AH

Cardiovascular System Notes

Cardiovascular System

Introduction

  • "Cardio" refers to the heart.
  • "Vascular" refers to vessels.
  • The lectures will cover the heart and its vessels.

Location and Size of Heart

  • The heart is about the size of a closed fist.
  • Weighs approximately 250g in adult women and 300g in adult men.
  • Rests on the diaphragm near the midline of the thorax within the mediastinum.
  • The mediastinum includes tissues like the heart, trachea, and esophagus, lying between the sternum and thoracic vertebrae.
  • The base of the heart is more superior and to the right.
  • The apex is more inferior and to the left.
  • Approximately 2/3 of the heart's mass lies to the left of the midline.

Pericardium

  • The pericardium surrounds and protects the heart.
  • Consists of two layers:
    • Fibrous Pericardium:
      • Superficial layer.
      • Composed of tough, inelastic tissue.
    • Serous Pericardium:
      • Thinner, more delicate membrane.
      • Double layer (Parietal and Visceral).
      • Pericardial cavity exists between the layers.
      • Pericardial fluid fills the pericardial cavity, reducing friction between the layers.
  • Analogy: The serous pericardium is like a flexible balloon filled with pericardial fluid, with the heart displacing into the balloon to create a double layer.

Layers of the Heart Wall

  • Three layers:
    • Epicardium:
      • Outermost layer; also the visceral layer of the serous pericardium.
      • Made of delicate connective tissue, giving it a smooth, slippery consistency.
    • Myocardium:
      • Middle layer.
      • Consists of cardiac muscle tissue (thickest layer).
    • Endocardium:
      • Innermost layer.
      • Thin layer of endothelium overlying connective tissue.

Chambers of the Heart

  • Four chambers:
    • Two superior chambers: Atria (Right & Left).
    • Two inferior chambers: Ventricles (Right & Left).

Right Atrium

  • Receives blood from:
    • Superior Vena Cava (from head and shoulders).
    • Inferior Vena Cava (from lower limbs).
    • Coronary Sinus (from the heart).
  • Blood passes from the right atrium to the right ventricle through the Tricuspid valve (Right Atrioventricular Valve).
  • The myocardial layer is thin due to the low pressures and small distance the right atrium has to pump.

Right Ventricle

  • Forms most of the Anterior Surface of the heart.
  • The cusps of the tricuspid valve are connected to papillary muscles via tendons called Chordae Tendinae.
  • Blood exits the right ventricle through the pulmonary valve into the pulmonary trunk, a large artery going to the lungs.
  • The myocardial layer is thicker due to larger pressures that it must overcome.

Left Atrium

  • Forms most of the base of the heart.
  • Receives oxygenated blood from the lungs through the pulmonary veins.
  • Blood passes from the left atrium into the left ventricle through the Bicuspid valve (Mitral valve or Left Atrioventricular Valve).
  • Fairly thin myocardial layer.

Left Ventricle

  • Contains Chordae Tendinae and papillary muscles, which anchor the cusps of the bicuspid valve.
  • Blood leaves the Left Ventricle through the aortic valve into the ascending aorta, which then goes to the rest of the body.
  • Much larger and thicker myocardial layer due to the greatest distances of blood pumped and pressures overcome.

Heart Valves

  • Pulmonary and Aortic valves are called "Semi Lunar Valves" because they resemble partial moons.

Systemic and Pulmonary Circulation

  • Two closed circuits where the output of one becomes the input for the other.

Systemic Circulation

  • The left side of the heart is the pump.
  • Blood leaves the left ventricle, enters the aorta, and is pumped to all the tissues of the body (except the lungs).
  • Returns to the Right atrium via the veins.
  • Sequence:
    • Left Atria
    • Left Ventricle
    • Aorta
    • Arteries
    • Arterioles
    • Systemic capillaries (gas and nutrient exchange).
    • Systemic venules
    • Systemic veins
    • Right Atrium

