Blood Vessels

Blood Vessels• hollow structures for

carrying blood• form closed system

beginning & ending at heart

• Arteries• Arterioles• Venules• Veins• Capillaries

Vessel Structure • 3 layers-tunicas• surround central

space or lumen• Tunica intima• Tunica media• Tunica adentitia

Tunica Intima or Tunica Interna• innermost layer

– in contact with blood

– consists of layer of simple squamous cells-endothelium

• fit closely together forming slick surface

• minimizes friction as blood moves through lumen

Tunica Media• middle layer• usually thickest• consists of smooth muscle,

collagen and in some cases elastic tissue

• strengthens vessels to prevent rupture

• provides vasomotion or changes in diameter of blood vessel

• impulses cause muscles to contract vasoconstriction– reduction in size of lumen

• impulses inhibitedmuscle fibers relaxdiameter increases-vasodilation

Tunica Adventitia or Tunica Externa• outermost layer• composed of

loose connective tissue

• responsible for attaching vessels to surrounding tissues

Arteries• carry blood away

from heart

• progressive diminution in diameter as recede from heart

• branch, diverge, & fork

Arteries• resistance vessels

• relatively thick muscular walls containing elastic & contractile fibers

• change diameters by expanding (elasticity) as pressure increases & by constricting under sympathetic nervous control (contractility)

• vasoconstriction & vasodilation affect: afterload, peripheral blood pressure & capillary blood flow

Types of Arteries• Elastic

– conducting arteries• largest• transport large amounts of blood away from

heart• elastin in all layers • withstand large pressure fluctuations• Muscular

– distributing arteries• deliver blood to organs & skeletal muscles• named arteries• thickest media• active in vasoconstriction• Resistance arteries

– arterioles• poorly defined external tunicas• diameters change in response to local

conditions, sympathetic innervations & hormonal stimulation

• more pressure is needed to push blood through constricted vessels

• force opposing blood flow is called resistance• major site of resistance to blood flow.

Capillaries • do work of cardiovascular system• walls permit exchange between blood interstitial

tissues• smallest blood vessels• consist of endothelium & basement membrane• several types• Continuous• Fenestrated• Sinusoids

Continuous Capillaries• complete endothelium

lining forming a continuous tube

• cells joined by tight junctions

• in all tissues except epithelia & cartilage

• permit diffusion of water, small solutes and lipid-soluble materials into surrounding interstitial fluid

Fenestrated Capillaries• have oval pores-

fenestrations• allow for rapid transport

of molecules through capillary wall

• found in organs that engage in rapid absorption or filtration

• kidneys, exocrine glands & choroid plexus of brain

Sinusoids• endothelial cells are

separated by wide gaps with no basal lamina

• proteins & blood cells can pass through

• found only in certain organs such as liver, bone marrow, & spleen

Capillaries• connect

arteries to veins

• do work of cardiovascular system

• found in beds

Capillary Beds• blood flow through beds-

micro circulation• one arterioledozens of

capillariesvenules• arterioles linked to capillaries

via metarterioles• surrounded by band of

smooth muscle-precapillary sphincter

• contraction-narrows diameter of capillary entrance reducing blood flow

• relaxation increases blood flow

Sphincter Open• capillaries

exchange materials with tissues

Sphincter Closed• blood bypasses

capillaries

• flows through thoroughfare channel to venule

Mechanisms of Movement

• Diffusion

• Bulk Flow

–Filtration

–Reabsorption

• Transcytosis

Diffusion• most important• net movement of ions &

molecules from areas of higher to areas of lower concentration

• difference between concentrations is- concentration gradient

• most rapid diffusion occurs where

• distances are small• concentration gradients

are large• molecules are small

Bulk Flow• Filtration & Reabsorption

• across capillary walls

• between blood & interstitial tissues

• due to hydrostatic & osmotic pressures

Bulk Flow• Filtration• direction of flow is out

of the capillary into the interstitial fluid

• at arterial end• Reabsorption• direction of flow is out

of interstitial fluid into the capillary

• at venous end

Capillary Exchange Pressures• two main factors promote filtration• blood hydrostatic pressure (BHP) & interstitial fluid osmotic pressure

(IFOP)• primary pressure promoting reabsorption-blood colloid osmotic pressure

(BCOP) • in vessels hydrostatic pressure is due to pressure that water in blood exerts

against vessels walls (BHP)• BHP is about 35mm of Hg at arterial end of capillary & 16mm Hg at venous end• BHP pushes fluid out of capillary into interstitial fluid.• Interstitial fluid hydrostatic pressure (IFHP) pushes fluid from interstitial

spaces back into capillaries- value is close to zero• Blood colloid osmotic pressure (BCOP) is determined by proteins present in

plasma• BCOP pulls fluid from interstitial spaces into capillaries-averages 26mm Hg• Opposing BCOP interstitial fluid osmotic pressure (IFOP)• IFOP pressure pulls fluid out of capillaries into interstitial fluids- value i0.1 –

