Gas exchange in the human body occur in lungs. it is described as follows:-
When we inhale atmospheric air, through the nose it go to the trachea and then via bronchus and bronchioles it ultimately go to the alveolus where exchange of different gas takes place. Alveolus is the sac filled with air and it is functional unit of lung. Gaseous exchange occur here. This alveolus separated from blood by a membrane called Alveo-capillary membrane. It is the barrier between air and blood. Different gaseous overcome this barrier by Diffusion method. In diffusion method gaseous go from high concentrated area to low concentrated area. So, Oxygen which is more in concentration in alveolus and low in blood, go to the blood from alveolus and carbon-di-oxide which is more in concentration in pulmonary capillary & low in alveolus go to the alveolus from pulmonary capillary blood.
So, we can see that oxygen and carbon-di-oxide, this 2 major gas exchanged from alveolus to blood and from blood to alveolus respectively by diffusion in the lung. This exchange happening maximum in base of the lung and minimum in apex of the lung.
The human gas-exchange process transfers the gases contained by the human blood in exchange for the gases found in atmosphere. The human gas exchange process happens in the lungs and this process is accomplished by the mechanisms of the heart. The process of movement of the air in and out of the lungs is called ventilation.
Humans have closed circulatory system or two-circuit circulatory systems, the pulmonary circuit and the systemic circuit.
Hence, the pulmonary circulation represents the circulation of the deoxygenated blood from the right ventricle of the heart to the lungs, through pulmonary artery, for oxygenation purposes, and then, back, from lungs, through pulmonary veins, to the left atrium of the heart.
The systemic circulation transports the oxygenated blood from the heart to the body and it carries back the deoxygenated blood from the body to the heart. In the systemic circulation the arteries carry the oxygenated blood to the tissues and the veins transport the deoxygenated blood back to the heart.
The advantage of the systemic circulation is that the blood that is delivered throughout the body has the same pressure at every point.
The process of gas exchange in the human body is also known as the process of inspiration and expiration.
Inspiration is when you are breathing in. The diaphragm moves downward, allowing for the lungs to expand. The lungs expand to be filled with air and the air pressure in the alveoli (the final point of inspiration as air passed through the lungs) becomes less than atmospheric pressure. Because the atmospheric pressure within the alveoli is less, air is forced in by the higher atmospheric pressure on the outside of the body.
Expiration is when you are breathing out. The diaphragm relaxes and moves back up, the lungs contract, and the air pressure in the alveoli is now more than the pressure externally which causes the air to be forced out of the lungs. When the air moves out of the lungs you breath out in what is known as an exhalation.