Text 5. Respiratory system

We usually think of respiration as the mechanical process of breathing, that is, the repetitive and, for the most part, unconscious exchange of air between the lungs and the external environment. This exchange of air at the lungs is also called external respiration. In external respiration, oxygen is inhaled (air inhaled contains about 21 per cent oxygen) into the air spaces (sacs) of the lungs and immediately passes into tiny capillary blood vessels surrounding the air spaces. Simultaneously, carbon dioxide, a waste product of the chemical combination of oxygen and food in cells, passes from the capillary blood vessels into the air spaces of the lungs to be exhaled (exhaled air contains about 16 per cent carbon dioxide).

While external respiration occurs between the outside environment and the capillary bloodstream of the lungs, another form of respiration is occurring simultaneously between the individual body cells and the tiny capillary blood vessels which surround them. This process is called internal (cellular) respiration. Internal respiration is the exchange of gases not at the lungs but at the cells within all the organs of the body. In this process, oxygen carried in the blood from the capillaries of the lung to the capillaries surrounding body cells passes out of the bloodstream and into the cells. At the same time, carbon dioxide, the waste produced in cells as oxygen chemically combines with food, passes from the tissue cells into the bloodstream and is carried by the blood back to the lungs to be exhaled.

Pathway of air from the nose to the capillaries of the lungs:

Nose

Nasal cavities and paranasal sinuses

Pharynx

Larynx

Trachea

Bronchi

Bronchioles

Alveoli

Lung capillaries (bloodstream)