Blood Vessels 

THE VASCULAR SYSTEM OF BLOOD VESSELS must:

	ensure delivery of blood to all tissues
	be flexible and adaptable so that blood flow can be varied according to the metabolic requirements of individual tissues or the body as a whole
	convert a pulsating blood flow in the arteries into a steady flow in the capillaries to allow optimum transfer of substances to and from the cells
	return blood to the heart

BLOOD VESSELS

There are three main types of blood vessels:

	arteries, which carry blood away from the heart at relatively high pressure.
	veins, which carry blood back to the heart at relatively low pressure.
	capillaries which provide the link between the arterial and venous blood vessels

The vascular system is composed of arteries, arterioles, capillaries, venules and veins. The structure of the vessels in the different parts of the vascular system varies and the differences relate directly to the function of each type of vessel. The walls of all the blood vessels, except the capillaries which are only one cell thick, have the same basic components but the proportion of the components varies with function.

Table 1. Comparison between different types of blood vessels

Vessel	aorta	artery	arteriole	capillary	venule	vein	vena cava
Wall	2mm	1mm	20mm	1mm	2mm	0.5mm	1.5mm
Lumen	25mm	4mm	30mm	8mm	20mm	5mm	30mm
Size in relation to each other. (n.b. this is not to scale for comparative purposes only)
Composition of cell wall endothelium (black)      elastic tissue (green)      smooth muscle (magenta) connective tissue (orange)
Number c. = approximately	1	c.100	c.108	c.1010	c.109	c.100	2
Pressure mmHg	100	100-40	40-30	30-18	18-10	10-5	5-0
Flow mm/sec	330-400	50	25	5	10	35	150
Cross sectional area cm2	8	20	700	5000	3000	20	14
Vessel	aorta	artery	arteriole	capillary	venule	vein	vena cava

All blood vessels (except capillaries as mentioned above) are composed of three layers surrounding a central blood carrying canal (known as the lumen). The three layers vary in prominence in different vessel types.

The outer layer of blood vessels is composed largely of collagen, but smooth muscle cells may be present, particularly in veins. This is often the most prominent layer in the walls of veins.

The middle layer in a blood vessel wall and is composed predominantly of smooth muscle reinforced by organised layers of elastic tissue which form elastic laminae. It is particularly prominent in arteries, being relatively indistinct in veins and virtually non-existent in very small vessels. In vessels which are close to the heart, receiving the full thrust of the systolic pressure wave, elastic tissue is very well developed, hence the term elastic arteries.

The inner layer is composed of a lining layer of highly specialised multifunctional flattened epithelial cells termed endothelium. It is the only layer that is present in all blood vessels.