One of the most characteristic symptoms of aging is shortness of breath, a sign of the reduced ability of the respiratory system to collect oxygen and deliver it to the bloodstream. Among the structural changes that lead to shortness of breath and other respiratory problems with aging are the decrease and weakening of chest wall muscles, modifications in collagen in lung tissue and blood vessel walls, increase in the size of alveoli (air cells in lungs) and bronchioles, calcification of the cartilage in the trachea and bronchial tubes, cross-linkages in the collagen contained in the walls of the alveoli, and thickening and stiffening of blood vessels that bring carbon dioxide to the lungs and take oxygen out. The result is reduced expansion of the lungs, rigidity of the trachea and bronchial tubes, and reduced elasticity of the alveoli walls. Measures such as vital capacity (maximum one-breath capacity), maximal oxygen uptake, oxygen content of the blood, basal oxygen consumption, and pulmonary tissue compliance (nonrelaxation after inspiration) decrease, whereas residual lung volume and time to return to normal breathing after exertion increase. The heart and lungs make up an interactive system, with decrements in the functioning of one organ leading to decrements in functioning of the other. Thus, carbon dioxide in blood entering the lungs from the heart by way of the pulmonary arteries is removed, and oxygen needed by the heart and other body structures is supplied by the lungs. In turn, the rate and volume with which blood reaches the lungs are controlled by the heart.
As is true of the cardiovascular system as well, the functioning of the respiratory system is affected not only by the rate of primary aging but also by lifestyle and environment. Cigarette smoking and living in a polluted, chemically hazardous environment, in particular, have debilitating effects on the efficiency with which the lungs function in getting carbon dioxide out of the body and oxygen into it.
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