The Heart 1

Anatomy

Anterior View:

  • Diastole: left image; blood filling the heart (relaxation)

  • Systole: right image; blood leaving the heart (contraction)

  • Basic flow of blood through the heart

    • Right atrium → right ventricle → lungs → left atrium → left ventricle → body

  • Know the basic anatomy of heart, especially these parts:

    • Right and Left Atrium

    • Right and Left Ventricle

    • Atrioventricular Valves (AV valves)

      • Tricuspid (right)

      • Bicuspid/Mitral (left)

    • Semilunar Valves

      • Pulmonary (right)

      • Aortic (left)

  • Valves: allow blood to flow in one direction 

    • As blood flows into the ventricle, it cannot flow back because the one-way valves will shut

    • AV valves open when heart is filling; AV valves close when contracting

    • AV valves prevent backflow into atria during contraction (systole)

    • Semilunar valves prevent backflow into ventricles during relaxation (diastole)

  • SA and AV Nodes

    • SA Node (sinoatrial): 

      • Specialized group of cardiac muscle cells that don’t contract (cardiomyocytes) 

      • Automatically ends impulses (acts as pacemaker)

      • Located in right atrium

      • Connected to AV 

    • AV Node (atrioventricular):

      • Slows impulses from SA

      • Can send impulses if SA doesn’t (back-up pacemaker)

      • Located between atria and ventricle 

Sounds: Lub & Dub

  • S1: Lub

    • Closing of AV valves - close at the start of systole (and end of diastole/end of relaxing); (heart was already filled with blood before S1)

  • S2: Dub

    • Closing of semilunar valves - close at the start of diastole (and end of systole); (heart is pumping out blood)

  • Abnormal sounds: (example: murmurs) caused by turbulent blood flow or backflow (regurgitation) 

    • Early, mid, late nature of problem

      • Early: the murmur starts right after a heart sound (like right after “lub” or “dub”)

        • Early systolic murmurs: murmur after “lub” - may indicate mitral valve regurgitation

        • Early diastolic murmur:  murmur after “dub” - may indicate aortic or pulmonary regurgitation

      • Mid (systolic): the murmur starts in the middle of the phase between heart sounds or in the middle of “lub-dub” phase; most associated with aortic stenosis 

      • Late (systolic): the murmur starts before the next heart sound; occur between the “dub” and the next “lub;” might indicate mitral valve prolapse

Pacemakers

  • Pacemaker System

    • SA node → initiates impulse (pacemaker)

    • AV node → delays signal (important for filling)

    • Bundle of His → Purkinje fibers → ventricles

  • Pacemaker Cell Physiology

    • High Na+ permeability → spontaneous depolarization

    • Ca2+ influx → action potential

    • K+ efflux → repolarization

  • Automatic Regulation

    • Epinephrine → inc of Na+ and Ca2+ → inc of HR

    • Acetylcholine → inc of K+ → hyperpolarization → dec of HR

Pacemaker Potential

  • Pacemaker Potential

    • Pacemaker cells:

      • Mainly located in SA node

      • Higher sodium concentration, lower potassium

      • Negatively charged by bringing in Na+ and leaking out K+

    • Adrenaline:

      • binds to beta-receptors on peacemaker cells 

      • increased Na+ and Ca+ ions intake

      • membrane potential increases much faster

      • heart rate increases

    • Acetylcholine:

      • binds to alpha-receptors on peacemaker cells

      • increases potassium output 

      • results in hyper-polarization

      • slower depolarization

      • heart rate decreases

Action Potentials and Contraction

  • Cardiac cells are ALL electrically connected (pacemaker cells and cardiomyocytes)

  • Examples:

    • Pacemaker cells: if one SA node cell (pacemaker cell) starts a signal, signal spreads

    • Cardiomyocytes: if one cell in ventricle is stimulated, signal spreads via gap junctions 

  • Conduction velocity (speed): 1 meter/sec from SA to AV node; slows down at AV node to 0.04m/s (slows down to let the valves close, ventricles fill with blood); and increases again at Purkinje fibers to 5m/s

  • Contraction process:

    • Blood first goes to atria → atria contracts

    • Want ventricles to get filled with blood from atria FIRST, then contract and send to the rest of the body (slowing down at AV node is important)

  • Key differences from Neurons

    • Cardiac AP lasts ~300 ms

    • Plateau phase due to Ca2+ influx

  • Why Plateau Matters

    • Prevents tetany

    • Ensures coordinated contraction

  • Excitation-Contraction Coupling

    • AP → Ca2+ entry → Ca2+ release from SR

    • Ca2+ → contraction

    • Ca2+ reuptake → relaxation

Cardiac Muscle Cell (Cardiomyocyte)

  • We will not see a resting potential for long in cardiac muscle cell

  • Blue: Depolarization; Voltage-gated Na+ channels open, resulting in opening of more of the same channels 

    • Not contracting yet until it becomes positive, just change in electrical charge 

    • Slightest impulse/charge from neighboring cells will take cardiac muscle cells from negative to positive charge (-70 to 0) → Na+ channels open

  • Orange: Calcium released from SR; prevents re-polarization; slows down process (plateau) 

    • Ca2+ stays in cytosol longer, keeping charge positive for longer; prolonging depolarization

    • Cardiac muscle cells want to stay positive longer because they want to stay contracted for a longer period of time to pump out all of the blood 

    • Plateau falls slightly because some K+ leakage, but most K+ channels are closed until end of plateau

  • Red: Repolarization of cardiac muscle cells; Ca2+ pumped back into SR; K+ channels open and rapid K+ outflow returns membrane to resting potential

ECG (electrocardiogram)

  • Records the electrical activity of the heart; tests for irregularities in heart function

  • Skin electrodes are placed on different parts of the body to detect electrical activity of the heart → heart depolarizes the entire body, so you can pick up the charge from even your hands and feet

ECG Wave (PQRST)

  • P wave: atrial depolarization (atria contract) (signal from SA and AV node); blood pushed from atria into ventricles

  • PR interval: the time from the start of atria contracting to the start of ventricles contracting → atria contraction and signal traveling through AV node (so technically a pause before ventricles contract)

  • PR segment: electrical signal is passing through the AV node, giving the ventricles time to fill with blood (delay at the AV node) 

  • QRS complex: ventricular depolarization (ventricles contract - pumping blood to lungs and the body); also atrial repolarization, but not shown in the wave

  • ST segment: time between ventricle depolarization and ventricular repolarization → ventricles are still contracted but are starting to recover (look at this segment for signs of a heart attack)

  • T wave: ventricular repolarization (ventricles rest) → ventricles are recovering and getting ready for the next beat

QT interval: covers the whole period when the ventricles are contracting (depolarizing) and then recovering (repolarizing)