Net Pressure Gradient Research Articles

Colloid osmotic pressure of the subcapsular interstitial fluid of rat kidneys during hydropenia and volume expansion.

To determine the colloid osmotic pressure of subcapsular interstitial fluid in rat kidneys two different methods were used. Collection of subcapsular fluid with glass pipettes or implanted microcatheters and subsequent protein analysis resulted in a protein concentration of 1.8g%±0.6 and 2.0g%±0.8, respectively. Lymph protein concentration was not significantly different from that of subcapsular fluid samples. During extracellular volume expansion both subcapsular and lymph protein concentration fell to 0.42g%±0.23 and 0.7g%±0.5. Application of anin vivo oncometric method resulted in an effective oncotic pressure about twice that estimated from protein determinations. Using average values for intratubular and intracapillary oncotic and hydrostatic pressures a tubulo-interstitial net driving force of 20 mm Hg and an interstitial-capillary net driving force of 13 mm Hg is estimated in hydropenic animals. During volume expansion net transtubular pressure gradient is reduced to about 60–70% of control while the transcapillary gradient is virtually unchanged.

Peristaltic carrying and mixing of chyme in the small intestine (An analysis of a mathematical model of peristalsis of the small intestine)

The motion of a fluid created by the peristalsis is a problem of importance for a better understanding of some physiological transport phenomena. As a mathematical model of a rather unusual type of peristalsis involved in small intestines, the low Reynolds number flow of a Newtonian fluid in a circular cylindrical tube involving a series of traveling nodal constrictions, which is axisymmetric and infinitely sharp, is studied. The solution is posed in a series form. Two fundamental solutions are (1) a peristaltic carrying in which the motion of the fluid is created by the peristalsis without a net pressure gradient and (2) a peristaltic compression without net transport of the fluid. A general solution is obtained by a linear combination of these two fundamental solutions. Numerical calculation is carried out for a set of combinations of internode distances and depth of nodal constrictions. The distribution of the fluid velocity and the effectiveness of the peristaltic carrying and compression is discussed. The physiological significance of carrying, mixing and compression, accompanied by peristalsis is discussed.

Postural Changes in Plasma Volume in Hypoalbuminemia

Egress and ingress of intravascular fluid are governed largely by the net pressure gradient of the opposing forces of hydrostatic pressure and plasma oncotic pressure. Although the concept of receptors is gaining wide acceptance, 1-3 their role in the moment-to-moment regulation of the blood volume is uncertain. It is generally agreed that in normal subjects the blood volume decreases with standing. 4-6 There are few studies, however, concerned with the magnitude of this change under circumstances of a disturbance in the forces controlling movements of fluid across the capillary membrane. In the present study, alterations in plasma volume on standing were observed in subjects with hypoalbuminemia in order to assess the role of lowered oncotic pressure in this reaction. Materials and Methods Radiochromium was employed as a reference for the determination of red cell mass; plasma volume was then estimated from this value and the venous hematocrit. This procedure