The normal function of parenchymal cells is largely dependent on the integrity of the interstitial tissues that make up the immediate microenvironment of the cells. Interstitial tissue is composed of cells, water and electrolytes, ground substance, and fibrillary elements (Figure 2-1). The pH and the electrolyte composition of interstitial tissue are maintained in equilibrium both with those of plasma in capillaries and with those of the intracellular fluid compartment. The ground substances and supporting fibers of interstitial tissue are produced by specialized connective tissue cells derived from the mesoderm (mesenchymal cells), mainly fibroblasts.
Composition of interstitial tissue. The interstitial fluid is in equilibrium with plasma on one hand and parenchymal cell cytoplasm on the other. Movement of water and electrolytes among plasma, interstitium, cells, and lymphatics is shown by arrows.
Interstitial injury may result from changes in plasma composition or from local changes in the tissue (eg, necrosis of parenchymal cells).
Accumulation of abnormal material in the interstitial tissue may cause structural abnormality without affecting the function of parenchymal cells (eg, increased numbers of fat cells; obesity, Chapter 10: Nutritional Diseases). More commonly, however, interstitial abnormalities result in secondary dysfunction of parenchymal cells.
Accumulation of Excess Fluid (Edema)
Edema may occur in all tissues but is most easily seen in the skin. The earliest clinical evidence of edema in the skin is the presence of pitting (the ability to produce a depression or pit in the skin by sustained finger pressure). Visible swelling of the skin occurs only when a large amount of excess fluid has collected (see Figure 3-1, which shows edema associated with an infected burn).
Edema also includes accumulation of fluid in body cavities such as the pleural cavity (hydrothorax, pleural effusion), peritoneal cavity (ascites), and pericardial cavity (pericardial effusion). Anasarca denotes massive edema of the whole body, including the body cavities.
Edema may be classified as localized (caused by local disturbance of the fluid exchange mechanism in the tissue) or generalized (caused by retention of sodium and water in the body). The distribution of the retained fluid in generalized edema is gravity-dependent, ie, around the ankles in ambulatory patients and the sacral region in bedridden patients.
Fluid exchange through the normal capillary wall is governed by the balance of opposing forces: capillary hydrostatic pressure forces fluid out; plasma colloid osmotic pressure draws it in. Normally, tissue hydrostatic and colloid osmotic pressures are near zero and do not affect this fluid exchange. Fluid passes out of the capillary mainly at the junctions between endothelial cells (pores), which permit only small nonprotein molecules to pass through (ultrafiltration). Almost all protein is retained in the vessel. The small amount of protein that escapes the capillary is ...