Key Points

• The microstructure of the capillary wall varies between tissues, with the highest rate of exchange of water and proteins noted in tissues perfused by discontinuous capillaries, followed by continuous and fenestrated capillaries, respectively.

• Capillaries in most tissues behave like negatively charged filters; that is, they retard the transport of negatively charged plasma proteins while enhancing the exchange of positively charged proteins.

• The magnitude of solute exchange across a vascular bed is determined by the number of perfused capillaries and the patency of the endothelial cell junctions.

• The rate and direction of fluid movement across capillaries is governed by the balance of hydrostatic and oncotic pressures exerted across the capillary wall.

• Conditions that elevate capillary hydrostatic pressure, such as arteriolar dilation or an increased venous pressure, favor the filtration of fluid out of the capillaries. Conditions that elevate plasma oncotic pressure favor the absorption of fluid from the interstitium to the capillary lumen.

• The lymphatic system provides the only means for removal of plasma proteins that escape the blood and enter the interstitium.

• The principal driving force for the entry of interstitial fluid into the terminal lymphatics is the interstitial fluid pressure.

• When capillary pressure is increased, the increased interstitial hydrostatic pressure and lymph flow and reduced interstitial oncotic pressure that result help prevent excessive fluid accumulation in the interstitium.

• Edema generally occurs when the rate of fluid filtration out of a capillary bed exceeds the ability of its lymphatic drainage to return the filtered fluid to the vascular system.

• The three most common vascular abnormalities that result in interstitial edema are an increased capillary hydrostatic pressure, a reduction in plasma oncotic pressure, and an increased capillary permeability.

Essential Medical Physiology, Third Edition

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