heart

• heart is a transport system consisting of two side-by-side pumps

  -right side receives oxygen-poor blood from tissues; pumps blood to lungs to get rid of CO2, pick up O2 via pulmonary circuit

  -left side receives oxygenated blood from lungs, pumps blood to body tissues via systemic circuit
• receiving chambers of heart

  -right atrium receives blood returning from systemic circuit

  -left atrium receives blood returning from pulmonary circuit
• pumping chambers of heart:

  -right ventricle pumps blood through pulmonary circuit

  -left ventricle pumps blood through systemic circuit
• size, location, and orientation of heart


  -approximately the size of a fist; weighs less than 1 pound

  -located in mediastinum between second rib and fifth intercostal space; on superior surface of diaphragm, two-thirds of heart to left of midsternal line, anterior to vertebral column, posterior to sternum
• size, location, and orientation of heart cont.

  -base (posterior surface) leans toward right shoulder

  -apex points toward left hip

  -apical impulse palpated between fifth and sixth ribs just below left nipple
• pericardium: double-walled sac that surrounds heart; made up of two layers that are separated by fluid-filled pericardial cavity (decreases friction)

  1. superficial fibrous pericardium: functions to protect, anchor heart to surrounding structures, and prevent overfilling
  2. deep two-layered serous pericardium; parietal layer lines internal surface of fibrous pericardium, visceral layer (epicardium) on external surface of heart
• pericarditis


  -inflammation of pericardium

  -roughens membrane surfaces, causing pericardial friction rub (creaking sound) heard with stethoscope
• cardiac tamponade


  excess fluid that leaks into pericardial space

  -can compress heart's pumping ability

  -treatment: fluid is drawn out of cavity (usually with syringe)
• three layers of heart wall

  1. epicardium: visceral layer of serous pericardium
  2. myocardium: circular or spiral bundles of contractile cardiac muscle cells
  3. endocardium: innermost layer; is continuous with endothelial lining of blood vessels, lines heart chambers and covers cardiac skeleton of valves

  -cardiac skeleton: crisscrossing, interlacing layer of connective tissue, anchors cardiac muscle fibers, supports great vessels and valves, and limits spread of action potentials to specific paths
• chambers and associated great vessels

  -internal features: four chambers (two superior atria and two inferior ventricles)

  -interatrial septum: separates atria

  -fossa ovalis: remnant of foramen ovale of fetal heart

  -interventricular septum: separates ventricles
• chambers and associated great vessels cont. surface features

  -coronary sulcus (atrioventricular groove), encircles junction of atria and ventricles

  -anterior interventricular sulcus, anterior position of interventricular septum

  -posterior interventricular sulcus, landmark on posteroinferior surface
• chambers and associated great vessels

  -atria: the receiving chambers, small thin-walled chambers that contribute little to propulsion of blood

  -auricles: appendages that increase atrial volume

  -right atrium: receives deoxygenated blood from body

  • anterior portion is smooth-walled
  • posterior portion contains ridges formed by pectinate muscles
  • posterior and anterior regions are separated by crista terminalis
• atria cont.

  -three veins empty into right atrium:

  • superior vena cava: returns blood from body regions above the diaphragm
  • inferior vena cava returns blood from body regions below the diaphragm
  • coronary sinus returns blood from coronary veins

  -left atrium receives oxygenated blood from lungs

  • pectinate muscles found only in auricles
  • four pulmonary veins return blood from lungs
• -ventricles: the discharging chambers

  • male up most of the volume of heart
  • right ventricle: most anterior surface, pumps blood into pulmonary truck
  • left ventricle: posteroinferior surface, pumps blood into aorta (largest artery in body)
  • trabeculae carneae: irregular ridges of muscle on ventricular walls
  • papillary muscles: project into ventricular cavity, anchor chordae tendineae that are attached to heart valves
  • thicker walls than atria
  • actual pumps of heart
• heart valves

  -ensure unidirectional blood flow through heart

  -open and close in response to pressure changes

  -two major types:

  1. atrioventricular valves located between atria and ventricles,
  2. semilunar valves located between ventricles and major arteries

