Final answer:
An anastomosis refers to a connection where an arteriovenous anastomosis connects an arteriole directly to a venule, allowing blood to bypass the capillary bed. Arterioles lead to capillaries but are themselves composed of three thinner layers, including the tunica media and tunica externa. So, the correct answer is d. an artery empties directly into a vein without passing through a capillary bed.
Step-by-step explanation:
An anastomosis in the context of the cardiovascular system is a connection between two blood vessels.
The correct description of an anastomosis as provided in the options is that it is a short vessel that connects an arteriole directly to a venule without passing through a capillary bed, which is known as an arteriovenous anastomosis.
This type of connection interestingly allows blood to bypass the capillary bed for various physiological reasons, such as thermoregulation, where it is important in controlling the flow of blood to the skin surface.
An arteriole is a very small artery that leads to a capillary and has the same three layers as larger vessels, though much diminished.
The tunica media is only one or two smooth muscle cell layers thick.
The tunica externa is also very thin.
Furthermore, in the typical circulatory route, arterioles give rise to metarterioles, which in turn supply a capillary bed.
Precapillary sphincters regulate the flow of blood into the capillaries, and a thoroughfare channel connects the metarteriole to a venule.
This is typically shown in figures describing the capillary bed.
So, the correct answer is d. an artery empties directly into a vein without passing through a capillary bed.