Veins Venules have little resistance or tone communicate chemically with nearby

Veins venules have little resistance or tone

This preview shows page 50 - 58 out of 58 pages.

Veins Venules (have little resistance or tone) communicate chemically with nearby arterioles to control capillary flow Veins serve as a blood reservoir containing 60% of blood volume in body Veins return blood back to the heart Large vein Capillaries → venules → small veins → large veins → Heart Endothelial cells Smooth Muscle (& elastin) valve
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Systemic veins 64% Heart 7% Pulmonary vessels 9% Systemic arteries 13% Systemic arterioles 2% Systemic capillaries 5% Percentage of total blood volume in different parts of circulatory system
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Venous Return Venous return : volume of blood per min entering each atrium from the veins Venous return is enhanced by: venous vasoconstriction by sympathetic system external compression of the veins from contraction of surrounding skeletal muscles one-way venous valves (spaced at 2-4 cm intervals) These actions help counter the effects of gravity on the venous system
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Venous return Cardiac suction effect ( pressure in heart pressure gradient) Venous valves (mechanically prevent backflow of blood) Pressure imparted to blood by cardiac contraction ( venous pressure pressure gradient) Salt and water retention Skeletal muscle pump ( venous pressure- pressure gradient) Respiratory pump ( pressure in chest veins pressure gradient) Blood volume ( venous pressure pressure gradient) Passive bulk-flow shift of fluid from interstitial fluid into plasma KEY = Short-term control measures = Long-term control measures Factors that facilitate venous return (by increasing the pressure gradient)
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Calf Foot vein supporting column of blood 34 cm in height Foot vein supporting column of blood 1.5 m (150 cm) in height Walking Standing Foot 27 mm Hg Venous pressure in foot 100 mm Hg 150 cm Heart Thigh 34 cm Effect of contraction of skeletal muscles of the legs in counteracting gravity Contraction empties a given vein segment, reducing amount of blood the lower veins must support
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Constriction forces fluid flow in both directions Fluid-filled tube Point of constriction Action of venous valves (permits flow of blood toward heart & prevents backflow) Closed venous valve prevents backflow of blood Contracted skeletal muscle Vein Open venous valve permits flow of blood toward heart Venous valves help counteract gravity forces
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Varicose veins
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Deep vein thrombosis (DVT) When a blood clot (thrombus) forms in a vein deep in the body “Economy class syndrome” most common in legs - lower leg & thigh More common >60 yrs old, but can occur at any age Blood clots can break loose & travel through bloodstream eventually blocking blood flow to tissues (lungs, brain, heart, etc.)
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Compression stockings Compression stockings/socks “flight socks” Medical Compression Stockings
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