here we'll learn how the body regulates blood flow and pressure first start at table denote that all fluids exert hydrostatic pressure to note that blood pressure specifically is the hydrostatic force that blood exerts against the vessel walls it propels blood throughout the vascular system with this definition as a background let's illustrate the direction of blood flow from the aorta which is the largest artery to the vena cava which is the largest Thane draw an aorta which carries blood away from the left ventricle of the heart show that the aorta branches into smaller arteries draw the arteries branching into arterioles show that the arterioles feed into networks of capillaries merging into venules which then merge into veins show that the veins merge into a vena cava the superior and inferior vena cava return blood to the right atrium of the heart the heart rhythmically pumps blood through this closed network of vessels which facilitates the continuous delivery of blood to all the body's tissues each level of the vascular system experiences a variable degree of pressure so now let's illustrate the relative blood pressures in the peripheral vessels draw a graph that aligns with our illustration of blood flow label the y-axis is pressure in millimeters of mercury and number it from 0 to 120 the unit millimeters of mercury refers to the force required to support a column of mercury X millimeters high so 120 millimeters mercury then is the force required to support a column of mercury 120 millimeters of mercury high shade our graph to correspond with the vessels in our illustration show that blood pressure fluctuates between a hundred and ten millimeters of mercury and 75 millimeters of mercury until the blood approaches the arterioles blood pressure fluctuates between ventricular relaxation or die astley and ventricular contraction or systole indicate that 75 millimeters of mercury is the average diastolic or relaxation pressure for a healthy young adult and 110 millimeters of mercury is the average systolic or contraction pressure show that blood pressure drops in the arterioles and gradually decreases until it reaches zero millimeters of mercury in the veins at zero millimeters of mercury the veins lack the pressure necessary to return blood to the heart so now let's write the two key mechanisms that promote venous return the first is skeletal muscle which surrounds the veins and contracts to force blood in a single direction when venous valves are closed if you wiggle your toes and raise and lower your feet now you can help move blood back to your heart the second is respiratory inspiration which increases the pressure in the abdomen and decreases the pressure on the thoracic cavity thus venous blood moves along this pressure gradient and returns from the abdomen to the thoracic cavity so take a deep breath to promote blood flow return to the heart during expiration the pressure gradient reverses direction so we have venous valves to prevent backflow of blood if you picture a giraffe bending gets head downward to reach some low-hanging leaves you can imagine that without venous valves so much blood would rush down the giraffe's neck that its head could theoretically explode now that we've learned mechanisms to promote venous return let's denote the following variables that influence blood pressure stroke volume which is the volume of blood that the heart ejects during each contraction heart rate which is the number of beats per minute cardiac output which is the stroke volume times the heart rate peripheral resistance which is a force produced by blood vessels that opposes blood pressure and impedes flow cardiac output heart rate and stroke volume are under nervous and hormonal control we discuss their effects on blood pressure elsewhere for now let's focus on peripheral resistance to note that peripheral resistance is influenced by total cross sectional area and the luminal diameter of the vessels so draw another graph below our pressure graph label the y-axis total cross sectional area this describes the cumulative cross-sectional area of all the vessels at a given level for example although the aorta has a larger diameter than an arteriole there is only one a Orta and many arterioles thus the arterioles have a greater total cross sectional area than the aorta indicate that the x axis of the graph corresponds with the vessels in our illustration of blood flow draw a symmetrical curve that begins to rise at the arterioles and peaks at the capillary bed indicate that the capillaries have the greatest total cross sectional area while the aorta and the vena cava have the smallest right that the total cross sectional area is directly proportional to peripheral resistance an increase in total cross sectional area implies an increase in the surface area of vessel walls right that total cross sectional area and peripheral resistance are inversely proportional to blood pressure thus an increase in surface area increases peripheral resistance and as a result reduces blood pressure total cross sectional area also relates to the velocity at which blood flows through the vessels so now draw a final graph with the y axis label the velocity and centimeters per second show that the blood flow velocity is highest in the aorta and drops gradually until blood reaches the capillaries a large total cross sectional area and slow blood flow velocity optimizes the role of capillary beds as sites of exchange between the circulation and bodily tissues right that the blood flow velocity is inversely proportional to the total cross sectional area illustrate that the velocity begins to increase at the venues until blood reaches the veins the same mechanisms that promote venous return which are skeletal muscle contractions and the pressure gradient created by respiratory inspiration produce this increase in velocity show that the blood flow velocity remains lower in the veins and the vena cava than in the aorta and the arteries the powerful left ventricle of the heart pumps blood into the aorta subsequent arteries and produces a greater blood flow velocity than the mechanisms that promote venous return so now let's explore how vessels with smooth muscle in their walls can adjust their luminal diameter and thus peripheral resistance to note that these vessels adjust their luminal diameter in the following ways vasoconstriction a contraction of smooth muscle that reduces the diameter of the vessel and increases peripheral resistance vasodilatation the relaxation of smooth muscle which increases the diameter of the vessel and decreases peripheral resistance all the vessels in our drawing can adjust peripheral resistance except for the capillaries because they lack smooth muscle our illustration does not include the meta arterioles which pass through capillary beds the meta arterioles have pre capillary sphincters comprised of smooth muscle which can also contract and relax to adjust peripheral resistance as a clinical correlation to note that a sphygmomanometer can measure a person's blood pressure the device comprises an inflatable cuff that wraps around the upper arm and attaches to a pressure gauge let's illustrate how a sphygmomanometer measures blood pressure draw an arm surrounded by a cuff show that the cuff inflates until it closes an artery in the arm and that no blood can flow past the inflated cup draw another arm surrounded by a cuff illustrate that the cuff deflates until a stethoscope detects the sound of blood passing through an artery in the arm right that at this point the gauge records a pressure of a hundred and ten millimeters of mercury or the systolic pressure draw final arm surrounded by a cuff show that the cuff loosens until blood passes freely through an artery in the arm right that at this point the gauge records 75 millimeters of mercury or the diastolic pressure blood pressure is recorded as systolic pressure over diastolic pressure this concludes our diagram