Lecture 2

The Heart Valves

The Four Valves of the Heart

  • There are 4 heart valves:
    • Atrioventricular (AV) valves:
      • Found between the atria and the ventricles on both the left and right sides of the heart.
      • AV valve located between the left atrium and left ventricle is the bicuspid or mitral valve.
      • AV valve located between the right atrium and the right ventricle is the tricuspid valve.
    • Semilunar (arterial) valves:
      • Found between the ventricles and the arteries into which the ventricles pump their blood.
      • Valve between the left ventricle and the aorta is the aortic valve.
      • Valve between the right ventricle and the pulmonary trunk is the pulmonary valve.
  • Valves:
    • Made of fibrous tissue (collagen) covered by endothelium.
    • The valve flaps are also called leaflets or cusps.
  • Valve rings:
    • Made of cartilage.
    • These are what the valves attach to.

The Valves and the Path of Blood Flow

  • Shows the position of the heart valves in the circulatory circuit.

How do the Valves Function?

  • Function of the heart valves:
    • Ensure unidirectional flow of blood through the heart.
    • Important so that blood flowing out of heart does not mix with new blood coming into the heart.
  • The valves open and close passively due to differences in pressure or pressure gradients.
    • Energy is not expended to open or close a valve.
    • Valves do not require muscles to open or close them.
    • A forward pressure gradient opens a one way valve; a backwards pressure gradient closes a one-way valve.
    • Valves normally do not open in the opposite direction.

Atrioventricular (AV) Valves

  • Found between the atrium and the ventricle
  • Prevent the backflow of blood into the atrium when the ventricle contracts.
  • When the pressure in the atrium exceeds the pressure in the ventricle, the atrioventricular valve will open, allowing blood to flow from the atrium into the ventricle so that the ventricle will fill with blood.
  • When the ventricle contracts and achieves a pressure greater than the pressure in the atrium, the valve will shut, preventing the backflow of blood from the ventricle into the atrium.
    • Tricuspid valve: AV valve located between the right atrium and the right ventricle; consists of three cusps or leaflets attached at the circumference to the valve rings.
    • Bicuspid or mitral valve: AV valve located between the left atrium and the left ventricle; consists of two cusps or leaflets attached at the circumference to the valve rings.
  • Each AV valve is part of an AV valve apparatus, which consists of the cusps or leaflets of the valve, chordae tendineae and papillary muscles.

Anatomy of the AV Valve Apparatus

  • Atrioventricular valve apparatus:
    • The edges of the AV valve leaflets are attached to tough, thin fibrous cords of tendinous-type tissue called chordae tendineae.
    • The chordae tendineae extend from the edges of the leaflets and attach to papillary muscles.
    • Papillary muscles: cone shaped muscles that protrude from the inner surface of the ventricular walls.
  • Papillary muscles do contract, and when they contract they pull on the chordae tendineae to become tight (taut).
  • This holds the valve in its closed position

Function of the AV Valve Apparatus

  • Heart valves open and close passively due to pressure gradients; there are no muscles involved in opening and closing the valve.
  • When the left ventricle is relaxed:
    • AV or bicuspid valve is open and the semilunar or aortic valve is closed.
    • Papillary muscles are also relaxed and chordae tendineae are slack or have low tension.
    • When the bicuspid valve is open, blood can flow from the left atrium into the left ventricle; the ventricle fills with blood as the aortic valve is closed; blood enters the ventricle but cannot leave.
  • When left ventricle has begun to contract:
    • The ventricle will squeeze its volume of blood as it contracts, increasing pressure inside the ventricle.
    • As the pressure in the ventricle rises above the pressure in the atrium, blood is pushed back towards the bicuspid valve; but the increased pressure causes the bicuspid valve to close as there is a greater pressure in front of this valve.
    • Closing of the bicuspid valve prevents the backflow of blood into the atrium as the ventricle is continuing to contract.
    • The papillary muscles also contract when the ventricle contracts.
      • This pulls the chordae tendineae downward or taut; chordae tendineae have tension.
      • Pulling of the chordae tendineae by the papillary muscles keeps the AV valve in a closed position in the face of a strong backward pressure gradient from the ventricle contracting.
      • The AV valve apparatus keeps the AV valves from everting, or opening backwards, into the atrium.
        • If the AV valves did evert, blood would flow the wrong way from the ventricle to the atrium.
  • Important: contraction of the papillary muscles does not open or close the valves; the valves open and close passively due to pressure differences across the valves.
  • As the pressure in the ventricle continues to increase as the ventricle continues to contract, the pressure in the ventricle will eventually exceed the pressure in the aorta, opening the aortic valve, allowing blood to flow out of the ventricle; this is a forward pressure gradient.

