Final answer:
The tunica responsible for active blood pressure regulation is the tunica media, not the tunica externa. The tunica externa primarily provides structural support, while the tunica media's smooth muscle adjusts vessel diameter to regulate pressure.
Step-by-step explanation:
The tunic that plays an active role in blood pressure regulation is the tunica media (option b). The tunica media is where the smooth muscle is located, and this muscle layer is responsible for vasoconstriction and vasodilation, which are crucial in regulating blood pressure. The tunica externa, also known as tunica adventitia, is primarily composed of connective tissue. It provides structural support and protection to the blood vessels, preventing blood cell adhesion to the wall of the vessel and is generally thicker in arteries due to the higher pressure. However, it is not actively involved in blood pressure regulation.
The endothelium, which is found in the tunica intima, plays a role in blood pressure regulation by releasing endothelins that can constrict the smooth muscle within the vessel walls. However, the overall regulation of blood pressure is largely attributed to the smooth muscle in the tunica media.