The behavior of gases can be explained by the principles of Dalton’s law and Henry’s law, both of which describe aspects of gas exchange. Dalton’s law states that each specific gas in a mixture of gases exerts force (its partial pressure) independently of the other gases in the mixture. Henry’s law states that the amount of a specific gas that dissolves in a liquid is a function of its partial pressure. The greater the partial pressure of a gas, the more of that gas will dissolve in a liquid, as the gas moves toward equilibrium. Gas molecules move down a pressure gradient; in other words, gas moves from a region of high pressure to a region of low pressure. The partial pressure of oxygen is high in the alveoli and low in the blood of the pulmonary capillaries. As a result, oxygen diffuses across the respiratory membrane from the alveoli into the blood. In contrast, the partial pressure of carbon dioxide is high in the pulmonary capillaries and low in the alveoli. Therefore, carbon dioxide diffuses across the respiratory membrane from the blood into the alveoli. The amount of oxygen and carbon dioxide that diffuses across the respiratory membrane is similar.
Ventilation is the process that moves air into and out of the alveoli, and perfusion affects the flow of blood in the capillaries. Both are important in gas exchange, as ventilation must be sufficient to create a high partial pressure of oxygen in the alveoli. If ventilation is insufficient and the partial pressure of oxygen drops in the alveolar air, the capillary is constricted and blood flow is redirected to alveoli with sufficient ventilation. External respiration refers to gas exchange that occurs in the alveoli, whereas internal respiration refers to gas exchange that occurs in the tissue. Both are driven by partial pressure differences.
Dalton’s law
statement of the principle that a specific gas type in a mixture exerts its own pressure, as if that specific gas type was not part of a mixture of gases
external respiration
gas exchange that occurs in the alveoli
Henry’s law
statement of the principle that the concentration of gas in a liquid is directly proportional to the solubility and partial pressure of that gas
internal respiration
gas exchange that occurs at the level of body tissues
partial pressure
force exerted by each gas in a mixture of gases
total pressure
sum of all the partial pressures of a gaseous mixture
ventilation
movement of air into and out of the lungs; consists of inspiration and expiration
1. Gas moves from an area of ________ partial pressure to an area of ________ partial pressure.
A) low; high
B) low; low
C) high; high
D) high; low
D
2. When ventilation is not sufficient, which of the following occurs?
A) The capillary constricts.
B) The capillary dilates.
C) The partial pressure of oxygen in the affected alveolus increases.
D) The bronchioles dilate.
A
3. Gas exchange that occurs at the level of the tissues is called ________.
A) external respiration
B) interpulmonary respiration
C) internal respiration
D) pulmonary ventilation
C
4. The partial pressure of carbon dioxide is 45 mm Hg in the blood and 40 mm Hg in the alveoli. What happens to the carbon dioxide?
A) It diffuses into the blood.
B) It diffuses into the alveoli.
C) The gradient is too small for carbon dioxide to diffuse.
D) It decomposes into carbon and oxygen.
B
1. Compare and contrast Dalton’s law and Henry’s law.
Both Dalton’s and Henry’s laws describe the behavior of gases. Dalton’s law states that any gas in a mixture of gases exerts force as if it were not in a mixture. Henry’s law states that gas molecules dissolve in a liquid proportional to their partial pressure.
2. A smoker develops damage to several alveoli that then can no longer function. How does this affect gas exchange?
The damaged alveoli will have insufficient ventilation, causing the partial pressure of oxygen in the alveoli to decrease. As a result, the pulmonary capillaries serving these alveoli will constrict, redirecting blood flow to other alveoli that are receiving sufficient ventilation.