Question

How do partial pressures of oxygen and CO2 vary around the body and how does this affect exchange (i.e.) direction of movement of gases in different parts of the body (lung capillary beds vs. systemic capillary beds)

Answer 1

Partial pressure of a gas is a measure of the how much amount of gas is present in this case, blood, tissues or alveoli. Air consists mainly of nitrogen, oxygen and carbon-dioxide, so the pressure created by oxygen in part is known as partial pressure of oxygen (PO₂). Similarly, pressure created by carbon-dioxide is known as partial pressure of carbon-dioxide (PCO₂).

The atmospheric pressure at sea level is about 760 mm Hg and the air contains about 21% of oxygen so the partial pressure is about 0.21 times that of 760 mm Hg.

In order to exchange gases between alveoli and blood, PO₂ and PCO₂ has to be maintained.

Normally,

• Alveoli maintains
  • PO₂ = 100 mm Hg
  • PCO₂ = 40 mm Hg

• Alveolar capillaries
  • Entering the alveolar capillaries (blood entering from systemic circulation)
    • PO₂ = 40 mm Hg (relatively low as the blood has just returned from the systemic circulation & deprived of oxygen)
    • PCO₂ = 45 mm Hg (relatively high because the blood returning from the systemic circulation has picked up carbon dioxide)

The low level of oxygen in the alveolar capillaries creates a pressure which draws oxygen into the blood from alveoli and moreover, hemoglobin has high affinity for oxygen to bind compared to CO_2. High level of CO₂ into the alveolar capillaries forces it to enter into alveoli and out in the atmosphere. Thus, partial pressure plays a very significant role in exchange of gases.