ch 21

blood vessels

largest blood vessels

pulmonary trunk (right ventricle → lungs) and aorta (out of left ventricle)

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characterized by high blood pressure

arteries

carry blood away from heart

arterioles

smallest branches of arteries that lead to capillary beds

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resistance vessels with no tunica externa and thin/incomplete tunica media

capillaries

smallest blood vessels with thin walls (for easy diffusion), networks permeate all active tissues

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location of exchange between blood and interstitial fluid (slow rate of exchange)

venules

smallest branches of veins that collect blood from capillaries

medium sized veins

very few smooth muscle cells, longitudinal bundles of elastic fibers

large veins

have all 3 tunica layers, thick tunica externa and thin tunica media

venous valves

folds of tunica intima that prevent blood from flowing backward; compression pushes blood toward the heart

varicose veins

may occur when walls of veins near the valves weaken

veins

return blood to heart

larger diameters, thinner walls, lower blood pressure

vessel wall layers

1. tunica intima
2. tunica media
3. tunica external

tunica intima

innermost layer and includes endothelial lining + internal elastic membrane (in arteries)

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contains elastic fibers that are particularly important in larger vessels (needs to be able to stretch and return back to size) during systole contraction to handle shock wave

tunica media

middle layer of vessel, contains smooth vessel

tunica externa

outermost layer of vessel, anchors vessel to adjacent tissues

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contains collagen fibers, elastic fibers and smooth muscle cells (in veins)

typical artery

usually round, with relatively thick wall

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rippled endothelium due to vessel constriction w/ internal elastic membrane

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thick tunica media dominated by smooth muscle cells and elastic fibers w/ external elastic membrane

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collagen and elastic fibers in tunica externa

typical vein

usually flattened or collapsed, with relatively thin wall

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smooth endothelium w/o internal elastic membrane (not dealing with huge forces)

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thin tunica media, dominated by smooth muscle cells and collagen fibers w/o external elastic membrane

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collagen, elastic fibers, smooth muscle cells in tunica externa

arteries vs. veins

arteries have thicker wall ands higher blood pressure

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constricted artery has a small, round lumen + endothelium is folded; more elastic

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veins have large, irregular lumen; have valves

vasoconstriction

contraction of arterial smooth muscle

vasodilation

relaxation of arterial smooth muscle, enlarges the lumen

artery contractility

elasticity allows arteries to absorb pressure waves that come with each heartbeat

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change in diameter controlled by sympathetic division of ANS

after load

the pressure that must be exceeded before ejection of blood from the ventricles can occur

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greater if aorta is constricted, also applies to arteries and arterioles

vasoconstriction/vasodilation

affects

1. afterload on heart
2. peripheral blood pressure
3. capillary blood flow

elastic arteries

conducting arteries

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large vessels (pulmonary trunk, aorta) with tunica medias that are predominantly elastic fibers

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elasticity evens out pulse force

muscular arteries

(distribution arteries), most arteries are medium size arteries

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tunica media has predominantly smooth muscle cells (deals less with pressure waves and more with vasoconstriction and dilation)

aneurysm

a bulge in an arterial wall, caused by a weak spot in elastic fibers

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pressure may rupture vessel

circle of Willis

a crown of arteries in the brain

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very common spot for an aneurysm

capillary structure

endothelial tube, inside thin basement membrane

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no tunica media or tunica externa

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diameter is similar to that of a red blood cell

capillary beds

connect one arteriole and one venule

precapillary sphincter

guards entrance to each capillary

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opens and closes, causing capillary blood to flow in pulses (made of muscle tissue)

collaterals

multiple arteries that contribute to one capillary bed, allow circulation if one artery is blocked

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aka redundant pathways to make sure tissue will get blood

arterial anastomosis

fusion of two collateral arteries

arteriovenous anastomosis

direct connection between arterioles and venules

bypass the capillary bed

blood vessel capacitance

ability to stretch

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veins are capacitance vessels → stretch more than arteries; act as blood reservoirs

venoconstriction

occurs in response to blood loss, increasing amount of blood in arterial system and capillaries

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to maintain blood pressure

blood pressure

arterial pressure (mm Hg)

capillary hydrostatic pressure

pressure within the capillary beds

venous pressure

pressure in the venous system

circulatory pressure

must overcome total peripheral resistance

total peripheral resistance

resistance of entire cardiovascular system

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affected by vascular resistance, blood viscosity, turbulence

vascular resistance

due to friction between blood and vessel walls

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depends on vessel diameter, which varies by vasodilation and vasoconstriction

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R increases as vessel diameter decreases

blood viscosity

resistance caused by thickness of liquid

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whole blood viscosity is about 4 times that of water

turbulence

swirling action that disturbs smooth flow of liquid, occurs in heart chambers and great vessels

