Anatomy Chapter 19: Blood Vessels

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blood vessels

a closed delivery system that starts and ends at the heart, three types of blood vessels (arteries, capillaries, veins)


carry blood away from the heart "branch", always carry oxygenated blood


carry blood to the heart "merge" always carry oxygen poor blood (with the exception of pulmonary veins)


central blood containing space of a blood vessel

tunica intima

inner most layer of a blood vessel, contains endothelium which minimizes friction


simple squanmous that lines the lumen of all vessels, it is continuous with the endocardial lining of the heart

tunica media

middle layer of blood vessel wall, mostly made of smooth muscle cells and elastin, its activities are critical in regulating circulatory dynamics, it also maintains blood pressure


reduction in lumen diameter as the smooth muscle contracts


increase in lumen diamneter as the smooth muscle relaxes

tunica externa

outer layer of the blood vessels, is composed largely of loosely woven collagen fibers, that protect and reinforce the vessel and anchor it to its surroundings- infiltrated with lymphatic vessels and nerve fibers

vasa vasorum

system of tiny vessels located in the tunica externa of large blood vessels to ensure that they get enough blood supply themselves

elastic arteries

thick walled arteries near the heat (the aorta and its major branches), largest in diameter from 2.5-1 cm, most elastic/low resistance, "conducting arteries", many elastin sheets among the smooth muscle of the tunica media, relatively inactive to vasoconstriction

conducting arteries

elastic arteries

muscular or distributing arteries

deliver blood to specific body organs, internal diameter ranges from 1cm-0.3mm, thickest tunica media of all vessels, there is an elastic lamina on each face of the tunica media, branch form elastin arteries


smallest of the arteries, have a lumen diameter ranging from 0.3mm-10micom, tunica media is mostly smooth muscle with few elastic fibers, lead to capillary beds


smallest of the blood vessels (microscopic)- just a tunica intima, length is about 1m, diameter 8-10microm (RBCs flow through single file), exchange materials (gases, nutrients, hormones) between blood and IF


spiper shaped smooth muscle cells on the outside of the capillary wall that help control capillary permeability

continuous capillaries

abundant in the skin and muscles, are the most common endothelial cell joined by tight junctions

intercellular clefts

gaps of unjoined membrane created by incomplete gap junctions- allow limited passage of fluids and small solutes

fenestrated capillaries

similar to continuous yet they contain pores, usually covered by a thin membrane, found wherever active capillary absorption or filtrate formation occurs, in the small intestines (allows nutrients to pass) in the endocrine organs (allows hormones to pass), kidneys (rapid filtration of plasma

sinusoids or sinusoidal capillaries

highly modified leaky capillaries found only in the liver, bone marrow, spleen and adrenal medulla- large irregular shaped lumens with pores- fewer tight junctions and larger intercellular clefts than ordinary capillaries- allow large molecules to pass

hepatic macrophages

located in the endothelium of sinusoids in the liver, remove and destroy any contained bacteria

capillary beds

interweaving networks of capillaries, consists of two types of vessels- a vascular shunt (metarteriole-thoroughfare cahnnel), a short vessel that directly connects the arteriole and venule at opposite ends- true capillaries, the actual vessel (exchange)


flow of blood from arteriole through capillary bed to venule

terminal arteriole

feeds the capillary bed


a vessel structurally intermediate between an arteriole and a capillary, continuous with the thouroughfare channel

thoroughfare channel

intermediate between a capillary and a venule, joins the postcapillary venule that drains the bed

post capillary venule

drains the bed

true capillary

branches off of the metarteriole proximal end of the shunt) and return to the thouroughfare channel (distal end)- about 10-100 per capillary bed

precapillary sphincter

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


ranging from 8-100 microm in diameter, are formed when capillaries unite, thick tunica media

capitance vessels

(or blood reservoirs) veins- because up to 65% of the bodies blood supply is formed in the veins at anytime

venous valves

folds of the tunica media, resemble semilunar valves, abundant in limbs- prevent blood from flowing backwards (against gravity)

varicose veins

veins that have become tortuous and dilated because of incompetent (leaky) valves, vein walls stretch and become "floppy" causing blood to pool, generally occurs in the legs

venous sinuses

highly specialized flattened veins with extremely thick walls consisting of only endothelium, supported by surrounding structures

vascular anastomes

interconnections where vascular channels unite

arterial anastomes

when arteries supplying the same area come together, provide alternate pathways, located around the joints

collateral channels

alternate pathways for blood to reach a given body region, can supply adequate blood supply if there is a cut or a clot

arteriovenous anastomoses

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

blood flow

is the volume of blood flowing through a vessel, an organ or the entire circulation in a given period of time (mL/min.)

