BIOLOGY Volume 2 - A Guide to General Biology - 2004

14. TRANSPORT IN ANIMALS

14.5. Blood vessels

14.5.1. General structure

Blood circulating through the body passes through Arteries, capillaries, and Veins. In principle, the walls of both arteries and veins consist of three layers: 1) an inner layer, or endothelium (tunica intima), formed of squamous epithelium; 2) a middle layer (tunica media), consisting of smooth Muscle Cells and elastic fibers; and 3) an outer layer (tunica externa), composed primarily of inelastic Collagen fibers.

Structural Features of arteries and veins are listed in Table 14.3 and shown in Fig. 14.3. Some of the Tissues that make up these vessels are discussed in Chapter 6 (see Fig. 6.14, 6.22, and 6.23).

Class="center">Table 14.3. Structure AND Functions of arteries, capillaries, and veins (diagrams are not to scale)

Arteries

Veins

Capillaries

Carry blood away from The Heart

Carry blood toward the heart1

Connect arteries and veins. Serve as the site of exchange of substances between blood and other tissues

Middle layer thick, consisting of elastic fibers and smooth muscle cells

Middle layer relatively thin, with few smooth muscle cells and elastic fibers

Middle layer absent. The only tissue is squamous endothelium (sec. 6.3.1). Elastic fibers absent

No internal Valves (except at the exit from the heart)

Large veins have valves at regular intervals that prevent backflow of blood

No internal valves

Blood pressure high, pulsatile

Blood pressure low, non-pulsatile

Blood pressure decreasing, non-pulsatile

Blood flows rapidly

Blood flows slowly

Blood flow slows down

Blood volume small

Blood volume much greater than in capillaries or arteries

Blood volume large

Blood is oxygenated, except in the pulmonary arteries

Blood is deoxygenated, except in the Pulmonary veins

Blood gradually becomes deoxygenated (the reverse occurs in the Lungs)

1 Portal veins are an exception.

Fig. 14.3. A. Cross-section of an artery and a vein. B. Cytology/practical/54.html">Longitudinal section of a vein showing a valve.



Last update: 06/08/2026

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