the heart

The blood, heart vessels and blood vessels make up the circulatory system.

Within the circulatory system, the heart and blood vessels make up the cardiovascular system.

 

Heart

  • Made up of a base, an apex and a diaphragm

    • Apex is located to the left

    • Base is located posteriorly

Fibrous skeleton:

  • Functions for electrical insulation & structural support

  • Made up of rings of connective tissue that surrounds valves

    • Separates atria & ventricles from each other

    • Left ventricle takes up most of the apex

Pericardium:

  • Double layered connective tissue sac

    • Holds heart in place

Layers:

  • Fibrous pericardium (superficial)

    • Anchors heart to anterior sternum

  • Serous pericardium

    • Parietal layer

      • Directly anchored to fibrous pericardium

      • Reflects inwards & continues as visceral layer

    • Visceral layer

      • Directly applied to surface of heart

    • Both layers are continuous with each other

Pericardial cavity:

  • Potential space between fibrous & serous layers

    • Not expected to be hollow

  • Pericardial fluid

    • Lubricates & cushions heart

Superficial layers: (superficial to deep)

Epicardium:

  • Also called the visceral pericardium

  • Directly adhered to external surface of heart

Myocardium:

  • Cardiac muscle

    • Thickest layer

Endocardium:

  • Endothelial lining within interior chambers of heart

  • Covers valves & continuous with blood vessels emerging from heart

Chambers:

L & R Atria

  • Most superior chambers

  • Receives blood from great veins

  • Separated by interatrial septum (wall of tissue)

L & R Ventricles:

  • Most inferior chambers

  • Receives blood from atria & sends to other regions of body through great arteries

  • Separated by interventricular septum

  • Left ventricle is thicker than right ventricle

    • R side is pulmonary (sends to lungs)

    • L side is systemic (sends to rest of body)

      • As a result of increased pressure on left side of heart

      • Intraventricular septum tends to bulge into right side of heart

  • Coronary & interventricular sulcus covered in fat & blood vessels

    • Where interatrial & interventricular septums are located

L & R Auricles:

  • Increases internal volume of L & R atria when blood is taken in

Valves:

  • Ensures one way blood flow

    • Opens in one directions

    • Snaps shut against backflow

  • 4 valves in heart"

    • Atrioventricular Valves:

      • Proper function requires papillary muscles & chordae tendineae

        • Papillary muscles

          • upwards projections from floor of ventricles

        • Chordae tendineae

          • collagen fibers that attach papillary muscles to AV valves

      • During contraction:

        • Papillary muscles contract & places downwards tension on chordae tendineae

          • Holds atrioventricular valves shut during contraction & increased pressure in ventricles

        • This prevents backflow of blood

      • Tricuspid

        • Between R atrium & R ventricle

        • 3 cusps

      • Mitral

        • Between L atrium & L ventricle

        • 2 cusps

    • Semilunar Valves:

      • Regulates blood flow from ventricles to great arteries

      • Catches backflow to prevent leaking back into ventricles

      • Pulmonary

        • 3 cusps

      • Aortic

        • 3 cusps

Flow of Blood:

  • Both circulations occur simultaneously

Pulmonary circuit:

  • On right side of heart

    • Receives blood from inferior & superior venae cavae to right atrium

      • Tricuspid valve to R ventricle

    • Oxygen-poor blood is transported to lungs via pulmonary trunk & arteries

      • Pulmonary valve

Systemic circuit:

  • On left side of heart

    • Receives oxygenated blood from pulmonary veins to L atrium

      • Mitral valve to L ventricle

    • Oxygenated blood sent to all tissues of body via aorta

      • Aortic valve

Coronary arteries:

  • Receives high oxygenated blood from backflow

  • Heart has high energetic needs

    • Receives ~5% of circulating blood (requires its own circulation)

  • R & L coronary arteries are primary arteries relied on

    • Branches off of base of aorta (superior to L ventricle)

Left coronary artery:

  • Travels through coronary sulcus

  • Anterior interventricular branch

    • Supplies anterior ventricles & 2/3 of interventricular septum

  • Circumflex branch

    • Supplies left atrium & posterior wall of left ventricle

Right coronary artery:

  • Supplies right atrium & sinoatrial node

  • Right marginal branch:

    • Supplies lateral portions of right atrium & ventricle

  • Posterior interventricular branch:

    • Supplies posterior walls of ventricles & 1/3 of interventricular septum

 

Cardiomyocytes:

  • Very high energetic need (many mitochondria)

