Chapter 20 - Cardiovascular System (CVS): The Heart

• Basic structural components of the CVS:

  • heart
  • arteries
  • capillaries
  • veins
• Heart:

  • hollow muscular pumping organ within the thoracic cavity
  • a dual pump: right side pumps blood to lungs = Pulmonary Circuit and left side pumps blood to the rest of body = Systemic Circuit
• Arteries:

  • vessels which conduct blood away from the heart
  • mostly carry O2 rich blood
• Capillaries:

  • microscopic vessels where the exchange of materials between the blood/tissues occurs
• Veins:

  • vessels which conduct blood towards the heart
  • mostly carry deoxygenated blood
• Heart Structure:

  • located within the thoracic cavity
  • lies 2/3's on the left side
  • enclosed by the pericardium - fibrous pericardium and serous pericardium
• 1. Fibrous pericardium: tough, dense, irregular connective tissue, outer part- prevents overstretching of heart, protects heart and helps to anchor heart in mediastinum
• Serous pericardium: a double-walled sac

  • parietal pericardium is fused to fibrous pericardium
  • visceral pericardium (epicardium) adheres to heart surface
  • located between the layers is the pericardial cavity filled with pericardial fluid
• Wall of the heart consists of 3 layers:

  1. outer epicardium
  2. middle myocardium
  3. inner endocardium
• Pulmonary arteries - carry deoxygenated blood to the lungs where O2 will be picked up
• Pulmonary veins - carry oxygenated blood from the lungs  back to the left side of the heart
• Outer epicardium (visceral pericardium): simple squamous epithelium and thin layer of areolar CT
• The heart has 4 chambers:

  1. two atria - thin-walled, upper collecting chambers
  2. two ventricles - thick-walled, lower pumping chambers
• Why is the left ventricle wall 3x as thick as the right ventricle wall?

  A thicker wall means a more powerful pump and blood has to travel further to the rest of the body from the left ventricle. The right ventricle just pumps blood a short distance to the lungs.
• Middle myocardium: consists of cardiac muscle arranged in bundles
• Inner endocardium: thin layer of endothelial cells (simple squamous cells) overlying a thin layer of CT

  • Smooth lining for the heart chambers and is continuous with the lining of the blood vessels and covers the valve surfaces
• Heart has 4 valves:

  • they consist of 2 or 3 fibrous flaps of tissue (cusps) covered with endocardium
  • promote the one-way flow of blood (prevent backflow)
  • operate based on pressure differences
• Atrioventricular (AV) valves:

  • located between an atrium and ventricle heart chamber
  • right AV valve located between right atrium and right ventricle - Tricuspid Valve (3 flaps/cusps)
  • left AV valve located between left atrium and left ventricle - Bicuspid Valve (2 flaps/cusps)
• Atrioventricular (AV) valves - continued:

  • free edges of flaps are attached to papillary muscles of ventricle wall by CT fibers - chorde tendineae
  • as the atria fills with blood (pA > pV), flaps of valve are pressed against the ventricle wall and the valve is open
  • ventricle fills and begins to contract
  • back pressure of blood pushes flap (pV > pA) and the valve is closed
• Chordae tendineae = collagen cords = heart strings
• AV valves open:

  1. blood returning to the heart fills atria, pressing against the AV valves. the increased pressure forces AV valves open
  2. as ventricles fill, AV valve flaps hang limply into ventricles
  3. atria contract, forcing additional blood into ventricles
• AV valves close:

  1. ventricles contract, forcing blood against AV valve cusps
  2. AV valves close
  3. papillary muscles contract and chord tendineae tighten, preventing valve flaps from everting into atria
• Semilunar (SL) valves:

  1. pulmonary semilunar valve - located between right ventricle and pulmonary trunk
  2. aortic semilunar valve - located between left ventricle and aorta
• Semilunar (SL) valves - continued:

  • made up of 3 pocket-like cusps attach to the artery wall
  • when ventricle contracts, pressure pushes valve against artery wall (pVent > pArt) and valve is open
  • when ventricle relaxes, blood flows back and fills pockets (pArt > pVent) and valve is closed
• Semilunar valves open: as ventricles contract and intraventricular pressure rises, blood is pushed up against semilunar valves forcing them open
• Semilunar valves closed: as ventricles relax and intraventricular pressure falls, blood flows back from arteries, filling the cusps of semilunar valves and forcing them to close
• Blood flow through the heart:

