BISC 307L 2nd Edition Lecture 24 Current Lecture Anatomy of Cardiovascular System The heart receives blood from veins into one of the atriums which pumps to a ventricle and then goes out through a system of arteries to veins We have two circulatory systems systemic and pulmonary The pulmonary circulation comes out the right side of the heart and returns to the left side of the heart which pumps out to the systemic circulation So the heart is two pumps in one The right side pumps to the lungs and the left side pumps to the rest of the body Two pumps in the heart are in series out put of one is input of the other These basic facts are important in understanding both the function and malfunction of the heart in pathological conditions In most cases capillaries normally drain into venules and veins which go to the heart But sometimes capillaries go back to the right side of the heart One example is the hypothalamic hypophyseal portal vein system where capillaries in the median eminence below the hypothalamus send blood into venules and veins those go into another bed of capillaries in the anterior pituitary and the pituitary veins drain the blood and send it back to the right heart Another example involves the liver The liver is supplied by a hepatic artery but also by a hepatic portal vein which starts with capillaries in the digestive tract They drain through the hepatic portal vein and break into a second bed of capillaries in the anterior pituitary and the pituitary veins drain the blood and send it back to the right heart This is the biggest portal vein system in the body and its function is to position the liver to handle the flood of nutrients that comes out of the liver in the absorptive state A third portal system is in the kidney Renal arteries supply the glomerular capillaries out of which fluid is filtered The venules draining the glomerular capillaries don t go to a vein and back for the heart they go into another bed of peritubular capillaries that wrap around the tubule Pressure Flow and Resistance There is a relationship between pressure flow and resistance Why does blood flow at all Well fluids flow down pressure gradients and fluids refer to liquids or gases In order for blood to circulate there needs to be pressure The units of pressure we are going to use are mmHg Blood flows in one direction and at every point in its pathway the pressure is less as you go forward so the pressure pushes the blood in that direction The flow rate Q is proportional to the pressure gradient Flow rate is the volume of blood moving past a point in a period of time The Pressure is pushing flow through a resistance and the rate of the flow is inversely proportional to the resistance Determinants of Vessel Resistance Blood vessel resistance is determined by length viscosity and radius For fluid flowing through a tube of uniform diameter we have a flow Q through a tube of length L and radius R Poiseuille figured out that the resistance of this cylindrical vessel to the flow is equal to 8 x length of tube x viscosity of blood by pi and radius 4 Resistance is ultimately proportional to the length and inversely proportional to the fourth power of the radius since the viscosity of the blood stays relatively the same We know that the total length of the blood vessel in the systemic circulation is way longer than in the pulmonary circulation So the total resistance based on the length of the blood vessel is much higher in the systemic circulation than in the pulmonary But the input of the second system has to be equal to the output of the first so the outputs of the two sides of the heart have to be matched over time In order for the left heart to force blood through this high resistance systemic network at the same rate at which the right heart is forcing blood through the much lower resistance pulmonary vessels the left side of the heart has to generate much higher pressures since flow rate is proportional to the pressure gradient So left side arteriole pressure is high and right side pressure is much lower As a result the left ventricle is meaty and thick walled while the right ventricle is thinly walled reflecting the difference in pressure each side has to generate Pictured above is a reservoir with two outlets The pressure driving the flow of the water out of the tubes and their lengths are the same but they differ in their radii Poiseuille s law says that the resistance of the tube is inversely proportional to radius 4 so if the left tube has r of 1 and the right tube tube has an r of 2 the resistance on the right is 1 16 th of the left A doubling of the radius drops the resistance 16x so the resistance of a blood vessel to flow is very sensitive to radius If the flow rate is proportional to the pressure gradient and inversely proportional to the resistance then Q must be proportional to pressure gradient resistance All the blood vessels except capillaries and venules have circular bands of smooth muscle in their walls That smooth muscle can cause vasoconstriction which reduces the radius of the vessel And even a small decrease increase in radius constriction has a big effect on resistance and therefore on flow of blood Also the whole circulation system is closed so regional differences in the state of vasoconstriction can redirect blood flow the heart is always pumping a certain amount of blood For example if you get embarrassed and blush there is increased blood flow to the skin of the face Vasodilation of blood vessels allows blood to flow in but it came at the expense of vasoconstriction at someplace else because you have a fixed amount of blood Vasodilation is the opposite of vasoconstriction it is simply the relaxation of smooth muscle never the active opening of smooth muscle Looking back at poiseuille s law The length of the vascular system doesn t change from moment to moment So that leaves only the radius as the direct determinant of changes in resistance and therefore blood flow And in a closed system if there is constriction in one area then there is increased flow elsewhere Regional vasoconstriction dilation can cause redirection of blood flow out of the heart to different parts of the body But what about systemic blood flow What if all the blood vessels contracted at the same time If you change the overall resistance of the whole system and the flow is the same then you change the pressure So a large contraction of many of the vessels in the body would change the blood