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The Heart. Chapter 18. Introduction. The heart - a muscular double pump with 2 functions 1) Its right side receives oxygen-poor blood from the body tissues and then pumps this blood to the lungs to pick up O 2 and dispel CO 2

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The heart l.jpg

The Heart

Chapter 18


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Introduction

  • The heart - a muscular double pump with 2 functions

    1) Its right side receives oxygen-poor blood from the body tissues and then pumps this blood to the lungs to pick up O2 and dispel CO2

    2) Its left side receives the oxygenated blood returning from the lungs and pumps this blood throughout the body to supply O2 and nutrients to the body tissues

  • BVs that carry blood to and from the lungs form the pulmonary (plumonos = lungs) circuit

  • BVs that transport blood to and from all body tissues form the systemic circuit



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The Heart

  • Has 2 receiving chambers: right and left atrium (‘entranceway’)

    - receives blood returning from the 2 circuits

  • Has 2 main pumping chambers: the right and left ventricles (‘hollow belly’)

    - pumps blood around the 2 circuits

  • Hollow cone-shaped organ – typical weight 250-350g

    - about the size of your fist

  • The largest organ in the mediastinum

    - located between the lungs

    - oblique position with apex lying left to the midline



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Four “Corners” of the Heart

  • Important for clinicians – imaginary lines that connect the 4 corner points delineate the normal size and location of the heart

  • Superior right – at costal cartilage of 3rd rib and sternum

  • Inferior right – at costal cartilage of 6th rib lateral to the sternum

  • Superior left – at costal cartilage of 2nd rib lateral to the sternum

  • Inferior left (apex point) – lies in the 5th intercostal space at the midclavicular line



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Structure of the Heart - Coverings

  • Pericardium – a triple-layered sac that encloses the heart

    - fibrous pericarium, strong outer layer of dense CT

    - serous pericardium, formed from 2 layers: the outer parietal pericardium adheres to the inner surface of the fibrous pericardium and is continuous with the visceral layer or epicardium

  • Pericardial cavity – between parietal and visceral layers

    - epithelial cells secrete a lubricating serous fluid

    - reduce friction between the heart and outer wall of the pericardial sac



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Layers of the Heart Wall

3 layers - a superficial, middle, deep

  • Epicardium – visceral layer of the serous pericardium (often infiltrated with fat)

  • Myocardium – forms the bulk of the heart

    - consists of cardiac muscle tissue (layer that contracts)

    - CTs bind the cells into elongated circular and spiral bundles

  • Endocardium – sheet of endothelium resting on a thin layer of CT

    - lines the heart chambers and covers the heart valves



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Bundles

  • Function to squeeze blood through the heart in the proper directions

    - inferiorly through the atria

    - superiorly through the ventricles

Figure 18.4


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Heart Chambers

  • Right and Left atria – superior chambers

    - divided longitudinally by the interatrial septum

  • Right and Left ventricles – inferior chambers

    - divided longitudinaly by the interventricular septum

  • External boundaries marked by 2 grooves

    1) Coronary sulcus forms a crown by circling the boundary between the atria and the ventricles

    2) Anterior and posterior interventricular sulci



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Right Atrium

  • Forms right border of heart - receives oxygen-poor blood returning from the systemic circuit via veins:

    - superior and inferior vena cava and the coronary sinus

  • Internally has 2 parts: smooth walled posterior and anterior part lined by pectinate muscles

    - separated by the crista terminalis –landmark used to locate sites where veins enter the right atrium

  • Fossa ovalis – depression in interatrial septum

    - remnant of foramen ovale of the fetal heart

  • Inferiorly and anteriorly opens into the right ventricle through tricuspid valve (rt. atrioventricular valve)


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Right Ventricle

  • Forms most of the anterior surface – receives blood from right atrium

    - pumps it into the pulmonary circuit via the pulmonary trunk

  • Internal walls of right ventricle marked by trabeculaecarneae (‘little beams of flesh’)

    - cone-shaped papillary muscles project from the walls

    - chordae tendineae, thin strong bands (‘tugging on my heartstrings’) from the papillary muscles to the flaps (cusps) of the tricuspid (right AV)

  • Pulmonary semilunar valve (pulmonary valve)

    - opening between the right ventricle and the pulmonary trunk



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Left Atrium

  • Most of the heart’s posterior surface - receives oxygen-rich blood returning from the lungs

    - through 2 right and 2 left pulmonary veins

    - anteriorly only left auricle is visible

  • Interior atrial wall is smooth

    - pectinate muscles line only the auricle

  • Opens into the left ventricle through the mitral valve (left atrioventricular valve)


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Left Ventricle

  • Forms the apex of the heart – dominates the inferior surface of the heart

  • Internal walls contain:

    - trabeculae carneae, papillary muscles, chordae tendineae, and cusps of an AV (mitral valve)