    • Relies on aerobic activity

    • Gives capacity to work nearly continuously for many decades without fatigue

    • Cardiac muscle is very vulnerable to oxygen deficiency

  • Connected by intercalated discs

    • Cell adhesion molecules prevents contracting cells from pulling apart

    • Gap junctions allows for electrical impulses to travel between cells

 

Heart conduction

Conduction system:

  1. Sinoatrial node (modified cardiomyocytes)

    • Close to base of superior vena cava

    • Sets timing of electrical activity & heart rate

  2. Atrioventricular node (modified cardiomyocytes)

    • Within R-IA septum

    • Transmits electrical stimulus from SA node to ventricles

  3. Atrioventricular bundle

    • Penetrates fiber skeleton of heart

    • The connection from atrioventricular node to interventricular septum

  4. R & L bundle branches

    • Transmits signal DOWN interventricular septum

  5. Subendocardial conducting network

    • At apex of heart

    • Turns superiorly & travels through L ventricle walls

 

Pacemaker cells:

  • Found in sinoatrial node & atrioventricular node

  • Can depolarize themselves rather than rely on external stimulus

  • Are NONCONTRACTILE

    • Modified cardiomyocytes that EMIT an ELECTRICAL STIMULUS when depolarized

  • Gradually takes in positively charged ions and internal cell charge increases

    • Called "pacemaker potential"

      • Auto-depolarizes & emits electrical discharge

        • Becomes electrical stimulus that leads to other cells' depolarization

      • Leads to HEARTBEAT

      • Resting heartbeat is generally 75bpm

        • (Depolarizes every ~0.8 seconds)

  • Hits certain voltage threshold

    • Voltage gated calcium ion channels open & calcium rushes into cell

      • Rapid influx depolarizes cell

  • PLATEAU:

    • Muscle cell has depolarized & contracted and takes in more calcium ions

      • Delays repolarization

      • Extends period of depolarization

      • Extends length of muscle contraction as a result

      • Facilitates ejection of blood from heart & allows it to more efficiently move blood from place to place

PLATEAU PHASE

Impulse Conduction to Myocardium:

  1. Sinoatrial node spontaneously depolarizes

  2. Spreads across atria & causes contraction (at same time)

  3. Signal reaches atrioventricular node in base of interventricular septum

    • Delayed transmission allows atria to fully complete contraction

    • Atrioventricular bundle allows for passage through fiber skeleton

  4. Depolarization continues down interventricular septum

  5. Depolarization occurs & causes contraction of ventricles

    • Depolarization begins at apex & spreads superiorly

      • Superior movement helps ejection of blood & through semilunar valves

    • Contraction occurs at same time

Cardiac innervation:

  • Autorhythmic

    • Does NOT require any external stimulus

  • Autonomic nervous system is able to modify heartbeat

    • Can only adjust activity

    • Originates in cardiac centers of medulla oblongata

Sympathetic modification:

  • Originates in cardioacceleratory center in medulla oblongata

  • Sympathetic fibers travel down spinal cord

    • Cardiac nerves innervate sinoatrial & atriovenventricular nodes, cardiac muscle & great / coronary arteries

  • Increases heartrate & contractile strength and can induce vasoconstriction

Parasympathetic modification:

  • From cardioinhibitory center of medulla oblongata

  • Vagus nerve innervates sinoatrial & atrioventricular nodes

  • Reduces heart rate

    • Decreases rate of depolarization of pacemaker cells

Cardiac rhythm:

  • Single atrial & ventricle contraction is a HEARTBEAT

    • Contraction = systole

    • Relaxed = diastole

  • Sinus rhythm:

    • Set by sinoatrial node

    • 70-80 bpm

  • Junctional rhythm:

    • Set by atrioventricular node

    • 40-60 bpm

    • Any slower wouldn't be sufficient enough to keep us alive

      • Blood is circulated through body too slowly

Fibrillation:

  • Rapid & irregular contractions

  • Atrial fibrillation:

    • Heart efficiency is GREATLY REDUCED

  • Ventricular fibrillation:

    • Heart function COMPLETELY STOPS

  • Defibrillation shocks heart with electricity to depolarize ENTIRE myocardium

  • Atrial repolarization happens DURING QRS segment (ventricle depolarization)

Commotio cordis:

  • Blunt force impact during ventricular repolarization

    • 15-30 ms BEFORE peak of T-wave

  • Sends heart into V-FIB

  • Most often associated with projectile sports

  • Most commonly seen in males age 10-18

  • Requires IMMEDIATE medical intervention

 