  -no valves are found between major veins and atria; not a problem because inertia of incoming blood prevents backflow, & heart contractions compress venous openings
• -two atrioventricular (AV) valves prevent backflow into atria when ventricles contract

  • tricuspid valve (right AV): made up of three cusps and lies between right atria and ventricle
  • mitral valve (left AV, bicuspid valve): made up of two cusps and lies between left atria and ventricle
  • chordae tendineae: anchor cusps of AV valves to papillary muscles that function to hold valve flaps in closed position, and prevent flaps from everting back into atria
• -two semilunar (SL) valves prevent backflow from major arteries back into ventricles

  • open and close in response to pressure changes
  • each valve consists of three cusps that roughly resemble a half moon
  • pulmonary semilunar valve: located between right ventricle and pulmonary trunk
  • aortic semilunar valve: located between left ventricle and aorta
• -two conditions severely weaken heart:

  • incompetent valve: blood back flows so heart re-pumps same blood over and over
  • valvular stenosis: stiff flaps that constrict opening, heart needs to exert more force to pump blood

  -defective valve can be replaced with mechanical, animal, or cadaver valve
• pathway of blood through the heart: right side edition

  -superior vena cava (SVC), inferior vena cava (IVC), and coronary sinus→ right atrium → tricuspid valve → right ventricle → pulmonary semilunar valve → pulmonary trunk → pulmonary arteries → lungs
• pathway of blood through heart: left side edition


  -four pulmonary veins → left atrium → mitral valve → left ventricle → aortic semilunar valve → aorta → systemic circulation
• -equal volumes of blood are pumped to pulmonary and systemic circuits

  -pulmonary circuit is short, low-pressure circulation

  -systemic circuit is long, high-friction circulation

  -anatomy of ventricles reflects differences; left ventricle walls are 3x thicker than right so they pump with greater pressure
• coronary circulation:

  -functional blood supply to heart muscle itself

  -shortest circulation in body

  -delivered when heart is relaxed

  -left ventricle receives most coronary blood supply
• coronary arteries:

  -both left and right CA arise from base of aorta and supply arterial blood to heart

  -both encircle heart in coronary sulcus

  -branching of coronary arteries varies among individuals

  -arteries contain many anastomoses (junctions): provide additional routes for blood delivery, cannot compensate for coronary artery occlusion

  -heart receives 1/20th of body's blood supply
• coronary arteries cont.

  -left coronary artery supplies interventricular septum, anterior ventricular walls, left atrium, and posterior wall of left ventricle; has two branches the anterior interventricular artery and circumflex artery

  -right coronary artery supplies right atrium and most of right ventricle; has two branches the right marginal artery and the posterior interventricular artery
• coronary veins:

  -cardiac veins collect blood from capillary beds

  -coronary sinus empties into right atrium: formed by merging cardiac veins

  • great cardiac vein of anterior interventricular sulcus
  • middle cardiac vein in posterior interventricular sulcus
  • small cardiac vein from inferior margin

  -several anterior cardiac veins empty directly into right atrium anteriorly
• -angina pectoris: thoracic pain caused by fleeting deficiency in blood delivery to myocardium, cells are weakened

  -myocardial infarction (heart attack): prolonged coronary blockage, areas of cell death are repaired with non-contractile scar tissue
• microscopic anatomy

  -cardiac muscle cells: striated, short, branched, fat, interconnected

  • one central nucleus (at most 2)
  • contain numerous large mitochondria (25-35% of cell volume) that afford resistance to fatigue
  • rest of volume composed of sarcomeres (Z discs, A bands, and I bands present)
  • T tubules are wider but less numerous (enter cell only once at Z disc)
  • SR simpler than in skeletal muscle; no triads
• microscopic anatomy cont.

