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Chapter 21

Nursing HAP201 Chapter Notes - Chapter 21: Common Iliac Artery, Internal Carotid Artery, Tunica Media

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Nursing HAP201
Judith Card

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HAP201 Week Five/Chapter 21 The Cardiovascular System: Blood Vessels and Hemodynamics
21.1: Structure and Function of Blood Vessels
Arteries: carry blood away from the heart to other organs
Arterioles: very small arteries that are able to enter a tissue and branch into numerous tiny vessels called capillaries
Capillaries: their thin walls allow the exchange of substances between blood and blood tissues. A group of these within a tissue
reunite to form small veins called venules.
Venules merge to form progressively larger blood vessels called veins. They convey blood from the tissues back to the heart
Basic Structure of a Blood Vessel
o A blood vessel wall consist of three layer: epithelial inner lining, a middle layer consisting of smooth muscle and elastic
connective tissue, and a connective tissue outer covering
o The three structural layers of a generalized blood vessel from innermost to outermost are the following
Tunica interna (intima): forms the inner lining of a blood vessel and is in direct contact with the blood as it flows
through the lumen of the vessel. It has three layers:
Endothelium: innermost layer continuous with the endocardial lining of the heart. These cells are active
participants in vessel-related activities, e.g. physical influences on blood flow, secretion of locally acting
chemical mediators that influence the contractile state of the vessel’s overlying smooth muscle and assistance
with capillary permeability. They also reduce surface friction
Basement membrane: second layer that provides physical support for the epithelial layer; anchors the
endothelium to the underlying connective tissue while regulating molecular movement. It also guides cell
movement during repair of blood vessel walls.
Internal elastic lamina: outermost layer; forms boundary between the tunica interna and tunica media. It has
openings that facilitate diffusion of material through the next layer
Tunica Media: a muscular and connective tissue layer that displays the greatest variation among the different vessel
types. It is relatively thick and comprises of smooth muscle cells (to regulate diameter of lumen) and substantial
amounts of elastic fibers. Sympathetic stimulation increase causes smooth muscle to contract, narrowing the lumen.
(vasoconstriction; opposite is vasodilation). It helps regulate flow and blood pressure, as well as initiate vascular
spasms when a small artery is damaged
Tunica Externa: this is the outer covering of a blood vessel; consists of elastic and collagen fibers. It contains
numerous nerves and tiny blood vessels that supply the tissue of the vessel wall. They also help anchor the vessels to
surrounding tissues.
o These are found empty at death
o The walls have three layers, similar to a typical blood vessel, but also has a thick muscular-to-elastic tunica media.
o They have a high compliance as they have a lot of elastic fibers. Thus, their walls stretch easily or expand without tearing in
response to a small increase in pressure
o Elastic arteries
These are the largest arteries in the body; they have the largest diameter but are relatively thin compared with the
overall size of the vessel. Examples include the two major trunks that exist the heart, the aorta and pulmonary trunk,
along with the aorta’s major initial branches (brachiocephalic, subclavian, common carotid, and common iliac
Elastic arteries help propel blood onward while the ventricles are relaxing. They are able to stretch to accommodate
the pressure change. When they stretch, they function as a pressure reservoir and elastic fibers store mechanical
energy, which is converted to kinetic energy for the blood.
Are also called conducting arteries, as they conduct blood from the heart to medium-sized, more muscular arteries
o Muscular Arteries
These are medium sized arteries, as their tunica media contains more smooth muscle and fewer elastic fibers than
elastic arteries.
They have thick walls, thus are capable of greater vasoconstriction and vasodilation to adjust the rate of blood flow
They continue to branch and ultimately distribute blood to each of the various organs, so they are called distributing
arteries, e.g. brachial artery and radial artery
There is a reduced amount of elastic tissue in the walls of muscular arteries, thus these vessels do not have the ability
to recoil and help propel blood like elastic arteries. Instead, they help maintain vascular tone (the ability of muscle to
