Hemodynamics and interstitial fluid pressure in the rat tail.

Blood flow in the rat was measured during pentobarbital anesthesia by plethysmographic and thermometric techniques. Tail arterial and venous pressures (Pa and Pv) were measured by glass micropipettes and interstitial fluid pressure (PIF) by wick-in-needle technique. Large pressure gradients were measured along the tail, Pa decreasing and Pv increasing toward the tip. In the vasoconstricted tail, distal arterial and venous pressures (Pad and Pvd, respectively, 10 cm from the tail root) were 55 and 11% of aortic pressure (PA), while PIF was 0-2 mmHg. Plasma volume expansion increased blood flow by a factor of 10 to 35. Pad rose to 74% and Pvd to 20% of PA. PIF increased to 15 mmHg, in parallel with Pv. Venous stasis (cuff pressure 14.7 mmHg) increased PIF and Pv by 3.5 and 9 mmHg, respectively, while tail volume increased by 0.4 to 1.2%. In conclusion, the large flow increase induced by plasma volume expansion depends strongly on dilation of the tail artery, with two- to threefold increase in internal radius. Simultaneously the tail veins relax and expand. Subcutaneous tissue is compressed between the expanding vessels and the tight skin, and PIF increases almost sufficiently to prevent a rise in net capillary filtration pressure. This immediate edema-preventing mechanism is less efficient during venous stasis, which presumably does not induce "active" dilation of the tail vessels. Similar mechanisms probably exist in other "encapsulated" tissues.