The act of breathing is done by the lungs expanding and contracting in order to draw air in and then push out of the lung.
The chest cavity, which contains the heart and lungs, is bounded on both sides by the ribs and, at the bottom, by the diaphragm. The diaphragm is a muscular plate which stretches from one side of the body to the other and divides the chest cavity from the abdominal cavity which contains the stomach and intestines.
Inside the chest cavity, there is a slight vacuum or negative pressure. By expanding the ribs outwards and pulling the diaphragm downwards, the size of the chest cavity is increased; the vacuum reduced; and the lungs can expand pulling air in. This process is known as inspiration. Try it for yourself. Take a deep breath and you will feel your ribs moving outwards. The whole movement, both of the ribs and the diaphragm, is muscular.
When more air is needed, as when running, the muscles between the ribs work harder and expand the ribs further. When both lungs have been filled with air, the diaphragm and ribs are relaxed and push inwards towards the lungs. This increases the vacuum inside the chest cavity by making it smaller. The lungs contract and push the air out - this process is known as expiration.
Blood which is low in oxygen and high in carbon dioxide is brought to the lungs by the pulmonary artery. This artery branches out and ends in capillaries surrounding the alveoli. The air in the alveoli, which has just been breathed in, is rich in oxygen.
A very rapid exchange now takes place. Oxygen passes through the thin walls of the alveoli into the capillaries while carbon dioxide passes in the opposite direction into the alveoli. The blood, now rich in oxygen, leaves the lungs through the pulmonary vein, which carries it to the heart from where it is pumped around the body and used up in the production of energy.
Rate and depth of breathing
Respiration is normally an involuntary movement which goes on without any conscious thought. However, humans do have some voluntary control (e.g. taking deep breaths, holding the breath and so on) although this can only be done for short periods.
The rate and depth of breathing is controlled by the respiratory centre situated in the brain. This centre is stimulated by the levels of carbon dioxide in the blood. When the carbon dioxide level rises, the speed and depth of breathing is increased to expel this and increase the amount of oxygen.
What are Lungs?
Each lung is a cone-shaped, soft, spongy mass of tissue consisting of tubes and air sacs. Lungs are elastic and can expand and contract as air is breathed in and out. Lungs are also well supplied with blood vessels which divide into many thread-like capillaries. The lungs lie within a special cavity in the body called the pleural cavity (pleural means "rib" in Greek). Outside each lung is a slippery membrane called the pleural membrane. This membrane allows the lungs to slide more easily during breathing.
At birth the lungs are pinkish white in color; in adult life the color is a dark gray, mottled in patches; and as age advances, this mottling assumes a black color. The coloring matter consists of granules of a carbonaceous substance deposited in the alveolar tissue near the surface of the organ. It increases in quantity as age advances, and is more abundant in males than in females. As a rule, the posterior border of the lung is darker than the anterior. The right lung usually weighs about 625 gm., the left 567 gm., but much variation is met with according to the amount of blood or serous fluid they may contain. The lungs are heavier in the male than in the female, their proportion to the body being, in the former, as 1 to 37, in the latter as 1 to 43.
The lungs are composed of an external serous coat, a subserous alveolar tissue and the pulmonary substance or parenchyma.
The serous coat is the pulmonary pleura. This is a thin, transparent, and invests the entire organ as far as the root. The subserous alveolar tissue contains a large proportion of elastic fibers; it invests the entire surface of the lung, and extends inward between the lobules. The parenchyma is composed of secondary lobules which, although closely connected together by an interlobular alveolar tissue, are quite distinct from one another, and may be teased asunder without much difficulty in the fetus. The secondary lobules vary in size; those on the surface are large, of pyramidal form, the base turned toward the surface; those in the interior smaller, and of various forms. Each secondary lobule is composed of several primary lobules, the anatomical units of the lung. The primary lobule consists of an alveolar duct, the air spaces connected with it and their blood and lymph vessels and nerves.