CBSE Class 10 Answered

The air that we breathe in comprises of oxygen, nitrogen and a mix of several different gases. All the air goes inside the lungs when we breathe in. It is only in the lungs that they get separated. Inhaled oxygen enters the lungs and reaches the alveoli. The layers of cells lining the alveoli and the surrounding capillaries are each only one cell thick and are in very close contact with each other. This barrier between air and blood averages about 1 micron in thickness. Oxygen passes quickly through this air-blood barrier into the blood in the capillaries.

Haemoglobin in our RBCs is an iron pigment that has high affinity for oxygen. Therefore the oxygen is extracted from the air and taken into the blood stream. Since there are no special pigments to absorb them, the other constituents of the air remain in the lungs and are then exhaled from the lungs , they are not stored anywhere else. Nitrogen is a relatively inert gas and doesn't readily react.

Another factor that governs the uptake and release of both Oxygen and Carbon Dioxide, is partial pressure of these gases in the lungs and blood stream.