Arterial (Semilunar) Valves

  • Semilunar or arterial valves:
    • Found between the ventricle and the artery into which the ventricle ejects its blood.
    • 3 leaflets or cusps (left and right semilunar valves).
      • Pulmonary valve: valve found between the right ventricle and the pulmonary trunk.
      • Aortic valve: valve found between the left ventricle and the aorta.
    • Do not have chordae tendineae or papillary muscles (no valve apparatus associated with these valves).
  • The pressure pushing back against the valve from the artery is not high enough to force the valve to evert or open backwards into the ventricle, as the artery does not contract.
  • Open when the pressure in the ventricle is greater than that in the artery into which the ventricle ejects its blood.
  • When the ventricle begins to relax, the pressure in the ventricle will decrease; when the pressure in the ventricle falls below the pressure in the artery, the semilunar valve will close, preventing the backflow of blood from the artery into the ventricle.
  • Semilunar valves open/close due to pressure differences across the valve; there are no muscles or energy expended to open/close these valves.

Pulmonary Valve

  • (This is an image of a valve)

Cardiac Skeleton

  • The fibrous skeleton of the heart:
    • Made of dense connective tissue.
    • Includes the heart valve rings and the dense connective tissue between the heart valves.
    • Functions:
      • Physically separates the atria from the ventricles.
      • Electrically inactive and blocks the direct spread of electrical impulses from the atria to the ventricles.
      • Provides support for the heart, providing a point of attachment for the valves leaflets and cardiac muscle.

Coronary Circulation

  • The heart, like other organs, receives its blood supply through arteries that branch from the aorta.
  • Coronary circulation is part of the systemic circulatory system and supplies blood to and provides drainage from the tissues of the heart.
    • Coronary arteries: arteries supplying the heart.
    • Aortic sinus is a dilation or out-pocketing of the ascending aorta; site where the left and right coronary arteries.
    • Cardiac veins: collect poorly oxygenated blood and empty into the coronary sinus, which returns blood to the right atrium.

Coronary Sinus

  • A collection of veins joined together to form a large vessel that collects blood from the myocardium of the heart and empties into the right atrium, returning the poorly oxygenated blood back to the right side of the heart.

Systole

  • Represents the time during which the left and right ventricles contract and eject blood into their respective artery.

Diastole

  • Represents the period of time when the ventricles are not contracting; relaxed.
  • Myocardial blood flow is not steady:
    • Blood flow almost ceases while the heart is contracted (systole) and peaks while the heart is relaxed (diastole).

Coronary Artery Disease

  • Caused by atherosclerosis of the coronary arteries supplying blood to the heart tissues.
  • Atherosclerosis is a condition in which the arteries become hardened and narrowed because of an excessive accumulation of plaque in the vessel wall.
    • Atherosclerotic plaque: made of fat, cholesterol, calcium and other substances in the blood.
    • When plaque builds up, the diameter of that artery is narrowed, providing resistance to blood flow, reducing flow through the arteries supplying the heart tissue.

Angina

  • Chest pain; when a plaque is present in a coronary artery, restricted blood flow to the heart muscle may result in chest pain.

