Читать книгу Understanding Anatomy and Physiology in Nursing - John Knight - Страница 107

Amazingly, the human body has between 60,000 and 100,000 miles of blood vessels which function as conduits through which our 5 litres of blood is continuously circulated. There are three major types of blood vessel: arteries, veins and capillaries.

Arteries and veins are the largest blood vessels and both consist of three distinct layers (tunics) of tissue, outlined in Figure 3.7.

 The tunica externa: This is the protective outer layer of the vessel composed predominantly of collagen-rich connective tissue. It is usually continuous with the surrounding tissues, serving to anchor the blood vessel in position within the body and prevent vessel movement following ejection of blood from the heart or during the physical movement of the body.

 The tunica media: Composed of involuntary smooth muscle, this middle layer can contract (vasoconstriction) or dilate (vasodilation) to change the diameter of the blood vessel and alter the rate of blood flow. The tunica media is much thicker in arteries than in veins since arteries are usually carrying blood under high pressure and their walls require extra reinforcement. The smooth muscle layers are innervated by sympathetic nerve fibres which are under the influence of the vasomotor centre within the medulla oblongata of the brain. This is the region of the brain that regulates vascular tone and therefore blood pressure by controlling the processes of vasoconstriction and vasodilation.

 The tunica intima: This is the thinnest and innermost layer of the blood vessel. It is composed of a single layer of incredibly smooth squamous epithelial cells (the endothelium) and is separated from the smooth muscle cells of the tunica media by a thin layer of collagen-rich tissue termed the lamina. In arteries the smooth, silky nature of this innermost layer affords minimal resistance, ensuring that blood flows rapidly in concentric layers (laminar blood flow).

Figure 3.7 The internal structure of an artery and vein