Pulmonary Circulation

  • The right side of the heart is the pump.
  • Blood leaves the right ventricle:
    • Enters the pulmonary trunk.
    • Then pulmonary arteries (deoxygenated blood).
    • Then to the lungs.
    • Returns to the left atrium via pulmonary veins (oxygenated blood).
  • Sequence:
    • Right Atrium
    • Right Ventricle
    • Pulmonary trunk
    • Pulmonary arteries
    • Pulmonary capillaries (in the lungs, where CO2 is exhaled and O2 is inhaled).
    • Pulmonary veins
    • Left Atrium

The Conduction System

  • The heart has a network of specialized cardiac muscle fibers called autorhythmic fibers.
    • They are self-excitable (don’t need external excitation).
    • Act as a pacemaker, setting the rate for the whole heart to contract.
    • Form the conduction system; a path for each cycle of cardiac excitation to progress in a coordinated manner through the heart, making it an effective pump.

The 5 Step Sequence of Cardiac Action Potential Propagation

  • Cardiac excitation occurs at the Sinoatrial node (SA) Node in the Right atrial wall.
    • Action potentials propagate through both atria, and they both contract.
  • The Action Potentials from the SA node reach the AV node located between the 2 atria.
  • From the AV node, the Action Potential reaches the Atrioventricular Bundle (Bundle of HIS), which propagates to the ventricles.
  • From the AV Bundle, the Action Potential enters both the Right and Left Bundle branches and continues toward the apex of the heart.
  • Finally, the Purkinje Fibers conduct the Action Potential from the Apex of the heart to the remainder of the ventricles, and the ventricles Contract!

Generation of Action Potentials

  • An action potential in skeletal muscle or nerve is a sequence of rapidly occurring events in two phases:
    • Depolarization Phase
    • Repolarization Phase

Depolarization Phase

  • The negative resting membrane potential becomes less negative, then positive.
  • This occurs because Na^+ channels open, allowing Na^+ to rush into the cell (down both electrical and chemical gradients).

Repolarization Phase

  • The membrane potential is restored to its resting state.
  • This occurs because the K^+ channels open, allowing potassium to flow out, which re-establishes the resting membrane potential.

Summary of Action Potential Sequence

  • Resting state: All voltage-gated Na^+ and K^+ channels are closed.
  • Depolarizing phase: Depolarization to threshold opens Na^+ channel activation gates; Na^+ inflow further depolarizes the membrane, opening more Na^+ channel activation gates.
  • Repolarizing phase: Na^+ channel inactivation gates close and K^+ channels open. Outflow of K^+ causes repolarization.
  • Repolarization continues: K^+ outflow restores resting membrane potential; Na^+ channel inactivation gates open. Return to resting state when K^+ gates close.

The Cardiac Action Potential

  • Initiated by the SA node, travelling along the Conduction System to allow the contractile fibers in the Atria and Ventricles to contract.
  • Three different phases of Action Potential propagation in the contractile fibers:
    • Depolarization
    • Plateau
    • Repolarization

Depolarization

  • Resting membrane potential is -90mv.
  • When the threshold is reached by an action potential, very rapid depolarization occurs.
  • Na^+ channels open, and a rapid Na^+ influx brings the membrane potential to +20mv.

Plateau

  • The maintained depolarization.
  • Ca^{2+} channels open and Ca^{2+} moves inside the cell (from a higher concentration in the interstitial fluid to a lower concentration in the cytosol).
  • The elevated Ca^{2+} concentration in the cytosol triggers contraction of the cardiac muscles.
  • K^+ outflow balances the Ca^{2+} inflow, which allows for sustained depolarization.

Repolarization

  • Ca^{2+} channels close, and more K^+ channels open, allowing the K^+ outflow to restore the negative resting membrane potential back to -90mv.

Refractory Period

  • The time when a second contraction cannot be triggered.
  • The refractory period of cardiac muscle lasts longer than the contraction itself so no other contraction can occur until relaxation is well under way.
  • Therefore, tetani (sustained contraction) cannot occur in cardiac muscle.
  • The absolute refractory period of skeletal muscle is shorter than in cardiac muscle, so other contractions can be stimulated. Tetani can occur.
  • Tetany is like a muscle spasm (an involuntary contraction).

