* Damping vessels: arteries. The conduit and large distributing arteries offer little resistance to blood flow and help in the propagation of the arterial pulse. They have a windkessel or air cushion effect by virtue of partial accommodation of the stroke volume with ventricular systole. They offer dynamic resistance to the oscillatory components of pulsatile flow, constituting vascular impedance. This damps the pressure oscillations caused by intermittent ventricular ejection. With distal movement of the arterial pressure pulse, there is a rise in systolic pressure, a fall in diastolic pressure and a widening of the pulse pressure. The peak systolic pressure is amplified with passage down the lower limbs.
* Resistance vessels: small arteries; arterioles. These vessels are largely responsible for vascular resistance and the maintenance of blood pressure. A micro-circulatory unit is a collection of vessels taking origin from an arteriole.
* Exchange vessels: capillaries, which are concerned with the transfer of nutrients and waste products between the blood and tissues. They are thin-walled, consisting of a single layer of endothelial cells (with no muscle or connective tissue) and have a large surface area. The endothelial cells are surrounded by a basement membrane and a fine network of reticular collagen fibres. Capillaries form branching networks and have a high density in metabolically active tissues such as glands, and cardiac and skeletal muscle. Capillary endothelial cells possess the property of forming new capillaries (angiogenesis).
Capillaries may vary in structure according to functional needs as follows: Fenestrated or visceral capillaries (glomerulus, choroid plexus, intestinal epithelium, ciliary bodies of the eye) with fenestrations or pores, 60-80 nm in diameter, with or without a thin diaphragm.
a Continuous or somatic capillaries (muscle; skin; lungs; brain; thymus; bone):
0 with tight junctions and a continuous basal lamina.
s Discontinuous capillaries (liver, bone marrow, spleen), with wide intercellu-
1 lar gaps.
y • Capacity vessels (collecting and reservoir system): veins and venules, vena e cavae, right atrium. Two thirds of the blood is contained within the venous system, which performs a reservoir function.
Arterio-venous shunts connect arterioles and venules and are common in the skin in certain parts of the body, including the fingertips and ear lobules. They are involved in thermoregulation.
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