External and Internal Respiration
Objective 7
State Dalton’s Law and Henry’s Law. Explain how each is relevant to external and internal respiration. Using these laws, compare and contrast human physiology at normal atmospheric pressure and at high pressure. Describe, in detail, the process of external respiration and movement of gases across the alveolar-capillary (A-C) membrane and explain ventilation-perfusion coupling. Describe the process of internal respiration. Compare and contrast the processes of external and internal respiration. Compare and contrast the partial pressures of oxygen and carbon dioxide during the process of external and internal respiration.
External and internal respiration refer to the exchange of gases at the alveoli and at the tissues. These processes are explained by two physical laws: Dalton’s Law and Henry’s Law.
Dalton’s Law
Dalton’s Law says that the particles in a gas don’t care about each other. In a mixture of different gases (gas A and gas B and gas C), the pressure due to gas A is exactly equal to its proportion in the mixture.
Atmospheric pressure is defined as 760 mm Hg at sea level. This is also called a pressure of 1 atmosphere (1 atm = 760 mm Hg). This is the pressure (collisions) created by a column of air 10 miles (16 km) high. These collisions, this pressure, can lift a column of mercury (Hg) 760 mm. When you listen to the weather report, they call this “30 inches” (30 inches Hg = 760 mm Hg). Remember from Unit 16 that systolic blood pressure is about 1/6 atm or 120 mm Hg.
Because 21% of the atmosphere is oxygen, 21% of the collisions (pressure) are due to oxygen. We call this the partial pressure of oxygen.
Henry’s Law
Henry’s Law says the amount of a gas that is dissolved in a liquid is directly proportional to the partial pressure of the gas. In the body much more CO2 is dissolved in blood plasma than O2 because it is 24x more soluble than oxygen. A hyperbaric chamber increases the atmospheric pressure of oxygen, and more dissolves in solution (the blood).
When Olympians came to Salt Lake City to compete in the 2002 Winter Olympics, they had to contend with the high altitude. Atmospheric pressure decreases as we ascend in altitude because the column of air above your head is 1500 m (5000 ft) shorter. Henry’s Law states that if the atmospheric pressure of oxygen is lower, less will dissolve in solution. Altitude sickness may occur because of the lack of oxygen dissolved in the blood. Ultimately pulmonary vessels may vasoconstrict increasing pulmonary pressure. Fluid may be pushed out of the vessels leading to the serious condition of pulmonary edema. Lack of oxygen to the brain may lead to cerebral edema and subsequent death.
The opposite condition may occur with scuba divers. For every 30 feet that a diver descends, atmospheric pressure increases by 1 atm (760 mmHg). A much larger amount of nitrogen than normal is now dissolved in the blood because of the high pressure. If a diver ascends too rapid