BISC 307L 2nd Edition Lecture 28 Current Lecture Blood Vessels Aneurysms only occur in arteries because only in arteries is the wall strain high enough to cause an aneurysm Only 3 out of these 5 vessels have smooth muscle in their walls arteries arterioles and veins Capillaries and venules don t they consist of a single layer of endothelial cells The vascular smooth muscle is capable of contracting or not when contracting it is called vasoconstriction When not contracting the pressure inside the vessel causes them to inflate and this is called vasodilation The vasoconstriction of the arteries and especially the arterioles is going to increase the resistance to blood flow These changes in resistance to blood flow make it possible to redirect blood flow in the systemic circulation which has many paths so regional vasoconstriction can redirect blood flow one way or another For example after lunch vasodilation causes blood to flow toward the digestive organs Microcirculation Circulation of small vessels Can see an artery feeding into an arteriole into capillaries flow is down from the top across to the right then back up Arteries are strongly reinforced by connective tissue sheaths Arterioles do not have this also they can generate force very efficiently for long periods of time They are always contracted to some extent 24 hours a day their level of contraction is called vascular tone Capillaries have no smooth muscle except at the place where the arterioles meet the capillaries there are bands of smooth muscles called pre capillary sphincters and they regulate how much blood is going through the capillary network In most tissues every cell in the tissue is no more than a few cells away from a capillary so there is dense vascularization of capillaries through most tissues Under most conditions most of the capillaries do not have blood flowing through them the precapillary sphincters are constricted Only 10 20 of capillaries at a time have blood flowing through them Blood has to get from arteriole side to venous side to get back to the heart so if the capillaries are sealed off how does the blood get by They get through thoroughfare channels formed by metarterioles or arteriovenous bypasses Metarterioles have some smooth muscle around it but not as much as a normal arteriole These are bigger vessels and tend to be thicker walled than capillaries so they don t contribute much to gas nutrient exchange Because they are bigger leukocytes which are larger than RBC s use them instead of capillaries to get around Angiogenesis There s a lot of research recently on angiogenesis the formation of new blood vessels This is important in tumor research a tumor attracts blood vessels so that it can grow So there has been a lot of advances on the growth factors that promote angiogenesis Important stimulants vascular endothelial growth factor VEGF this can happen in the retina due to vascular damage caused by diabetes that forces VEGF to be used to vascularize the area in order to fix it fibroblast growth factor FGF matrix metalloproteinases MMPs these are extracellular proteases that digest molecules in the extracellular matrices These are important in angiogenesis because things like VEGF stimulate endothelial cells to de differentiate from their normal mature phenotype to proliferate and migrate to form new blood vessels The migration of endothelial cells to tissue to a site that needs vascularization requires these MMP s to digest their way through the space a dozen other factors Inhibitors of angiogenesis angiostatin endostatin a proteolytic fragment of a blood clotting protein called plasminogen many others Pressure in Blood Vessels Looking at pressure change in systemic circulation Graph shows pressure in mmHg in various places in the systemic circulation starting from left ventricle to right atrium In general pressure fluctuates from 120 to 0 During systole left ventricle pressure is high and during diastole it is at or close to 0 Blood comes out of LV and enters aorta There are still pressure fluctuations here but they are much less between 120 80 Why the arterial pressure fluctuates less is because arteries are moderately compliant tendency of a material to deform under pressure a rubber balloon would have much less compliance than a plastic bag which would take much less air pressure to blow up and highly elastic property of a material to recoil to its original size after its been deformed rubber balloon would be highly elastic and plastic bag not at all This means when blood comes into artery they expand due to the moderate compliance During isovolumic ventricular relaxation and the pressure in the ventricle is falling because it is in diastole there is no more blood being ejected into the aorta because the higher backpressure has closed the semilunar valve The stretching of the elastic wall had stored some of the energy that the blood and during diastole of the ventricle the aorta squeezes down on the blood and keeps the pressure from going as low as it is in the ventricle hence the 120 80 instead of the 120 0 The pulse pressure is the difference between systolic pressure and diastolic pressure MAP mean arterial pressure is not the halfway between the systolic and diastolic pressure because the systolic and diastolic phases are different in length and because the falling phase of pressure is slower than the rising phase Result Mean Arterial Pressure MAP diastolic P 1 3 pulse P 80 1 3 120 80 93 mm Hg 93 can be thought of as the arterial pressure that is pushing blood through the vascular system We have two ways of calculating mean arterial pressure now The way above and the formula MAP CO x peripheral resistance As the blood leaves the arteries and enters the arteriole the pulses dampen out because the arterioles are much smaller Resistance of the arterioles are high which diminishes the pressure fluctuations Deeper you go into the arterioles they get more damped By the time you reach the capillaries there are no more fluctuations This allows for more efficient exchange of nutrients gases in capillaries If you were to lose elasticity due to disease like arteriosclerosis hardening of arteries the pressure fluctuations on the artery would be bigger because it was the elasticity that dampened the pressure pulse and would pulse further into the vascular system even to the capillaries which would mess with their function The causes of arteriosclerosis are associated with advanced age and