Abstract
There are two pressures in the tissues which affect the flow of fluid across the capillary wall: the interstitial hydrostatic and oncotic pressures. The methods used to measure these pressures have been the subject of controversy and this has led to some doubt about the correct values. The manometer described below was designed to minimise the problem of scar tissue formation, slow equilibration and inflammation encountered in other techniques.
Two types of membrane are used. The first is a large pore membrane which allows the measurement of the sum of the hydrostatic pressure and the pressure due to the interstitial matrix. The second is a small pore membrane which permits the determination of the sum of the hydrostatic and oncotic pressures in the interstitium.
The aim of the manometer design is to achieve extremely rapid measurement of pressures. Equilibration can usually be obtained in 1–2 minutes, although with some membranes as little as 30 seconds is required.
When the subcutaneous fascia of the ventral abdomen of the rat is rapidly exposed and the membranes place on it, the following pressures are found. The large pore membrane gives a mean pressure of −1.85 cmH2O (S.D. 0.7, n = 20) in the superficial fascia and a mean pressure of −1.85 cmH2O (S.D. 0.8, n = 7) on the fibrous tissue overlying the abdominal muscles. The small pore membrane gives a mean pressure of −12.53 cmH2O (S.D. 2.57, n = 13) on the superficial fascia and the same pressure on the fibrous tissue overlying the muscles. Subtraction of the pressure found with the large pore membranes from that found with the small pore membranes gives the oncotic pressure due to plasma proteins in the tissues. This pressure was −10.7 cmH2O.
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