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Biological Sciences
Peter A.Bowler

e109 11/9/12 -Circulatory system branches -5 L/min is the typical cardiac output leaving the left ventricle and going to the rest of the body -Blood flow distribution will change with demand (fig 15-12 shows at rest) -Can have increase in one branch with decrease in another -liver and digestive tract -different parts of the body are different sizes, so different amount of blood needed i.e. liver and digestive tract require the most blood (1.35 L/min) i.e. maxima per chunk of tissue (in the middle for mL/[min x (100 gram)], then the kidneys are the maxima in processing the most blood b/c of their function (processing wastes so large of amounts of blood are needed to allow the kidneys to function) -Neural tissues has special metabolic requirements (pg. 297 Ch. 9) -Neurons require constant supply of oxygen and glucose; sensitive to decreases in oxygen and glucose -Brain function diminishes almost immediately without oxygen or glucose (damage may occur within minutes) -The brain consumes a large fraction of the total body glucose consumption (~50%); high consumption organ Fig 15-13 -Total cardiac output is distributed among branches—distribution can change with changing resistance in different branches -The downstream vessels must have the same flow rate as the upstream flow rate if blood is not disappearing or changing in volume [blood flow through four identical vessels (A-D) is equal. Total flow into vessels equals total flow out] -When vessel B constricts, resistance of B increases and flow through B decreases. Flow diverted from B is divided among the lower-resistance vessels A, C, and D, but the total flow remains unchanged if the flow rate remains the same. Capillaries differ in how leaky they are -thin wall and very leaky -Capillaries have very small diameters—no molecules are very far from the wall and this promotes exchange vs. big veins where the blood is far from the wall +the blood moving through the capillaries moves very slowly (low velocity), so this promotes exchange in the capillaries as well -Exchange between the plasma and the interstitutal fluid takes place either 1. by movement in between endothelial cells (the paracellular pathway) 2. by movement through/across the cells (endothelial transport) by transcytosis -Most common: continuous capillaries, which have leaky junctions +mechanism of transport: 1. Paracellular pathway (“in between”)  diffusion (noATP required): oxygen and carbon dioxide diffuse across cell membranes, moving down their concentration gradients a. endothelial cell junctions allow water solutes, small dissolved solutes, and gases to move by diffusion b. Paracellular pathway depends on how leaky the cell junctions are 2. Endothelial transport (“across the cell”)  transcytosis: moving across the endothelial cells [endocytosis + vesicular transport + exocytosis] a. transcytosis brings proteins and macromolecules across the endothelium b. some vesicles may fuse to create temporary channels or the transient formation of pores -Fenestrated capillaries have large pores and are found in the kidney and intestine The “blood-brain barrier” 400 miles of capillaries in the brain -these capillaries are NOT very permeable compared to the previous capillaries -these capillaries protect the brain from toxins -Transporters in membranes move nutrients and wastes, since diffusion across these capillaries is limited -permeable exceptions (most of the brain is extremely impermeable, except for the hypothalamus that is permeable): hypothalamus, vomiting center in the brain stem +the hypothalamus portion of the brain is permeable because it makes it easier for the hypothalamus to dump trophic hormones into the blood stream (portal system) +the vomiting center in the brain stem also has a permeable region in the brain in order to detect any toxins in the blood, so that the brain gets the sign to induce vomiting; the brain is not detoxifying (it is just sensing the toxins) Velocity of blood is slowest in the capillaries, which has the HIGHEST cross-sectional area -Velocity of blood flow is slower with increasing cross-sectional area (assuming equal volume flow rates) Fig 14-6 and Fig 15-17 -Velocity of blood is fastest in the aorta, which has the lowest cross-sectional area -Velocity of blood flow depends on the total cross-sectional area -As cross-sectional area increases, the velocity decreases (inverse relationship) -the total flow is the same, but it is just slowing down as it reaches the larger cross-sectional areas and speeds up if it reaches the more narrower areas (smaller cross-sectional areas) Filtration (fluid movement [water with small solutes] across capillary walls) in systemic capillaries Net pressure = hydrostatic pressure – colloid osmotic pressure -there is net filtration at the arterial end and net absorption at the venous end -Two phe
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