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