 This video will cover the following objective for the anatomy of blood vessels Compare and contrast the structure and function of arteries arterioles capillaries venules and veins blood flows out of the heart into arteries the largest arteries known as elastic arteries are able to stretch out in order to receive the increased volume of blood as Blood is pumped out of the ventricles into the arteries the aorta is the major elastic artery that receives oxygen-rich blood at high pressure from the left ventricle and then blood flows out of the aorta into smaller branches and These smaller branches will then distribute blood to different regions of the body there these Medium-sized arteries are known as the muscular arteries For example, there's a brachial artery that carries blood through your arm And this is an example of a muscular artery or on the diagram here We see the hepatic artery that would be an example of a muscular artery that's Distributing blood to the liver or the renal arteries are muscular arteries that deliver blood to the kidneys arteries branch into smaller and smaller arterioles and the arterioles then Deliver blood into capillaries so arterioles are the smallest arteries and Because they have a relatively small diameter, but they receive blood That's coming from larger arteries at relatively high pressure the arterioles play a major function in regulating the blood pressure and regulating the Distribution of blood to different capillary beds in the body So the arterioles are the primary resistance vessels that are critical for slowing down blood flow And as blood flows through the arterioles into the capillaries the blood pressure drops so Arterials deliver blood into capillaries, which are the smallest blood vessels The capillaries have very thin walls and are specialized for exchange nutrients and waste move Out of and into the blood at the capillaries throughout the body and then blood collects from the capillaries into Venuals, which are the smallest veins and the veins then drain blood Into the heart draining blood into the atria of the heart the the largest veins That are draining blood from the systemic circuit are the the vena cava that drain the deoxygenated blood into the right atrium Then the the pulmonary veins drain oxygen rich blood Coming back from the lungs Into the left atrium there are three layers To the blood vessel wall in an artery or vein The outermost layer known as tunica externa or tunica adventitia Consists primarily of Fibrous connective tissue lots of bundles of collagen fibers This helps to provide Structural support for the blood vessel wall and anchor the blood vessel to its surroundings the middle layer known as tunica media Contains smooth muscle tissue to regulate the diameter of the blood vessel contributing to regulation of blood flow and blood pressure The deepest layer of the blood vessel wall known as tunica intima contains a sheet of cells known as a simple squamous epithelium called the endothelium Which functions to create a barrier containing the blood within the blood vessel The largest arteries known as elastic arteries Have a large amount of elastic fibers enabling them to stretch and recoil in response to high blood pressure The elastic fibers are illustrated with the dark blue squiggly lines in the drawing the illustration below Muscular arteries are important for regulating the distribution of blood to large regions of the body And muscular arteries contain a relatively large proportion of smooth muscle in their wall Arterioles are the smallest arteries. They also have a thick tunica media relative to the three layers of their wall And this smooth muscle of the tunica media also allows the small arterioles to constrict decreasing their diameter Arterioles because they have a small diameter Contribute the majority of resistance the majority of total peripheral resistance which helps to maintain blood pressure And arterioles can regulate the distribution of blood into capillary beds The heart functions as a pump driving blood flow through the blood vessels As the heart beats with each contraction The blood forced into the arteries causes an increase in the blood pressure inside of the arteries then blood flows from An area of high pressure to low pressure From the elastic arteries into muscular arteries then into arterioles and capillaries Then from the capillaries blood continues flowing into lower pressured veins That then drain into the atria of the heart Where the pressure is the lowest and so blood pressure drives the flow of blood Blood flows from areas of high pressure to low pressure So every time the heart ejects blood There's an increase in the arterial blood pressure from the lowest value of blood pressure in an artery to the highest value So the lowest value we call diastolic pressure and the highest value we call systolic pressure And the difference between systolic and diastolic is what's known as the pulse pressure That is The pulse that you can feel if you place your finger on an artery and feel the expansion of an artery with each heart beat When the blood pressure is measured, it's usually measured in a muscular arteries such as the brachial artery in the arm And so we can measure the values of the systolic and diastolic pressure and then calculate the corresponding pulse pressure We can also calculate the mean arterial pressure Which is a time-weighted average of blood pressure. It's not just systolic plus diastolic divided by two Instead, we'll calculate this Mean arterial pressure as one third systolic pressure plus two thirds diastolic pressure Reflecting the longer amount of time that the heart spends in diastole compared to systole This image shows us a cross section through an artery and a vein that are of comparable size A muscular artery and a medium sized vein that are running parallel to one another On the bottom left, you can see the artery has a nice round shape with a thick muscular wall to withstand high blood pressure In contrast, the vein has a relatively thinner wall and has an irregular shape While arteries receive blood at high pressure, veins carry blood at relatively low pressure However, veins do contain the majority of the blood volume And veins have a high capacity to stretch out to expand in order to store this large volume of blood even at low pressure Because the arteries are involved in regulating the distribution of high pressure blood to different regions of the body They have that thick tunica media smooth muscle layer that regulates the diameter of the artery And the tunica media then is the thickest of the three layers in the wall of an artery In contrast, in a vein, you can see tunica externa is the largest of the three layers in the wall of the vein Veins have the same