BISC 307L 2nd Edition Lecture 29 Current Lecture Blood Flow to Organs at Rest Coming out of the lungs to the left side of the heart is a CO of 5L min Red arterial blood coming in Blue venous blood draining out Venous return comes back into the right heart The Figure shows the fraction that goes to each organ in terms of L min and in percentages on the left hand side And in black on the graph below you can see the of total CO that goes to the brain heart etc The three center black points receive a greater percentage of the left side cardiac output than other organs The red 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 cardiac output and this can go as high as 80 of cardiac output which is redirected to the muscles during strenuous exercise Blood Pressure Velocity and CSA On the left is a diagram showing a tube whose diameter varies along its length and fluid is flowing through the tube The rate at which fluid is flowing is the same at all points in the tube The velocity of the flow therefore has to vary it is fastest in the small diameter 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 Capillaries Capillaries 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 all the 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 liquid part 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 along the 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 Edema Filtration of fluid occurs at the upstream end and absorption at the downstream end Any extra fluid percolates into lymph vessels The lymph system is venous starts with blind ended porous capillaries which drain 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 especially in the lower body where blood would accumulate 2 Increased central venous pressure e g heart failure normally central venous pressure is low But if it