Arteries, arterioles and the distribution of blood flow

===Lecture Details===
Roger Evans; Week 11 MED1022; Physiology
===Lecture Content===
Inner layer is tunica interna which is the endothelium, close endothelial lining, separates from underlying muscle (media), controls diameter by releasing substances that constrict or widen the vessel. Middle layer is media, consists of smooth muscle and sheets of elastic tissue. Circular muscle can change diameter, elastic layer contributes to elastic capacity. Tunica externica is loosely woven outer collagen layer, protects, reinforces and anchors. Capillaries only have endothelium. Pressure in a tube depends on volume of fluid in it and distensibility of walls. Elastic nature of large arteries allows continuous flow in systole and diastole- systole 1/3 flows out, diastole 2/3. Pulse pressure is systolic - diastolic pressure. MAP is average pressure over one cycle; is diastolic pressure + 1/3 pulse pressure.
Arteries have low resistance to flow, not much loss of pressure going along the arteries (pressure in femoral artery similar to aorta, carotids). Arterioles regulate flow, they control proportional distribution of flow to different organs and tissues of body through changes in diameter. Arterioles usually are at some level of continuous contraction, this allows for dilation by relaxing contraction or constriction.
Blood flow increases when local metabolic activity increases (active hyperaemia). Is due to dilatation of arterioles caused by local chemical factors (decreased O2, increased CO2, increased metabolites, increased hydrogen). Especially important in skeletal muscles and the heart. Hormones and factors related to the endothelium also play a part in local chemical changes. Local physical influences are myogenic response to stretch (increased stretch has increased contraction, decreased stretch has decreased contraction); particularly good in the brain. Local physical influences are temperature, reactive hyperemia (after period of occlusion, increased blood flow).
Most arterioles are sympathetically innervated, density of nerves differs between vascular beds. They release NA; acts on a adrenoreceptors in smooth muscle, causes contraction. There is no significant parasympathetic except in the genitals. NA causes dilation when acting on areas with beta adrenoreceptors. Endothelial cells release NO, prostacyclin and vasoconstrictor substances- shear stress can cause.
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