5mm Hg• Net Filtration Pressure = (NFP) = (BHP + IFOP) – (BCOP + IFHP)• NFP is equal to pressures promoting pressure minus pressures promoting

reabsorption

Net Filtration Pressure• Net Parterial end = (35 mm Hg + 1)

- (26mm Hg + 1) = 10mm Hg• value tells there is a net

outward pressure• fluid moves out of capillaries

into interstitial fluid-net filtration

• Net P venous end = (16mm Hg )+ 1) + (26 mm Hg + 1) = -9 mm Hg

• value tells net absorption is taking place

• there is a net inward pressure forcing fluid into capillaries from interstitial fluid

Veins• carry blood back to

heart

• join, merge, & converge

• diameters small in venules

• become progressively larger as approach heart

Veins• thin walls• can distend• capable of holding a great

deal of blood• capacitance vessels- blood

reservoirs which can be drawn upon in time of need

• many especially in arms & legs, have flaps or valves

• composed of 2 leaflets • close if blood begins to

back up in veins

Distribution of Blood• blood volume

unevenly distributed

• heart, arteries & capillaries account for 30-35% total volume

• venous system contains 64% total volume

Cardiovascular Physiology• blood must circulate• heart provides force &blood vessels are conduits• blood flow is the volume of blood that flows

through a tissue in a given time (ml/min)• total blood flow is CO

– volume of blood that circulates through systemic or pulmonary circuits each minute

• CO = SV X HR• how CO becomes distributed into circulatory

routes depends on two more factors• pressure differences & resistance to flow

Pressure Differences & Resistance• pressure gradient• difference in pressure from one end of vessel to

another-P• largest-from base of aorta to proximal ends of

peripheral capillaries• greater P more blood flow• resistance• any force opposing movement• due mainly to friction of blood with blood vessel

walls • greater resistanceslower blood flow

Blood Pressure• produced by contraction of

ventricles• determined by: CO,

resistance & blood volume• measured by using a

sphygmomanometer & brachial artery

• highest in arteries during systole

• lowest in arteries during diastole– expressed as mmHg

Blood Pressure• systemic arterial pressure

ranges from 100mm Hg-aorta to 35mm Hg-capillaries

• venous end of capillaries, pressure 16 mmHg

• pressure continues to drop as blood enters systemic veins

• reaches zero mm Hg as blood flows into right ventricle

MAP• Mean arterial pressure• value for arterial pressure representing it driving

process• average blood pressure in the arteries• MAP = diastolic pressure + 1/3(systolic pressure

– diastolic pressure)• If normal: MAP = 80mm Hg + 1/3(120-80mm Hg)

= 93mm Hg• CO = MAP/R or MAP = CO X R• shows that if CO rises due to HR or SV then so

does MAP since CO = SV x HR

Blood Pressure & Blood Volume• blood pressure also

depends on total volume of blood in cardiovascular system

• anything that increases blood volume will increase blood pressure

• kidney helps with long term regulation of blood pressure

Peripheral ResistanceSystemic Vascular Resistance

• resistance of entire arterial system• F = P/R• Flow = change in pressure divided by resistance• equation shows blood flow is directly proportional to

pressure gradient & inversely proportional to resistance• higher PRlower rate of blood flow• pressure gradient must be greater than total peripheral

resistance for blood to flow• vascular resistance is the opposition to blood flow due to

friction between blood vessel walls & blood• depends on• vessel lumen• blood viscosity• total vessel length

Blood Viscositythickness of bloodgreater viscositymore friction

greater resistanceanemia & polycythemia will change

hematocrit changes viscosity changes resistance

under normal conditions negligible

Vessel Length• longer vesselsgreater resistance

• length increases friction

• two vessels-equal diameters

• if one is twice length of other-longer vessel has twice resistance of shorter vessel

• factor is usually constant

Vessel Diameter• most important factor

contributing to resistance• significant effects• smaller vesselsgreater friction• more fluid in contact with vessel

wallmore frictionmore resistance

• friction in larger vessels is low because blood comes into contact with vessel wall less oftenless less friction less resistance

Vessel Diameter• two vessels-equal

lengths• One- twice diameter of

other• using formula: R- 1/r4

• vessel with twice diameter of other has 1/16 as much resistance as smaller diameter vessel

• or-smaller vessel has 16X as much resistance

Blood Velocity• depends on flow rate & cross sectional area• flow rate• volume of blood passing one point in

system/unit time• given as Liters/minute or ml/min• Velocity• distance fixed volume of blood travels in

given time period• measured in cm/sec• inversely related to cross sectional area• slowest where total cross sectional area is

greatest• fastest where cross sectional area is leas• cross sectional area of aorta-3 – 5cm2 • average velocity - 40cm/sec• Capillaries-total cross sectional area-4500 –

6000cm2 - velocity is less than 0.1 cm/sec• slows down for capillary exchange

Control of Blood Pressure & Flow

• Nervous System

• Hormones

• Autoregulation

Neural Mechanisms• CV center regulates HR & SV

• controls blood vessels via ANS• exert sympathetic &

parasympathetic control over blood vessels throughout body

• Sympathetic input reaches heart via cardiac accelerator nerves

• increase in sympathetic stimulation increases HR & contractility.

