Partial Pressures of O2 and CO2 in Respiratory and Cardiovascular Systems

Partial Pressures of Oxygen and Carbon Dioxide

To understand the respiratory and cardiovascular system, it's crucial to know the partial pressures of oxygen ($O<em>2$) and carbon dioxide ($CO</em>2$) in different areas:

• Alveolar air
• Oxygenated blood
• Deoxygenated blood
• Target metabolizing tissues

Understanding Relative Differences

Instead of memorizing numbers immediately, focus on understanding the relative differences in partial pressures.

• Compare $O_2$ in alveolar air vs. deoxygenated blood and oxygenated blood.

Alveolar Air:

• Partial pressure of oxygen ($pO_2$) is approximately 100 mm Hg.

Deoxygenated Blood:

• $pO_2$ is around 40 mm Hg.
• This blood is flowing into the lungs to be oxygenated.
• There is a much lower concentration/partial pressure of oxygen compared to alveolar air.

Gas Exchange

Because of the difference in partial pressures:

• Oxygen moves out of the alveolar air and diffuses across the respiratory membranes into the bloodstream.
  • No active transport is needed because oxygen and carbon dioxide are lipid-soluble.
  • They move down the diffusion gradient (partial pressure gradient).
• Carbon dioxide moves out of the deoxygenated blood and into the alveolar air.
  • Partial pressure differential of $CO_2$ is about 5 mm Hg.
  • The partial pressure differential of $O_2$ is about 60 mm Hg when comparing alveolar air and deoxygenated blood.
  • Carbon dioxide is much more soluble than oxygen.

Oxygenated Blood:

• $pO_2$ is around 95 mm Hg, slightly lower than in alveolar air.
• This difference is due to:
  • Shunts: Blood passes through the lungs but doesn't get fully oxygenated.
  • Some oxygen gets consumed along the way.

Target Metabolizing Tissues:

• Oxygenated blood carries oxygen to these tissues.
• Roles are switched: $pO_2$ in oxygenated blood is about 95 mm Hg, while in target cells, it's around 40 mm Hg.
• Oxygen diffuses out of hemoglobin, across the capillary endothelium, and into the target cells down its diffusion gradient.
• Partial pressure of Carbon dioxide in metabolizing tissues is higher than in oxygenated blood (40 mmHg).
• Carbon dioxide moves out of the tissues and into the blood.

The Cycle

1. Blood becomes deoxygenated (oxygen moves out, carbon dioxide moves in).
2. Blood returns to the heart.
3. Blood is pumped to the lungs.
4. The process repeats.