contract and maintain a state of partial contraction). This stiffens the vessel wall and is needed to maintain vessel
pressure and efficient blood flow
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o Abundant microscopic vessels that regulate the flow of blood into the capillary networks of the body’s tissue
o They have a thin tunica interna with a thin internal elastic lamina that disappears at the terminal end, called metarteriole; it
tapers towards the capillary junction.
o At this junction, the distal-most muscle cell forms the precapillary sphincter, which monitors the blood flow into the
capillary; other muscle cells in the arteriole regulate the resistance to blood flow
o The tunica exertna consist of areolar connective tissue abundant with sympathetic nerves that alter the diameter of arterioles
and influence the rate of blood flow and resistance through these vessels
o They are important in regulating blood flow from arteries into capillaries by regulating resistance, the opposition to blood
flow due to friction between blood and the walls of blood vessels. Thus, they are known as resistance vessels
o These are the smallest blood vessels; they form the U-turns that connect the arterial outflow to venous return
o They form an extrinsic network of short, branched interconnecting vessels that course among the individual cells of the body
o Microcirculation is the flow of blood from a metarteriole through capillaries and into a post-capillary venule (venule that
receives blood from a capillary)
o They participate in the exchange of substances between blood and interstitial fluid, and are thus called exchange vessels
o The number of capillaries varies with the metabolic activity of the tissue they serve. Body tissues with high metabolic
activity have extensive capillary networks. Tissues with lower metabolic requirements have fewer capillaries
o They lack both a tunica media and tunica externa. They are only composed of a single layer of endothelial cells and a
basement membrane. Thus, a substance in the blood must pass through just one cell layer to reach the interstitial fluid and
tissue cells.
o Exchange of materials occurs through the walls of capillaries and the beginning of venules; the walls of arteries, arterioles,
most venules and veins present too thick of a barrier.
o Venules and veins have thin walls that do not readily maintain their shape.
o Venules drain the capillary blood and begin the return flow of blood back to the heart. They also function as significant sites
of exchange of nutrients and wastes and white blood cell emigration.
o These have very thin walls relative to their total diameter.
o They are composed of the same three layers as the arteries: the tuninca interna is thinner, the tunica media is thinner with
little smooth musucle and the tunica externa is the thickest layers
o The average BP in veins is lower than arteries, due to a pressure difference. Venous blood has a slow flow and this is due to
structural differences.
o Many veins contain valves, thin folds of tunica interna that form flap-like cusps. The low BP in veins allows blood returning
to the heart to slow and even back up; the valves aid in venous return by preventing the backflow of blood
o Anastomotic veins: double set veins that escort arteries and connect with one another. They cross the accompanying artery
to form rungs between the paired veins. The greatest number of paired veins are in the limbs
o Superficial veins: these veins course through the subQ layer and are unaccompanied by parallel arteries.
o Deep veins travel between skeletal muscle.
o The amount of blood flow through superficial veins varies form location within the body. In the upper limb, the superficial
veins are much larger than the deep veins and serve as the major pathways from the capillaries of the upper limb back to
heart. In the lower limb, deep veins serve as the principal return pathways
Blood Distribution
o Systemic veins and venules contain a large percentage of the blood volume, functioning as blood reservoirs from which
blood can be diverted quickly if the need arises.
21.2: Capillary Exchange
This is the movement of substances between blood and interstitial fluid.
o Many substances enter and leave capillaries by simple diffusion.
o Oxygen and nutrients are present in high concentrations, so they diffuse down their concentration gradients and into
interstitial fluid and then to body cells.
o Carbon dioxide and other wastes released by body cells are present in higher concentrations in this fluid, so they
diffuse into blood
o Substances in blood or interstitial fluid can cross the walls of a capillary by diffusing through the
intercellular clefts or fenestrations or by diffusing through the endothelial cells
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