• Oxygen and carbon dioxide diffuse between:
o The alveoli and pulmonary capillaries in the lungs
o The systemic capillaries and cells throughout the body.
• The diffusion of these gases, moving in opposite directions, is called gas exchange.
Dalton's Law of Partial Pressures
• In order to understand gas exchange, we must first understand the air we breathe.
• The atmosphere is a mixture of gases, including oxygen, carbon dioxide, nitrogen, and water.
o The combined pressure of these gases equals atmospheric pressure.
o At sea level, atmospheric pressure is 760 mm Hg
• Each gas within the atmosphere is responsible for part of that pressure in proportion to its percentage in
o Oxygen comprises 20.9% of the atmosphere
The pressure exerted by oxygen is 20.9% of the total pressure of 760 millimeters of
mercury, which equals 159 millimeters of mercury
This value is known as the partial pressure of oxygen, and is written as "P" with the
• The partial pressures of the four gases add up to 760 millimeters of mercury
o The total atmospheric pressure
• This demonstrates Dalton's Law of Partial Pressures
o States that in a mixture of gases, the total pressure equals the sum of the partial pressures exerted
by each gas.
o The partial pressure of each gas is directly proportional to its percentage in the total gas mixture.
Effect of High Altitude on Partial Pressures
• Atmospheric pressure decreases with increasing altitude.
o For example, on the top of Mt. Whitney, atmospheric pressure drops to approximately 440
millimeters of mercury.
o Oxygen still makes up 20.9% of the atmosphere, but the PO2 is 20.9% of 440 millimeters of
mercury, or about 92 millimeters of mercury.
o Compare that to the PO2 at sea level of 159 millimeters of mercury.
• Lower atmospheric pressure means fewer gas molecules
o Therefore fewer oxygen molecules are available.
o That explains why you may gasp for breath at high altitudes.
• As you can see, at high altitudes the partial pressures of all gases are lower than at sea level.
• •Within the lungs, oxygen and carbon dioxide diffuse between the air in the alveoli and the blood, that is
between a gas and a liquid.
• This movement is governed by Henry's Law
o States that the amount of gas which dissolves in a liquid is proportional to:
The partial pressure of the gas
The solubility of the gas
• In a container, the oxygen in the air is at equilibrium with the oxygen in the liquid
o At equilibrium, the pressure of the oxygen in the air is the same as in the liquid, with the gas
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molecules diffusing at the same rate in both directions.
o If you increase the pressure in the container more oxygen molecules dissolve in the liquid,
moving from a region of high pressure to a region of low pressure.
Diffusion continues until a new equilibrium is reached.
This is what happens when oxygen moves from the alveoli into the blood.
• Now let's look at the diffusion of carbon dioxide.
o Although both gases are at the same pressure, far more carbon dioxide dissolves in the liquid
o This occurs because carbon dioxide is much more soluble than oxygen.
o As stated in Henry's Law, the amount of oxygen and carbon dioxide that dissolves is
proportional to the partial pressure and the solubility of each gas.
Sites of Gas Exchange
• Sites of gas exchange in the body:
o External Respiration.
Blood that is low in oxygen is pumped from the R side of the heart, through the
pulmonary arteries to the lungs.
External respiration occurs within the lungs, as carbon dioxide diffuses from the
pulmonary capillaries into the alveoli, and oxygen diffuses from the alveoli into the
Oxygenrich blood leaves the lungs and is transported through the pulmonary veins to
the left side of the heart.
o Internal Respiration.
From there it is pumped through the systemic circuit to tissues throughout the body.
Internal respiration occurs within tissues, as oxygen diffuses from the systemic
capillaries into the cells, and carbon dioxide diffuses from the cells into the systemic
Factors Influencing External Respiration
• Efficient external respiration depends on three main factors:
o The surface area and structure of the respiratory membrane.
The 300 million alveoli, covered with a dense network of pulmonary capillaries, provide
an enormous surface area for efficient gas exchange.
In addition, the thinness of the respiratory membrane increases efficiency.
o The partial pressure gradients between the alveoli and capillaries.
o Efficient gas exchange requires matching alveolar airflow to pulmonary capillary blood flow.
External Respiration: Partial Pressures
• Partial pressure gradients affect gas exchange between alveoli & pulmonary capillaries.
• Notice that the partial pressures in the alveoli differ from those in the atmosphere.
o This difference is caused by a combination of several factors:
Humidification of inhaled air
• As it travels through the respiratory passageways to the alveoli, air is humidified,
picking up water molecules
• This greatly increases the partial pressure of water.
Gas exchange between the alveoli and pulmonary capillaries.
• A continuous exchange of oxygen and carbon dioxide occurs between the alveoli
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and pulmonary capillaries, changing the partial pressures of both gases.
• Oxygen diffuses out of the alveoli into the pulmonary capillaries and carbon
dioxide diffuses from the pulmonary capillaries into the alveoli.
Mixing of new and old air
• Since the alveoli do not completely empty between breaths, the air in the alveoli