PHYSIOLOGY OF MICROCIRCULATION.

The microcirculation refers to the smallest blood vessels in the body: the smallest arterioles the metarterioles the precapillary sphincters the capillaries the small venules

Functional element of microcirculation Microcirculatory part of vascular system performs all blood functions. There are such types of vessels: arterioles, metarterioles, capillaries and venuls. Mean diameter of these vessels is less than 100 mcm. Arterioles, capillary bed venuls and lymphatic capillaries compose functional element of microcirculation. Main processes as blood- tissue exchange or lymph production are performed there. Mean diameter of capillaries is 3- 6 mcm. The length of capillary vessel is near 750 mcm. Capillaries perform exchange in surface near mkm2. Blood flow velocity in capillaries consists near 0.3 mm/s, which permits passing erythrocytes through capillary in 2-3 s.

Microcirculatory bed and functional types of capillaries Depending on structure it distinguished three types of capillaries: somatic, visceral and sinusoidal. Capillaries walls are composed from one layer of endothelial cells and basal membrane. Endothelial cells are active elements of capillary bed. Endothelial cell may produce enzymes as antithrombin III endothelial relaxing factor, endothelial contracting factor, which may activate function of hormones and neurotransmitters on vessel's wall or cause some physiological effects by it. It was determined that endotheliocites may contract and become voluminous. Endoteliocytes contain microfibrills, composed from actin, myosin and other contractive elements. Such structures are directed along cell basis and binds to cytoplasm in places of intracellular contacts.

When microfibrills contracting two kinds of effects may be produced: both increasing intracellular split after contraction and increasing cell height and its' prominence inside the vessel. Capillary wall has small splits and a lot of pores. In certain organs capillary walls have some specialties. In kidneys glomeruls, intestinal epithelium, capillaries are fenestrated. This specialty permits passing through endothelial cells water, ions and other even rather large molecules as aminoacids or fructose. In red bone marrow, liver and spleen capillaries have interrupted walls, which let passing even blood cells.

Interstitial spaces Intracellular substance surrounds microcirculatory bed and lymphatic capillaries. Intracellular substance is composed by net of collagen and elastic fibers, which form small cavities filled in by gelatin-like substance including proteins, ions and water. Intracellular space has filter system and reabsorbtive system. Filter system in composed by capillary bed. Reabsorbtive system includes lymphatic capillaries and venules. Due to convection and diffusion in fluid surroundings, intracellular fluid streams from blood capillaries to lymphatic capillaries.

Transport of substances through capillary membrane Substances are transported through capillary membrane are lipid soluble as O2 or CO2 and water-soluble as ions or glucose. Substances of molecule size more than 6-7 nm cannot diffuse through intra-endothelial pores. The greater the concentration difference of a given substance on two sides of capillary membrane, the greater will bi net rate of diffusion. Forces that determine fluid movement through capillary membrane are capillary pressure, interstitial fluid pressure, plasma colloid osmotic pressure and interstitial fluid colloid osmotic pressure. At arterial end of capillary pressure is higher than interstitial fluid pressure, which causes filtration. At venous end of capillary plasma colloid osmotic pressure is lower than interstitial pressure, which cause reabsorbtion.

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