Question

How does exchange of respiratory gases take place in the alveoli of lungs?

Answer 1

Gas exchange is the intake of oxygen and release of carbon dioxide. Oxygen goes into the blood and carbon dioxide moves out if the blood. Gas exchange is surface dependent and alveoli in lungs is the site of it.

Complete answer:
Red blood takes up oxygen and releases carbon dioxide back to the lungs. RBCs transport oxygen to the rest of the body. Alveoli plays a major role in exchange of gases. They are tiny air sacs present in the lungs that take up the oxygen. They have a large surface area and are moist for gas exchange.
In the human body, tissues utilize the oxygen to produce ATP and carbon dioxide is released as a waste product. The ratio of carbon dioxide production to oxygen consumption is called respiratory quotient (RQ). The RQ is an important factor because it is used to calculate partial pressure of oxygen. The inhaled air mixes with residual air inside which lowers the partial pressure of oxygen within the alveoli which results in lower concentration of oxygen in the lungs than the atmosphere outside the body.
In the lungs, oxygen diffuses out of alveoli and into capillaries around it. Oxygen binds to the hemoglobin which is respiratory pigment found in RBCs. These RBCs carry oxygen to the tissues. Oxygen and carbon dioxide move independently of each other, they diffuse down their own partial gradients.
Simply, the change in partial pressure from the alveoli to the capillaries drives the oxygen into the tissues and carbon dioxide out of the tissues. The blood is then transported to lungs where differences in partial pressure result in movement of carbon dioxide out of the blood.

Note: Hemoglobin is an iron containing substance present in red blood cells. It has affinity to bind with oxygen and thus helps to transport oxygen from lungs to the rest of the body. One gram of hemoglobin has the affinity to combine with 1.39ml of blood.