What does systemic vascular resistance refer to?

Systemic vascular resistance refers to the resistance encountered by blood as it flows through the blood vessels in the systemic circulation. This resistance is primarily influenced by the diameter of the blood vessels; as the diameter decreases, resistance increases, and vice versa. The concept underscores that smaller vessels have a greater resistance to blood flow compared to larger vessels, which is crucial for understanding how blood pressure is regulated and how the cardiovascular system responds to various physiological demands.

The other answers relate to different aspects of cardiovascular physiology. While the pressure in major arteries is important for understanding blood flow dynamics, it does not specifically define resistance. The efficiency of the heart in pumping blood refers to cardiac output rather than vascular resistance. Lastly, the volume of blood in circulation is a measure of blood volume, which can affect pressure but not the intrinsic resistance characteristics of the blood vessels themselves. Thus, B accurately encapsulates the concept of systemic vascular resistance.