8.3 - Gaseous Exchange in Humans

Partial Pressure of Oxygen and Carbon Dioxide

The partial pressure gradient is what determines the rate of gaseous exchange by diffusion. Remember the concept of simple diffusion? Respiratory gases will diffuse from a higher partial pressure area to a lower partial area, down the partial pressure gradient.

Figure 11 - Recall Chapter 3, gas particles tend to move from a higher concentration region to a lower concentration region.

Gaseous Exchange and the Transport of Respiratory Gases

Transport of Carbon Dioxide in the Blood Circulatory System

Carbon dioxide released by the respiring cells can be transported in 3 ways:

• 7% of the carbon dioxide is transported as dissolved carbon dioxide in the plasma.

• 23% is transported as carbaminohemoglobin (bound to hemoglobin).

• 70% is transported as bicarbonate ions (HCO₃⁻) in the blood.

• Respiring body cells produce carbon dioxide.

• Carbon dioxide diffuses into blood plasma and combines with red blood cells.

• Carbon dioxide reacts with water to form carbonic acid (H₂CO₃), catalyzed by carbonic anhydrase in the red blood cells.

• The carbonic acid dissociates into hydrogen (H+) and bicarbonate ions (HCO₃⁻).

• Most HCO₃⁻ ions diffuse into the blood plasma and are carried to the lungs.

Figure 12 - This process happens in the tissue capillaries.

• The HCO₃⁻ ions diffuse from the blood plasma into the red blood cells, forming carbonic acid again.
• The carbonic acid breaks down to form water and carbon dioxide.
• Carbon dioxide eventually diffuses into the alveoli to be carried to the lungs for expulsion.

Figure 13 - This process takes place in the lungs. It is the reverse of what happens in the tissue capillaries.