NPB 101: Lecture 20 - Cardiovascular Physiology 3: Cardiac Cycle & Regulation

Learning Objectives: Trace the cardiac cycle, relating chamber pressues to valve opening and closure, blood flow, and ventricular volume. Interpret a ventricular pressure-volume loop, identifying filling, isovolumetric contraction, ejection, and isovolumetric relaxation. Calculate stroke volume, cardiac output, and ejection fraction from ventricular volumes and heart rate. Predict how changes in preload, afterload, and contractility affect stroke volume and ventricular empyting. Explain how sympathetic and parasympathetic signaling alter pacemaker activity and heart rate. Explain how sympathetic stimulation affects cardiac contraction and relaxation, and integrate changes in heart rate and stroke volume to predict cardiac output.

Cardiac cycle

All the events involved with blood flow through the heart during one heart beat.

Two primary phases:

• Systole

• Diastole

Systole

The phase of the cardiac cycle when the ventricles contract, pumping out blood into the arteries— sending blood to the lungs through the pulmonary artery and to the rest of the body through the aorta.

Diastole

The relaxation phase of the cardiac cycle when the ventricles relax, allowing for its chambers to refill with blood passed down from the atria.

Isovolumetric contraction

First step in Cardiac Cycle Sequence.

• Ventricular contraction increases pressure shutting the AV valves

Ejection

Second step in Cardiac Cycle Sequence.

• Left ventricle pressure > Aortic pressure

• Aortic valve opens

• Ventricular volume decreases

Isovolumetric relaxation

Third step in Cardiac Cycle Sequence.

• Left ventricle pressure drops below aortic pressure

• Aortic valve shuts

Passive filling

Fourth step in Cardiac Cycle Sequence.

• Pressure in the ventricles falls below that of the atria

• AV valve opens and ventricle fills

Active filling

Fifth step in Cardiac Cycle Sequence.

• Atrial contraction delivers the final blood volume to the ventricles

End-diastolic volume (EDV)

Volume of blood in the ventricles at the end of diastole (relaxation/filling)

End-systolic volume (ESV)

The amount of blood remaining in the chamber at the end of systole (contraction/emptying)

Stroke Volume (SV)

The amount of blood pumped out with each contraction.

• Equation: SV = EDV - ESV

• Regulated by…

  • Amount of blood coming back to the heart

    • Preload ~ EDV

  • How hard it is to push blood out of the heart

    • Afterload ~ aortic blood pressure

  • Contractility of the cardiac muscle

    • Inotropy

Cardiac Output (CO)

The amount of blood pumped by each ventricle per unit of time

• Equation: CO = HR x SV

  • Factors that affect heart rate or stroke volume will thus affect cardiac output

Heart rate

Cardiac cycles per unit time (beats per minute; 1/ cardiac cycle time).

• Regulated by autonomic nervous system

  • Sympathetic (Fight-or-Flight)

  • Parasympathetic (Rest-and-digest)

Ejection Fraction (EF)

Fraction of blood pumped from ventricle.

• Equation: SV/EDV

• Measure of contractility

Pressure Volume Loops

A real-time graphical plot of left (or right) ventricular pressure against ventricular volume during a single complete cardiac cycle (heartbeat).

• Ventricular pressure (mmHg)

• Ventricular volume (mL)

Preload (Intrinsic)

The degree to which the cardiac muscle cells are stretched before they contract.

• Determined by the volume of blood in ventricle when full before contraction (EDV)

• Increased preload = increased stroke volume

• Related to length tension curve

  • Operated ascending limb

Afterload (Intrinsic)

The pressure ventricles must produce to open aortic/pulmonary valves.

• Anything that increases systemic or pulmonary pressure can increase afterload

• Increased afterload decreases Ejection Fraction

Extrinsic Regulation of Heart Contractility

The control of myocardial force generation by factors originating outside the heart.

Two divisions of autonomic nervous system (ANS):

• Sympathetic

• Parasympathetic

Sympathetic Nervous System

Division of the ANS that drives the heart’s “fight-or-flight” response, increasing cardiac output to meet physical or emotional demands.

• Pathway:

  • Sympathetic cardiac nerves (thoracic spinal nerves) release norepinephrine

  • Adrenal medulla releases circulating epinephrine via bloodstream

• Receptors:

  • They bind to beta (β₁) adrenergic receptors on the atria and ventricles

    • Increases Ca²⁺ release and contraction

    • Faster Ca²⁺ re-uptake and relaxation

• Effects:

  • Inotropic effects on contractility

  • Causes heart rate to increase

    • Increase in pacemaker AP

Parasympathetic Nervous System

Division of the ANS that acts as the heart’s natural “rest-and-digest” brake petal, slowing down cardiac activity.

• Pathway:

  • The vagus nerve delivers acetylcholine (ACh) to the heart

• Receptors:

  • ACh binds to muscarinic (M₂) receptors

• Effects:

  • Slows depolarization in the heart

  • Induces hyperpolarization

    • ACh increases K⁺ outflow of SA and AV (pacemaker) nodes

    • Decreases slow Na⁺ and Ca²⁺ influx

  • Slows heart rate

    • Decrease in pacemaker AP

Autonomic Nervous System

The ANS affects the activity of the pacemaker neurons by affecting the function of individual channels.

Regulation of Cardiac Output