  3 main circuits of blood flow -

  1. pulmonary circulation
  2. systemic circulation
  3. coronary circulation
• Pulmonary circulation:

  • functions: to bring blood to the lungs for gas exchange
  • RV -> pulmonary trunk -> pulmonary arteries -> capillaries in lungs (blood releases CO2 and picks up O2) -> pulmonary veins-> LA
• Systemic circulation:

  • functions: to bring blood to other body organs
  • LV -> aorta -> arteries in body ->capillaries in body (blood releases O2 and picks up CO2) -> veins in body -> superior or inferior vena cava -> RA
• Coronary circulation:

  • functions: to supply blood to the myocardium
  • aorta -> coronary arteries -> capillaries in myocardium (blood releases O2 and picks up CO2) -> coronary veins -> coronary sinus -> RA
• • narrowing of coronary vessels decreases O2 supply of myocardium
  • O2 deprived myocardium switches to anaerobic metabolism which produces lactic acid
  • accompanied by pain in heart that radiates down left arm (angina pectoris)
  • forewarning of impending heart attack
  • prolonged coronary blockage can lea to the sudden death of a patch of myocardium (myocardial infarction or heart attack)
• Cardiac Conduction System:

  • same banding pattern and arrangement of actin and myosin myofilaments as skeletal muscle
  • relies on aerobic metabolism for ATP
  • rich in myoglobin = O2 binding pigment in muscle (source of stored O2)
  • more mitochondria that are larger and resistant to fatigue
  • muscle cells (fibers) are branched for added strength
  • smaller sarcoplasmic reticulum and less reserve of intracellular Ca2+
• Cardiac Conduction System - Continued:

  • special junctions - intercalated discs are found between cells
  • intercalated discs are thickenings of sarcolemma and consist of:
  • desmosomes (physical connection) that will anchor cells so they don't separate when contract
  • gap junctions (electrical connection) which allow ions to pass freely from cell to cell so cells contract as a coordinated unit
• Cardiac Conduction System:

  • specialized cardiac muscle cells known as autorhythmic fibers
  • make up 1% of cardiac muscle fibers
  • are more like nerve cells
  • self-excitable and initiate an electrical impulse
  • form an impulse conduction system through-out the heart
  • electrical impulse spreads to the "working" contractile fibers which in turn, generate an action potential and then contract
• Conduction Pathway: Non-contractile Cardiac Cells

  • cardiac excitation begins in sine-atrial (SA) node - pacemaker of the heart
  • located in right atrial wall near opening of superior vena cava
  • "pacemaker cells" do not maintain a stable resting membrane potential
  • fibers spontaneously depolarize to threshold and generate an action potential
• Action potential of Conduction pathway:

  1. starts at -60mV and slowly drifts upwards due to slow inflow of Na+ - gradual depolarization called pacemaker potential
  2. reaches a threshold of -40mV and action potential spike occurs due to Ca2+ influx through Ca2+ channels
  3. action potential spread through atria and then both atria contract
  4. depolarization occurs due to the closing of Na+ and Ca2+ channels and the opening of K+ channels
• Conduction Pathway:

  • conduction proceeds to atrioventricular (AV) node
  • AV node is located in the interatrial septum
  • delays the spread of the action potential by 0.1 sec to allow atria to finish contraction so that the ventricles have time to fill with blood
• Conduction Pathway:

  • from AV node, impulse goes to the bundle of His and R and L bundle branches (in interventricular septum)
  • branches give rise to numerous Purkinje fibers (subendocardial conducting network) which extend into the ventricle walls
  • action potential spreads throughout the ventricular myocardium
  • ventricles contract beginning at the apex and moving toward the atria
• What is an artificial pacemaker?

  a device that can be surgically implanted and send out small electrical currents to stimulate the heart to contract. can be used to restore a normal heart rhythm in an individual whose SA node has become damaged or diseased
• Conduction pathway:

  1. sinoatrial node
  2. atrioventricular node
  3. atrioventricular bundle
  4. right and left bundle branches
  5. Purkinje fibers