Transport of Respiratory Gases
Breathing involves inspiration and expiration, but gas exchange at the respiratory membrane and in tissues is continuous. Diffusion of oxygen (O2) and carbon dioxide (CO2) depends on pressure differences between the air, blood, and tissues.
Composition of Air
Atmospheric pressure at sea level is 101.3 kPa (760 mmHg). With altitude, atmospheric pressure decreases, while underwater it increases by ~1 atmosphere per 10 m depth.
Air contains nitrogen, oxygen, carbon dioxide, water vapor, and small amounts of inert gases. Each gas exerts a partial pressure proportional to its concentration (PO2, PCO2).
Alveolar Air
Alveolar air differs from atmospheric air, being saturated with water vapor and containing more CO2 and less O2. Gas exchange is continuous because alveoli are never empty, and only a portion of gases are exchanged during each inspiration.
Diffusion of Gases
Gases diffuse from higher to lower partial pressure across semipermeable membranes until equilibrium is reached. Nitrogen is not used by the body and remains unchanged.
External Respiration
Exchange of gases between alveoli and blood in alveolar capillaries occurs across the respiratory membrane. Alveolar walls are one cell thick and surrounded by capillaries. Venous blood arriving at the lungs has high CO2 and low O2. CO2 diffuses into alveoli, and O2 diffuses into blood, achieving equilibrium.
Internal Respiration
Exchange of gases between blood in capillaries and body cells. Arteries carry oxygenated blood to tissues, creating concentration gradients: O2 moves from blood to cells, and CO2 moves from cells to blood.
Transport of Gases in the Bloodstream
Oxygen Transport
- Bound to hemoglobin as oxyhemoglobin (98.5%).
- Dissolved in plasma (1.5%).
Oxyhemoglobin dissociates under low O2, low pH, or high temperature. Active tissues with more CO2 and heat promote oxygen release. Oxyhemoglobin is bright red, while deoxygenated blood is bluish-purple.
Carbon Dioxide Transport
- As bicarbonate ions (HCO3−) in plasma (70%).
- Loosely bound to hemoglobin as carbaminohemoglobin (23%).
- Dissolved in plasma (7%).
CO2 levels are crucial for acid-base balance. Excess CO2 lowers blood pH by forming carbonic acid, while sufficient CO2 supports the bicarbonate buffering system to maintain pH.
Detailed Notes
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