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what do blood vessels do?

they are dynamic structures that pulsate, constrict, relax and proliferate.

blood vessels definition

a closed delivery system that begins and ends at the heart

old theory about how blood went through the body?

blood moved through the body like an ocean tide, moving out through the heart and ebbing back into the same vessels
Greek physician Galen

what are the three major types of blood vessels?


movement of blood through blood vessels description

heart contracts, blood is forced into the large arteries leaving the ventricles. blood then moves into successively smaller arteries and reaching the smallest branches, the arterioles that blood drains from the capillaries into venules (smallest veins) and then into larger and larger veins that merge to form the large veins that empty into the heart.


the smallest branches of the arteries


the smallest veins

how many km/miles do the blood vessels collectively stretch?

60,000 miles or 100,000 km


carry blood away from the heart
are said to branch, diverge or fork as they form smaller divisions


carry blood towards the heart and are said to join, merge and converge into successively larger vessels approaching the heart.

explain the arteries and veins of the systemic circulation

arteries carry oxygenated blood and veins carry oxygen poor blood.

explain the arteries and veins of the pulmonary circuit

arteries carry oxygen poor blood to the lungs and the veins carry oxygen rich blood to the heart.

only the __ have intimate contact with tissue cells and directly serve cellular needs.

this occurs through the gossamer-thin capillary walls


distinct layers of the walls of all blood vessels


the central blood containing space of the blood vessels

what are the three tunics of blood vessels?

tunica intima
tinica media
tunica externa

tunica intima

the innermost tunic
has intimate contact with the blood in the lumen
contains the endothelium that lines the lumen of all vessels

tell me about the endothelium that the tunica intima contains?

it is simple squamous endothelium
it is contrinuous with the endocardial lining of the heart, and its flat cells fit closely together, forming a slick surface that minimizes friction as blood moves through the lumen.

what the a subendothelial layer? where is it found?

it is a layer consisting of a basement membrane and loose connective tissue that supports the endotheium
this can be found in vessels larger than 1 mm in diameter

tunica media

the middle tunic that is mostly circulary arranged smooth muscle cells and sheets of elastin.

what regulates the smooth muscle of the tunica media?

it is regulated by sympathetic vasomotor nerve fibers of the autonomic nervous system and chemicals.
either vasoconstriction or vasodialation ca be effected by this.


reduction in lumen diameter as the smooth muscle contracts


increase in lumen diameter as the smooth muscle relaxes

what is the role of the tunica media?

is critical in regulation of circulatory dynamics bc small changes in vessels diameter greatly influence blood flow and blood pressure.

sum up the tunica media

it is the bulkiest layer in arteries and bears the chief responsibility for maintaining blood pressure and contrinuous blood circulation.


outermost layer of a blood vessel wall
composed largerly of loosely woven collagen fibers that protect and reinforce the vessel and anchor it to surrounding structures.
is infiltrated with nerve fibers, lymphatic vessels and in larger veins, a network of elastic fibers.

vasa vasorum

a system of tiny blood vessels found in larger vessels that nourish the more exernal tissues of the blood vessel walls.

how is the innermost (or luminal) portion of the vessel nourished?

it obtains nutrients directly from blood in the lumen.

what are the three types of arteries in terms of relative size and function?

elastic arteries
muscular arteries

elastic arteries

thick walled arteries near the heart (aorta and its major branches).
largest in diameter and the most elastic.

conducting arteries, where does this name come from

elastic arteries are sometimes referred to as conducting arteries bc their large lumen make them low resistance pathways that conduct blood from the heart to medium sized arteries.

elastic arteries contain more __ than any other vessel type


which tunic contains the lost elastin?

tunica media
where the elastin constructs holey laminae (sheets) of elastic conn tissue that look like slices of Swiss cheese between the layers of smooth muscle.

elastic arteries also contain...

substantial amounts of smooth muscle

although they have smooth muscle, elastic arteries...

are inactive in vasoconstriction

function of elastic arteries

they are simple elastic tubes

elastic arteries are...

pressure reservoirs, explanding and recoiling as blood is ejected from the heart.

in elastic arteries, blood flows...

