Chapter 18: The Heart

Size, location, & orientation • Heart is about 250-350 grams (less than 1 lb)• About the size of a persons fist • Location –

– found in the mediastinum – medial cavity of thorax – 2/3 of mass is left of midsternal– Rests on superior surface of the diaphragm– Anterior to the vertebral column– Posterior to the sternum – Flanked by the lungs

• Base – – Posterior surface (top)– Broad & flat– 9 cm wide– Directed towards right shoulder

• Apex –– Pointed end– Base of both ventricles – Point towards the left hip

Coverings • Heart is enclosed in the pericardium:

– Protects the heart– Anchors it to surrounding structures (diaphragm)– Prevents over filling

• Serous pericardium: – Thin serous membrane – Two layers:

• Parietal –– Lines the inside of the pericardium – internal surface– Tough fibrous connective tissue layer – Anchors to diaphragm & sternum

• Visceral –– Covers the external surface of the heart – Serous layer turns downward to cover the heart– Part of heart wall – laced with fat

• Pericardial cavity – – Potential space – space between visceral & serous

layer of parietal pericardium– Contains pericardial fluid – allows serous membranes

to glide past one another

Layers of the heart wall • 3 layers:

– Superficial epicardium (viseral pericardium):• Visceral layer of the serious pericardium • Infiltrated with fat

– Middle myocardium:• Mostly cardiac muscle – bulk of the heart • Layer that actually contracts • Branching cardiac cells held together by crisscrossing

connective tissue fibers & arranged in bundles – Connective tissue fibers form network – fibrous skeleton

– reinforcement of heart muscle especially around valve areas & where vessels attach to the heart

– Deep endocardium:• White sheet of endothelium resting on connective

tissue layer• Located – inner myocardial surface, lining of

chambers, & covers connective tissue skeleton of valves

• Continuous w/ the endothelial linings of blood vessels leaving & entering the heart

Chambers• 4 chambers – 2 atria & 2 ventricles • Internal septum –

– divides the atria internally longitudinally

• Interventricular septum – – divides the ventricle internally

longitudinally• Right ventricle =

– most of the anterior surface • Left ventricle =

– inferoposterior aspect & forms most of the apex

Chambers • Atria –

– Superior – Receiving chambers for blood returning to the heart

from circulation– Small chambers– Push blood to ventricles – Auricles – ear-like flap – from outside of each atria –

increase atrial volume – Internal walls have ridges of muscles – pectinate – Fossa ovalis – shallow depression in interatrial septum

– residual from fetal heart– Blood enters the right atrium from –

• Superior vena cava – returns blood from above the diaphragm (upper body)

• Inferior vena cava – blood returning from below the diaphragm (below the heart)

• Coronary sinus – collects blood draining from myocardium– Blood enters the left atrium from –

• 4 pulmonary veins • Seen from posterior view of the heart • Transport blood from the lungs

Chambers • Ventricles –

– Inferior– Most of the mass of the heart – Right ventricle = anterior surface – Left ventricle = apex – Trabecuae carneae –

• irregular ridges of muscles – Papillary muscles –

• project into the ventricular cavity – play a role in valve function

• (muscle folds = trabeculae carnae – some are stalklike and attach to valves = papillary muscles)

– Discharging chambers – pumps of the heart – Blood propels out of the heart into circulation– Walls much thicker than atrial walls – Right ventricle – pulmonary trunk – routs blood to

lungs (gass exchange occurs)– Left ventricle - aorta – body

Blood pathway through heart• Pulmonary circuit –

– Right side of the heart– Low pressure– Blood to and from the lungs – Blood flow path-

•Blood in right atrium from body (oxygen poor, CO2 rich) via superior and inferior venae cavae

• right ventricle•pumps into lungs (for oxygen pickup) via

pulmonary trunk• from lungs to heart through pulmonary veins • left atrium • left ventricle

Blood pathway cont

• Systemic circuit – – Left side of the heart – High pressure – Supplies body w/ oxygenated blood– Pathway –

•From lungs•Pulmonary veins •Left atrium•Left ventricle – contracts •Aorta •Body

Valves • Atrioventricular valves (AV) –

– Atrial-ventricular junction– Prevent backflow into the atria when ventricles are

contracting – Attached to AV valves – collagen cords – chordae

tendineane – anchor papillary muscles protruding from ventricular walls

– Right AV valve = tricuspid valve • 3 flaps (tri) – endocardium & connective tissue

– Left AV valve = bicuspid valve • 2 flaps (bi)