blood pressure

the force per unit area exerted on a vessel wall, by the contained blood, is expressed in mm of mercury (Hg), unless stated otherwise is considered systemic, a change in BP though out the body keeps our blood flowing


is the opposition of flow and is a measure of the amount of friction blood encounters as it passes through the vessels- three sources of resistance: blood viscosity, vessel length, vessel diameter

peripheral resistance

where most resistance occurs, away from the heart

blood viscosity

the internal resistance to flow that exists in all fluids, thickness or stickyness, the greater the viscosity the more difficult it is to keep moving

total blood vessel length

the longer the length the more resistance

blood vessel diameter

changes frequency, smaller diameter greater friction, varies inversely with the fourth power of the vessel radius, small arteries constrict more than large arteries

laminar flow or steamlining

smooth constant flow

turbulent flow

irregular fluid motion which increases resistance

systolic pressure

pressure peak- BP in the aorta, averages 120 mm HG in healthy adults

diastolic pressure

lowest pressure 70-80 mm Hg

pulse pressure

difference between systolic and diastolic pressure, pulse is felt during systolic

mean arteriole pressure (MAP)

the pressure that propels the blood to the tissues

respiratory pump

pressure changes occurring in the ventral body cavity during breathing create the respiratory pump that moves blood towards the heart

muscular pump

consists of skeletal activity, muscles surrounding the deep veins contract and relax squeezing blood to the heart with the help from the valves

vasomotor center

a cluster of neurons in the medulla, neural center that oversees changes in diameter of blood vessels

cardiovascular center

integrates blood pressure control by altering cardiac output and blood vessel diameter

vasomotor fibers

sympathetic efferents which exit from the T1-L2 levels of the spinal cord, receive impulses from vasomotor center and innervate the smooth muscle of the blood vessels

vasomotor tone

constant state of moderate constriction, almost always arterioles


pressure sensitive mechonoreceptors that respond to changes in arteriole pressure and stretch- neural receptors located in the carotid sinuses (dilations in the internal carotid arteries which provide the major blood supply to the brain) in the aortic arch, and in the walls of nearly every large artery of the neck and throrax

carotid sinus reflex

protects blood supply to the brain

aortic reflex

maintains adequate BP in the systemic circuit as a whole


located in the aortic arch and large arteries of the neck- transmit impulses to the cardioacceletory center, which then increases cardiac output and vasocontriction- causes BP to rise and speeds return of blood- carotid and aortic bodies

norepinephrine (NE) and epinephrine

adrenal medulla hormones, are released at times of stress into the blood and enhance fight or flight response- both promote vasocontriction, epinephrine increases cardiac output

angiotensin II

stimulates intense vasocontriction, promoting a rapid rise in systemic BP, also stimulates release of aldosterone and ADH, which acts in long-term regulation of BP by enhancing blood volume

atrial natriuretic peptide (ANP)

hormone produced by the atria of the heart- causes BP and blood volume to decline, causes general vasodialation

antidiuretic hormone (ADH) or vasopressin

(vasopressin), produced by the hypothalamus, stimulates the kidneys to conserve water- can cause intense vasocontriction in emergencies

direct renal mechanism

alters blood volume independently of hormones, when BP or BV is high kidneys filtrate out extra fluid, and it is excreted in the urine , lowering BP and BV

indirect renal mechanism

renin- angiotensin mechanism

renin-angiotensin mechanism

when arterial BP declines , the kidneys release the enzymatic hormone renin into blood, renin triggers a series of reactions that produce angiotensin II


