The lungs are two cone-shaped organs located in the thoracic cavity, or chest, and are separated by the heart. The right lung is somewhat larger than the left. The pleural membrane surrounds and protects the lungs. One layer of the pleural membrane attaches to the wall of the thoracic cavity, and the other layer encloses the lungs. A fluid between the two membrane layers reduces friction and allows smooth movement of the lungs during breathing. The lungs are divided into lobes, each one of which receives its own bronchial branch. The bronchial branch subdivides and eventually leads to the terminal bronchi. These tiny airways lead into structures called respiratory bronchioles.
The respiratory bronchioles branch into alveolar ducts that lead into outpocketings called alveolar sacs. Alveoli, tiny expansions of the wall of the sacs, form clusters that resemble bunches of grapes. The average person has a total of about 300 million gas-filled alveoli in the lungs. These provide an enormous surface area for gas exchange. Spread flat, the average adult male's respiratory surface would be about 750 sq ft (70 m2), approximately the size of a handball court. Arterioles and venules make up a capillary network that surrounds the alveoli. Gas diffusion occurs rapidly across the walls of the alveoli and nearby capillaries. The alveolar-capillary membrane together is extremely thin, about 0.5 in (1.3 cm) thick.
The rate of external respiration in the lungs depends on several factors. One is the difference in concentration (partial pressure) of the respiratory gases in the alveolus and in the blood. Oxygen diffuses out of the alveolus into the blood because its partial pressure is greater in the alveolus than in the capillary. In the capillary, oxygen binds reversibly to hemoglobin in red blood cells and is transported to body tissues. Carbon dioxide diffuses out of the capillary and into the alveolus because its partial pressure is greater in the capillary than in the alveolus. In addition, the rate of gas exchange is higher as the surface area is larger and the membrane thinner. Finally, the diffusion rate depends on airflow. Rapid breathing brings in more air and speeds up the gas exchange.
The result of external respiration is that blood leaves the lungs laden with oxygen and cleared of carbon dioxide. When this blood reaches the cells of the body, internal respiration takes place. Under a higher partial pressure in the capillaries, oxygen breaks away from hemoglobin, diffuses into the tissue fluid, and then into the cells. Conversely, concentrated carbon dioxide under higher partial pressure in the cells diffuses into the tissue fluid and then into the capillaries. The deoxygenated blood carrying carbon dioxide then returns to the lungs for another cycle.
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