Microcirculation
The microcirculation—comprising arterioles, capillaries, and venules—is the site of nutrient exchange, waste removal, and regulation of peripheral resistance.
1. Key Components
- Arterioles: The primary resistance vessels; “gatekeepers” of blood flow into capillary beds via precapillary sphincters.
- Capillaries: The primary exchange vessels; single endothelial layer, largest total cross-sectional area, lowest velocity of blood flow.
- Venules: Primary site of leukocyte extravasation (diapedesis) and fluid/solute collection.
2. Governing Forces (Starling Law)
| Force | Effect |
|---|---|
Capillary Hydrostatic ( ) |
Favors filtration (out of the capillary). |
Plasma Oncotic ( ) |
Favors reabsorption (into the capillary). |
Interstitial Hydrostatic ( ) |
Usually slightly negative; favors filtration. |
Interstitial Oncotic ( ) |
Favors filtration (pulls water into the interstitium). |
3. Exam Must-Knows
- Edema Factors: Increased
- Autoregulation: The ability of tissues to maintain constant blood flow despite changes in perfusion pressure (Myogenic and Metabolic hypotheses).
Physiology Board Hint: Always remember that the arterioles provide the greatest resistance in the circulatory system. In pathology questions, if a substance is “vasoactive,” it acts primarily on the arterioles to adjust peripheral resistance, whereas “capillary” questions will always test your application of Starling forces.