24.3: Arteries and Arterioles
Arteries, the vasculature responsible for transporting blood from the heart, possess robust walls capable of enduring the elevated pressures exerted by the heartbeat. Arteries near the heart are especially thick-walled and enriched with elastic fibers across their three tunics, classifying them as elastic or conducting arteries. These arteries, usually with a diameter exceeding 10 mm, are characterized by their ability to dilate in response to the blood pumped from the heart's ventricles and recoil. This critical feature prevents an unhealthy increase in blood pressure and resistance, which would necessitate additional effort from the heart to maintain optimum blood flow. The elastic recoil also plays a crucial role in preserving the pressure gradient necessary for blood flow throughout the arterial system.
The arterial composition changes from the heart, where the pulsatile blood flow has attenuated. The tunica intima's (or interna) elastic fiber content reduces while the smooth muscle within the tunica media increases. At this juncture, the artery is labeled a muscular artery, typically varying from 0.1 mm to 10 mm in diameter. These arteries, with their thick tunica media, are instrumental in vasoconstriction, even as their reduced elastic content restricts their dilation capacity. However, the need for elasticity diminishes as the blood pressure subsides when it reaches these vessels.
It is crucial to note that the transition from an elastic to a muscular artery is not abrupt but occurs gradually as the vasculature continually branches out. These muscular arteries further branch to form arterioles, thereby earning their alternative name, distributing arteries.
Arterioles, the miniature arteries leading to capillaries, possess the same three tunics as larger vessels but demonstrate considerably reduced thickness. Despite their minimal lumen diameter (30 micrometers or less), arterioles are pivotal in resisting blood flow and causing a substantial decrease in blood pressure, earning them the reference as resistance vessels. The smooth muscle in arterioles exhibits a constant semi-contracted state, known as vascular tone, akin to the muscular tone observed in skeletal muscles. All blood vessels display this vascular tone due to the semi-contraction of their smooth muscles. The arterioles' significance lies in their principal role in blood pressure regulation and resistance. The arteriole lumen's exact diameter at any given time depends on neural and chemical controls, and the arterioles' vasoconstriction and vasodilation are vital in controlling blood flow distribution.
