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Microcirculatory Physiology

  • The microcirculation refers to the highly-distributed beds of capillaries which exist throughout nearly all the body's tissues. The basic functions of the microcirculation are to provide a source of nutrients and fluid for tissues and carry away metabolic wastes. These exchange processes are also designed to be selective, thus preventing escape of critical blood elements into the interstitial fluid. Much of these functions are dependent on the unique histological architecture of the microcirculation which is discussed in capillary histology and should be reviewed.
Basic Mechanism
  • Exchange of molecules and water between the microcirculation and surrounding tissues is largely governed by passive thermodynamic diffusion and is not an active, energy-dependent process.
  • Non-polar Molecules
    • Small, non-polar molecules can easily diffuse through endothelial cells and thus directly across the capillary barrier. This is especially true of gases such as O2 and CO2 as well as non-polar small molecules such as steroid hormones.
  • Polar Molecules
    • Polar molecules must diffuse through clefts between adjacent endothelial cells and across the endothelial basement membrane. Because these clefts are relatively small and the basement membrane acts like a fine sieve, only relatively small polar molecules such as ions, glucose, and so on can diffuse in measurable quantities. Large polar molecules, especially proteins, largely remain within the microcirculation.
  • Small Molecules
    • For most molecules the direction and rate of passive diffusion between the microcirculation and the interstitial fluid is governed by the concentration gradient for that molecule as well as the permeability of that particular capillary bed. For example, oxygen rapidly diffuses from the microcirculation into the interstitium because its concentration is higher within the microcirculation and the oxygen permeability of endothelia is very high. It should be pointed out that in the case of polar molecules, different capillary beds possess varying levels of permeability depending on the nature of their endothelial architecture and basement membrane. For example, glomerular capillaries are extraordinarily permeable to small polar molecules because of their fenestrated architecture, thus allowing high rates of selective diffusion for these molecules.
  • Water
    • The regulation of water exchange between the microcirculation and the interstitial fluid is somewhat more complex than that for most molecules. This is because the thermodynamic equivalent for water "concentration" involves both osmotic and hydorstatic pressure components. The thermodynamic regulation of water exchange between the microcirculation and the interstitial fluid is governed by the Starling Forces and is discussed on that page.