Gaseous Exchange in Humans

8.3

Gaseous Exchange in Humans

Partial Pressure of Oxygen and Carbon Dioxide

Gas diffusion depends on the partial pressure difference between two areas.
The gas diffuses from an area where its partial pressure is higher to an area where its partial pressure is lower, which is down a partial pressure gradient.

Gaseous Exchange and The Transport of Respiratory Gases

In the lungs, blood that enters the lung capillaries contains a higher partial pressure of carbon dioxide compared to the alveolus.
Carbon dioxide diffuses out from the lung capillaries and into the alveolus before it is expelled into the atmosphere through the nose and mouth.
The partial pressure of oxygen in the alveolus is higher than the lung capillaries.
Oxygen diffuses into the lung capillaries.
Oxygen combines with haemoglobin in erythrocytes to form oxyhaemoglobin.
Oxyhaemoglobin reaches body tissues for cellular respiration.
Cellular respiration releases carbon dioxide.
The partial pressure of carbon dioxide in cells is higher than tissue capillaries.
Carbon dioxide diffuses out from the body cells into the tissue capillaries and is transported back to the lungs.
When blood reaches the tissue through the tissue capillaries, the partial pressure of oxygen in the blood is higher than that of body cells because cellular respiration uses oxygen.
Oxyhaemoglobin breaks down and oxygen diffuses from the tissue capillaries to the body cells.

Transport of Carbon Dioxide in The Blood Circulatory System

The Transport of Carbon Dioxide from Body Cells to Tissue Capillaries

Carbon dioxide, \(\text{CO}_2\) released by the body cells binds with water, \(\text{H}_2\text{O}\) in the erythrocyte to form carbonic acid, \(\text{H}_2\text{CO}_3\).
The carbonic anhydrase enzyme in erythrocyte catalyses this reaction.
Carbonic acid, \(\text{H}_2\text{CO}_3\) will break down into bicarbonate ion, \(\text{HCO}_3^{-}\) and hydrogen ion, \(\text{H}^{+}\).
Then \(\text{HCO}_3^{-}\) diffuses into the blood plasma and is carried to the lungs.

The Transport of Carbon Dioxide from Lung Capillaries to The Alveolus

When the bicarbonate ion in blood plasma reaches the lung capillaries, it diffuses back into the erythrocyte.
The bicarbonate ion combines again with a hydrogen ion, \(\text{H}^{+}\) to form carbonic acid, \(\text{H}_2\text{CO}_3\).
Carbonic acid, \(\text{H}_2\text{CO}_3\) then breaks down into carbon dioxide and water.
Carbon dioxide diffuses through the lung capillaries into the alveolus and is expelled during exhalation.