Cardiac Cycle & Output

Fluid flow:

  • Requires pressure gradient

    • High to low

    • Until it's equalized

  • Volume & pressure increase together

  • Blood flows OUT of heart when ventricular pressure is GREATER than great vessel pressure

Pressure & flow:

  • Ventricular diastole DECREASES pressure

    • Atrioventricular valves open

      • Blood flows FREELY

      • Semilunar valves PREVENT backflow

  • Filling of ventricles pushes AV valve CLOSED

  • Ventricular systole INCREASES internal pressure

    • Ventricular pressure is GREATER than arterial pressure

    • semilunar valves opening

Cardiac cycle:

  • A complete contraction & relaxation of ALL CHAMBERS of heart

  • TOTAL DURATION of cardiac cycle:

    • 0.8s

    • Leads to average heart rate of 75 bpm

Quiescent period: (around 0.4s)

  • All four chambers are RELAXED at the same time

Phases:

  1. Ventricular filling

    • Rapid ventricular filling

      • Occurs DURING quiescent period

      • ~70% of filling occurs PASSIVELY during atrial diastole

    • Diastasis

      • Atrial pressure is the SAME as ventricular pressure

    • Atrial systole (around 0.1s)

      • Creates pressure gradient for continues flow

  2. Isovolumetric contraction (ventricular systole is around 0.3s)

    • Ventricular pressure is GREATER than atrial pressure

      • Leads to closure of AV valves

    • Ventricular pressure is LESS THAN arterial pressure

      • Semilunar valves remain CLOSED

    • NO BLOOD is ejected

  3. Ventricular ejection

    • Ventricular pressure is GREATER than arterial pressure

    • Stroke volume:

      • The total amount of ejected blood

      • Preload:

        • Tension in ventricular muscle fibers IMMEDIATELY BEFORE contraction

        • Main influence occurs in VENOUS RETURN

          • Increased blood volume

          • Increased ventricular stretch

          • Increased contractile force

          • Increased SV

      • Contractility:

        • Affects how hard myocardium contracts for a given preload

        • Higher contractility ejects MORE BLOOD from the heart

          • Sympathetic input

          • Electrolyte imbalances

          • Certain drugs & hormones

      • AFTERLOAD:

        • Forces ventricles MUST OVERCOME to eject blood

          • SV decreases AS afterload increases

          • Main influence is on arterial blood pressure

        • Usually CONSTANT

        • Blockage of arterial circulation CAN INCREASE afterload

    • Ejection fraction:

      • % of blood ejected

      • Measure of cardiac health

      • NEVER 100%

    • END OF VENTRICULAR SYSTOLE

  4. Isovolumetric relaxation

    • Ventricular pressure is LESS THAN arterial pressure

      • Semilunar valves CLOSE

    • Ventricular pressure is GREATER THAN atrial pressure

      • AV valves REMAIN closed

    • NO BLOOD FLOW INTO VENTRICLES

Heart sounds:

Auscultation:

  • Listening to sounds made by body

S1

  • "lub"

  • AV valves close as ventricular pressure is GREATER THAN atrial pressure

S2

  • "dub"

  • Semilunar valves CLOSE at beginning of VENTRICULAR DIASTOLE

S3

  • Sometimes

  • The filling of ventricles

  • Indicates VENTRICULAR ABNORMALITY in adults 40+

Cardiac output:

  • The amount of blood pumped by each ventricle in 1 minute

  • Cardiac Output = heart rate * stroke volume

  • Typical values:

    • 75 bpm * 70mL/beat = ~5L per minutes

  • Any changes to heart rate or stroke volume changes cardiac output

  • Cardiac reserve:

    • The difference between MAX & RESTING cardiac output

      • Indicates the heart's capacity for increased work

      • Can differ over a person's lifespan

Autonomic modulation of HEART RATE:

  • Sympathetic stimulation INCREASES heart rate

    • Faster sinoatrial node depolarization

  • Parasympathetic stimulates REDUCES heart rate

    • Hyperpolarizes cardiomyocytes

    • Vagal tone:

      • Consistent parasympathetic input to SA node

        • Suppresses intrinsic depolarization rate

    • Cardiac centers:

      • Communicates with..

      • Proprioceptors:

        • Changes to physical activity

      • Baroreceptors:

        • Blood pressure

      • Chemoreceptors:

        • Blood pH * CO2 & O2 levels

      • Higher brain centers:

        • Sensory & emotional stimuli