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Why are the precapillary sphincters of the body normally held closed?
AOpen beds would raise blood viscosity beyond safe limits
BOpen beds would strip the endothelium of its tight junctions
COpen beds together would hold the entire blood volume
DOpen beds would drive lymph backwards into the tissues
Answer & Solution
Correct answer: C. Open beds together would hold the entire blood volume
1. A metarteriole feeds a capillary bed that may hold 10 to 100 capillaries.
2. Rings of smooth muscle at each capillary origin, the precapillary sphincters, gate that entry.
3. If every bed in the body opened at once they would together hold every drop of blood available.
4. Nothing would then be left in the arteries, veins or the heart, so the sphincters stay shut by default.
5. They open only where the local tissue is short of oxygen and loaded with waste.
6. Viscosity depends on formed elements and plasma proteins, not on how many beds are open.
7. Lymphatic valves are one way, so backflow into tissue is not the constraint here.
_Source: OpenStax Anatomy and Physiology (CC BY 4.0), Ch 20 "The Cardiovascular System: Blood Vessels and Circulation", section 20.1 Structure and Function of Blood Vessels_
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