– How it works – • When heart is relaxed – AV valves hang into ventricle• Blood into atria & into ventricle (through open AV valve) • Ventricle contracts • Ventricular pressure rises – forces blood (superiorly)

against AV valve• Valve edges meet – closing the valve • Chordae tendineae & papillary muscles – anchor valves

while they are closed

Valves

• Semilunar (half moon) valves – – Major arteries leaving the heart– Prevent backflow into the ventricles – Aortic semilunar –

•Valve at the base of the aorta

– Pulmonary semilunar –•Valve at the base of the pulmonary trunk

– No chordae tendinae – valve movement caused by force of blood•Heart is relaxed – valves are closed •Heart contracts – valves are forced open

Microscopic anatomy • Cardiac muscle –

– Striated, short fat, branched, & interconnected muscle – One or two nuclei – Intracellular space filled w/ loose connective tissue

matrix & capillaries – connected to fibrous skeleton – allows cardiac cells to exert force

• Contraction occurs via sliding filaments – Adjacent muscles interconnect @ intercalated discs

• Disc contains desmosomes (hold cardiac cells together) & gap junctions (allow ions to pass from cell to cell)

• Allow cardiac cells to electrically behave as a unit – High concentration of mitochondria

• Metabolize fatty acids for ATP• Can switch nutrient pathways to use whatever nutrient

supply that is available – Depends on a continual supply of oxygen

• Aerobic respiration – can’t have oxygen deprivation & still operate

Heart contraction

• Sodium & calcium needs:– Sodium ions enter cardiac muscle cells

from extracellular fluid (sodium ion channels)

– Causes a depolarization that causes the sarcoplasmic reticulum (specialized ER) to release calcium

– Calcium enters the sarcoplasm (cytoplasm of cardiac cells)

– Calcium signals myofilaments (individual muscle fibers) to contract

– Cardiac muscles contract as a unit or not at all

Electrical events

• Intrinsic cardiac conduction system – made up of specialized cardiac cells –nodal system

• Initiate & distribute impulses• Ensures that the heart depolarizes

in sequential order• Contracts because of gap junction

(allows signals to pass between cells)

Electrical events• Sequence of excitation –

– 1. SA node –• Sinoatrial • Pacemaker - sets pace for the heart • In right atrial wall• Minute cell mass • Depolarizes 70-80 times per min• Called sinus rhythm – determines heart rate

– 2. AV node –• Atrioventricular • In interatrial septum – above the tricuspid valve • Depolarization spreads via gap junctions • From SA node to AV node • Impulse delayed 1 sec to allow atria to completely contract

– 3. AV bundle –• Atrioventricular• “Bundle of His”• Superior part of interventricular septum • Connects atria & ventricles electronically

Sequence of excitation cont. – 4. right & left bundles –

• Along interventricular septum toward apex of the heart

• Conduct signal through ventricles

– 5. ventricular walls –• Penetrate the heart apex• Turn superiorly into ventricular walls • Bundle branches excite septal cells • Contraction depends on cell-to-cell transmission via

gap junctions

• Total elapsed time from SA node to ventricular node = .22 sec

• Ventricles contract w/ wringing motion beginning at apex, moving toward atria

Nodes

Excitation

http://www.youtube.com/watch?v=MGxxRyJTmwU&feature=related

Heart sounds & CO• Lub –

– 1st heart sound – AV valves close– Beginning of systole (ventricular pressure rises above

atrial pressure)• Dup –

– 2nd heart sound– Closure of semilunar valves – During ventricular diastole

• Cardiac output (CO) –– Amount of blood pumped out each ventricle in one

minute (one cycle)– CO = HR x SV

• Stroke volume (SV) – – Volume of blood pumped by a ventricle with each beat

(during one contraction)– Correlated w/ force of ventricular contraction

Defects• Murmurs –

– Abnormal heart sounds – Indicate valve problems

• Tachycardia –– Abnormally fast heart rate– Over 100 beats per min

• Arrhythmias –– Uncoordinated contractions – Irregular heart rhythms

• Fibrillation –– Rapid, irregular, out of phase contractions– Heart is useless– Must be defibrillated immediately before brain death

(electric shock)• Abnormal pacemaker –

– Ectopic pacemaker – AV node may take over – but at a slower pace– Caffeine can cause irregular rhythms separate from

the SA node

Cardiac cycle • Ventricular filling –

– Flows passively through atria into ventricles (70% of blood)

– Atria contract propelling left over blood into ventricles

• Ventricular systole –– Atria relax– Ventricles contract– Pressure rises closing the AV valves – Semilunar valves forced open – Blood passes into the aorta & pulmonary trunk

• Isovolumetric relaxation –– Ventricular pressure drops – Closes the semilunar valves – Blood rebounds off the valves & continues its

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