What is the procedure of Gas Exchange in human body?

Exchange of oxygen with the blood is a specialized process. If oxygen were carried in solution in blood plasma (the liquid part of the blood), the blood could carry only about three-tenths of a milliliter of oxygen per hundred milliliters. The heart would be required to pump about 5000 liters of blood plasma per hour. To increase the oxygen-carrying capacity of the circulation system, red blood cells contain hemoglobin. Oxygen diffusing across the membrane is bound to hemoglobin and carried out to the different parts of the body in peripheral circulation.

Whether or not hemoglobin picks up oxygen depends on the partial pressure of oxygen, or pO₂ in the plasma. When pO2 is high, hemoglobin carries a maximum load of four molecules of oxygen. When the blood circulates to the muscle tissues and cells where oxygen has been used up and pO₂ is low, hemoglobin releases the oxygen it is carrying. Hemoglobin transports about 95% of the oxygen in the blood.

Some of the carbon dioxide produced from the cellular respiration pathway binds to hemoglobin, and is returned to the lungs, but most is converted into bicarbonate, which dissolves in the plasma.

CO₂ + H₂O → H₂CO₃^-

When blood enters the lungs, CO₂ leaves the capillaries by diffusion, and is exhaled. Some CO₂ is carried by chemical combination with oxygenated hemoglobin and is called carboxyhemoglobin.