continuously rather than starting and stopping with the pulsating rhythm of the heart.

muscular (distributing) arteries definition

deliver blood to specific body organs and account for most of the named arteries from lab.
are distal to the elastic arteries.

muscular arteries have the thickesd tunica __ of all vessels


what are muscular arteries active in?

they are active in vasoconstriction and are less distensible bc they have more smooth muscle and less elastic tissue than elastic arterioes.

in muscle arteries there is a...

elastic lamina on each face of the tunica media


smallest of the arteries.
larger arterioles have all 3 tunics but their tunica media is chiefly smooth muscle with a few scattered elastic fibers
smaller arterioles are a little more than a single layer of smooth muscle cells spiraling around the endothelial lining

what is minute to minute blood flow into the capillary beds determined by?

arteriole diamter, which varies in response to changing neural, hormonal and local chemical influences.

when arterioles constrict...

the tissue served are largely bypassed

when arterioles dialate...

blood flow into the local capillaries increases dramatically


smallest blood vessels.
have thin walls consisting of just a thin tunica intima

pericytes definition

spider shaped smooth muscle cells that stabilize the capillary wall and help control capillary permeability.
they are on the outer surface of some capillaries

capillary length and width is just large enough for...

RBCs to slip through in single file.

__ and __ are poorly vascularized

tendons and ligaments
cartilage and epithelial lack capillaries but recieve nutrients by nearby conn tissue. the avascular cornea and lens of the eye receive nutrients from aqueous humor

capillaries are the...

back alleys and driveways that provide direct access to nearly every cell in the body

what is the role of capillaries?

bc of their location and the thinness of their walls they are well suited for exchange of materials (gases, nutrients, hormones, etc) between the blood and the interstitial fluid.

what are the three types of capillaries?


continuous capillaries

they are abundant in the skin and muscles.
are the most common type of capillaries
they are continuous in the sense that their endothelial cells provide an uninterrupted lining, adjacent cells being joint laterally by tight junctions--but there are intercellular clefts.

intercellular clefts definition

gaps of unjoined membrane that are caused by the tight junctions being incomplete.
they allow limited passage of fluids and small solutes.

how are brain capillaries unique?

the tight junctions of their continuous capillaries are complete and are the structural basis of the blood brain barrier

fenestrated capillaries

capillaries were some of the endothelial cells are riddled with oval pores. the pores are usually covered with a delicate membrane (or diaphragm)
are much more permeable to fluids and small solutes than continuous capillaries.

what are pores also called?


fenstrated capillaries are found wherever...

active capillary absorption or filtration formation occurs.

ex- small intestines, endocrine organs, kidneys


also called sinusoidal capillaries
highly modified leaky capillaries
have large irregularly shaped lumens and are usually fenestrated.
have few tight junctions and larger intrercellular clefts than ordinary capillaries

sinusoids are only found in

the liver, bone marrow, spleed and adrenal medulla.

structurally, sinusoids allow...

large molecules and even blood cells to pass between the blood and surrounding tissue.

describe the sinusoids in the liver

the endothelium is discontinuous and its lining includes large hepatic marcrophages that remove and destroy any contained bacteria.

capillaries do not function...


capillary beds

interweaving networks of capillaries.


the flow of blood from an arteriole to a venule through a capillary bed.

what are the two types of vessels a capillary bed consists of

1. a vascular shunt (metarteriole-thoroughfare channel)
2. true capillaries

vascular shunt definition

a short vessel that directly connects the arteriole and venule at opposite ends of the bed

true capillaries definition

the actual exchange vessels

describe the flow of blood in microcirculation

a terminal arteriole feeding the bed leads into a metarteriole which is continuous with a thoroughfare channel. the thoroughfare channel joins the postcapillary venule that drains the bed.

metarteriole definition

a vessel structurally intermediate between the arteriole and a capillary

thoroughfare channel definition

intermediate between a capillary and a venule

true capillaries description

10-100 per capillary bed
branch off the metarteriole and return to the thoroughfare channel.
occassionally spring from the terminal arteriole and empty directly into the venule.

