10 The Respiratory System

Airways and breathing

Air usually enters through the nose or mouth, passes through the throat and larynx, and travels down the trachea, or windpipe. The trachea branches into two bronchi, one leading to each lung. Within the lungs, the bronchi divide into smaller bronchioles that end in clusters of tiny air sacs called alveoli. The nose helps warm and moisten incoming air.

During inhalation, the diaphragm contracts and moves downward as the chest expands. This increases the volume of the chest cavity and draws air into the lungs. When the diaphragm relaxes, the chest cavity becomes smaller and air flows out.

Gas exchange in lungs and tissues

Each alveolus is surrounded by a network of tiny blood vessels called capillaries. The walls of the alveoli and capillaries are very thin, allowing gases to diffuse across them. Because oxygen is more concentrated in alveolar air than in the incoming blood, $\mathrm{O_2}$ diffuses into the blood. Carbon dioxide moves in the opposite direction, from the blood into the alveoli, and is then exhaled. This exchange between alveoli and blood is called external respiration.

Most oxygen travels through the bloodstream attached to hemoglobin, a protein in red blood cells. At body tissues, oxygen leaves the blood and enters cells, while $\mathrm{CO_2}$ produced by cells enters the blood. This exchange between blood and tissues is called internal respiration. Much of the carbon dioxide is carried in the blood as bicarbonate; in the lungs it is converted back to $\mathrm{CO_2}$ and exhaled.

How circulation transports gases

Blood low in oxygen and carrying $\mathrm{CO_2}$ returns to the right side of the heart. The heart pumps it through the pulmonary arteries to capillaries around the alveoli. After gas exchange, oxygen-rich blood returns to the left side of the heart through the pulmonary veins. The heart then pumps this blood through the systemic circulation to the body. Pulmonary arteries carry blood toward the lungs, while pulmonary veins carry blood away from them.

The lungs exchange gases with the blood, and the heart and blood vessels carry those gases between the lungs and body tissues. Together, the respiratory and circulatory systems deliver oxygen to tissues and return carbon dioxide to the lungs for removal.

Breathing during exercise

During exercise, working muscles use more oxygen and produce more $\mathrm{CO_2}$. Breathing and heart rate increase, helping deliver oxygen to those muscles and transport additional carbon dioxide back to the lungs for removal. The brain also adjusts breathing in response to changes in blood gases, especially carbon dioxide.