BISC 307L 2nd Edition Lecture 24 Current LectureAnatomy of Cardiovascular SystemThe 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 circulatorysystems – systemic andpulmonary. The pulmonarycirculation comes out theright side of the heart andreturns to the left side of theheart, which pumps out tothe systemic circulation. Sothe heart is two pumps inone. The right side pumps tothe lungs and the left sidepumps to the rest of thebody. Two pumps in theheart are in series – out putof one is input of the other.These basic facts areimportant in understandingboth the function andmalfunction of the heart inpathological conditions.In most cases, capillariesnormally 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 thehepatic 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 ResistanceThere 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 resistanceDeterminants of VesselResistanceBlood vessel resistance isdetermined by length,viscosity, and radius. For fluidflowing through a tube ofuniform diameter, we have aflow Q through a tube oflength L and radius R.Poiseuille figured out that theresistance of this cylindricalvessel to the flow is equal to 8x length of tube x viscosity ofblood / 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 1/16th 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 redirectionof 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