8.3 - Gaseous Exchange in Humans

Partial Pressure of Oxygen and Carbon Dioxide

• Gas diffusion depends on the partial-pressure difference between two areas.
• Gas diffuses from an area of higher partial pressure to an area of lower partial pressure, down its partial-pressure gradient.
• At sea level, atmospheric pressure is $760\text{ mm Hg}$:
  • Oxygen forms 21% of the atmosphere; its partial pressure is $0.21\times 760=160\text{ mm Hg}$.
  • The partial pressure of carbon dioxide is $0.23\text{ mm Hg}$.

Gaseous Exchange Between the Lungs and Blood

• Blood entering the lung capillaries through the pulmonary artery has low oxygen partial pressure and high carbon-dioxide partial pressure.
• Carbon-dioxide partial pressure is higher in the lung capillaries than in the alveolus; carbon dioxide diffuses from the lung capillaries into the alveolus and is expelled through the nose and mouth.
• Oxygen partial pressure is higher in the alveolus than in the lung capillaries; oxygen diffuses into the lung capillaries.
• Oxygen combines with haemoglobin in erythrocytes to form oxyhaemoglobin:

$\text{Haemoglobin}+\text{Oxygen}\rightleftharpoons \text{Oxyhaemoglobin}$

• Blood leaving the lungs through the pulmonary vein has high oxygen partial pressure and low carbon-dioxide partial pressure.

Transport of Oxygen and Gaseous Exchange at Body Tissues

• Oxygen is transported from the lungs to body tissues as oxyhaemoglobin.
• In tissue capillaries, oxygen partial pressure in the blood is higher than in body cells because cellular respiration uses oxygen.
• Oxyhaemoglobin dissociates; oxygen diffuses from the tissue capillaries into body cells:

$\text{Oxyhaemoglobin}\to \text{Haemoglobin}+\text{Oxygen}$

• Cellular respiration releases carbon dioxide.
• Carbon-dioxide partial pressure is higher in body cells than in tissue capillaries; carbon dioxide diffuses from body cells into the tissue capillaries and is transported to the lungs.

Transport of Carbon Dioxide from Body Tissues

• Carbon dioxide is transported in three forms:
  • 70% as bicarbonate ions ($HC{O}_3^{-}$).
  • 23% combines with haemoglobin to form carbaminohaemoglobin.
  • 7% is dissolved and transported as carbonic acid ($H_{2}C{O}_3$).
• In erythrocytes, carbon dioxide released by body cells combines with water to form carbonic acid, catalysed by carbonic anhydrase:

$C{O}_2+H_{2}O\stackrel{\text{carbonic anhydrase}}{\to }{H}_{2}C{O}_3$

• Carbonic acid dissociates into bicarbonate ions and hydrogen ions:

$H_{2}C{O}_3\rightleftharpoons HC{O}_3^{-}+H^{+}$

• Bicarbonate ions diffuse into the blood plasma and are transported to the lungs.

Transport of Carbon Dioxide from Lung Capillaries to the Alveolus

• Bicarbonate ions in blood plasma diffuse back into erythrocytes at the lung capillaries.
• Bicarbonate ions recombine with hydrogen ions to form carbonic acid.
• Carbonic acid dissociates into carbon dioxide and water.
• Carbon dioxide diffuses from the lung capillaries into the alveolus and is expelled during exhalation.