[Campbell Biology P.981] Referring to Figure 42.13, which statement accurately describes the... | Practice Question
Referring to Figure 42.13, which statement accurately describes the regulation of blood flow within a capillary bed?
A: When precapillary sphincters are contracted, blood bypasses the true capillaries via thoroughfare channels, which remain open.
B: Thoroughfare channels regulate the passage of blood into specific capillary beds by constricting and dilating.
C: Capillaries are always open and have blood flowing through them, regardless of the state of the precapillary sphincters.
D: Precapillary sphincters are located at the venule end of the capillary bed and control outflow.
Explanation
Figure 42.13 b shows that when precapillary sphincters are contracted, blood flow is diverted away from the true capillaries and instead flows directly through the thoroughfare channel to the venule. The text confirms that thoroughfare channels are 'always open'.