precapillary sphincter

a cuff of smooth muscle fibers surrounding the root of each true capillary at the metarteriole and acts as a valve to regulate blood flow into the capillary.

blood flowing through a terminal arteriole may go either through __ or __

the true capillaries
through the shunt

when the precapillary sphincters are relaxed (open)...

blood flows through the true capillaries and takes part in exchanges with tissue cells.

when the precapillary sphincters are contracted (closed)...

blood flows through the shunts and bypasses the tissue cells.

the amount of blood entering a capillary bed is regulated by...

local chemical conditions and arteriolar vasomotor nerve fibers.

how is blood carried to the heart from capillary beds?

by veins

venules definition

are formed when capillaries unite.

postcapillary venules

the smallest venules
consist entirely of endothelium with pericytes contracting around them.
extrememly porous- fluid and WBC move easily from the bloodstream through their walls.

larger venules have (structually)

have one or two layers of smooth muscle cells and thin external as well

venules joing to form __


veins usually have __ distinct tunics

but their walls are thinner and their lumens larger than those of arteries.
lumens normally look slitlike when in prepared slides

structural makeup of veins descriptions

little smooth muscle or elastin in tunica media (which is poorly developed anyway and is thin)
tunica externa is the heaviest wall layer--consisting of thick longitudinal bundles of collagen fibers and elastic networks

capacitance vessels/blood reservoirs

a name for veins bc they accommodate a large blood volume due to their large lumens and thin walls.
up to 65% of the body's blood supply is found in the veins at any time.

explain the difference between vein wall thickness and that of corresponding arteries

walls of veins can be much thinner than arterial walls without danger of bursting bc the blood pressure in veins is low.

adaptations to the low pressure condition to ensure blood returns in the same amount of time it was pumped out (2)

1. large diameter lumens of veins (offers low resistance to blood flow)
2. valves that prevent blood from flowing backward

venous valves

formed from folds of the tunica intima. resemble the SL valves of the heart in structure and function

prevent blood from flowing backward.

where are venous valves most abundant?

in the limbs where the upward flow is opposed by gravity.

where are venous veins absent?

absent in veins of the thoracic and abdominal cavities.

vascular anastomoses

interconnections where vascular channels unite.

arterial anastomoses

name for the merged arteries that supply the same territory.

collateral channels

alternate pathways provided by anastomoses for blood to reach a given body region.
provides an area that has been blocked/cut off with adequate blood supply

arterial anastomoses occur around...

joints- where active movement may hinder blood flow through one channel
also common in abdominal organs, the brain and the heart.

arteries that dont have anastomoses or have a poorly developed collateral circulation supply are in the

retina, kidneys, and spleen.
if their blood flow is interrupted, cells supplied by these vessels die.

arteriovenous anastomoses

shunts of capillary beds that connect arterioles and venules
ex- metarteriole thoroughfare channel

venous anastomoses

interconnect veins

do arteries or veins interconnect more readily?

venous anastomose are abundant so occlusions of a vein rarely block blood flow or lead to tissue death

breakdown of circulation

heart is the pump, the arteries are pressure reservoirs and conduits, the arterioles are resistance vessels that control distribution, the capillaries are exchange sites and the veins are conduits and blood reserviors.

blood flow definition

the volume of blood flowing through a vessel, an organ, or the entire circulation in a given period (ml/min)
under resting conditions its relatively constant.

blood flow is equivalent to...

cardiac output (CO)

blow flow through ___ body organs may vary eidely and is related to their immediates needs


blood pressure (BP) definition

the force per unit area exerted on a vessel wall by the contained blood
expressed in millimeters of mercury (mm Hg)

blood pressure means...

systemic arterial blood pressure in the largest arteries near the heart.

what is a pressure gradient?

the differences in blood pressure within the vascular system always from an area of high pressure to an area of low pressure through the body

the pressure gradient provides what for blood?

it provides the driving force that keeps blood moving

resistance definition

the opposition to flow and is a measure of the amount of friction blood encounters as it passes through the vessels.

peripheral resistance

another name for resistance because most friction is encountered in the peripheral (systemic) circulation

what are the 3 important sources of resistance?