  -intercalated discs are connecting junctions between cardiac cells that contain:

  • desmosomes: hold cells together; prevent cells from separating during contraction
  • gap junctions: allow ions to pass from cell to cell; electrically couple adjacent cells; allows heart to be a functional syncytium a single coordinated unit
• microscopic anatomy cont. 2

  intercellular space between cells has connective tissue matrix (endomysium)

  • connects numerous capillaries
  • connects cardiac muscle to cardiac skeleton, giving cells something to pull against
• how physiology of skeletal and cardiac muscle differ

  -similarities with skeletal muscle:

  • muscle contraction is preceded by depolarizing action potential
  • depolarization wave travels down T tubules; causes sarcoplasmic reticulum (SR) to release Ca2+
  • excitation-contraction coupling occurs, Ca2+ binds troponin causing filaments to slide
• how physiology of skeletal and cardiac muscle differcont.


  -differences between the two

  -some cardiac muscle cells are self-excitable, two kinds of myocytes:

  • contractile cells: responsible for contraction
  • pacemaker cells: non-contractile cells that spontaneously depolarize, initiate depolarization of entire heart, do not need nervous system stimulation in contrast to skeletal muscle fibers
• how physiology of skeletal and cardiac muscle differcont. 2

  -heart contracts as a unit:

  • all cardiomyocytes contract as a unit (functional syncytium), or none contract
  • contraction of all cardiac myocytes ensures effective pumping action
  • skeletal muscles contract independently

  -influx of Ca2+ from extracellular fluid triggers Ca2+ release from SR:

  • depolarization opens slow Ca2+ channels in sarcolemma, allowing Ca2+ to enter cell
  • extracellular Ca2+ then causes SR to release its intracellular Ca2+
  • all cardiomyocytes contract as a unit (functional syncytium), or none contract
• how physiology of skeletal and cardiac muscle differcont 3


  -tetanic contractions canot occur in cardiac muscles:

  -cardiac muscle fibers have longer absolute refractory period than skeletal muscle fibers

  • absolute refractory period is almost as long as contraction itself
  • prevents tetanic contractions
  • allows heart to relax and fill as needed to be an efficient pump
• how physiology of skeletal and cardiac muscle differ cont 4

  -the heart relies almost exclusively on aerobic respiration:

  • cardiac muscle has more mitochondria than skeletal muscle so has greater dependence on oxygen (can't function w/o oxygen)
  • skeletal muscle can go through fermentation when oxygen not present
  • both types of tissues can use other fuel sources, cardiac is more adaptable to other fuels including lactic acid but must have oxygen
• electrical events of the heart

  -heart depolarizes and contracts without nervous system stimulation, although rhythm can be altered by autonomic nervous system
• intrinsic conduction system

  -coordinated heartbeat is a function of:

  1. presence of gap junction
  2. insintrinsic cardiac conduction system

  • network of non-contractile (autorhythmic) cells
  • initiate and distribute impulses to coordinate depolarization and contraction of heart
• intrinsic conduction system


  -action potential initiation by pacemaker cells

  • cardiac pacemakers cells have unstable resting membrane potentials called pacemaker potentials or pre-potentials
• three parts of action potential in intrinsic conduction system

  1. pacemaker potential: K+ channels are closed, but slow Na+ channels open, causing interior to become more positive
  2. depolarization: Ca2+ channels open (around -40 mV), allowing huge influx of Ca2+, leading to rising phase of action potential
  3. repolarization: K+ channels open, allowing efflux of K+, and cell becomes more negative
• intrinsic conduction system cont.

  -sequence of excitation:

  -cardiac pacemaker cells pass impulses, in following order across the heart in ~0.22 seconds:

  1. sinoatrial node →
  2. atrioventricular node →
  3. atrioventricular bundle →
  4. right and left bundle branches →
  5. subendocardial conducting network (purkinje fibers)
• sequence of excitation explained:

  1 sinoatrial (SA) node:

  -pacemaker of heart in right atrial wall, depolarizes faster than rest of myocardium

  -generates impulses about 75 x/minute (sinus rhythm), inherent rate of 100 x/minute tempered by extrinsic factors

  -impulses spreads across atria and to AV node

  2 atrioventricular (AV) node:

  -in inferior interatrial septum

  -delays impulses approximately 0.1 second because fibers are smaller in diameter, have fewer gap junctions, allows atrial contraction prior to ventricular contraction

  -inherent rate of 50 x/minute in absence of SA node input