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USC BISC 307L - Vessels and Pressure, Blood Figs
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BISC 307L 2nd Edition Lecture 29 Current LectureBlood Flow to Organs at RestComing out of the lungs to the left side of the heart is a CO of5L/min. Red = arterial blood coming in. Blue = venous blooddraining out. Venous return comes back into the right heart. The Figure shows the fraction that goes to each organ in terms ofL/min and in percentages on the left hand side. And in black on thegraph below, you can see the % of total CO that goes to the brain,heart, etc. The three center black points receive a greaterpercentage of the left side cardiac output than other organs. Thered points are the same points normalized to mL/100g tissue/min. You can see that the kidney receives a disproportionate and massive amount of blood flow, because its function is to continuously regulate and filter the blood. The kidney receives a lot of flow, and at high pressure. Skeletal muscle under resting conditions receives 21% of cardiacoutput, and this can go as high as 80% of cardiac output, which isredirected to the muscles during strenuous exercise. Blood Pressure, Velocity, and CSAOn the left is a diagram showing a tube whosediameter varies along its length, and fluid is flowingthrough the tube. The rate at which fluid is flowing isthe same at all points in the tube. The velocity of theflow therefore has to vary – it is fastest in the smalldiameter part, and slower in the big part. The velocity of flow depends on the CSA through which the flow occurs. Consequence of this is shown on the right. This shows the total CSA of all the blood vessels, and on the left are the big arteries. Their total CSA is rather small. And capillaries have the biggest CSA. So the velocity of flow is fastest in the arteries, and slowest in the capillaries. The fact that the blood flow is the slowest in the capillaries is advantageous because that is where exchange of material/nutrients occurs. CapillariesCapillaries have a single layer of endothelial cells, and around that are basement membranes, a mesh of fibrous proteins that surround the outside and fill up the gaps between the cells. The gaps between adjacent cells are porous – fluid leaks out. You can see junctions in the figure below, through which materials leak. A fenestrated capillary has the same structure as a continues one, but the endothelial cells actually have pores. In addition to the junctions between adjacent endothelial cells, there are pores that go through one side or another – these are leakier than the continuous capillaries. Fenestrated capillaries’ endothelial cells can actually transport material across the cells through vesicles – endocytosis on one side, translocation of the vesicle, and exocytosis to dump the contents out the other.Most capillaries, however, are continuous. Capillaries are usually fenestrated in tissues where it is important to be porous – places like the digestive tract and the kidney. The one exception are capillaries in the CNS, where there are not even open junctions between the basement endothelial cells – in the spinal cord and brain, they are sealed together with tight junctions to form the blood-brain barrier to protect the brain from harmful substances. Any material getting into or out of the brain either has to diffuse across the endothelial cells or be actively transported by transcytosis or by active transporters. I in this way, the endothelial cells control what gets into or out of the brain. Fluid Exchange Across Capillaries Consider the forces that push fluid through the holes in capillaries. These capillaries leak fluid allthe time due to a pressure gradient. This is called bulk flow. Shown below is a capillary, flowing from left to right. Typical values for BP at upstream end = 32 mmHg, and 15 mmHg at the downstream end. The difference between them is the pressure gradient pushing blood through the capillaries. There are three main forces that are at work here. 1. Blood pressure is constantly pushing blood from the left to the right.2. Because the blood is under pressure, fluid filters out – this is filtration because the liquidpart of the blood, or plasma, is being pushed out and that mesh is fine enough to exclude protein but not small solutes like anions or small organic molecules like glucose. That outward movement of water carrying small solutes with it is proportional to the pressure flowing out. So upstream where the BP is higher, there is a bigger outward gradient than at the downstream end.3. There are forces that push the fluid inward – that is, protein in the plasma does not leak out. Protein in the plasma exerts an osmotic pressure, and draws fluid inward. This colloid osmotic pressure is = 25 mmHg, labeled pi. Since there is very little protein outside of capillaries, pi = ~0 outside. Concentration of protein stays about the same along the length of the capillary, so we have a constant 25 mmHg drawing water in alongthe blood vessels. At upstream end, you have a 7 mmHg gradient pushing fluid out, and net 10 coming in at the downstream end. In between, the fluid is moving. It is the fluid coming out of the upstream end that supplies the fluid getting sucked in at the downstream end, and that fluid is interstitial fluid. It is in circulation, and is being renewed by oozing out of upstream end and coming back in downstream, it is not stagnant. Is the amount of fluid filtered out of capillaries in a day equal to the amount reabsorbed? No. The filtration(non-kidney) is about 20 L/day. And being sucked back is 18 L/day. What happens the extra 2 L a day? The extra fluid gets drained out of tissue by the lymphatic system. Capillary Lymph Exchange and EdemaFiltration of fluid occurs at theupstream end, and absorption at thedownstream end. Any extra fluidpercolates into lymph vessels. Thelymph system is venous, starts withblind ended, porous capillaries, whichdrain into venules, into lymph nodes, and eventually coalesce into a big duct that empties into the vena cava near the right atrium, and that fluid is being emptied out back to the heart. It’s a low pressure system, and skeletal muscle pumping is how the fluid gets moved around. Problems can happen – resulting in swelling or edema. Causes:1. Poor lymph drainage (parasites, tumors, surgical removal) – blockage of lymphatic vessels due to parasitic infection of lymph nodes. Tumor cells can grow and block them and cause severe swelling, especially in the extremities, and that can create a condition called elephantiasis,


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