4.6: Arteries - Structure and Functions

Arteries are flexible tubes that carry blood from the heart to every region of the body. Arteries have special properties that ensure that they perform this task effectively. These properties derive from the three layers composing the arterial wall.

Inner Layer

The innermost layer of an artery is called the endothelium. It is supported by a thin underlying layer that contains collagen and a mat of elastin fibers (Figure 4.9a). Like the endocardium in the heart, the endothelium provides smoothness by forming a continuous glistening layer that coats the collagen and other materials in the arterial wall. In so doing, it permits blood to flow easily without clotting.

Figure 4.9a Structure of a large artery and vein. (Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland. Used with permission).

The endothelium also secretes several signaling materials including nitric oxide (*NO); prostacyclin, endothelin, and angiotensin converting enzyme (ACE), and ACE inhibitors. Nitric oxide and prostacyclin promote vasodilation in many arteries. Nitric oxide also limits vessel thickening by inhibiting the growth of smooth muscle, and it inhibits clot formation and plaque formation. Endothelin and ACE promote vasoconstriction. ACE inhibitors reduce blood pressure by inhibiting vasoconstriction, thus allowing natural vasodilators to have more effect. The effects of *NO usually dominate, keeping vessels adequately dilated.

Middle Layer: Large Arteries

The middle layer of the largest arteries consists mostly of elastic fibers that make the arteries strong. Most of the elastic fibers in the aorta are produced before birth or during childhood, but some new elastin is produced throughout life. Since these arteries are closest to the heart, strength is necessary to withstand the high blood pressure produced by each heartbeat.

This layer also provides elasticity, which allows the arteries to be stretched outward somewhat each time the ventricles pump blood into them. The extra space provided by the stretching prevents the systolic pressure from rising too high, and the work the heart must perform is kept reasonably low, just as it is easier to blow up an easily stretched balloon than a stiff one. Preventing excessive pressure also keeps the arteries from being injured by the accompanying extreme forces.

The elasticity of large arteries helps to prevent blood pressure from rising too high in yet another way. Unusually high blood pressure immediately causes normal arteries to be stretched outward excessively. Nerve cells in the walls of arteries detect this abnormal stretching and send signaling impulses to the blood pressure control center in the brain. Other factors from the artery that influence the sensory neurons include prostacyclin, which increases the signals, and reactive oxygen species, which reduce them. The brain then sends impulses to the heart telling it to pump less blood. It also tells blood vessels in various areas of the body to dilate to provide more space for the blood coming from the heart. As a result, blood pressure decreases and the large arteries return to their normal size. The nerve cells are then no longer activated, and blood pressure stabilizes at the normal level. Note that this is a negative feedback system that maintains proper and fairly stable conditions in the body.

The nerves in blood vessels send different impulses to the brain when blood pressure is too low and arteries are not stretched enough. The result is the sending of norepinephrine and related substances to the heart and vessels. These substances raise blood pressure by several means, including stimulating the heart and causing the smaller arteries to constrict.

Elasticity also causes the arteries to snap back to their original diameters when the ventricles are relaxing. This elastic recoil helps maintain diastolic pressure between beats by squeezing the blood. Diastolic pressure keeps the blood moving forward steadily while the heart rests briefly after each beat. Thus, elastic recoil serves the same purpose as the spring that keeps a watch ticking between windings.

Middle Layer: Smaller Arteries

The middle layer of smaller arteries contains some elastic fibers but is composed mostly of smooth muscle (Figure 4.9b). When the muscle contracts, it causes the arteries to constrict, reducing the flow of blood.

Figure 4.9b Structure of a smaller artery. (Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland. Used with permission).

As a rule, the smooth muscle in most smaller arteries contracts weakly, providing some resistance to flow while allowing ample blood flow through the arteries. This resistance is important because without it, blood would flow from the arteries into the capillaries so quickly that blood pressure would drop too low, especially between heartbeats. The same effect is observed with tires. A tire with a tiny hole loses pressure so slowly that it can be reinflated before any harm is done, while a tire with a large leak loses all its pressure and goes flat very quickly.

In normal situations the constriction of small arteries increases whenever blood pressure begins to drop too far. This additional constriction increases resistance and raises blood pressure back to the proper level. Conversely, if blood pressure rises too high, the smooth muscle relaxes, the arteries dilate, resistance drops, and blood pressure decreases back to normal levels.

In addition to regulating blood pressure by constricting or dilating as a group, individual arteries can constrict to reduce blood flow to organs that need little flow while others dilate to increase flow to more active organs. Thus, the smaller arteries act like a set of valves or traffic signals to make sure that each part of the body receives only as much blood flow as it needs.

The constriction and dilation of smaller arteries are controlled by negative feedback systems. The arteries respond to several factors, including nerve impulses, hormones, temperature, and chemical conditions in their vicinity. These mechanisms help maintain normal blood pressure and blood flow to each body structure.

Outer Layer

The outer layer of arteries consists largely of loose connective tissue containing soft gel and scattered fibers. This layer loosely attaches arteries to other structures, enabling arteries to be shifted as parts of the body move while preventing the arteries from moving too far out of position.