Overview
This lecture explains the processes of external and internal respiration, focusing on how oxygen and carbon dioxide are exchanged between the lungs, blood, and tissues based on differences in partial pressure.
External Respiration
- External respiration is the gas exchange between alveoli (air sacs) in the lungs and blood capillaries.
- Oxygen moves from alveoli (high partial pressure, ~104 mm Hg) to blood capillaries (low partial pressure, ~40 mm Hg).
- Carbon dioxide moves from deoxygenated blood (high partial pressure, ~45 mm Hg) to alveoli (low partial pressure, ~40 mm Hg).
- Both gases follow the rule: move from high to low partial pressure.
- These transfers occur simultaneously; oxygen enters the blood, while carbon dioxide leaves to be exhaled.
Internal Respiration
- Internal respiration is the gas exchange between the blood and body tissues.
- Oxygen moves from oxygenated blood (high partial pressure, ~104 mm Hg) to tissues (low partial pressure, ~40 mm Hg or less if exercising).
- About 1.5% of oxygen travels as dissolved gas; most is released from hemoglobin before diffusing into tissues.
- Carbon dioxide moves from tissues (high partial pressure, ~45 mm Hg or more if exercising) to blood (lower partial pressure, ~40 mm Hg).
- Oxygen leaves the blood for tissues while carbon dioxide enters the blood from tissues, both happening at the same time.
Key Terms & Definitions
- External respiration — exchange of gases between lung alveoli and blood capillaries.
- Internal respiration — exchange of gases between blood and body tissues.
- Alveoli — tiny air sacs in the lungs where gas exchange occurs.
- Partial pressure — the pressure exerted by a single type of gas in a mixture, driving diffusion.
- Hemoglobin — a protein in red blood cells that binds and carries oxygen.
Action Items / Next Steps
- Review the concepts of external and internal respiration.
- Memorize the typical partial pressures for oxygen and carbon dioxide in lungs, blood, and tissues.
- Understand how partial pressure gradients drive gas exchange.