In response to tissue hypoxia, the kidneys secrete the hormone erythropoietin (chapter 13). Erythropoietin stimulates the bone marrow to increase its production of hemoglobin and red blood cells. In the Peruvian Andes, for example, people have a total hemoglobin concentration that is increased from 15 g per 100 ml (at sea level) to 19.8 g per 100 ml. Although the percent oxyhemoglobin saturation is still lower than at sea level, the total oxygen content of the blood is actually greater—22.4 ml O2 per 100 ml compared to a sea-level value of about 20 ml O2 per 100 ml. These adjustments of the respiratory system to high altitude are summarized in table 16.13.
It should be noted that these changes are not unalloyed benefits. Polycythemia (high red blood cell count) increases the viscosity of blood; hematocrits of 55% to 60% have been measured in people who live in the Himalayas, and higher values are reached if dehydration accompanies the polycythemia. The increased blood viscosity contributes to pulmonary hypertension, which can cause accompanying edema and ventricular hypertrophy that can lead to heart failure.
Test Yourself Before You Continue
1. Describe the effect of exercise on the Po , PC0j, and pH blood values and explain how ventilation might be increased during exercise.
2. Explain why endurance-trained athletes have a higher than average anaerobic threshold.
3. Describe the changes that occur in the respiratory system during acclimatization to life at a high altitude.
• Nasal hairs and mucus prevent dust and other foreign material from damaging respiratory passageways (p. 482)
• The lungs are protected by the rib cage, and bones of the rib cage serve as levers for the action of respiratory muscles (p. 488)
• Red blood cells, needed for oxygen transport, are produced in the bone marrow (p. 371)
• The respiratory system provides all organs, including the bones, with oxygen and eliminates carbon dioxide (p. 480)
• Contractions of skeletal muscles are needed for ventilation (p. 488)
• Muscles consume large amounts of oxygen and produce large amounts of carbon dioxide during exercise (p. 342)
• The nervous system regulates the rate and depth of breathing (p. 499)
• Autonomic nerves regulate blood flow, and hence the delivery of blood to tissues for gas exchange (p. 420)
• Epinephrine dilates bronchioles, reducing airway resistance (p. 231)
• Thyroxine and epinephrine stimulate the rate of cell respiration (p. 609)
• The heart and arterial system delivers oxygen from the lungs to the body tissues, and veins transport carbon dioxide from the body tissues to the lungs (p. 366)
• Blood capillaries allow gas exchange for cell respiration in the tissues and lungs (p. 392)
• The immune system protects against infections that could damage the respiratory system (p. 446)
• Alveolar macrophages and the action of cilia in the airways help to protect the lungs from infection (p. 483)
• The kidneys regulate the volume and electrolyte balance of the blood . .(p. 524)
• The kidneys participate with the lungs in the regulation of blood pH (p. 548)
• The GI tract provides nutrients to be used by cells of the lungs and other organs (p. 561)
• The respiratory system provides oxygen for cell respiration of glucose and other nutrients brought into the blood by the digestive system (p. 480)
• The lungs provide oxygen for cell respiration of reproductive organs and eliminate carbon dioxide produced by these organs (p. 480)
• Changes in breathing and cell respiration occur during sexual arousal (p. 643)
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