blood viscosity
vessel length
vessel diameter

blood viscosity definition

the internal resistance to flow that exists in all fluids and is related to the thickness of a fluid.
remains fairly constant

the greater the viscosity...

the less easily molecules slide past one another and the more difficult it is to get and keep the fluid moving.

why is blood more viscous than water?

it contains formed elements and plasma proteins.

total blood vessel length

the longer the vessel, the greater the resistance.

what two of the three sources of resistance normally remain constant?

blood viscosity and vessel length

changes in __ __ __ are frequent and significantly alter peripheral resistance

blood vessel diameter

fluid close to the wall of a tube or channel is what? what is to be said of the opposite?

fluid close to the wall/tube is slowed by friction as it passes along the wall, whereas fluid in the center of the tube flows more freely and faster.

laminar flow
(also called streamlining)

the relative speed and position of fluid in the different regions of the tube's cross section remain constant.

smaller the tube, ___ the friction

bc more of the fluid contacts the tube wall where its movement is impeded.

resistance varies ___ with the 4th power of the vessel radius

meaning the larger arteries do not contribute much to peripheral resistance.

smaller-diameter arterioles, which can enlarge or constrict in response to neural and chemical controls, are the...

major determinants of peripheral resistance.

turbulent flow definition

irregular fluid motion where blood from the different laminae mexes.
dramatically increases resistance
occurs when blood encounters an abrupt change in the tube size or protruding areas of the tube wall

blood flow is directly proportional to the difference in...

blood pressure between 2 points in the circulation

when blood pressure (triangle P) increases, blood flow (F)...

speeds up
when blood pressure decreases, blood flow declines.

blood flow is inversely proportional to the

peripheral resistance (R) in the systemic circulation

if peripheral resistance increases, blood flow...


formula for blood flow

F = blood pressure (triangle P)/peripheral resistance (R)

which factor affects blood flow more?

peripheral resistance (R) is more important in influencing local blood flow because R can easily be changed by altering blood vessel diameter.

any fluid driven by a pump through a circuit of closed channels operates under...


the nearer the fluid is to the pump...

the greater the pressure exerted on the fluid

blood flows through the blood vessels along a..

pressure gradient, always moving from higher to lower pressure areas.

the pumping action of the heart generates

blood flow

pressure results when...

flow is opposed by resistance.

___ blood pressure is highest in the aorta

and declines throughout the pathway to finally reach 0 mm Hg in the right atrium.

where does the steepest drop in blood pressure occur?

the arterioles
they offer the greatest resistance to blood flow

as long as a pressure gradient exists...

blood continues to flow into it completes the circuit back to the heart.

arterial blood pressure reflects 2 factors:

1. how much the elastic arteries close to the heart can be stretched (their compliance or distensibility)
2. the volume of blood forced into them at any time

if the amounts of blood entering and exiting the elastic arteries were equal,

arterial pressure would be constant
however blood pressure rises and falls in a regular fashion

as the left ventricule contracts and expels blood into the aorta, it imparts...

kinetic energy to the blood, which stretches the elastic aorta as aortic pressure reaches its peak

systolic pressure

averages 120 mm Hg in healthy adults
the pressure in the aorta at its peak.

why does blood move forward into the arterial bed?

bc the pressure in the aorta is higher than the pressure in the more distal vessels

what happens during diastole?

the aortic valve closes, preventing backflow of blood into the heart, the walls of the aorta recoil which maintains sufficient pressure to keep the blood flowing foward into the smaller vessels.

diastolic pressure

during diastole when aortic pressure drops to its lowest (70-80 mm Hg)

elastic arteries are the

pressure reservoirs.
keep the blood circulting during diastolic pressure, when the heart is relaxing.

pulse pressure

the difference between the systolic and diastolic pressure.
it is felt as a throbbing pulsation in an artery during systole

increased whats cause temporary increases in the pulse pressure?

increased stroke volume and faster blood ejection from the heart (results from increased contractibility)

aortic pressure ___ with each heartbeat


mean arterial pressure (MAP)

the pressure that propels the blood to the tissue

which phase lasts longer? diastole or systole?


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