Myocardial infarction:

  • Heart attack; atherosclerotic plaques can grow so large that they completely block arterial blood flow, causing a heart attack; heart muscle dies due to loss of blood supply.

The Cardiac Syncytium

  • Cardiac muscle cells = myocytes
    • Joined by intercalated discs which contain gap junctions and desmosomes
    • Mechanically, chemically, and electrically connect myocytes to one another
  • The entire heart tissue resembles a single, enormous muscle cell and the cardiac muscle is called a syncytium (a set of cells that act together)
  • Gap junctions allow excitation, or action potentials, to spread quickly from one myocyte to another by cell-to-cell contact
    • Cardiac muscle cells are so tightly connected that when one of the myocytes becomes excited, the action potential spreads to all of them through gap junctions
  • The heart is a functional syncytium:
    • Cardiac muscle has 2 syncytia: the left and right atria act as one functional syncytia and the left and right ventricles also act as another functional syncytia
    • This gives the heart an all or none property- either all of the myocytes respond and are excited or none of the myocytes respond

Cardiac Muscle

  • The heart contracts in series: first the left and right atria contract together and then the left and right ventricles contract together
  • Action potentials lead to contraction of heart muscle cells and ejection of blood
  • Autorhythmicity (automaticity): the heart contracts or beats rhythmically as a result of action potentials that it generates itself
    • Action potentials in the heart are generated without nervous or hormonal stimulation
    • The rhythmicity of the heart is myogenic in origin, which means muscular in origin
  • 2 types of specialized cardiac muscle cells or myocytes: contractile cells and conducting cells
Contractile cells:
  • Perform the mechanical work of pumping or contracting to propel blood forward; generate pressure to move blood
  • ~ 99 % of myocytes are contractile cells
  • Do not normally initiate their own action potentials, but contract when stimulated by an action potential passed to them through gap junctions from an adjacent contractile cell that has been stimulated by an action potential or an adjacent conducting cell.
Conducting cells:
  • Autorhythmic cells which initiate and conduct action potentials which are responsible for contraction of the contractile cells
  • Conducting cells are myocytes (muscle cells) which initiate and conduct action potentials without nervous or hormonal stimuli
  • Have very few myofibrils (protein filaments needed for contraction) and do not contribute to the heart’s contraction and the movement of blood.
  • ~ 1 % of myocytes are conducting cells
  • Part of the conducting system of the heart
  • Are in electrical contact with each other and the cardiac contractile cells through the gap junctions

Membrane Potential

  • Membrane potential: a separation of positive and negative charges across a membrane or a difference in the electrical potential across a cell’s membrane (mV)
  • Cells at rest maintain a non-zero transmembrane potential, with a negative voltage in the cell interior as compared to the cell exterior
  • Membrane potential: influences the movement of ions across the membrane
  • Action potential: when the resting membrane potential is transiently reversed from negative values inside the cell to values more positive than outside,
  • In muscle cells, action potentials are the electrical events that cause the mechanical event of contraction
  • The action potential is brought about by changes in the membrane permeability to certain ions

The Action Potential

  • (This is just a review of the nerve/muscle action potential)

The Heart’s Conducting System

  • The spread of action potentials through the myocardium leads to the contraction of the heart muscle cells
  • The heart contracts in series: first both atria depolarize and contract as a unit before both ventricles depolarize and contract as a unit
  • The conducting system contains myocytes, or cardiac muscle cells, that are capable of initiating and propagating action potentials
    • Autorhythmic myocytes
    • The conducting myocytes are found in: sinoatrial node (SAN), internodal pathways, atrioventricular node (AVN), the bundle of His (AV bundle), the left and right bundle branches, Purkinje fibers
      • SAN located in the wall of the right atrium; AVN located at the base of the right atrium; internodal pathways extend from the SAN to the AVN and also cross the interatrial septum to the left atrium; bundle of His passes through the cardiac skeleton; left and right bundle branches travel along the interventricular septum; left and right bundle branches make contact with Purkinje fibers, which extend into the myocardium of the ventricles

