Veins and venous return

===Lecture Details===
Roger Evans; Week 11 MED1022; Physiology
===Lecture Content===
60% of TBV is in the veins, at venules pressure is 15mmHg, in right atria pressure is 0. Veins must provide a low resistance pathway back to the heart. Veins have large diameters and have a low resistance to flow. They are thin walled and very distensible, so make good volume reservoirs. Right atrial pressure is sometimes known as central venous pressure; pressure in peripheral veins is peripheral venous pressure. Venous pressure is determined by volume of blood and distensibility of vein walls. Smooth muscle is present in walls of veins, recieves sympathetic input, contraction of smooth muscle stiffens the vein walls (doesn't really contract it much); increases venomotor tone. Increased venous pressure increases venous return, increases EDV, increases stroke volume (Frank-Starling), increases CO.
Muscle pump helps venous return, particularly in exercise. Respiratory pump is when alternating changes in abdominal pressure (increase)- expansion is due to reduced intrathoracic pressure during inspiration and there is compression of veins due to skeletal muscle contraction and descent of diaphragm during inspiration. Cardiac suction is when the AV valve is pulled down during contraction of the ventricle which expands the atrial space and sucks the blood from the veins to the atria. Ventricles suck blood from the atria when they relax. Venous valves are one way valves, located periodically in most veins, allow flow towards the heart but prevent backflow, important for skeletal muscle and respiratory pump. Important to counteract the effects of gravity. Varicose veins are tortuous commonly seen in the legs, blood pools in superficial veins and they become distended. If venous return falls too low there is an inadequate SV. If this happens CO falls and not enough blood is delivered. 
Vascular function curve relates venous return is related to right atrial pressure. When right atrial pressure is low, venous return is high. Negative right atrial pressure collapses veins. If venous return and CO are plotted together, intersection shows operating state of CV system under those conditions. Vascular function curve can be changed by venomotor tone, blood volume, changes in constriction/dilation of 'upstream' arterioles. Cardiac function curve is changed by contractility.
Response to haemorrhage has two responses- vasoconstriction (to stop blood loss) and vasodilation (to get more blood back to the heart). Hypotension can occur from standing up.
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