a hormone that enhances renal absorption of sodium, as sodium moves into the blood stream water flows, as a result blood volume is conserved

vital signs

body temp., pulse, blood pressure, respiratory rate


the alternating expansion and recoil of arteries during each cardiac cycle- a pressure wave is created, the radial pulse is located on the wrist

pressure points

arterial pulse, they are compressed to stop blood flow into distal tissues during hemorrhage

auscultatory method

systemic blood pressure is measured indirectly in the brachial artery of the arm, a blood pressure cuff or sphygmomanometer is wrapped around arm, just superior to the elbow and inflated until cuff pressure exceeds systolic pressure- blood flow stops, cuff pressure is gradually reduced and brachial artery is heard


low blood pressure-systolic is below 100 mmHg, generally a good thing

orthostatic hypotension

temporary low blood pressure and dizziness, prone in elderly when they rise suddenly from a reclined position

chronic hypotension

due to poor nutrition, inadequate levels of blood protein, hints to Addisons disease, hypothyroidism or severe tissue wasting- acute hypotension hints to poor circulation


high blood pressure that may be transient or persistant- transient- elevations in systolic pressure occur as normal adaptations during fever, physical exertion, or emotional upset -persistent- (in obese people) hormones released from adipocytes both increase sympathetic tone and interfere with ability of endothelium to induce vasodialation

chronic hypertension

a common dangerous disease, 30% of people greater than 50 have it, strains the heart and damages arteries, organs do not get proper nourishment- sustained arteriole pressure 140/90 or greater

primary or essential hypertension

when no underlying cause is identified, about 90% or hypertensive people- hereditary predispositions, an environmental factors such as: diet, diabetes mellitus, obesity, age, stress, smoking

secondary hypertension

10% of cases, due to identifiable disorders such as obstruction of the renal arteries, kidney disease and endocrine disorders

tissue perfusion

blood flow through body tissues involves: delivery of O2 and nutrients to, and removal of waste from tissues (gas exchange in lungs, absorption or nutrients in digestive tract, urine formation in the kidneys


the automatic adjustment of blood flow to each tissue on proportion to the tissues requirements at any instant





nitric oxide (NO)

is a powerful vasodialator which acts via cyclic GMP second messenger system


potent vasocontrictors that belong to a family of peptides

myogenic responses

an increase in smooth muscle tone that compensates for changes on systemic pressure

reactive hyperemia

dramatic increased blood flow into a tissue that occurs after the blood supply to the area has been temporarily blocked


when a number of blood cells in a particular region increase and existing muscles enlarge

active or exercise hyperemia

when muscles become active, blood flow increases (hyperemia) in direct proportion to their greater metabolic activity


produced by a perspiration enzyme, it stimulates the vessels endothelial cells to release the potent vasodialator NO


blood flow through capillary networks- reflects the on/off opening and closing of precapillary sphincters in response to local autoregulatory controls


movement along a concentrated gradient, from and area of higher concentration to lower

hydrostatic pressure

is the force exerted by a fluid pressing against a wall- in capillaries it is the capillary blood pressure

capillary hydrostatic pressure (HPc)

tends to force fluids through the capillary walls (process called filtration) leaving behind cells and most proteins- BP drops as blood flows through the capillaries (from 35mm Hg-17mm Hg)

interstitial fluid hydrostatic pressure (HPif)

opposes hydrostatic (HPc)- acting outside of the capillaries and pushing in fluid, however there is usually very little fluid int he interstitial space because lymphatic vessels constantly withdraw it- HPif has an assumed value of 0

colloid osmotic pressure

the force opposing hydrostatic pressure, is created by the presence of fluid of large non-diffusable molecules such as proteins that are unable to cross the wall, these molecules draw water towards themselves (osmosis)

capillary colloid osmotic pressure (OPc)

pressure created by blood proteins (mostly albumin), also called oncotic pressure, about 26 mmHg, interstitial fluid contains few proteins- colliod osmotic pressure (OPif) about .1-5 mm Hg

net filtration pressure (NFP)

considers all forces acting at capillary bed, fluids will leave capillary bed if net HP is greater than net OP and fluids will enter capillary if net OP exceeds net HP

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