Exchanging Oxygen and Carbon DioxideThe primary function of the respiratory system is to exchange oxygen and carbon dioxide. Inhaled oxygen enters the lungs and reaches the alveoli. The cells lining the alveoli and the surrounding capillaries are each only one cell thick and are in very close contact with each other. This barrier averages about 1 micron (1/10,000 of a centimeter) in thickness. Oxygen passes through this air-blood barrier quickly and into the blood in the capillaries. Similarly, carbon dioxide passes from the blood into the alveoli and is then exhaled. Oxygenated blood travels from the lungs through the pulmonary veins and into the left side of the heart, which pumps the blood to the rest of the body (see Section 3, Chapter 20). Oxygen-depleted, carbon dioxide-rich blood returns to the right side of the heart through two large veins, the superior vena cava and the inferior vena cava. Then the blood is pumped through the pulmonary artery to the lungs, where it picks up oxygen and releases carbon dioxide (see Section 3, Chapter 20). To support the exchange of oxygen and carbon dioxide, about 6 to 10 liters of fresh air per minute are brought into the lungs, and about three tenths of a liter of oxygen is transferred from the alveoli to the blood each minute, even when the person is at rest. At the same time, a similar volume of carbon dioxide moves from the blood to the alveoli and is exhaled. During exercise, it is possible to breathe in as much as 100 liters of air per minute and extract 3 liters of oxygen from this air per minute. The rate at which oxygen enters the body is one useful measure of the total amount of energy expended by the body.
Three processes are essential for the transfer of oxygen from the outside air to the blood flowing through the lungs: ventilation, diffusion, and perfusion. Ventilation is the process by which air moves in and out of the lungs. Diffusion is the spontaneous movement of gases, without the use of any energy or effort by the body, between the gas in the alveoli and the blood in the pulmonary capillaries. Perfusion is the action by which the cardiovascular system pumps blood throughout the lungs. The body's circulation is an essential link between the atmosphere, which contains oxygen, and the cells of the body, which consume oxygen. For example, the delivery of oxygen to the muscle cells throughout the body depends not only on the lungs but also on the ability of the blood to carry oxygen and on the ability of the circulation to transport it.
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