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The Vascular System Basic components of a blood vessel wall:
- 1. Be able to describe the organization of the vascular system
- 2. Know the layers of blood vessel wall
- 3. Describe why the arterial system has high pressure
- 4. Describe why the venous system haslow pressure
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Basic components of a blood vessel wall:
- intima, internal elastic lamina
- media, external elastic lamina
- adventitia
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components of a blood vessel:
- connective tissue: provide tensile and elastic strength via collagen and elastin
- vascular smooth muscle: maintain vascular tone to regulate resistance and blood pressure and consists of contractile smooth muscle cells ( nitric oxide, prostacyclin and endothelin that can cause relaxation or contraction of vascular smooth muscle)
- endothelium: selective barrier to adjust permeability. It also secretes factors that stimulate new vessel growth (angiogenesis) and regulate proliferation of vascular smooth muscle cells in the vessel wall. Also prevents platelet adhesion and thrombus formation.
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Arteries:
- serve as pressure reservoir and have a large radius, muscular wall, abundance of fibrous material, elastic wall.
- Cardiac contraction stretches the arterial wall depending on the amount of blood volume and distensibility of the wall. The elastic fibres then recoil allowing continuous blood flow during diastole.
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Steps to elastic recoil:
- Ventricle contracts: Semilunar valve opens. Aorta and arteries expand and store pressure in elastic walls.
- Isovolumic ventricular relaxation: Elastic recoil of arteries sends blood forward into rest of circulatory system. Semilunar valve shuts, preventing flow back intoventricle.
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Arterioles:
- highly muscularwalls, smaller radii, main site of resistance, drop in pressure gradient (for flow into capillaries)
- pulsatile systolic/diastolic swings become non-pulsatile as blood pressure is attenuated in the arterioles, and pressure continues to drop on the way to capillaries.
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Capillaries:
- Most abundant vessel, exhange occurs at this site
- consists of 1 cell layer apposed to a basement membrane.
- lack smooth muscle and elastic tissue to facilitate exchange.
- 1 um thick and 7-8 um in diameter
- capillaries in brain have pericytes which help establish blood brain barrier.
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Types of capillaries:
- continuous: the junctions between endothelial cells are closely apposed together, preventing substances from crossing
- fenestrated: abundant in the kidney glomerulus. Large gaps between endothelial cells allow passage of large molecules
- sinusoids: very porous capillaries present in the liver
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Veins and venules:
- have a large radii, and thus low resistance to blood flow.
- Capacitance vessels. Serve both as a passage way to the heart and as blood reservoir.
- Less smooth muscle than arteries but more collagen fibers. (has little myogenic tone)
- Have little elasticity and thus no recoil
- coalesce in to larger veins as they approach the heart and as the cross sectional area decreases(bigger but far fewer vessels), the blood flow increases as veins approach the heart?
- Contain valves to prevent back flow.
- richly supplied by sympathetic nerve fibres causing vaso constriction and increases in pressure which decreases the capacity but resistance is affected insignificantly because the diameter is still large.
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