  • Superiorly opens into the aorta of the systemic circulation

    - pumps blood through systemic circuit via the aortic semilunar valve (aortic valve)



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Pathway of Blood Through the Heart

Path of a single drop of blood around the circuits

  • Beginning with oxygen-poor systemic blood as it arrives at the right side of the heart

    - superior to the diaphragm enters the rt atrium via the SVC

    - body regions inferior to the diaphragm enters via the IVC

    - from heart wall collected by and enters through the coronary sinus

  • Passes from the right atrium through the tricuspid valve to the right ventricle

    – propelled by gravity and right atrium contraction

    - ventricle contracts propels blood through the pulmonary semilunar valve into the pulmonary trunk and to the lungs through the pulmonary circuit for oxygenation


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  • Oxygenated blood returns via the 4 pulmonary veins to the left atrium

    - passes through the mitral valve to the left ventricle propelled by gravity and left atrium contraction

  • Left ventricle contracts propels blood through the aortic semilunar valve into the aorta and its branches

    - delivers oxygen and nutrients to the body tissues through systemic capillaries

    - oxygen-poor blood returns through the systemic veins to the right atrium

  • Whole cycle repeats continuously

  • 2 atria always contract together, followed by the simultaneous contraction of the 2 ventricle


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Figure 18.6 left atrium


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Figure 18.6b left atrium


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Heartbeat left atrium

  • Single sequence of atrial contraction followed by ventricular contraction

    - 70 – 80 beats per minute at rest

  • Systole - contraction of a heart chamber

  • Diastole (‘expansion’) – when a heart chamber relaxes and fills with blood

  • Systole and diastole also refer to the stage of heartbeat when ventricles contract and expand


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Structure of Heart Wall left atrium

  • Walls differ in thickness

    - atria are thinner (filling by gravity)

    - left ventricle (systemic pump) has thicker walls than the right ventricle (pulmonary pump)

  • Left ventricle generates more force and pumps blood at a higher pressure

    - systemic circuit is longer than the pulmonary circuit

    - offers greater resistance to blood flow


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Structure of the Heart Wall left atrium

  • Left ventricle – three times thicker than right

    • Exerts more pumping force

    • Flattens right ventricle into a crescent shape

Figure 18.7


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Heart Valves – Valve Structure left atrium

  • Paired AV and semilunar valves enforce the one-way flow of blood through the heart

    - from the atria to the ventricles and into the great arteries that leave the superior part of the heart

  • Each valve consists of 2 or 3 cusps

    - flaps of endocardium reinforced by cores of dense CT

  • AV valves – between atria and ventricles

    - rt AV is the tricuspid and lt AV is the bicuspid (aka mitral)

  • Aortic and pulmonary (semilunar) valves – at junction of ventricles and great arteries


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Fibrous Skeleton left atrium

  • Lies in the plane between the atria and the ventricles and surrounds all 4 heart valves

    - composed of dense CT

  • Functions:

    1. Anchors the valve cusps

    2. Prevents overdilation of valve openings

    3. Main point of insertion for bundles of cardiac muscle

    4. Blocks direct spread of electrical impulses from the atria to the ventricles

    - critical for proper coordination of atrial and ventrticular contractions



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Function of the AV Valves left atrium

a) When the ventricles are relaxed, the valves are forced open by the blood pressure exerted on their atrial side

Figure 18.9a


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Function of the AV Valves left atrium

b) When the ventricles contract, forcing the contained blood superiorly, the valves are pushed shut

Figure 18.9b


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Function of the Semilunar Valves left atrium

a) During ventricular contraction, the valves are pushed open, and their cusps are flattened against the artery walls

b) When the ventricles relax, the backflowing blood closes the valves

Figure 18.10a, b


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Heart Sounds left atrium

  • Closing of the valves causes vibrations in the adjacent blood and heart walls

    - ‘lub-dup’ is the sound of valves closing

  • ‘Lub’ sound produced by closing of the AV valves at the start of ventricular systole

  • ‘Dup sound produced by the closing semilunar valves at the end of ventricular systole


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Heart Sounds left atrium

Figure 18.8a


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Heart Sounds left atrium

  • Each valve sound is best heard near a different heart corner

  • Pulmonary valve – superior left corner

  • Aortic valve – superior right corner

  • Mitral (bicuspid) valve – at the apex

  • Tricupsid valve – inferior right corner


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Figure 18.11 left atrium


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Cardiac Muscle Tissue left atrium

  • Forms the thick layer called myocardium

    - contains cardiac muscle cells and CT

  • Contractions of the cells pump blood through the heart and into blood vessels

    - contract by sliding filament mechanism

  • Cardiac muscle cells areshortand branching with 1 or 2 nuclei - striated like skeletal muscle

    - cells are joined at intercalated discs – complex junctions that form cellular networks (cardiac fibers)

