Substance Transported


Carbon dioxide

Combines with iron atoms of hemoglobin molecules About 7% dissolves in plasma

About 23% combines with the amino groups of hemoglobin molecules About 70% reacts with water to form carbonic acid; the carbonic acid then dissociates to release hydrogen ions and bicarbonate ions

Oxyhemoglobin Carbon dioxide Carbaminohemoglobin Bicarbonate ions

How do bicarbonate ions help buffer the blood (maintain its pH)?

As blood passes through the capillaries of the lungs, the dissolved carbon dioxide diffuses into the alveoli, in response to the relatively low PCo2 of the alveolar air. As the plasma PCO2 drops, hydrogen ions and bicarbonate ions in the red blood cells recombine to form carbonic acid, and under the influence of carbonic anhydrase, the carbonic acid quickly yields new carbon dioxide and water:

How do bicarbonate ions help buffer the blood (maintain its pH)?

What is the chloride shift?

How is carbon dioxide released from the blood into the lungs?

Carbaminohemoglobin also releases its carbon dioxide, and both of these events contribute to the PCO2 of the alveolar capillary blood. Carbon dioxide diffuses out of the blood until an equilibrium is established between the PCO)2 of the blood and the PCO2 of the alveolar air. Figure 19.43 summarizes this process, and table 19.7 summarizes the transport of blood gases.

D Describe three ways carbon dioxide can be transported from cells to the lungs.

How can hemoglobin carry oxygen and carbon dioxide at the same time?

-Span Changes

Changes in the respiratory system over a lifetime reflect both the accumulation of environmental influences and the effects of aging in other organ systems. The lungs and respiratory passageways of a person who has breathed only clean air are pinker and can exchange gases much more efficiently as the years pass than can the respiratory system of a person who has breathed polluted air and smoked for many years. Those who have been exposed to foul air are more likely to develop chronic bronchitis, emphysema, and/or lung cancer. Long-term exposure to particulates in the workplace can also raise the risk of developing these conditions. Still, many age-associated changes in the respiratory system are unavoidable.

With age, protection of the lungs and airways falters, as ciliated epithelial cells become fewer, and their cilia less active or gone. At the same time, mucus thickens; the swallowing, gagging, and coughing reflexes slow; and macrophages lose their efficiency in phagocy-tizing bacteria. These changes combine to slow the clearance of pathogens from the lungs and respiratory passages, which increases susceptibility to and severity of respiratory infections.

Several changes contribute to an overall increase in effort required to breathe that accompanies aging. Cartilage between the sternum and ribs calcifies and stiffens, and skeletal shifts change the shape of the thoracic cavity into a "barrel chest" as posture too changes with age. In the bronchioles, fibrous connective tissue replaces some smooth muscle, decreasing contractility. As muscles lose strength, breathing comes to depend more upon the diaphragm. The vital capacity, which reaches a maximum by age 40, may drop by a third by the age of 70 years.

Keeping fresh air in the lungs becomes more difficult with age. As the farthest reaches of the bronchiole walls thin, perhaps in response to years of gravity, they do not stay as open as they once did, trapping residual air in the lower portions of the lungs. Widening of the bronchi and alveolar ducts increases dead space. The lungs can still handle the same volume of air, but a greater proportion of that air is "stale," reflecting lessened ability to move air in and out. The maximum minute ventilation drops by 50 percent from age 20 to age 80.

Aging-associated changes occur at the microscopic level too. The number of alveoli is about 24 million at birth, peaking at 300 million by age eight years. The number remains constant throughout life, but the alveoli expand. Alveolar walls thin and may coalesce, and the depth of alveoli begins to diminish by age 40, decreasing the surface area available for gas exchange—about three square feet per year. In addition, an increase in the proportion of collagen to elastin, and a tendency of the collagen to crosslink, impairs the ability of alveoli to expand fully. As a result, oxygen transport from the alveoli to the blood, as well as oxygen loading onto hemoglobin in red blood cells, becomes less efficient. Diffusion of CO2 out of the blood and through the alveolar walls slows too.

As with other organ systems, the respiratory system undergoes specific changes, but these may be unnotice-able at the whole-body level. A person who is sedentary or engages only in light activity would probably not be aware of the slowing of air flow in and out of the respiratory system. Unaccustomed exercise, however, would quickly reveal how difficult breathing has become compared to years past.

99 How does the environment influence the effects of aging on the respiratory system?

^9 What aging-related changes raise the risk of respiratory infection?

^9 How do alveoli change with age?

Clinical Terms Related to the Respiratory System anoxia (ah-nok'se-ah) Absence or a deficiency of oxygen within tissues. asphyxia (as-fik'se-ah) Deficiency of oxygen and excess carbon dioxide in the blood and tissues. atelectasis (at"e-lek'tah-sis) Collapse of a lung or some portion of it.

bradypnea (brad"e-ne'ah) Abnormally slow breathing. bronchitis (brong-ki'tis) Inflammation of the bronchial lining. Cheyne-Stokes respiration (chain stoks res"pi-ra'shun)

Irregular breathing pattern of a series of shallow breaths that increase in depth and rate, followed by breaths that decrease in depth and rate. dyspnea (disp'ne-ah) Difficulty in breathing. eupnea (up-ne'ah) Normal breathing.

hemothorax (he"mo-tho'raks) Blood in the pleural cavity. hypercapnia (hi"per-kap'ne-ah) Excess carbon dioxide in the blood.

hyperoxia (hi"per-ok'se-ah) Excess oxygenation of the blood. hyperpnea (hi"perp-ne'ah) Increase in the depth and rate of breathing.

hyperventilation (hi"per-ven"ti-la'shun) Prolonged, rapid, and deep breathing.

hypoxemia (hi"pok-se'me-ah) Deficiency in the oxygenation of the blood.

hypoxia (hi-pok'se-ah) Diminished availability of oxygen in the tissues.

lobar pneumonia (lo'ber nu-mo'ne-ah) Pneumonia that affects an entire lobe of a lung. pleurisy (ploo'ri se) Inflammation of the pleural membranes. pneumoconiosis (nu"mo-ko"ne-o'sis) Accumulation of particles from the environment in the lungs and the reaction of the tissues. pneumothorax (nu"mo-tho'raks) Entrance of air into the space between the pleural membranes, followed by collapse of the lung.

rhinitis (ri-ni'tis) Inflammation of the nasal cavity lining. sinusitis (si"nu-si'tis) Inflammation of the sinus cavity lining. tachypnea (tak"ip-ne'ah) Rapid, shallow breathing. tracheotomy (tra"ke-ot'o-me) Incision in the trachea for exploration or the removal of a foreign object.

Respiratory System

Respiratory System

The respiratory system provides oxygen for the internal environment and excretes carbon dioxide.

Integumentary System Cardiovascular System

Stimulation of skin receptors may alter respiratory rate.

Stimulation of skin receptors may alter respiratory rate.

As the heart pumps blood through the lungs, the lungs oxygenate the blood and excrete carbon dioxide.

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