three layers that we saw in the walls of arteries Tunica externa is the outermost superficial layer of fibrous connective tissue anchoring the blood vessel to its surroundings And the tunica externa is the majority of the wall of a vein the smallest veins are known as venules and The venules have a very thin tunica media. They are primarily The endothelium and a surrounding layer of connective tissue The venules drain into medium-sized veins A distinctive feature of medium-sized veins is that they contain valves to prevent backward flow of blood The valves are inward folds of the endothelium in tunica intima And the valves function to create compartments inside of veins where blood can only move towards the heart And blood cannot flow backward towards the capillaries Medium-sized veins then drain into large veins and the Largest veins will drain into the atria of the heart so the Vows inside veins enable a skeletal muscle pumping function where As a skeletal muscle contracts it squeezes on the blood In a vein that's deep to that muscle As that Muscle is squeezing on the vein it increases pressure In a compartment of that vein in between two valves this Increased pressure causes the blood to be forced Back towards the heart through the valve because blood is not able to go backwards The valve prevents blood from flowing backwards and so in the leg blood would be forced up towards the heart Capillaries are the smallest blood vessels that provide the site for exchange of substances between the blood and other tissues For example oxygen moves from the air of the alveoli of the lungs into the capillaries of the pulmonary circuit And carbon dioxide moves from the blood of the capillaries out into the air in the alveoli in the lungs And then in the systemic capillaries we have the opposite thing happening where Oxygen will move from the blood out of the capillary into the tissues to provide oxygen to support the metabolism of cells and carbon dioxide and metabolic waste produced by cells moves from the tissue into the blood inside of the capillaries other substances such as hormones Can move in and out of the blood at the capillaries So the capillaries primary function is exchange Blood flows into the capillary from an arterial and then capillaries drain blood into venules while the arterioles have a thick smooth muscle layer in their tunica media that helps to Control blood flow and regulate the distribution of blood to capillaries There are also pre capillary sphincters that are smooth muscles wrapped around the Entrance of the capillaries These pre capillaries sphincters can contract in order to reduce blood flow through the capillary bed allowing blood to bypass as it flows through the meta arterial Directly as a thoroughfare channel into a venule This would occur if the tissue Has sufficient oxygen and has a low metabolic rate isn't producing a large amount of carbon dioxide So that the blood can be bypassed through this capillary bed Or this would occur in the capillaries of the digestive system when You're no longer absorbing nutrients from the diet or if you're having a fight-or-flight stress response But in contrast one a tissue needs an increased Exchange of nutrients and waste For example when a skeletal muscle is contracting in order to Produce motion The high metabolic rate will produce lots of carbon dioxide and rapidly use oxygen And this will stimulate relaxation of the smooth muscle in the arterioles and relaxation of the pre capillaries sphincters To increase the blood flow through the capillary bed helping to provide nutrients and carry away waste Capillaries are the smallest blood vessels, which are specialized for the exchange of substances between the blood and the surrounding tissues Fluid is able to flow From the blood through the Capillary wall Through the intercellular clefts of continuous capillaries and through the fenestration pores of fenestrated capillaries As well as through the intercellular gaps of sinusoids Capillaries are the smallest blood vessels and they have a simple structure with just an inner lining of endothelium Surrounded by a basement membrane of loose connective tissue There are three major types of capillaries continuous capillaries are the most common type found in the majority of organs in the body Capillaries inside skeletal muscles would be continuous capillaries or the capillaries found within the lungs in the pulmonary circuit Would also be examples of continuous capillaries These are known as continuous capillaries because the endothelium Is consisting of cells that are held tightly together by Tight junctions which help to prevent large substances from moving through the wall of the capillary Small non-polar molecules like carbon dioxide and oxygen can move through the capillary wall by simple diffusion There are intercellular clefts Little spaces in between adjacent endothelial cells Which can open to allow larger substances to pass through the wall of a continuous capillary, but continuous capillaries are the least permeable of the capillaries So fenestrated capillaries have numerous small pores known as fenestrations Which enable fenestrated capillaries to be highly permeable to fluid So there are fenestrated capillaries found in the kidneys in a special capillary bed known as the glomerular capillary bed where The blood is filtered Inside of the kidney another place where fenestrated capillaries are found is in the coroid plexus inside of the Ventricles in the brain where cerebrospinal fluid is produced So these fenestrations make the fenestrated capillaries very leaky very permeable to pressure or to liquid being forced out of the blood by the blood pressure by the hydrostatic pressure or the capillary hydrostatic pressure the force of the blood that's pushing the liquid Out through the wall of the capillary into the surrounding tissue Sinusoids are the most permeable of the capillaries Sinusoids have an incomplete basement membrane and large intercellular gaps in the endothelium This enables large Particles such as cells to move in and out of the blood Sinusoids are found in the liver and in the bone marrow in the spleen and in the lymph nodes And so the sinusoids in the bone marrow enable the cells Of the blood to move into blood as they're being produced in the process of hematopoiesis Where the stem cells are dividing and differentiating to form all the various Cells of blood those cells are formed in the bone marrow and then migrate through the intercellular gaps Of these sinusoids to get into the blood Similarly at the at the spleen the sinusoids enable worn out erythrocytes to be removed from the blood And in the in the lymph nodes And in the liver These sinusoids are also going to enable leukocytes to move in and out of the blood