• decrease in sympathetic stimulation decreases HR & contractility

• Parasympathetic stimulation is conveyed along vagus nerve

• results in decreased HR

Nervous System Control• cardiovascular center integrates

nerve impulses from cerebral cortex, limbic system & hypothalamus

• Proprioceptors– monitor joint movements

• Barorecepetors– monitor pressure changes

• Chemoreceptors– monitor chemical changes in

blood• regulates via negative feedback

loops & 2 major reflexes• Baroreflexes• Chemoreflexes

Baroreflex• autonomic negative feedback response to

change in blood pressure• detected by baroreceptors• located in carotid arteries & aorta• monitor stretch in walls due to pressure of

blood flowing to brain• two types-carotid sinus reflex & aortic reflex• carotid reflex• regulates bp in brain• aortic reflex regulates systemic bp• blood pressure rises walls stretch

increases rate of baroreceptors signals over glossopharygeal nervesinhibits sympathetic neurons & increases parasympathetic firing reduces HR & force of contraction which decreases CO

• blood pressure drops back to normal• slows rate sympathetic stimulation is sent to

vasomotor nerves that cause vasoconstriction results in vasodilation

• SVR, lowers CO & lowers blood pressure

Baroreflex• Baroreceptors in walls of ascending aorta

&aortic arch begin aortic reflex

• Once stimulated send impulses over vagus nerve to CV center.

• blood pressure decreases baroreceptors stretch lesssend impulses at slower rate to CV center decreases parasympathetic stimulation & increases sympathetic stimulation via cardiac accelerator nerves

• Sympathetic nervous stimulation increases secretion of epinephrine & norepinephrine from adrenal medulla

• causes the heart to beat faster & more forcefully increases SVR, CO & blood pressure

Baroreflexes• important in short term

regulation of blood pressure• keep BP stable when

moving from reclining to standing position

• quickly adapt to prolonged or chronic episodes of high or low blood pressure

• kidneys come in to restore & maintain BP by regulating blood volume

• major determinant of CO through influences on venous pressure, venous return, EDV & SV

Chemoreflex• autonomic response to changes in

blood chemistry– especially pH, O2 & CO2

• Chemoreceptors-aotic & carotid bodies

• negative feedback• abnormal conditions cardiovascular

centers noticerespond in ways to counteract abnormal condition homeostasis restored

• low O2 (hypoxia), high CO2 (hypercapnia) & low pH (acidosis) stimulate chemoreceptors CV centerwidespread vasoconstriction increases BP

• also sends impulses to the respiratory center

• primary response is to adjust respiration

Hormones & Blood Pressure• hormones help regulate blood pressure

& blood flow by altering CO, changing SVR or adjusting total blood volume

• Renin-angiotensin aldosterone system (RAA)

• Epinephrine & Norepinephrine

• ADH

• ANP-atrial natriuretic peptide

Renin-Angiotensin Aldosterone system (RAA)

• blood volume falls or blood flow to kidneys decreasesjuxtaglomerular cells in kidney secrete renin

• Renin & ACE (angiotensin-converting enzyme) make angiotensin II

• affects blood pressure in two ways• Angiotensin II is a

vasoconstrictor• increases BP by increasing

systemic vascular resistance• stimulates secretion of

aldosterone form adrenal cortex• causes increase reabsorption of

sodium & water by kidneys • increases blood volume

increases blood pressure

Epinephrine & Norepinephrine• stresshypothalamusfight-or-

flight responseadrenal medulla norepinephrine & epinephrine

• cause vasoconstriction of arterioles in skin veins & in abdominal organ veins

• increase CO by increasing HR & force of heart contraction via generalized vasoconstriction

• increases blood pressure

• except skeletal & cardiac muscle where they produce vasodilation

• enhances blood flow to heart & skeletal muscle

ADH• from posterior pituitary• released due to lowered

blood volume• increase in osmotic

concentration of plasma• immediate effectperipheral

vasoconstrictionincreases BP

• causes kidneys to conserve water increases blood volumeincreases BP

ANP • made by right atrium in

response to excessive stretchingdecreases BP

• increases Na excretion at kidneyspromoting water loss

• generalized vasodilation effect– helps to lower blood

pressure

Autoregulation-Myogenic Regulation

• ability of tissues to regulate their own blood supply• local factors changechanges pattern of blood flow in capillary beds• changes diameter of precapillary sphincters that feed capillaries• changing diameter varies resistance• Vasodilators:

– NO-nitric oxide– increased CO2

– decreased O2

– lactic acid– increased K– increased H– increased histamine– high temperatures

• relax smooth muscle cells of precapillary sphincters• Vasoconstrictors:

– vasopressin– norepinephrine– angiotensin II– serotonin

• contract smooth muscle cells in sphincters