4 Circulatory System

8/14/2019 4 Circulatory System

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Right side pumps blood through

the lungs, maintaining thepulmonary circulation.

Both sides of the heart have an

atrium that collects blood & aventricle that ejects it.

The coordinated action of all 4chambers enables the heart to

maintain these 2 circulationsconcurrently.


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Blood ejected by the heartunder substantialhydrostatic pressure is distributed by abranch system of thick-walled arteries, the

finest branches of which (arterioles) have aproportionately narrow lumen.

Mainly because blood acts as a viscous fluid,the relatively narrow lumen of arteriolesreduces the hydrostatic pressure of blood

entering open capillaries.

The thin fragile walls of capillaries facilitateexchange of O2, nutrients, waste products, &

other substances between blood & tissue

fluid.


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Capillaries also produce & can resorb tissuefluid, & they facilitate O2 uptake & CO2

clearance from the lungs, nutrient absorption

from the intestine, & toxic waste excretionfrom the kidneys.

Blood flow through capillaries is regulated byarterioles & also intermediate vessels called

metarterioles, which open into thin-walledthoroughfare channels passes into thin-walled venules, which resemble widecapillaries.


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Venules lead into small veins, which open into

larger veins & so on.

The largest veins return deoxygenated bloodto the heart.

The arteries of the systemic circulationdistribute oxygenated blood, whereas thepulmonary artery takes deoxygenated bloodto the lungs where it becomes reoxygented.


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Blood Vessel & Heart Walls: General Organization

The walls of the blood vessels, heart andlymphatics are conventionally regarded as

being constructed of 3 coats or tunicae, asfollows:

1) Tunica Intima the innermost coat.

- in blood vessels, it consists of:

a) a lining membrane (endothelium),b) an underlying basement membrane,c) a variable amount of subendothelial CT, &d) an internal elastic lamina (layer of elastin)

- not present in some small vessels


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- tunica intima of the heart is its lining, theendocardium, w/c is made up primarily ofendothelium & CT of various kinds.

2) Tunica media the middle coat.- generally includes 2 components arrangedin concentric layers:

a) muscle fibers w/c are smooth muscle

fibers in most blood vessels but cardiacmuscle fibers in the heart & certain ofits large associated vessels, and

b) elastin.


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Some small vessels, however, lack musclefibers & elastic laminae in their media.

In the heart, coat is highly developed as the myocardium (the substantial muscular layer

of the heart wall.

3) Tunica adventitia outermost coat.- predominantly loose CT, together w/ somesmooth muscle in certain large vessels.

- supplied by tiny blood vessels of its own(vasa vasorum L, vessels of the vessels),w/c in some cases supply the outer part ofthe media also.


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Myocardium

muscular walls of the heart, representing thetunica media, are primarily constructed ofcardiac muscle fibers.

the endomysial loose CT that lies between themuscle fibers supplies them w/ abundantcapillaries and also w/ lymphatics.


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Endocardium

lines the heart chambers and covers heartvalves.

Endothelium, with its underlying basementmembrane & adjacent loose CT, lies at the

luminal surface of the endocardium.

Deep to the loose CT lies a layer of denseordinary CT, & beneath this some fat cellsare generally present.


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Epicardium

made up of:

1) a layer of fibroelastic CT, w/ blood vessels,lymphatics, nerve fibers, and a certainamount of fat tissue, that merges w/ theendomysium of the myocardium, and

2) a superficial mesothelial membraneconsisting of squamous epithelial cells.


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Impulse-Conducting System of the Heart

- Plays a key role in coordinating cardiaccontraction.

- It is a conduction pathway that consistsentirely of cardiac muscle cells, somespecialized for initiating the impulses forcontraction & others adapted for conductingthese impulses rapidly throughout the heart.


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Sinoatrial Node

- Is a small mass of specialized cardiac musclefibers & associated fibroelastic CT.

- It lies in the wall of the superior vena cava,close to the site where this borders on the

right atrium.

- Each wave of excitation eliciting a heartbeatoriginates as a spontaneous depolarizationof cardiac muscle pacemaker cells in the

SA node.


