The Respiratory System / The Respiratory System - Introduction

The Alveoli

The bronchioles divide many more times in the lungs to create an impressive tree with smaller and smaller branches, some no larger than 0.5 mm (0.02 in) in diameter. These branches dead-end into tiny air sacs called alveoli. The alveoli deliver oxygen to the circulatory system and remove carbon dioxide. Interspersed among the alveoli are numerous macrophages, large white blood cells that patrol the alveoli and remove foreign substances that have not been filtered out earlier. The macrophages are the last line of defense of the respiratory system; their presence helps ensure that the alveoli are protected from infection so that they can carry out their vital role.

The alveoli number about 150 million per lung and comprise most of the lung tissue. Alveoli resemble tiny, collapsed balloons with thin elastic walls that expand as air flows into them and collapse when the air is exhaled. Alveoli are arranged in grapelike clusters, and each cluster is surrounded by a dense hairnet of tiny, thin-walled capillaries. The alveoli and capillaries are arranged in such a way that air in the wall of the alveoli is only about 0.1 to 0.2 microns from the blood in the capillary. Since the concentration of oxygen is much higher in the alveoli than in the capillaries, the oxygen diffuses from the alveoli to the capillaries. The oxygen flows through the capillaries to larger vessels, which carry the oxygenated blood to the heart, where it is pumped to the rest of the body.

Carbon dioxide that has been dumped into the bloodstream as a waste product from cells throughout the body flows through the bloodstream to the heart, and then to the alveolar capillaries. The concentration of carbon dioxide in the capillaries is much higher than in the alveoli, causing carbon dioxide to diffuse into the alveoli. Exhalation forces the carbon dioxide back through the respiratory passages and then to the outside of the body.

Regulation

The flow of air in and out of the lungs is controlled by the nervous system, which ensures that humans breathe in a regular pattern and at a regular rate. Breathing is carried out day and night by an unconscious process. It begins with a cluster of nerve cells in the brain stem called the respiratory center. These cells send simultaneous signals to the diaphragm and rib muscles, the muscles involved in inhalation. The diaphragm is a large, dome-shaped muscle that lies just under the lungs. When the diaphragm is stimulated by a nervous impulse, it flattens. The downward movement of the diaphragm expands the volume of the cavity that contains the lungs, the thoracic cavity. When the rib muscles are stimulated, they also contract, pulling the rib cage up and out like the handle of a pail. This movement also expands the thoracic cavity. The increased volume of the thoracic cavity causes air to rush into the lungs. The nervous stimulation is brief, and when it ceases, the diaphragm and rib muscles relax and exhalation occurs. Under normal conditions, the respiratory center emits signals 12 to 20 times a minute, causing a person to take 12 to 20 breaths a minute. Newborns breathe at a faster rate, about 30 to 50 breaths a minute.

The rhythm set by the respiratory center can be altered by conscious control. The breathing pattern changes when a person sings or whistles, for example. A person also can alter the breathing pattern by holding the breath. The cerebral cortex, the part of the brain involved in thinking, can send signals to the diaphragm and rib muscles that temporarily override the signals from the respiratory center. The ability to hold one's breath has survival value. If a person encounters noxious fumes, for example, it is possible to avoid inhaling the fumes.

A person cannot hold the breath indefinitely, however. If exhalation does not occur, carbon dioxide accumulates in the blood, which, in turn, causes the blood to become more acidic. Increased acidity interferes with the action of enzymes, the specialized proteins that participate in virtually all biochemical reaction in the body. To prevent the blood from becoming too acidic, the blood is monitored by special receptors called chemoreceptors, located in the brainstem and in the blood vessels of the neck. If acid builds up in the blood, the chemoreceptors send nervous signals to the respiratory center, which overrides the signals from the cerebral cortex and causes a person to exhale and then resume breathing. These exhalations expel the carbon dioxide and bring the blood acid level back to normal.

A person can exert some degree of control over the amount of air inhaled, with some limitations. To prevent the lungs from bursting from overinflation, specialized cells in the lungs called stretch receptors measure the volume of air in the lungs. When the volume reaches an unsafe threshold, the stretch receptors send signals to the respiratory center, which shuts down the muscles of inhalation and halts the intake of air.

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