    - cells are separated by endomysium – binds adjacent cardiac fibers and contains blood vessels and nerves


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Intercalated Discs left atrium

  • Complex junctions where the sarcolemmas of adjacent cells interlock through meshing ‘fingers’

  • Transverse regions contain long desmosome-like junctions called fasciae adherens

    - bind adjacent cells together and transmit the contractile force

  • Longitudinal regions contain gap junctions

    - allow ions to pass between cells, transmitting the contractile signal to adjacent cells


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Figure 18.12a, b left atrium


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Figure 18.12c, d left atrium


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Cardiac Muscle Tissue left atrium

  • Triggered to contract by Ca2+ entering the sarcoplasm in response to an action potential

    - rise in intracellular calcium signals the SR to release its stored Ca2+

    - ions diffuse into the sarcomeres triggering the sliding filament mechanism

  • Not all cardiac cells are innervated

    - isolated cardiac cells will still contract rhythmically

    - inherent rhythmicity is the basis for rhythmic heartbeat


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Conducting System left atrium

  • Cardiac muscle tissue - intrinsic ability to generate and conduct impulses

    - stimulate these same cells to contract rhythmically

    - do not depend on extrinsic nerve impulses

  • Conducting system

    - a series of specialized cardiac muscle cells carry impulses throughout the heart musculature

    - signals heart chambers to contract and initiating each contraction sequence thereby setting the heart rate

  • Components include the:

    - sinoatrial node, internodal fibers, AV node, AV bundle, right and left bundle branches and subendocardial branches (Purkinje fibers)


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  • Sequence that controls each heartbeat – atrial contraction followed by ventricular contraction

    1. SA node – impulses spread along the cardiac fibers of the atria to signal the atria to contract

    - some travel along an internodal pathway to the AV node

    2. At the AV node impulses are delayed for a fraction of a second

    3. Impulses race through the AV bundle (bundle of His) and enter the interventricular septum and divide

    4. Right and left bundle branches (crura)

    5. Purkinje fibers – halfway down the crura become subendocardial branches


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Figure 18.14 followed by ventricular contraction


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Innervation followed by ventricular contraction

  • Heart rate is altered by external controls

  • Nerves to the heart include

    - Visceral sensory fibers

    - Parasympathetic branches of the vagus nerve

    - Sympathetic fibers from cervical and upper thoracic chain ganglia

Figure 18.15


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Blood Supply to the Heart followed by ventricular contraction

  • Heart filled with blood - walls are too thick to obtain nutrients by diffusion

  • Blood supply to the muscular walls and tissues of the heart is delivered by the coronary arteries

    - left and right coronary arteries arise from the aorta

    - located in the coronary sulcus

  • Left coronary artery divides into the anterior interventricular and circumflex arteries

  • Right coronary artery descends in the coronary sulcus and branches at the inferior border

    - anterior becomes the marginal artery and posterior the posterior interventricular artery


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Cardiac Veins followed by ventricular contraction

  • Carry deoxygenated blood from the heart wall into the right atrium

    - also occupy the sulci on the heart surface

  • Coronary sinus – the largest

    - on the posterior part of the coronary

    - returns most of the venous blood from the heart to the right atrium

  • 3 large tributaries drain into the coronary sinus

    - great cardiac vein, middle cardiac vein, small cardiac vein

  • Anterior cardiac veins – empty directly into the right atrium


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Figure 18.16 followed by ventricular contraction


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Disorders of the Heart followed by ventricular contraction

Coronary artery disease (CAD)

  • Artherosclerosis – fatty deposits in the inner lining of the heart

  • Angina pectoris – chest pain

  • Myocardial infarction – blocked coronary artery

    - heart attack

  • Silent ischemia – blood flow is interrupted often

    - no pain or warning


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  • Heart failure followed by ventricular contraction - progressive weakening of the heart due to weakened or damaged ventricles

    • cannot meet the body’s demands for oxygenated blood

  • Congestive heart failure (CHF)

    - aka as dilated cardiomyopathy

    - heart enlarges while its pumping efficiency declines

    - cause unknown but may involve a destructive positive feedback loop (sympathetic stimulation)

  • Cor pulmonale (‘heart lung’)

    - enlargement and potential failure of the right ventricle

    - resulting from elevated BP in the pulmonary circuit

    - blockage or vessel constriction of the lungs increases resistance to blood flow  increased BP

    - acute cases may develop from an embolism


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The Heart in Adulthood and Old Age followed by ventricular contraction

  • Resilient heart usually functions well throughout life

    - individuals who exercise regularly and vigorously the heart adapts by increasing in strength and size which persist until old age

  • Age-related changes:

    1) Hardening and thickening of the heart valve cusps

    2) Decline in cardiac reserve – sympathetic control of the heart becomes less efficient

    3) Fibrosis of cardiac muscle – as one ages, more and more cardiac cells die and are replaced by fibrous scar tissue