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Phisiology-3 - Physiology 3 Capillaries Anatomy The flat...

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Physiology 3 Capillaries: Anatomy: The flat cells that constitute the endothelial tube are not attached tightly to each other but are separated by narror water filled intercellular clefts . o Endothelial cells have lots of vesicles and they form continuous fused vesicle channels . In some tissues, blood from the heart passes through metarterioles before reaching capillaries. o These have smooth muscle o The site at which the capillary exits from a matarteriole is surruounded by a ring of smooth muscle, the precapillary sphincter , which is controlled by metabolic factors. Velocity of Capillary Blood Flow: When a continuous stream moves through consecutivbe sets of tubes, the velovity of flow decreases as the sume of the cross sectional areas of the tubes increases. Blood from the aorta is fast and slows down as the tubes shrink in diameter By the time they reach the capillaries the blood cells are moving slowly enough to enable exchanged b/w the blood and ISF. The velocity of flow then increases as the blood is retured into the heart since 1 vein has less cross sectional area than 100 venules that aggregate into that vein. Blood velocity is not dependent on proximity to the heart, but rather on total cross sectional area of the vessel. Because each vapillary is very narrow, it offers considerable resistance to flow, but the huge total number of capillaries provides such a large cross sectional area that the toal resistance of all the capillaries is much lower than the arterioles. Diffusion Across the Capillary Wall: Three mechanisms allow substance to move b/w the ISF and Plasma: o Diffusion: With the exception of the brain, it is the only important means by which net movment of nutrients, oxygen and metabolic end products occurs across the capillary walls. Capillaries contain watter filled channels as waell as the intercellular clefts that allow movement of ions and small polar molecules. In the brain, there are no intercellular clefts, so ions don’t ‘leak’ out. Molecules must exit by carrier mediated transport. Liver cells have large clefts. o Vesicle transport: Used to transport proteins. Occurs in the endocytosis of plasma at the luminal border and exocytosis of the endocytotic vesicle at the interstitial side. o Bulk Flow Fluids diffuse from the capillary wall into the interstitial fluid, where they enter the cells. Metabolism- utilization or production- is what establishes the transcapillary diffusion gradients. Bulk Flow: 394-399, 401-403, 425-437
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Physiology 3 Normally there is 4 times as much ISF than plasma. Hydrostatic Force: the pressure exerted by a fluid due to the force of gravity. In the presence of a hydrostatic pressure difference across it, the capillary wall behaves like a porous filter, permitting protein free plasma to move by bulk flow from capillary plasma to ISF through the water filled channels ( filtration ) o The concentrations of all the plasma solutes except protein are virually the same in the filtering fluid as in the plasma.
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