Blood Flow

• 5% of blood pumped by heart is pumped to heart itself
• Abundant cardiac muscles need lots of oxygen and nutrients

• Branches off Aorta
• Anterior interventricular branch: supplies ventricles and parts of interventricular septum
• Circumflex branch: lies in coronary sulcus; supplies left atrium and posterior left ventricle

• Branches off Aorta
• Supplies sinoastrial node (pacemaker)
• Right Marginal Branch: supplies lateral right atrium and ventricle
• Poster Interventricular Branch: supplies posterior ventricles

Some blood drains directly into right atrium and right ventricle

• Huge vein in coronary sulcus on posterior side that collects blood and empties into right atrium
• Great Cardiac Vein: collects blood from anterior heart, empties into coronary sinus
• Middle Cardiac Vein: collects blood from posterior heart, empties into coronary sinus
• Left Marginal Vein: Empties into coronary sinus

Blockage of coronary arteries (ischemia) typically caused by atherosclerotic plaque formation

• Caused by coronary ischemia
• Cardiac muscle downstream of blockage dies
• Cardiac cells can die within minutes
• Built-in protection from MI: arterial anastomoses provide many routes for blood to reach all parts of heart (collateral circulation)

• Chest pain from partial obstruction of coronary blood flow
• Pain caused by ischemia of cardiac muscle
• Myocardium shifts to fermentation and produces lactic acid, which causes pain.

• Reduce risk factors: stop smoking, modify diet, reduce stress, increase exercise
• Drug treatment: reduce coagulation (aspirin, coumadin), block sympathetic stimulation (propranolol), cause vasodilation (nitroglycerin)
• Surgery:
• Artherectomy: plaque removal through catheter
• Balloon Angioplasty: Pressing plaque against vessel walls
• Coronary Artery Bypass: create detour around blockage

• Contractile Muscle Cells: produce heart contractions
• Conducting System: nodes generate electrical signals; conducting cells distribute electrical signals between nodes and to remainder of heart

• Cardiocytes: short muscle cells, striated, one nucleus, lots of glycogen surrounding it.
• Intercalated disks: join cardiocytes together
• Interdigitating folds: interlock cells together
• Mechanical junctions: tightly join cardiocytes
• Gap junctions: allow ions to flow between cells

• Coordinates heartbeat
• Internal pacemaker and conduction pathways through myocardium
• Pathway of electrical signal: sinaotrial node, atrioventricular node, atrioventricular bundle, the purkinje fibers
• Signals pass for cell to cell through gap junctions

• Modified cardiocytes in right atrium, near superior vena cava
• Pacemaker cells initiate heartbeat
• Pace determines heart rate
• Spreads signal throughout both atria

• Near right AV valve
• Excited by SA node firing, then allows for the excitement of cardiocytes in ventricles
• Provides delay: don't want atria and ventricles contracting at same time

• Bundle of cells from AV node to apex of heart
• Transmits electrical signal to apex

Nerve-like projections that spread throughout ventricular myocardium

• Vagus nerves terminate in SA and AV nodes
• Slow heart rate
• At rest, parasympathetic NS is active: without activity, heart beat is 80-100 bpm instead of the typical 60-80 bpm
• Parasympathetic released ACh onto SA node, which opens K+ channels and decreases frequency of spontaneous depolarizations

• Fibers terminate is SA and AV nodes; in atrial and ventricular myocardium; in aorta, pulmonary trunk, coronary arteries
• Increases heart rate and dilate coronary arteries (more blood flow to heart)
• Norepinephrine released by sympathetic NS makes spontaneous depolarization in SA node more frequent

Monitor blood volume for autonomic nervous system

• Cycle of Events
• Systole: atrial or ventricular contraction
• Diastole: arial or ventricular relaxation

• Normal heartbeat triggered by SA node
• Typically 60-100 beats per minute

• Spontaneous firing from something other than the SA node
• Nodal Rhythm: AV node sets rate at 40-50 bpm if SA node is damaged
• Intrinsic Ventricular Rhythm: if both nodes are damaged, heart rate is 20-40 bpm (need pacemaker)

• Any abnormal rhythm
• Atrial flutter: atria beat 200-400 bpm (atrial fibrillation)
• Premature ventricular contractions caused by stimulants, stress, and lack of sleep
• Ventricular Fibrillation: electrical signals reach parts of ventricles at different times and can't pump blood
• Defibrillation: electrical shock to depolarize entire heart and reset SA nodes to sinus rhythm

• No resting potential
• Repeating cycle of depolarizations

• Initially depolarizes due to influx of Na+ (“pacemaker potential”)
• At threshold (-40 mV), fast Ca2+ and Na+ channels open
• Fast depolarization peaks at 0 mV
• Opening of K channels hyperpolarizes membrane
• K channels close, then cycle starts over
• Each depolarization causes heart contraction

• SA node depolarization spreads to atrial muscle cells and AV node through gap junctions
• AV node also has pacemaker-like activity, but it’s slow and SA node speeds it up

• Action potential spreads down AV bundle and Purkinje fibers
• Papillary muscles contract just before ventricles to tighten slack in chordae tendinae
• Contraction begins at apex, then twisting upwards

• Stable resting potential of -90mV
• Depolarize only when stimulated
• Depolarization phase: stimulus opens Na+ channels, quick depolarization, then Na+ channels close
• Plateau Phase: Ca2+ channels open, cause sarcoplasmic reticulum to release more Ca2+, sustains contraction
• Repolarization Phase: Ca2+ channels close, K+ close to return to resting potential