Cardiac Skeleton

  • Non-conducting or it will not allow action potentials to travel across it
  • Physically separates the atria from the ventricles: stimuli cannot cross from the atria to the ventricles through the cardiac skeleton
  • The only electrical connection between the atria and ventricles in a normal heart is the AVN and the Bundle of His

Sinoatrial (SA) Node

  • All the cells in the conducting system are capable of initiating action potentials
    • The rate at which each region action potentials differs
  • Conducting myocytes in the SAN generate action potentials at the fastest rate; 60 to 100 action potentials per minute
  • This stimulus is then passed on to the other regions of the conducting system through gap junctions, generating action potentials in these other regions before they have time to initiate their own action potentials
  • SAN generates action potentials that drive the rest the conducting system.
    • Cardiac pacemaker: initiates action potentials that set the heart rate
    • SAN generates action potentials → internodal pathways → contractile cells of both the left and the right atria → left and right atria contract at same time → stimulus is also passed by the internodal pathways to AVN → the wave of depolarization must passthrough the AVN and the Bundle of His to excite the ventricles due to presence of cardiac skeleton
  • Cells of conducting system are muscle cells

Atrioventricular (AV) Node

  • Stimulus passes to the AVN through the internodal pathways from the SAN
  • AV nodal delay: the propagation of action potentials through the AVN is relatively slow: takes ~ 100 milliseconds for the stimulus to pass through the AVN to the Bundle of His
    • This delay ensures that the atria depolarize and contract before the ventricles depolarize and contract
  • The ventricular myocardium must be relaxed to fill with blood from the atria
    • Ensures ventricles are relaxed and have time to fill with blood before they contract

Excitation of the Ventricles

  • AVN and Bundle of His are the only electrical connection between the atria and ventricles in a normal heart
  • Left and right bundle branches travel along intraventricular septum and make contact with Purkinje fibers

Purkinje fibers

  • Large number, diffuse distribution (ie. all over the ventricles), fast conduction velocity
  • Stimulus depolarized left and right ventricular myocytes and causes contraction nearly simultaneously due to Purkinje fibers

Sequence of Excitation

  • Shows the spread of excitation from the sinoatrial node to the atrioventricular node and the left and right atria
Cells are connected by gap junctions, which offer a low resistance pathway for excitation to spread from cell to cell
  • Ions move from cell to cell through gap junctions

Summary: Conducting System of the Heart

Demonstrates how the wave of depolarization spreads across the heart
  • Myocytes in the SAN initiate the action potential; action potential passes through the conducting myocytes in the internodal pathways to the contractile myocytes of both the left and right atria causing depolarization and contraction of the atrial myocardial cells; action potential also passes through the conducting myocytes in the internodal pathways to the AVN; it takes 100 msec for the stimulus to pass through the AVN; stimulus passes to the conducting cells of the Bundle of His which divides into the left and right bundle branches; wave of depolarization spreads down the interventricular septum; the bundle branches separate at the apex of the heart and enter the ventricles and make contact with the Purkinje fibers; Purkinje fibers spread the depolarizing stimulus to the ventricles
  • The chambers of the heart contract in series: first both atria depolarize and contract, and then both ventricles depolarize and contract, leading to the movement of blood through the heart

Wolff-Parkinson-White Syndrome

  • There is an extra connection in the heart called an accessory pathway
    • An accessory pathway is an abnormal piece of muscle that connects directly between the atria and the ventricles
    • Allows electrical signals to bypass the AVN and move from the atria to the ventricles faster than usual
    • Electrical impulses may also be transmitted abnormally from the ventricles back to the atria
    • Disrupts the coordinated movement of electrical signals through the heart, leading to an abnormally fast heartbeat, called tachycardia, and other arrhythmias