5.5: Age Changes Affecting Ventilation

Because everyone's airways are subjected to some air pollution and other environmental insults, it is difficult to know which age‑related changes in airways are due to aging and which are due to other factors. However, certain changes seem to occur in all people. These universal changes are considered age changes and thus are included in this section. Let us examine how age changes affect the five requirements for ventilation.

Open Airways

Mucus and Cilia

All airways from the nasal cavities to the smallest bronchioles produce mucus continuously. With aging, the mucus produced is more viscous and therefore more difficult to move. In addition, both the number and the rate of motion of the cilia decrease. As the clearing out of mucus slows, the accumulation of mucus narrows airways, and this inhibits ventilation. When ventilation becomes more difficult, the work of breathing increases and extra CO₂ is produced by the muscles of ventilation, making respiration less efficient. Narrower airways also reduce the rate of airflow and therefore reduce the maximum possible minute volume.

Airway Structure

Age changes in the walls of bronchioles cause them to become even narrower, amplifying the effect of mucus accumulation. In addition, the bronchioles close earlier during expiration, trapping more air in the smaller airways and in the alveoli, especially in the lower parts of the lung. One result is an increase in residual air. This causes the fresh air entering with each inspiration to be mixed with a larger amount of stale residual air, decreasing the rate of diffusion. A second result is uneven lung ventilation.

While the bronchioles become narrower, the larger airways in the lungs and the alveolar ducts increase in diameter. These changes compound the negative effects by increasing the dead space. Thus, fresh inspired air is mixed not only with more residual air in the smallest airways and alveoli but also with more dead space air. This further decreases the rate of diffusion.

The increase in tidal volume with age may help minimize the expected drop in the diffusion rate during tidal breathing by mixing more fresh air with the increasing amounts of stale air remaining in the respiratory system. The rate of diffusion remains high because the O₂ concentration in the lungs is kept high while the CO₂ levels are kept low.

Defense Mechanisms

Since age changes in the mucus and cilia cause slower movement of mucus, harmful materials such as microbes, particles, and noxious chemicals trapped by the mucus stay in the respiratory system longer. Aging also decreases the functioning of other defense mechanisms, including reflexes (see below), white blood cells, and the immune system. All these age changes cause an increase in the risk of developing respiratory infections and other respiratory problems.

Proper Pressure Changes

Muscles

As with most muscles, aging causes respiratory muscles to become weaker. The decrease in muscle strength is not enough to detract from performing tidal breathing or ventilating at moderately increased minute volumes. However, it slowly decreases the maximum pressure changes that the muscles can produce and thus decreases the maximum rate of airflow attainable.

Skeletal System

Age changes in the cartilage, bones, and joints of the thorax also reduce a person's ability to produce large pressure changes in the thoracic cavity. The cartilage attaching the ribs to the sternum becomes more calcified and stiff, and the ribs become less elastic. Age changes in the cartilage and ligaments of other joints, such as those between the ribs and the vertebrae, result in decreases in the ease and range of motion of the bones they connect.

Aging also leads to slight alterations in the positions of the bones of the chest. The chest becomes deeper from front to back, making deep inspiration more difficult. Altered posture from other changes in the skeletal system further reduces a person's ability to inspire quickly and fully.

Because of these skeletal age changes, there is a decline in the maximum minute volume attainable and an increase in the work of breathing. Older people partially compensate for these effects by relying more on diaphragmatic breathing.

Lungs

Though there are no important age changes in the elastic fibers or surfactant, other age changes in the lungs significantly affect pressure changes. For example, aging causes the coiled collagen fibers in the lungs to become somewhat limp and less resilient. Also, aging causes the alveoli to become shallower, and this reduces the amount of surface area present. The resulting reduction in surface tension decreases elastic recoil. Both of these age changes reduce the maximum rate of expiration attainable and add to the work of breathing (Figure 5.8).

Figure 5.8 Effects of aging on alveoli: (a) young alveoli. (b) old alveoli. (Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland. Used with permission.)

Compliance

Aging causes the coiled collagen fibers to become somewhat limp and stretch more easily. These changes increase the compliance of the lungs and tend to make inspiration easier. Note, however, that the increase in lung compliance is much less than the increase in chest stiffness caused by skeletal age changes. Thus, there is a net increase in stiffness of the respiratory system, resulting in decreased ability to inspire.

Control Systems

Aging does not seem to affect the contributions of the nervous system to rhythmic breathing under resting conditions. However, three types of age change reduce the ability of the nervous system and endocrine system to cause adaptive changes in ventilation:

1. Neurons monitoring O₂, CO₂, acid/base balance, and muscle activity seem to become less sensitive to changes in these parameters.
2. There may be changes in the nervous pathways through which all their impulses are sent, resulting in altered ventilation.
3. The lungs become less sensitive to norepinephrine from sympathetic nerves and the endocrine system.

These age changes result in a slower and smaller increase in minute volume when there is a decrease in O₂ or an increase in CO₂, acids, or body activity. As a result, individuals who begin vigorous activity feel out of breath and tire more quickly as they get older.

Age changes in other parts of the nervous system reduce its ability to provide the swallowing, gag, and cough reflexes that defend the respiratory system (Chapter 6). Because of these changes, it takes a greater amount of material and a longer time to start a defensive reflex. Once it begins, the response is slower and weaker.

Therefore, older individuals must avoid situations that raise the risk of choking. These include eating quickly; talking or laughing while eating; eating while lying on one's back; and eating after consuming alcoholic beverages or medications that slow the reflexes.

Consequences

Reductions in pressure changes caused by weakening of muscles, stiffening of the respiratory system, and decreased alveolar surface area combine with narrowing of the airways to produce two effects. First, they cause a decrease in the rate at which air can flow in the system. Second, they make ventilation more difficult and therefore increase the work of breathing. This reduces the amount of available O₂ and increases CO₂ in the blood.

Although aging does not change the total lung capacity, age changes affect the volumes of air that can be moved. The more rapid closing of bronchioles, together with stiffening of the system, causes a decrease in both inspiratory and expiratory reserve volume. At the same time, tidal volume increases somewhat, and the age changes cause an increase in residual volume both at rest and during increased ventilation. These changes in volumes cause the vital capacity to decrease.

These changes in respiratory volumes have two effects. First, they further decrease maximum minute volume. Second, the decrease in vital capacity, combined with the increase in residual capacity, means that less fresh inspired air is mixing with more stale air remaining in the lungs. This change decreases the rate of diffusion. The problem is compounded by the increase in dead space. The age‑related increase in tidal volume may help compensate for this problem during quiet breathing (Figure 5.9).

Figure 5.9 Age changes in respiratory volumes (Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland.)

The force of gravity on the lungs causes the lower bronchioles to close sooner than do those in the upper regions. Therefore, the lower parts have a higher proportion of the residual air than do the upper regions. This unevenness in ventilation increases with age. As seen below in the discussion of perfusion and diffusion, this further decreases the efficiency of the system. Thus, as people get older, they must ventilate more air to get the same amount of gas exchange, and this adds to the work of breathing. Breathing more deeply can partially overcome the deleterious effects of uneven ventilation. Aging also reduces the maximum respiratory rate (breaths per minute) because of age changes that slow airflow and age changes in the nervous system.

These decreases in maximum flow rate, maximum volume per breath, and maximum respiratory rate combine to cause a decrease in the maximum minute volume. Many individuals can expect their maximum minute volumes to decline by 50 percent as they pass from their twenties to very old age. This change makes a major contribution to the decrease in the maximum rate of gas exchange as people age. Age changes in perfusion and diffusion, discussed below, cause additional decreases in gas exchange.