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Humans have roughly 5 to 6 liters of blood at any given time in their  body  . Out of  this*

50% (2500ml)  is located in the systemic venous compartment.
18% is within the pulmonary circulation participating in the vital oxygenation
12% (500-600ml) is within the cardiac chambers.
8%  is in the arterial tree of  the body.
5%  is  within the  capillaries.
2%  is in the aorta.
* Source : Best & Taylor Physiological basis of  medical practice 1966, 8th edition

What is the implication of this predominantly venous distribution of blood  at rest ?

  • A competent venous tone is essential  for the human beings to maintain the erect posture.
  • Bulk of the cause of syncope in humans is due to peripheral  mechanism like loss of vascular tone and resultant venous pooling.
  • The  concept of venous reservoir is so important in emergency situations like  hypotension  as  simple elevation of legs  is equivalent to  infusing 500 -800 ml of intravenous saline .
  • Similarly during acute left ventricular failure trunk elevation and legs dangling down can reduce the pulmonary congestion very significantly and reduce pulmonary capillary wedge pressure (LVEDP)

 Autonomic dysfunction and venous insufficiency

 Autonomic dysfunction and resultant  orthostatic hypotension is directly related  to venous reservoir dysfunction.Increasing effective circulatory volume by elastic stockings or administration of mineralocorticosteroids like fludrocortisone (.5mg/day ) can be useful in this condition

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