Page 111 - Textbook of Pathology, 6th Edition

Normal Fluid Pressures There is considerable pressure gradient at the two ends of 95
1. OSMOTIC PRESSURE. This is the pressure exerted by capillary loop—being higher at the arteriolar end (average
the chemical constituents of the body fluids. Accordingly, 32 mmHg) than at the venular end (average 12 mmHg).
osmotic pressure may be of the following types (Fig. 5.3,A): Tissue tension is the hydrostatic pressure of interstitial
Crystalloid osmotic pressure exerted by electrolytes fluid and is lower than the hydrostatic pressure in the
present in the ECF and comprises the major portion of the capillary at either end (average 4 mmHg). CHAPTER 5
total osmotic pressure. Effective hydrostatic pressure is the difference between
Colloid osmotic pressure (Oncotic pressure) exerted by the higher hydrostatic pressure in the capillary and the lower
proteins present in the ECF and constitutes a small part of tissue tension; it is the force that drives fluid through the capillary
the total osmotic pressure but is more significant wall into the interstitial space.
physiologically. Since the protein content of the plasma is
higher than that of interstitial fluid, oncotic pressure of Normal Fluid Exchanges
plasma is higher (average 25 mmHg) than that of interstitial Normally, the fluid exchanges between the body
fluid (average 8 mmHg). compartments take place as under:
Effective oncotic pressure is the difference between the At the arteriolar end of the capillary, the balance between
higher oncotic pressure of plasma and the lower oncotic the hydrostatic pressure (32 mmHg) and plasma oncotic
pressure of interstitial fluid and is the force that tends to draw pressure (25 mmHg) is the hydrostatic pressure of 7 mmHg
fluid into the vessels.
which is the outward-driving force so that a small quantity
2. HYDROSTATIC PRESSURE. This is the capillary blood of fluid and solutes leave the vessel to enter the interstitial
pressure. space. Derangements of Homeostasis and Haemodynamics

Figure 5.3 Diagrammatic representation of pathogenesis of oedema (OP = oncotic pressure; HP = hydrostatic pressure). A, Normal pressure
gradients and fluid exchanges between plasma, interstitial space and lymphatics. B, Mechanism of oedema by decreased plasma oncotic pressure
and hypoproteinaemia. C, Mechanism of oedema by increased hydrostatic pressure in the capillary. D, Mechanism of lymphoedema.
E, Mechanism by tissue factors (increased oncotic pressure of interstitial fluid and lowered tissue tension). F, Mechanism of oedema by increased
capillary permeability.