The Cardiovascular System: Blood Vessels Mr. Tsigaridis

Introduction The blood vessels of the body form a closed delivery system that begins and ends at the heart Often compared to a plumbing system, it is a far more dynamic system of structures that pulse, constrict and relax and even proliferate to meet changing body needs

Blood Vessel Structure & Function The major types of blood vessels are Arteries The large distributing vessels that bring blood to the body Capillaries The tiny vessels that distribute blood to the cells Veins The large collecting vessels that bring blood back to the heart Intermediate vessels connect Arterioles bring blood to the capillaries Venules drain blood from the capillaries

Blood Vessel Structure & Function The pattern of distribution starts with arteries to arterioles to capillaries to venules to veins The blood vessels in the adult human body carry blood in a distribution network that is approximately 60,000 miles in length Only capillaries come into intimate contact with tissue cells and serve cellular needs

Structure of Blood Vessel Walls

Blood Vessel Walls The walls of blood vessels are composed of three distinct layers or tunics The tunics surround a central opening called a lumen

Arteries Arteries are vessels that carry blood away from the heart All arteries carry oxygen rich blood with the exception of those in the pulmonary circuit Blood proceeds to the tissues through Elastic arteries Muscular arteries Arterioles

Elastic (Conducting) Arteries Elastic arteries are thick walled arteries near the heart - the aorta and its major branches These arteries are the largest in diameter and the most elastic A large lumen allows them to serve as low resistance pathways that conduct blood from the heart to medium-sized arteries and thus are called conducting arteries

Elastic (Conducting) Arteries The elastic arteries contain more elastin than any other type of vessel While present in all three layers, the tunica media contains the most The abundant elastin enables these arteries to withstand and smooth out large pressure fluctuations by expanding when the heart forces blood into them and then recoiling to propel blood onward into the circulation when the heart relaxes

Muscular (Distributing) Arteries The muscular distributing arteries deliver blood to specific body organs and account for most of the named arteries Proportionately, they have the thickest media of all vessels Their tunica media contains relatively more smooth muscle and less elastic tissue than that of elastic arteries They are more active in vasoconstriction and are less distensible

Muscular (Distributing) Arteries As in all vessels, concentric sheets of elastin occur within the tunica media of muscular arteries although these sheets are not as thick or abundant as those of elastic arteries

Muscular (Distributing) Arteries A feature unique to muscular arteries, especially thick sheets of elastin lie on each side of the tunica media An external elastic lamina lies between the tunica media and tunica externa

Muscular (Distributing) Arteries The elastin in muscular arteries, like that in elastic arteries, helps dampen the pulsatile pressure produced by the heartbeat

Arterioles Arterioles have a lumen diameter from 0.3 mm to 10 m, and are the smallest of the arteries Larger arterioles exhibit all three tunics, but their tunica media is chiefly smooth muscle with a few scattered muscle fibers The smaller arterioles that lead into capillary beds, are little more than a single layer of smooth muscle cells spiraling around the endothelial lining

Arterioles The diameter of each arteriole is regulated in two ways: Local factors in the tissues signal the smooth musculature to contract or relax, thus regulating the amount of blood sent downstream to each capillary bed Sympathetic nervous system adjusts the diameter of arterioles throughout the body to regulate systemic blood pressure

Capillaries The microscopic capillaries are the smallest blood vessels In some cases, one endothelial cell forms the entire circum- ference of the capillary wall The average length of a capillary is 1 mm and the average diameter is 8-10 m

Capillaries Capillaries have a lumen just large enough for blood cells to slip through in single file

Capillaries Capillaries are the body’s most important blood vessels because they renew and refresh the surrounding tissue fluid (interstitial fluid) with which all cells in the body are in contract Capillaries deliver to interstitial fluid the oxygen and nutrients that cells need while removing carbon dioxide and nitrogenous wastes that cells deposit in the fluid

Capillaries Given their location and the thinness of their walls capillaries are ideally suited for their role of providing access to nearly every cell Along with the universal functions just described some capillaries also perform site-specific functions Lungs: gas exchanges Endocrine glands: pick up hormones Small intestine: nutrients Kidneys: removal of nitrogenous wastes

Capillary Beds A capillary bed is a network of the body’s smallest vessels that run throughout almost all tissues, especially the loose connective tissue This flow is also called a microcirculation

Capillary Beds In most body regions, a capillary bed consists of two types of vessel a vascular shunt (meta- arteriole) and true capillaries

Capillary Beds The terminal arteriole leads into a metarteriole which is directly continuous with the thorough- fare channel

Capillary Beds The thoroughfare channel joins the post- capillary venule that drains the capillary bed

Capillary Beds The true capillaries number 10 to 100 per capillary bed, depending on the organ served Branch from metarteriole to thoroughfare channel

Capillary Beds A cuff of smooth muscle fibers, called a pre- capillary sphincter surrounds the root of each capillary at the metarteriole and acts as a valve to regulate the flow of blood into the capillary

Capillary Beds When the precapillary sphincters are relaxed, blood flows through the true capillaries and takes part in exchanges with tissue cells

Capillary Beds When the precapillary sphincters are contracted, blood flows through the shunts and bypasses the tissue cells

