood produce steep diffusion gradients, driving oxygen into the blood stream and carbon dioxide into the alveolar air space. The two layers of cells that separate the air in the alveolar spaces and the blood in the capillaries offer minimal resistance to this gaseous diffusion.

     Oxygen dissolves in the film of moisture lining the alveolar wall before it diffuses into the blood. Diffusion of oxygen is greatly enhanced by the presence of haemoglobin in the red blood cells. This is because as soon as the oxygen diffuses into the blood, it enters the red blood cells and form a loose complex with hemoglobin known as oxyhaemoglobin. This keeps the concentration of free oxygen in blood low, thereby maintaining a steep diffusion gradients even after a large amount of oxygen has diffused into the blood. In fact , blood containing haemoglobin can absorb about 70 times more oxygen than blood without the oxygen carrying pigment.

   The carbon dioxide waste from the cells is transported in the blood plasma ad hydrogen carbonate ions. In the capillaries lining the alveoli, the hydrogen carbonate ions breaks down to carbon dioxide and water. The dissolved carbon dioxide diffuses into the alveolar air space and escapes as a gas.

      Inhaled air contains about 21% oxygen. However, not all this oxygen is removed during gaseous exchange on the alveoli. Exhaled air contains more carbon dioxide, water and best than inhaled air.

 Gaseous exchange in body cells

, Oxygenerated blood from the lungs is pumped by the heart to all parts of the body. In the body cells, which carry out cellular respiration continuously, the concentration of oxygen is low and that of carbon dioxide is high. Oxyhaemoglobin breaks down under these condition to release oxygen. As a result , the diffusion gradients for carbon dioxide and oxygen between the body cells and blood is steep. This causes the oxygen in the blood to diffuse into the body cells and carbon dioxide in the cells to diffuse out into the blood.

 Lung capacity and breathing rate

 Our lungs do not become completely empty. In an average adult, after a normal expiration about 3000cm3 of air still remains in them. Of this volume , about 1500 cm3 can be expelled forcibly. The remaining 1500 cm3 can never be expelled naturally and is known as residual air. Each time a normal inspiration or expiration occurs, about 500cm3 of air moves in or out of the lungs. This is the tidal air, after a normal inspiration, a further 200cm3 of air may be taken in forcibly. Hence, our lungs are capable of holding up to to 5500cm3 of air this volume is known ad the total lung capacity.

    An average person breathes about 16 times a minute. During exercise, this can increase to about 20 to 30 breathe per minute. The depth of breathing can also increase. ( the heart too pumps blood more rapidly during exercise). This allows more oxygen to be absorbed and transported to the active muscles. At the same time. The extra amount of carbon dioxide formed in the muscle cells can also ne got rid of rapidly by the increased rage and depth of breathing.