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- Frequency of SA nodal depolarization dependson w/c of the autonomic divisions is supplyingefferent impulses.

- Each wave of depolarization that originates inthe SA node is conducted by way of gap

junctions along internodal atrial pathways &spreads through ordinary atrial muscle fibers,triggering their contraction.

- Muscle fibers of the SA node are somewhatnarrower than ordinary atrial fibers, & sincetheir role is to initiate & conduct impulses, notto undergo strong contractions, they contain

fewer myofibrils.


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Atrioventricular Node & AV Bundle

- Lies in the interatrial septum, near theopening of the coronary sinus.

- When the wave of depolarization reachesthis node from the internodal pathways, itis momentarily delayed by the nodal cells &then conducted to the ventricles.

- From the AV node, the wave ofdepolarization is conducted along a bundleof specialized conducting cardiac musclefibers known as the AV bundle or bundle ofHis.


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his part of the conduction path extends

nteriorly from the AV node, penetrates theibrous partition lying between atrial &entricular muscle, & enters the intervetriculeptum, where it divides into left & rightranches.

oughly halfway down the septum, its two

ranches become bundles of typicalPurkinjeibers.


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Purkinje Fibers

- of the heart lie in the deepest layers of theendocardium.

- specialized conducting-fibers.

- Contain fewer myofibrils, & these have a

peripheral arrangement around the fiberscentral sarcoplasmic core.

- This wide central core contains abundantglycogen, giving the fiber a pale, empty look

except for central nuclei.


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Cardiac Valves

- Each ventricle has inlet & outlet valves,both of the flap (leaflet) type.

- The heart valve leaflet is an intimal sheetw/ a core of irregular dense ordinary CT

that is mostly avascular.

- Elastic fibers are distributedasymmetrically in the leaflet, andendothelium covers its entire surface.


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Large & medium-sized blood vessels presentin sections are generally arteries or veins.

These vessels are usually distinguishable byobserving:

1) wall thickness and2) size & general shape of the lumen.

Arterial lumen looks more or less round bec.there is only a minor tendency for such thick-walled vessels to collapse at death.


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Veins have relatively thin walls because theyhave only to accommodate blood undervenous blood pressure.

the relatively wide venous lumen tends toflatten at death, when veins collapse becausethe venous blood pressure drops.

the three coats of the blood vessel wall arenot always discernible, but they are clearlyrecognizable in muscular arteries.


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Major function:distribution of blood from theheart to the capillary beds ofthe body.

3 major categories of vessels in the arterial system:

a) Muscular or medium-sized arteries

- are branches of the elastic arteries and

constitute the great majority of thearteries in the body.


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outer border of the media is indicated by theexternal elastic lamina, w/c is a somewhatless conspicuous fenestrated layer of elastin.

thickness of the adventitia varies, but in manycases it is comparable to the thickness of themedia.

adventitia consists chiefly of elastic fibers,together w/ some collagen.

- it is provided w/ vasa vasorum thatcommonly extend into the periphery of themedia & supply this as well. Lymphatics arepresent.


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Stretch is maximal in the aorta & pulmonaryartery (elastic arteries), i.e., both of the greatarteries conveying blood from the heart.

This is because their walls consist primarily of

elastin.

blood flow is regulated by luminal diameter ofthese arteries. (hormones & other factors)

b) Elastic Arteries include the aorta & itsmajor branches.


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Main fxn: to transport the blood from theheart & move it along the vascular path.

As a result of the increased amount of

smooth muscles in their walls, thesearteries control the blood flow & bloodpressure through vasoconstriction orvasodilation of their lumina.


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intima = substantially thicker than that seenin muscular arteries.

- of the aorta accounts for about one quarter

of the thickness of the whole wall.

- in H&E-stained sections, it appears palerthan the media.

- many fenestrated elastic laminae & elastic

fibers are present in the so-called sub-endothelial layer between the endothelium& the internal elastic lamina.

- intimal cells are mostly smooth muscle cellsthat produced the interstitial matrix of thevessel, but some fibroblasts also arepresent.


