Which of the following is NOT one of the four systems that regulate blood pressure?

The correct answer is skin temperature because it is not directly involved in the regulation of blood pressure. Blood pressure regulation is primarily managed by four key systems: the heart, which pumps blood; blood vessel tone, which involves the constriction and dilation of the blood vessels to regulate flow and pressure; and the kidneys, which help control blood volume and electrolyte balance, further influencing blood pressure.

While skin temperature can reflect changes in blood flow and peripheral circulation, it does not have a direct regulatory role in the mechanisms that maintain blood pressure. The primary systems that actively contribute to maintaining blood pressure involve cardiovascular dynamics, vascular resistance, and renal function, rather than thermoregulation.