Chapter 20 Notes
The Heart

1. Embryonic development

• origin: mesoderm
• week 5: septa develop
• week 7: four chambers deliver nutrients exchange wastes
• foramen ovale: interatrial opening
• ductus arteriosus: pulmonary trunk to aorta opening
• these bypass fetal lungs

The Anatomy of the Heart

2. Location

• mediastinum
• pericardial cavity
• right border, is right atrium
• left border is ventricle
• apex is the pointed end
• base: blunt superior end from which blood vessels emerge
• heart online

3. Layers

• parietal pericardium
• - outer fibrous connective tissue attached to blood vessels but not to heart,
     anchors and protects against friction
  - inner serous membrane secretes pericardial fluid

• pericardial space
• epicardium is visceral pericardium
• myocardium is the bulk of the heart made of cardiac muscle
• endocardium inner lining continuous with valves and blood vessels

4.Chambers

    Atria

• upper, thin walled flap-like appendages "auricles"
• separated by interatrial septum
• right receives blood from SVC, IVC, coronary sinus
• left receives blood from pulmonary veins
• coronary sulcus marks the separation between atria and ventricles

    Ventricles

• thicker walls due to pumping burden
• separated internally by interventricular septum and externally by coronary sulcus
• right pumps deoxygenated blood to lungs
• left pumps oxygenated blood to the systemic circulation, very thick muscular walls
• interventricular sulci mark the separation of the ventricles anteriorly and posteriorly

5. Valves

• cuspid valves
• - leaf-like flaps pointed into ventricles
  - prevent backfill as ventricles contract
  - open most of the time and close superiorly when ventricles contract
  - chordae tendineae anchor valves to papillary muscles of ventricles
  - right is tricuspid
  - left is bicuspid or mitral valve
  - closure produces first heart sound LUBB
  - online valve location

• semilunar valves
• - half-moon depressions that fill with blood
  - closed except when pressure in ventricles exceeds pressure in arteries
  - open superiorly when ventricles contract
  - right is pulmonary valve
  - left is aortic valve
  - closure produces second heart sound dupp

6. Defects

• congenital
• - foramen ovale: interatrial septal defect
  - ductus arteriosus: patent ductus arteriosus
  - tetrology of Fallot

1. pulmonary trunk too narrow
2. pulmonary valve stenosed
3. ventricular septal defect
4. aorta opens from both ventricles

• valvular
• - heart murmur: leaky valves allow blood to flow back from the direction it came
  - mitral stenosis: narrow diseased fibrotic valve hinders systemic blood flow
  - aortic stenosis: pressure build up in left ventricle

7. Coronary circulation

• the blood supply to the myocardium
• right coronary artery
                              -->anterior marginal artery
                              -->posterior descending artery
• left coronary artery
                              -->posterior circumflex artery
                              -->anterior descending artery
• cardiac veins drain into coronary sinus

8. Clinical

• ischemia: reduced oxygen supply to heart due to narrowed coronary blood vessels
• angina pectoris: chest pain or pressure radiating from chest and labored breathing due to inadequate blood flow
• myocardial infarction: "heart attack"
• - coronary arteries are totally obstructed
  - cardiac muscle is replaced by scar tissue

• anticoagulants: prevent blood clotting, heparin and coumadin
• antithrombolytic agents: "clot busters" dissolve clots plasmin, streptokinase, and TP-A (tissue plasminogen activator)
• online coronary blood vessels 

The Conduction System of the Heart

9. Inherent contractility

• heart responds to changes in capillaries
• cardiac muscle contractions have their own rhythm and pace
• contractions arise spontaneously
• cells meet end to end at intercalated disks
• electrical current flows from cell to cell so that atria and then ventricles contract as a unit
• one group of cells has the fastest Action Potential and sets the pace for other cells
• click her for picture

10. Conduction pathway

• there is specialized myocardium for self-excitation
• this tissue generates and distributes an electrical impulse but does not contract
• Sino-Atrial (SA) Node
  - cells in right atrium near SVC
  - establish the rate of contraction
  - Pacemaker, begins cardiac cycle of 72 beats per minute
  - naturally permeable to Na⁺ which flows in and produce action potentials
  - depolarization of SA node causes the exchange of ions across the
     cell membranes of cardiac muscle cells

- 90 mV    1. Na⁺ enters, depolarization

+20 mV    2. Ca⁺⁺ enters, maintains plateau

-90mV      3. K⁺ leaves, repolarization

- action potential spreads across atria and cardiac muscle contracts for .1 seconds

• Action potential cannot reach ventricles due to a fibrous ring,
• Atrioventricular (AV) Node
  - serves as a bridge between the atria and ventricles
  - depolarization her is slow so that atria can complete their contraction

• bundle of His
  - conduction fibers in interventricular septum
  - receive action potential next
  - right and left bundle branches are conduction fibers for the cardiac muscle    
    cells in each ventricle

• Purkinje fibers
  - penetrate and pass action potential to each ventricle which then contracts for .3 seconds

11. Refractory period

• period when another contraction cannot occur
• long period of rest, .7 seconds for atria and .4 seconds for ventricles
• allows heart chambers to fill with blood
• allows muscle to relax and prevents tetany

The Electrocardiogram

12. Electrocardiogram

• graphic record of heart’s electrical activity
• note: not a record of contraction but movement of ions
• takes advantage of "battery" effect of ions as electrical currents are sent through the body fluids
• P wave
  - spread of atrial depolarization as impulse travels from SA node over atria, .1 seconds

• QRS complex
  - spread of depolarization into the ventricles, .06 sec.

• T wave
  - ventricular repolarization as K+ channels open
  - varies the most, .12 seconds

• P-R Interval
  - beginning of P to beginning of QRS
  - beginning of atrial depolarization to the beginning of ventricular depolarization, .16 seconds
  - longer in coronary artery disease or any delay in conduction

• S-T segment
  - end of S to beginning of T
  – time between depolarization and repolarization of ventricles
  - elevated in MI’s, depressed in ischemia

  online electrocardiograms

• reasons for performing a resting EKG
• - evaluate symptoms of heart disease
  - detect abnormal rhythms
  - follow recovery of a heart attack
  - drug monitoring
  - check an artificial pacemaker
  
• online Cardiology topics