Heart

• Size and location:

  fist-sized

  70 pbm

  apex (bottom part) points down and to the left

  mediastinum - space where the heart is found, between the lungs
• Chambers of the heart:

  divided into 4 chambers

  • 2 atria (atrium singular)
  • superior
  • each as an auricle - expansion area
  • function: receive blood from the lungs of the body
  • 2 ventricles
  • inferior
  • walls of the left are thicker than those of the rights
  • functions: to pump blood to the lungs (right/pulmonary) of body (left/systemic)
• Coverings:

  Pericardial sac: fluid filled sac that surrounds heart

  • fibrous pericardium - outer layer
  • visceral pericardium - inner layer
  • epicardium (sometimes referred as)
  • pericardial fluid (between two layers)
  • acts as a lubricant (helps reduce friction, heat, wear & tear, etc)
• Layers:

  Epicardium (outer)

  • visceral pericardium

  Myocardium (middle)

  • cardiac muscle
  • coronary blood vessels

  endocardium (inner)

  • endothelium (tissue, lines walls of atria & ventricles, provides smooth surface for blood to come in contact with)
• Heart Microscopic Anatomy:

  Muscle cells are striated, similar to skeletal muscle, but cells…

  • are short and branched
  • contain a single nucleus
  • contain a large number of mitochondria
  • intracellular space is filled with loose connective tissue and many capillaries
• Intercalated discs:

  heart cells are connected by intercalated discs

  • specialized cell junctions
  • desmosomes - physically holds cells together
  • gap junctions - allows communication between cells via cytosol, physically connected
• Heart valves:

  prevent the backflow of blood

  • regurgitation - when blood leaks back into the valve

  atrioventricular valves

  • separate the atria from the ventricles
  • tricuspid valve - right side
  • bicuspid valve - left side
  • chordae tendineae and papillary muscles - helps valves stay closed during contraction
• Valves:

  separate heart from arteries

  • pulmonary semilunar valve - in between right ventricle and pulmonary trunk
  • aortic semilunar valve - in between left ventricle and aorta

  *valves close when heart relaxes
• fibrous skeleton:

  dense connective tissue - separates atria and ventricles

  • form rings around valves

  functions:

  • foundation for valves
  • prevention of overstretching
  • insertion point from muscle
  • electrical insulator between atria and ventricles → no gap junctions in the layer
• Coronary Vessels:

  coronary arteries branch from ascending aorta

  • these branch into smaller arteries

  after flowing through coronary veins, blood collects in coronary sinus before entering the right atrium

  function: supply heart muscle with O2 and nutrients and remove waste products (CO2)
• Pathway:

  Note: arteries carry blood away from heart, veins return blood back to heart

  Blood, low in oxygen returns from body via the vena cava (inferior and superior)

  • Right atrium
  • Tricuspid valve
  • Right ventricle
  • pulmonary semilunar valve
  • pulmonary trunk/arteries blood low in O2
  • Lungs to pick up O2 and release CO2
  • pulmonary vein
  • left atrium
  • bicuspid valve
  • left ventricle
  • aortic semilunar valve
  • aorta
• Two circulatory systems:

  Heart is 2 pumps

  • Pulmonary circuit (right side, much shorter circuit, only goes to lungs)
  • Systemic circuit (left side, body system)
• Conduction and pace maker system:

  Heart is myogenic

  • Autorymthic cells - generate signal (electrical) that tells the heart to contract, generated by itself→ not brain
  • Specialized cardiac cells
  • Unstable membrane potentials - generates action potential without outside stimulus
  • Pacemaker potentials - sends signals at regular intervals
  • Establish fundamental rhythm of heart
• Pacemaker Groups:

  3 groups groups

  • Sinoatrial node (SA node) (right atrium)
  • “Pacemaker” (primary, controls all others)
  • Atrioventricular node (AV node)
  • Conducting cells - in between R&L ventricle, come off AV node
  • AV bundle and branches
  • Purkinje fibers - stimulate ventricles to contract, found in the walls to the ventricle
• Pacemakers:

  Rate of heart is determined by fastest group of cells (SV node)

  Signal delayed through AV node *signal goes through SA to AV node → causes delay

  • (Delay causes) separate atrial and ventricular contractions

  ANS and endocrine system only modify rate
• Spread of the signal and contraction:

  Signal begins at SA node

  • Cells depolarize (ion channels open, becomes positive)
  • This creates action potential (crosses threshold)

  Signal moves through the cells of atria via gap junctions

  This wave of depolarization causes atria to contract

  Signal spreads to AV node

  Signal travels through AV node into AV bundle

  Signal travels down the intraventricular septum (divides ventricles?) via the right and left bundle branches

  Signal travels up the walls of the ventricles via the Purkinje fibers
• Physiology of Contraction:

  Action potential if generated by SA node and travels through the conduction system

  Action potential spreads through the contractile cells

  This occurs in 3 Phases:

  • depolarization
  • plateau
  • repolarization
• Phase 1: Depolarization

  contractile fibers are brought to threshold by excitation of neighboring fibers

  voltage gated Na+ gates open (sodium)

  rapid influx of Na+

  • depolarization

  Na+ gates close
• Phase 2: Plateau

  Voltage gated slow Ca+2 (calcium) gates open in cell membrane and SR

  Ca+2 enters the cell from ECF and SR

  Ca+2 help uncover myosin binding site on actin

  Na+ pumps turn on

  • Na+ leaves cell, balancing the Ca+2 entering, causing a plateau

  depolarization last 175 msec
• Phase 3: repolarization

  slow voltage-gated K+ channels open

  • K+ rapidly effluxes

  Ca+2 gates shut

  K+ leaves the cell, repolarizing cell

  • return to resting potential
• Refractory Period:

  longer than the contraction period

  result is no contraction until relaxation is underway

  prevention of tetanus

  Functional reason: in order for heart to pump blood, ventricles must alternately fill (relaxation) and the empty (contraction) - (if heart doesn’t relax, blood can’t enter)

  *prevents prolonged contraction