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atherosclerotic plaques cause abnormal turbulence

lumen

inside of cavity

lumen vs. area

cross sectional area of vessel lumens are inversely related to vessel diameter

systemic blood pressure

pressure from aorta to capillaries

systolic pressure

peak arterial pressure during ventricular systole (contraction phase)

diastolic pressure

minimum arterial pressure at end of ventricular diastole

pulse pressure

difference between systolic and diastolic pressure

mean arterial pressure

diastolic pressure + 1/3 pulse pressure

normal blood pressure

120/80

hypertension

abnormally high blood pressure, greater than 140/90

cause for concern if symptoms accompany high BP

hypertension effects

heart muscles get larger (fighting against greater resistance) → left ventricular hypertrophy

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wear and tear on blood vessels: endothelium damage → deposition of plaques and lipids; lesions of atherosclerotic plaques

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end-organ damage

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heart attack, stroke (ischemic, hemorrhagic)

hypotension

abnormally low blood pressure

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not getting oxygen/nutrients in any tissues → dangerous for brain

elastic rebound

arterial walls stretch during systole and rebound during diastole

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keep blood moving during diastole

return of blood to heart

assisted by skeletal muscular compression of veins and respiratory pump

respiratory pump

thoracic cavity expands during inhalation, decreasing venous pressure in the chest

capillary exchange

vital to homeostasis; materials move across capillary walls by diffusion, filtration, reabsorption

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filter more than they reabsorb → excess fluid enters lymphatic vessels

filtration and reabsorption

1. ensures that plasma and interstitial fluid are in constant communication and mutual exchange
2. accelerates distribution of nutrients, hormones, dissolved gases throughout tissues
3. assists in transport of insoluble lipids and tissue proteins that cannot cross capillary walls
4. carries bacterial toxins and other chemical stimuli to lymphatic tissue and organs (becomes diluted)

hydrostatic pressure

physical force pushing a fluid out (used to negate osmosis)

tissue perfusion

blood flow through the tissues; carries O2 and nutrients to tissues and organs, carries CO2 and wastes away

vasomotion/autoregulation

contraction and relaxation cycle of precapillary sphincters

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causes blood flow in capillary beds to constantly change routes (depends on time of day, position, etc.)

autoregulation

affects precapillary sphincters (depends on what chemicals are nearby)

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causes immediate, localized homeostatic adjustments

neural mechanisms

respond quickly to changes at specific sites

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from brain (cardiovascular center of medulla oblongata) → increases or decreases cardiac output (stroke volume and heart rate)

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mostly detects CO2

endocrine mechanisms

direct long-term changes

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output of epinephrine, norepinephrine, cortisol, thyroid hormone

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E and NE from adrenal medullae stimulate cardiac output and peripheral vasoconstriction

vasodilators

factors that promote dilatation of precapillary sphincters, increasing blood flow

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local ones include low O2 or high CO2 levels, nitric oxide

vasomotor center

control of vasoconstriction → controlled by adrenergic nerves (NE) and stimulates contraction in arterioles walls

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control of vasodilation → controlled by cholinergic nerves (NO) and relaxes smooth muscle

vasomotor tone

produced by constant action of sympathetic vasoconstrictor nerves

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level of tension from both types of nerves on the blood vessels

reflex control of cardiovascular function

baroreceptors (respond to changes in blood pressure) and chemoreceptors (respond to changes in chemical composition: pH and dissolved gases)

when blood pressure rises

CV centers decrease cardiac output (cardioinhibitory center stimulated, cardioacceleratory center inhibited) and cause peripheral vasodilation (vasomotor center inhibited)

when blood pressure falls

CV centers increase cardiac output (cardioacceleratory center stimulated, cardioinhibitory center inhibited) and cause peripheral vasoconstriction (vasomotor center stimulated)

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increased contractility = increased stroke volume + HR = increased cardiac output

atrial baroreceptors

monitor blood pressure at the end of the systemic circuit

chemoreceptor reflexes

peripheral chemoreceptors in carotid bodies and aortic bodies monitor blood

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respond to changes in pH (measure of hydrogen, too high → increase HR), O2 and CO2

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coordinate cardiovascular and respiratory activites

antidiuretic hormone

secreted by posterior lobe of pituitary and elevates blood pressure (over a long period of time)

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reduces water loss at kidneys (keeping water inside body = more fluid going through vessels)

angiotensin II

released in response to a decrease in renal blood pressure

heavy exercise

activates sympathetic nervous system and cardiac output increases to max (4x resting level)

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restricts blood flow to nonessential organs and redirects blood to skeletal muscles, lungs and heart

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blood supply to brain is unaffected

hemorrhaging

entire cardiovascular system adjusts to maintain blood pressure and restore blood volume

carotid and aortic reflexes

(short term elevation of blood pressure)

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increase cardiac output (increasing HR) and cause peripheral vasoconstriction

sympathetic nervous system

(short term elevation of blood pressure)

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further constricts arterioles and venoconstriction improves venous return

hormonal effects

(short term elevation of blood pressure)

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increase cardiac output and increase peripheral vasoconstriction