Arteries and arterioles

The cardiovascular system

The cardiovascular system is divided for descriptive purposes into two main parts.

• The circulatory system, consisting of the heart, which acts as a pump, and the blood vessels through which the blood circulates

• The lymphatic system, consisting of lymph nodes and lymph vessels, through which colourless lymph flows (see Ch. 6).

The two systems communicate with one another and are intimately associated.

The heart pumps blood into two anatomically separate systems of blood vessels (Fig. 5.1).

• the pulmonary circulation

• the systemic circulation.

The right side of the heart pumps blood to the lungs (the pulmonary circulation) where gas exchange occurs; i.e. C0₂ leaves the blood and enters the lungs, and 0₂ leaves the lungs and enters the blood. The left side of the heart pumps blood into the systemic circulation, which supplies the rest of the body. Here, tissue wastes are passed into the blood for excretion, and body cells extract nutrients and 0₂.

The circulatory system ensures a continuous flow of blood to all body cells, and its function is subject to continual physiological adjustments in order to maintain

an adequate blood supply. Should the supply of oxygen and nutrients to body cells become inadequate, tissue damage occurs and cell death may follow.

Blood vessels

Arteries and arterioles

These are the blood vessels that transport blood away from the heart. They vary considerably in size and their walls consist of three layers of tissue (Fig. 5.3):

• tunica adventitia or outer layer of fibrous tissue

• tunica media or middle layer of smooth muscle and elastic tissue

• tunica intima or inner lining of squamous epithelium called endothelium.

The amount of muscular and elastic tissue varies in the arteries depending upon their size and function. In the large arteries, sometimes called elastic arteries, the tunica media consists of more elastic tissue and less smooth muscle. This allows the vessel wall to stretch, absorbing the pressure wave generated by the heart as it beats. These proportions gradually change as the arteries branch many times and become smaller until in the arterioles (the smallest arteries) the tunica media consists almost entirely of smooth muscle. This enables their diameter to be precisely controlled, which regulates the pressure within them. Systemic blood pressure is mainly determined by the resistance these tiny arteries offer to blood flow, and for this reason they are called resistance vessels.

Arteries have thicker walls than veins and this enables them to withstand the high pressure of arterial blood.

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