NORMAL VENTILATION

The primary function of the lungs is to provide oxygen to the blood and to remove carbon dioxide from it. The lungs are within the chest, which is enclosed on all sides by the ribs, cartilages, and the muscles between them.

To reach the lungs, air enters through the mouth and nose and then travels through the pharynx, larynx, and down trachea. As you know the trachea branches into two main bronchial tubes, or air passageways. Each bronchial tube then branches into smaller passageways (bronchi), which divide several times more, finally forming much smaller tubes (bronchioles). The branching creates the appearance of an upside-down tree. The smallest bronchioles end in tiny closed elastic air sacs called alveoli. The blood is carried to these air sacs by tiny blood vessels. The vessels, called pulmonary capillaries, release carbon dioxide from the blood into air sacs and at the same time absorb oxygen from the air sacs into the blood.

The pleura covers the outside of the lungs and the adjacent chest wall. This membrane allows the lungs to move easily within the chest cavity as the human breathes.

When you inhale, the muscles of ribs contract, causing the ribs to move upward and outward. At the same time, the diaphragm contracts, pushing down toward the abdomen. These two actions increase the size of the chest cavity and thus cause the lungs to expand and air to be sucked into them.

When you exhale, the diaphragm and rib muscles relax and return to their original positions. This decreases the size of the chest cavity, compressing the lungs slightly and forcing the air, now carrying carbon dioxide, out of the body. The whole process of breathing in and out occurs automatically without your thinking about it.

The lungs are connected to the heart by the pulmonary veins and arteries (the term "pulmonary" comes from the Latin word pulmo for "lung"). The blood travels throughout the body, returns to the heart, and is pumped by the right ventricle through the pulmonary artery to the lungs. The blood then passes through the arteries of the lungs into smaller vessels, similar to the branching of the bronchi. Finally, it flows into the smallest blood vessels, which are located in the membranes lining the alveoli. Capillaries are so tiny that often the cells of the blood have to pass through them single file.

After gases are exchanged in the alveoli, the blood, now carrying oxygen, passes into the smallest veins. These veins merge to form larger and larger vessels until the pulmonary veins are formed. The pulmonary veins carry oxygenated blood from the lungs back to the heart, from which it is pumped out again to deliver oxygen to the cells of the body and to remove carbon dioxide.