Differences between Cardiac and Skeletal Muscle Action Potentials

  • Cardiac action potentials have a plateau phase.
  • Tetany can’t occur in heart muscle due to the long refractory period.

EKG (Electrocardiogram)

  • The propagation of the Action Potential through the heart generates electrical currents that can be detected at the surface of the body.
  • An EKG (or ECG) is a recording of these electrical currents produced by all the heart muscle fibers during each heartbeat.
  • Through an EKG, heart pathology can be detected.
  • Three waves appear on the EKG tracing with each heartbeat:
    • P wave
    • QRS Complex
    • T wave

P wave

  • Small deflection on the EKG.
  • Represents atrial depolarization.

QRS complex

  • Large upward, then downward deflection.
  • Represents ventricular depolarization.
  • Atrial repolarization occurs at this time but is masked by the ventricular depolarization.

T wave

  • Small upward deflection.
  • Represents ventricular repolarization.

Ten Steps in the Cardiac Cycle

  • A cardiac cycle is all the events associated with one heartbeat.
  • The events occur simultaneously in the left and right sides of the heart, although the pressures generated are greater on the left side.
  • Three main phases:
    • Atrial systole (atria contract, ventricles relax) - Steps 1-4
    • Ventricular Systole (ventricles contract, atria relax) - Steps 5-8
    • Relaxation Period (both atria and ventricles relax) - Steps 9-10

Atrial Systole (Atria contract and ventricles relax)

  • First 4 steps of the cardiac cycle.
    • Step 1: Depolarization of the SA node causes atrial depolarization (indicated by the P wave on ECG).
    • Step 2: Atrial Depolarization causes atrial contraction (systole); blood is forced through the atria into the ventricles.
    • Step 3: Completion of Atrial Systole contributes another 25 ml of blood to each ventricle, so end diastolic volume (the volume of blood in each ventricle at the end of ventricular relaxation) is about 130 ml. End Diastolic Volume is the Fullest the Ventricles get
    • Step 4: Onset of Ventricular depolarization (indicated by the QRS complex of the ECG).

Ventricular Systole

  • Second phase of the cardiac cycle (5th through 8th steps).
  • Ventricles contract, and atria relax.
    • Step 5: Ventricular Depolarization causes ventricular systole. As the ventricles contract, blood is pushed against the AV valves, forcing them to close. The ventricles continue to contract isometrically, yet both the semilunar and AV valves are closed. This is called the Isovolumetric contraction. During this time, pressure rises in the ventricles.
    • Step 6: The ventricles continue to contract. When the ventricular pressure is greater than aortic pressure (80 mmHg), then the semilunar valves open.
    • Step 7: The ventricles eject their blood (70ml). The volume remaining in each ventricle is the end systolic volume (ESV). Stroke volume, the volume of blood ejected per beat from each ventricle, equals the EDV – ESV. At rest, SV = 70 ml.
    • Step 8: The ventricles repolarize (marked by the T wave of the ECG).

Relaxation Period

  • The final phase of the contraction cycle (9th and 10th steps).
  • Both the atria and ventricles relax.
  • This is the time that shortens as the HR increases, not the atrial or ventricular systole.
    • Step 9: Ventricular Repolarization causes ventricular diastole. The ventricular pressure falls, causing blood to start flowing backwards into the ventricles, which closes the semilunar valves. When all four valves are closed, this is the period of isovolumetric relaxation
    • Step 10: Pressure continues to decrease in the ventricles. When ventricular pressure decreases below atrial pressure, the AV valves open, and ventricular filling starts. At the end of the relaxation period, the ventricles are ¾ full (105 ml). The p wave appears again, signalling the start of another cardiac cycle.

Cardiac Cycle Summary:

  • Step 1: Atrial depolarization (p wave)
  • Step 2: Atrial contraction
  • Step 3: Completion of contraction
  • Step 4: Ventricular depolarization (QRS complex)
  • Step 5: Ventricular contraction
  • Step 6: Semilunar Valves open
  • Step 7: Ventricular ejection
  • Step 8: Ventricular repolarization (T wave)
  • Step 9: Closure of semilunar valves
  • Step 10: AV valves open