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http://wiki.answers.com/Q/What_does_the_venous_return_depend_on
http://www.askabiologist.org.uk/answers/viewtopic.php?id=1882
how this happens I always think that increasing the blood pressure in veins would help movement
thanks
Blood pressure at venous end of capillary: An increased pressure at the venule end of the capillary reduces venous return, because the pressure difference between the right atrium and the peripheral veins is reduced. Similarly a decrease in pressure at the venule end would increase venous return.
http://www.askabiologist.org.uk/answers/viewtopic.php?id=1882
Surely increasing the blood pressure in veins would help movement, and obviate the need for valves?
No. Because the pressure gradient is reduced, there would be a problem with adequate circulation. In addition, the capillaries, where the business of gas and nutrient exchange takes place, have permeable walls. An increase in venous pressure would increase the pressure inside the capillaries leading to 'leak' of fluid from the blood into the tissue (a bit like tea being squeezed from a tea bag). The tissue becomes 'water-logged' (clinicians and scientists call this 'oedema') and it is a problem because it increases the distances that gases and nutrients must diffuse to meet the metabolic needs of the tissue. This problem happens in congestive heart failure, where the venous pressure increases due to inadequate pumping by the heart.
how this happens I always think that increasing the blood pressure in veins would help movement
thanks