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internal elastic lamina that delineates theperiphery of the intima is less conspicuousthan in muscular arteries.

Thick media constitutes the major part ofthe aortic wall.

- consists primarily of a great manyfenestrated elastic laminae that becomeincreasingly numerous w/ age.

outer half to two thirds of the media issupplied by vasa vasorum, the inner third orso of the media, & all of the intima, dependon diffusion of O2 & nutrients from the lumen.


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thin adventitia consists largelyof elastic & collagen fibers.

- provided with lymphaticcapillaries as well as vasa

vasorum.


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c. Arterioles - the smallest arterial vessels

- their walls consist of 1 5 layers ofsmooth muscle

- by constricting or dilating their lumen,they regulatethe flow of blood intothe capillary beds.

- the terminal arterioles give rise to

capillaries.


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In distinguishing these vessels from smallmuscular arteries, it is helpful to rememberthat the wall of an arteriole is thick relative

to the diameter of the lumen & appearsalmost as wide as the lumen.

intima = consists of the endothelium & itsbasement membrane w/ an apposedinternal elastic lamina.

media = represented by only 1 or 2 layers ofsmooth muscle cells (the easiest wayto recognize an arteriole!), togetherw/ an inconspicuous external elasticlamina.


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adventitia = contains a few elastic & collagenfibers.

internal & external elastic laminae are

lacking in the smallest arterioles supplyingblood to capillaries.


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Smallest blood vessels in the body.

Average diameter of their lumina isabout 8 um, which almost equals thesize of the RBCs.


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3 types of capillaries:

1. Continuous capillaries - the most common

type & are found in the connective tissue,all muscle tissue, the CNS, & other organs.

- Are continuous in the sense that thecytoplasm of their endothelial cellsconstitutes an uninterrupted(i.e., unfenestrated) lining.

- This is the common type of capillary thatcharacterizes most body tissues.


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2. Fenestrated capillaries - allows a more rapidexchange of molecular substances between

blood & tissues.

- Are more permeable because the endothelialcell cytoplasm is perforated by circularfenestrations.

- primarily found in the endocrine organs, thesmall intestine, & the glomerulus of thekidneys.


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3. Sinusoids - found in the liver, spleen, &

bone marrow

- Are thin-walled, capillary-like venousblood vessels that have an unusuallywide lumen & an associated population

of macrophages.


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Are classified as small, medium & large.

Are generally more numerous & havethinner walls & larger diameters thanthe arteries.

Blood pressure is lower than in thearteries

= as a result, blood flow in the veins ispassive


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wide lumen that generally appearsflattened in sections.

internal & external elastic laminae arepoorly represented here & hence

inconspicuous in veins.

W ll t t f i i i th ff t


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Wall structures of veins varies in the ff aspects:

Small & medium-sized veins

thin intima consists of an endothelium, itsthin basement membrane, & only a trace ofunderlying CT w/ a very meager internalelastic lamina.

media = much thinner than that of thearteries

in most veins, the media consists of a fewcircular layers of smooth muscleinterspersed w/ collagen & elastic fibers.


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However, cerebral, dural, & certain otherveins lack smooth muscle cells in theirmedia.

In contrast, certain veins of the limbs,notably the superficial veins of the legs, havea thick muscular media that resist the

distension resulting from venous returnagainst gravity.

the thickest coat of veins is their adventitia,made up of wide collagen fibers & elasticfibers, fibroblasts, & smooth muscle cells.


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Large Veins

intima= is similar to that of other veins, w/

an incomplete internal elastic lamina

media = contains a few circular layers ofsmooth muscle but in general ispoorly developed.

adventitia = the thickest coat; containslongitudinal bundles of collagen fibersalong with some elastic fibers, smoothmuscle cells, & fibroblasts. (well-developed of the inferior vena cava)


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Walls of large vessels are too thick to receivenourishment by direct diffusion from the

lumen. As a result, the walls of these vesselshave their own small vessels called vasavasorum (vessels of the vessel).

- More extensive in veins than in arteries.

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4 Circulatory System