Term
| who was the first person to describe capillaries? |
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Definition
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Term
| what is the major determinant of resistance? how is this manifested in capillaries? how is this overcome in capillaries? |
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Definition
| radius - capillaries have a very small radius and therefore high resistance, however this is spread out over a huge cross-sectional area, so resistance in total is lower |
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Term
| what is the major mechanism for exchange of exchange of materials in the body? |
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Definition
| diffusion down a concentation gradient - distances for this are very low, so a tremendous amount of exchange occurs over the length of a capillary |
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Term
| what is the movement of fluid in response to the net hydrostatic and osmotic forces across the capillary endothelium? |
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Definition
| filtration and absorption, the minor means of exhanging materials in the body |
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Term
| what is the rate of diffusion determined by? |
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Definition
| the concentration gradient (higher = faster), which is the concentration difference of a substance between 2 sides of a membrane |
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Term
| what is an example of a concentration gradient in the human body? |
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Definition
| the concentration of oxygen is higher in capillary blood than interstitial fluid, allowing O2 to move to the tissues. CO2 is higher in these tissue than in the capillary blood, and can therefore diffuse out |
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Term
| what is the major way of controlling blood pressure? |
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Definition
| arterioles are surrounded by a thick coat of smooth muscle and are under hormonal control |
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Term
| what does the arteriole divide into? |
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Definition
| arterioles divide ~5x into metarterioles, which are inbetween arterioles and capillaries. they have intermittent smooth muscle and divide 5-8x before they reach the structure of a capillary |
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Term
| what is a pre-capillary sphincter? |
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Definition
| a patch of smooth muscle that controls the flow down the structure of the capillary |
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Term
| what do capillaries eventually become? |
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Definition
| venules, which have sparse patches of smooth muscle (which works b/c not much muscle is needed to contract these vessels - as venules become larger vessels, more smooth muscle is seen and compliance goes down) |
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Term
| how many cells thick is a capillary? what is their diameter? |
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Definition
| capillaries have 1 cell thick walls and are about 1/2 a micron in diameter |
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Term
| where do most of the exchanges occur in capillaries? |
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Definition
| in the intracellular cleft, the gap between cells - the size of which is 50-90 nm (smaller than albumin – it cannot diffuse through pore) |
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Term
| what are plasmalemmal vesicles? |
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Definition
| these do not have much function in terms of transport (quantity) but possibly form vesicular channels |
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Term
| can capillaries in beds of different organs have different structures? |
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Definition
| yes, for example, the capillaries for the glomerulus have lots of gaps - to the point where they is referred to as a "fenestrated capillaries" structured so b/c their function is set up for filtering blood |
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Term
| how can capillaries resist significant pressure? |
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Definition
| the law of laplace states that as radius drops, wall tension drops (T=Pr) |
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Term
| what determines capillary pressure? |
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Definition
| the volume of blood and the compliance of the capillary |
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Term
| is compliance an important determinant of capillary pressure? |
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Definition
| while it is a determinant, capillaries are relatively stiff, so compliance does not play a large role in determinin pressure |
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Term
| what is the major determinant of capillary pressure? |
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Definition
| capillary volume is determined by inflow (influenced by arterial pressure and precapillary resistance) vs outflow (influenced by venous pressure and postcapillary resistance). the resistance of venous structures is approximately 1/4th that of the arterial structures and thus even small **changes in venous pressure affect capillary pressure more than a comparable change in arterial pressure |
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Term
| what are the factors that determine net filtration pressure in the capillary? |
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Definition
| capillary hydrostatic pressure (Pc - pushes fluid out of capillary and into interstitium), interstitial fluid pressure (Pif - driving fluid back into capillary), plasma colloid pressure (PIp - absorbing fluid from interstium), and interstitial fluid colloid osmotic pressure (PIif - pulls fluid out of capillary) |
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Term
| what is starling's law of the capillary? |
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Definition
| Pc-Pif-PIp+PIif = NFP (net filtration pressure, which is usually slightly positive, resulting in filtration out of the capillaries). whatever pushes things out of the capillary has a (+) sign and whatever takes fluid back into the capillary has a (–) sign |
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Term
| what is Kf? how does this value affect NFP? |
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Definition
| Kf is the capillary filtration coefficient for a certain bed which expresses the # and size of pores as well as the # of capillaries (measure of capacity of capillary membranes to filter water for a given NFP). filtration is then = Kf x NFP |
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Term
| does net hydrostatic pressure differ depending on where the capillary is? |
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Definition
| yes, glomerular capillaries' net filtration is 10 mm Hg while in the peritubular capillaries reabsorbtion is 10 mm Hg. in general, mesenteric capillaries absorb and net filtration only occurs in a few places such as the kidneys and glands |
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Term
| what determines the capillary filtration equilibrium point, and how different capillaries fall on either side of it? |
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Definition
| capillary colloid osmotic pressure generally remains the same, but in the arterial end of capillaries, capillary hydrostatic pressure is high (favoring net filtration) which decreases as the capillaries become venous, dropping below capillary colloid osmotic pressure (favoring net absorption). the point where capillary hydrostatic pressure and capillary colloid osmotic pressure meet is the capillary filtration-pressure equilibrium point |
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Term
| how do plasma proteins affect colloid osmotic pressure? |
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Definition
| about 80% of total plasma colloid osmotic pressure results from albumin blood fraction, 20% from globulins, and almost none from fibrinogen |
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