Plasma colloid osmotic pressure in venous blood from the human foot in orthostasis.
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Several theories could explain the slow rate of edema formation in the feet of sitting or standing man. One possible mechanism is a rise in local plasma colloid osmotic pressure (pi p). We measured pi p in blood from superficial veins of warm and cold feet during orthostasis. The difference in venous pi p between the foot and the arm averaged 12.8 mmHg (range 9.4-16.1 mmHg) in subjects sitting in a cold room. Hemoconcentration was also reflected as increased hematocrit. These findings support the view that a local increase of pi p in foot capillaries contributes considerably to edema prevention during orthostasis.