Review of Medical Physiology - William F. Ganong 2002

Introduction
General Principles and Cellular Basis of Human Physiology
Capillary Wall

Filtration

The capillary wall separating plasma from the interstitial fluid differs from the cellular membranes separating interstitial fluid from intracellular fluid because they are subject to different pressure gradients. As a result, filtration plays a major role in the Movement of Water and solutes. By definition, filtration is the process whereby fluid is driven across a membrane or other barrier due to a pressure difference between the two sides. The volume of fluid filtered per unit time is directly proportional to the pressure gradient, membrane surface area, and membrane permeability.

Oncotic Pressure

The Structure of the capillary wall varies across different vascular beds (see Chapter 30). However, in Skeletal Muscle and many other Organs, only water and relatively small solutes pass freely through the wall. The apertures in the capillary wall (the junctions between endothelial Cells) are too small to allow significant amounts of Plasma Proteins and other colloids to pass. Small quantities do cross the capillary wall via transcytosis (see below), but their effect is negligible. Consequently, the capillary wall acts as a semipermeable membrane to colloids, exerting an Osmotic Pressure of approximately 25 mmHg. The colloid osmotic pressure generated by plasma colloids is referred to as oncotic pressure. Filtration across the capillary membrane, driven by vascular hydrostatic pressure, is counteracted by oncotic pressure. The exact way in which the balance between hydrostatic and oncotic pressures regulates exchange across the capillary wall is discussed in detail in Chapter 30.

Transcytosis

Vesicles are present within the Cytoplasm of endothelial cells. Circulating tracer proteins introduced into the bloodstream have been detected within these vesicles and the interstitium, indicating that small amounts of protein are transported via endocytosis followed by exocytosis from the capillaries through the endothelial cells to the interstitial side of The Cell. This transport mechanism involves coated vesicles and is known as transcytosis, Vesicular Transport, or cytopempsis.