- Arterial pH does not change during moderate exercise, although it may decrease during strenuous exercise because of lactic acidosis.
3. On the other hand, venous Pco2 increases during exercise because the excess C02 produced by the exercising muscle is carried to the lungs in venous blood.
Table 4-7. Summary of Respiratory Responses to Exercise
02 consumption C02 production Ventilation rate
T (matches 02 consumption/C02 production) No change
No change in moderate exercise 4- in strenuous exercise (lactic acidosis)
Arterial Po2 and PcO; Arterial pH
Venous Pco2 Pulmonary blood flow (cardiac output) V/Q ratios
More evenly distributed in lung
V/Q = ventilation/perfusion.
4. Pulmonary blood flow increases because cardiac output increases during exercise. As a result, more pulmonary capillaries are perfused, and more gas exchange occurs. The distribution of V/Q ratios throughout the lung is more even during exercise than when at rest, and there is a resulting decrease in the physiologic dead space.
B. Adaptation to high altitude (Table 4-8)
1. Alveolar Po2 is decreased at high altitude because the barometric pressure is decreased. As a result, arterial Po2 is also decreased (hypoxemia).
2. Hypoxemia stimulates the peripheral chemoreceptors and increases the ventilation rate (hyperventilation). This hyperventilation produces respiratory alkalosis, which can be treated by administering acetazol-amide.
3. Hypoxemia also stimulates renal production of erythropoietin, which increases the production of RBCs. As a result, there is increased hemoglobin concentration, increased 02-carrying capacity of blood, and increased 02 content of blood.
4. 2,3-DPG concentrations are increased, shifting the hemoglobin-02 dissociation curve to the right. There is a resulting decrease in affinity of hemoglobin for 02 that facilitates unloading of 02 in the tissues.
5. Pulmonary vasoconstriction is another result of hypoxemia (hypoxic vasoconstriction). Consequently, there is an increase in pulmonary arterial pressure, increased work of the right side of the heart against the higher resistance, and hypertrophy of the right ventricle.
Table 4-8. Summary of Adaptation to High Altitude
Alveolar Po2 Arterial Po2 Ventilation rate Arterial pH
Hemoglobin concentration 2,3-DPG concentration Hemoglobin-02 curve Pulmonary vascular resistance
4- (resulting from -1- barometric pressure) I (hypoxemia) T (hyperventilation) T (respiratory alkalosis) T (polycythemia) t
Shift to right; I affinity
DPG = diphosphoglycerate.
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