Capillary Beds Most tissues have a rich supply, but there are a few exceptions Tendons and ligaments / poorly vascularized Cartilage / from adjacent connective tissue Epithelia / from adjacent connective tissue Cornea / nourished by aqueous humor

Capillary Beds The relative amount of blood entering a capillary bed is regulated by vasomotor nerve fibers and local chemical conditions A capillary bed may be flooded with blood or almost completely bypassed, depending on conditions in the body or in that specific organ Example of shunting blood from digestive organs to skeletal muscles

Capillary Permeability The structure of capillaries is well suited for their function in the exchange of nutrients and wastes between the blood and the tissues through the tissue fluid A capillary is a tube consisting of thin endothelial cells surrounded by a basal lamina The endothelial cells are held together by tight junctions and occasional desmosomes

Capillary Permeability Tight junctions block the passage of small molecules, but such junctions do not surround the whole perimeter of the endothelial cells Instead, gaps of unjoined membrane called intercellular clefts occur through which small molecules exit and enter the capillary

Routes of Capillary Permeability Molecules pass into and out of capillaries via four routes Direct diffusion through endothelial cell membranes Through the intercellular clefts Through cytoplasmic vesicles Through fenestrations in fenestrated capillaries

Routes of Capillary Permeability Most exchange of small molecules is thought to occur through intercellular clefts Carbon dioxide and oxygen seem to be the only important molecules that diffuse directly through endothelial cells because these uncharged molecules easily diffuse through lipid containing membranes of cells

Low Permeability Capillaries The blood-brain barrier prevents all but the most vital molecules(even leukocytes) from leaving the blood and entering brain tissue The blood-brain barrier derives its structure from the capillaries of the brain Brain capillaries have complete tight junctions, so intercellular clefts are absent

Low Permeability Capillaries Brain capillaries are continuous, not fenestrated and they also lack caveolae Vital capillaries that must cross brain capillaries are “ushered through” by highly selective transport mechanisms in the plasma membranes of the endothelial cells

Veins Veins are the blood vessels that conduct blood from the capillaries back to the heart Because blood pressure declines substantially while passing through the high-resistance arterioles and capillary beds, blood pressure in the venous part of the circulation is much lower than in the arterial part

Veins Because they need not withstand as much pressure, the walls of veins are thinner than those of comparable arteries The venous vessels increase in diameter, and their walls gradually thicken as they progress from venules to the larger and larger veins leading to the heart

Venules Venules, ranging from 8 to 100 m in diameter are formed when capillaries unite The smallest venules, the postcapillary venules, consist of endothelium on which lie pericytes

Venules Venules join to form veins With their large lumens and thin walls, veins can accommodate a fairly large blood volume Up to 65%of the body’s total blood supply is found in the veins at any one time although the veins are normally only partially filled with blood

Veins externa Veins have three distinct tunics, but their walls are always thinner and their lumens larger than those of corresponding arteries There is little smooth muscle even in the largest veins

Veins The tunica externa is the heaviest wall layer and is often several times thicker than the tunica media In the venae cavae, the largest veins, which return blood directly to the heart the tunica externa is further thickened by longitudinal bands of smooth muscle Tunica externa

Veins Veins have less elastin in their walls than do arteries, because veins do not dampen any pulsations (these have been smoothed out by the arteries) Because blood pressure within veins is low, they can be much thinner walled than arterioles without danger of bursting

Veins Low-pressure conditions demand some special adaptations to help return blood to the heart at the same rate as it was pumped into circulation One structural feature that prevents the backflow of blood away from the heart is the presence of valves within veins

Veins Venous valves are formed from folds of the tunica intima and they resemble the semilunar valves of the heart in structure and function Venous valves are most abundant in the veins of the limbs, where the upward flow of blood is opposed by gravity

Veins A few valves occur in the veins of the head and neck, but none are located in veins of the thoracic and abdominal cavities A functional mechanism that aids the return of venous blood to the heart is the normal movement of our body and limbs

Veins Another mechanism of venous return is called the skeletal muscular pump Here contracting muscles press against the thin-walled veins forcing valves proximal to the contraction to open and propelling the blood toward the heart

Vascular Anastomoses Where vessels unite or interconnect, they form vascular anastomoses Most organ receive blood from more than one arterial branch and arteries supplying the same area often merge, forming arterial anastomoses Arterial anastomoses provide alternative pathways called collateral channels for blood to reach a given body region

Vascular Anastomoses If one arterial branch is blocked arterial anastomoses provide the region with an adequate blood supply Arterial anastomoses are abundant in abdominal organs and around joints, where active movement may hinder blood flow through one channel

Vascular Anastomoses Anastomoses are also prevalent in the abdominal organs, brain, and heart Because of the many anastomoses among the smaller branches of the coronary artery in the heart wall, a coronary artery can be 90% occluded by atherosclerosis (plaque) before a myocardial infarction (heart attack) occurs

Vasa Vasorum The wall of the blood vessels contain living cells and therefore require a blood supply of their own For this reason the larger arteries and veins have tiny arteries, capillaries and veins in their tunica externa These tiny vessels the vasa vasorum nourish the outer half of the wall of a large vessel with the inner half being nourished by the blood in the lumen

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