Chapter 13

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Chapter 13

Blood, Heart

and Circulation

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Components of Circulatory System

Include cardiovascular and lymphatic systems Heart pumps blood thru cardiovascular system Blood vessels carry blood from heart to cells and back

Includes arteries, arterioles, capillaries, venules, veins Lymphatic system picks up excess fluid filtered out in

capillary beds and returns it to veins Its lymph nodes are part of immune system

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Structure of Heart

Heart has 4 chambers 2 atria receive blood from venous system 2 ventricles pump blood to arteries 2 sides of heart are 2 pumps separated by muscular septum

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Cardiac Cycle

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Cardiac Cycle

Is repeating pattern of contraction and relaxation of heart Systole refers to contraction phase Diastole refers to relaxation phase Both atria contract simultaneously; ventricles follow 0.1-

0.2 sec later

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Cardiac Cycle

End-diastolic volume is volume of blood in ventricles at end of diastole

Stroke volume is amount of blood ejected from ventricles during systole

End-systolic volume is amount of blood left in ventricles at end of systole

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As ventricles contract, pressure rises, closing AV valves Called isovolumetric contraction because all valves are closed

When pressure in ventricles exceeds that in aorta, semilunar valves open and ejection begins

As pressure in ventricle falls below that in aorta, back pressure closes semilunars

All valves are closed and ventricles undergo isovolumetric relaxation

When pressure in ventricles falls below atria, AVs open and ventricles fill

Atrial systole sends its blood into ventricles

Cardiac Cycle continued

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Electrical Activity of Heart

Myocardial cells are short, branched, and interconnected by gap junctions

Entire muscle that forms a chamber is called a myocardium or functional syncytium Because APs originating in any cell are transmitted to all

others Chambers separated by nonconductive tissue

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SA Node Pacemaker

In normal heart, SA node functions as pacemaker Depolarizes

spontaneously to threshold (= pacemaker potential)

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Membrane voltage begins at -60mV and gradually depolarizes to -40 threshold

Spontaneous depolarization is caused by Na+ flowing through channel that opens when hyperpolarized (HCN channel)

At threshold V-gated Ca2+ channels open, creating upstroke and contraction

Repolarization is via opening of V-gated K+ channels

SA Node Pacemaker continued

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Conducting Tissues of Heart continued

In septum of ventricles, His divides into right and left bundle branches Which give rise to

Purkinje fibers in walls of ventricles These stimulate

contraction of ventricles

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Conduction of APs

APs from SA node spread at rate of 0.8 -1 m/sec Time delay occurs as APs pass through AV node

Has slow conduction of 0.03– 0.05 m/sec AP speed increases in Purkinje fibers to 5 m/sec

Ventricular contraction begins 0.1–0.2 sec after contraction of atria

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Refractory Periods

Heart contracts as syncytium and thus cannot sustain force

Its AP lasts about 250 msec

Has a refractory period almost as long as AP

Cannot be stimulated to contract again until has relaxed

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Arteries

Large arteries are muscular and elastic Contain lots of elastin Thicker compared to veins Part of autonomic nervous system Expand during systole and recoil during diastole

Helps maintain smooth blood flow during diastole

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Arteries Small arteries and arterioles are muscular

Provide most resistance in circulatory system Arterioles cause greatest pressure drop

Mostly connect to capillary beds Some connect directly to veins to form arteriovenous

anastomoses

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Provide extensive surface area for exchange Blood flow through a capillary bed is determined by state of

precapillary sphincters of arteriole supplying it

Capillaries

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In continuous capillaries, endothelial cells are tightly joined together Have narrow intercellular channels that permit exchange of

molecules smaller than proteins Present in muscle, lungs, adipose tissue

Fenestrated capillaries have wide intercellular pores Very permeable Present in kidneys, endocrine glands, intestines.

Discontinuous capillaries have large gaps in endothelium Are large and leaky Present in liver, spleen, bone marrow

Types of Capillaries

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Contain majority of blood in circulatory system Very compliant (expand readily) Contain very low pressure (about 2mm Hg)

Insufficient to return blood to heart

Veins

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Blood is moved toward heart by contraction of surrounding skeletal muscles (skeletal muscle pump) And pressure drops

in chest during breathing

1-way venous valves ensure blood moves only toward heart

Veins

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Heart Disease

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Atherosclerosis

Is most common form of arteriosclerosis (hardening of arteries) Accounts for 50% of

deaths in US Localized plaques

(atheromas) reduce flow in an artery And act as sites for

thrombus (blood clots)

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Atherosclerosis

Plaques begin at sites of damage to endothelium E.g. from

hypertension, smoking, high cholesterol, or diabetes

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Atherosclerosis

Plaques begin at sites of damage to endothelium E.g. from

hypertension, smoking, high cholesterol, or diabetes

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Cholesterol and Plasma Lipoproteins

High blood cholesterol is associated with risk of atherosclerosis

Lipids, including cholesterol, are carried in blood attached to LDLs (low-density lipoproteins) and HDLs (high-density lipoproteins)

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Cholesterol and Plasma Lipoproteins

LDLs and HDLs are produced in liver and taken into cells by receptor-mediated endocytosis In cells LDL is oxidized

Oxidized LDL can injure endothelial cells facilitating plaque formation

Arteries have receptors for LDL but not HDLWhich is why HDL isn't atherosclerotic

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Lymphatic System

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Lymphatic System

Has 3 basic functions: Transports interstitial fluid (lymph) back to blood Transports absorbed fat from small intestine to blood Helps provide immunological defenses against pathogens

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Lymphatic System continued

Lymphatic capillaries are closed-end tubes that form vast networks in intercellular spaces Very porous, absorb

proteins, microorganisms, fat

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Lymph is carried from lymph capillaries to lymph ducts to lymph nodes


Lymph nodes filter lymph before returning it to veins via thoracic duct or right lymphatic duct

Nodes make lymphocytes and contain phagocytic cells that remove pathogens

Lymphocytes also made in tonsils, spleen, thymus


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Chapter 13

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