The Heart: Cardiac Cycle, Cardiac Output, and Regulation

The Heart

Introduction

The heart is a muscular organ responsible for pumping blood throughout the body, supplying tissues with oxygen and nutrients while removing waste products. Understanding the mechanical and regulatory events of the heart is essential for students of Anatomy & Physiology.

Phases of the Cardiac Cycle

Overview of the Cardiac Cycle

The cardiac cycle refers to the sequence of mechanical and electrical events that occur during one heartbeat. It includes alternating periods of contraction (systole) and relaxation (diastole) in the atria and ventricles, resulting in blood flow through the heart and into the circulatory system.

• Systole: Period of contraction when blood is ejected from the chambers.

• Diastole: Period of relaxation when chambers fill with blood.

• Pressure and Volume Changes: Each phase is characterized by changes in pressure and blood volume within the heart chambers.

Phases of the Cardiac Cycle

• Atrial Systole: Atria contract, pushing blood into the ventricles.

• Ventricular Systole (First Phase): Ventricles contract, closing atrioventricular (AV) valves.

• Ventricular Systole (Second Phase): Semilunar valves open, and blood is ejected into the arteries.

• Early Ventricular Diastole: Semilunar valves close, and ventricles begin to relax.

• Late Ventricular Diastole: Chambers relax and fill with blood as AV valves open.

Electrocardiogram (ECG) Correlation: The cardiac cycle phases correspond to specific waves on the ECG, such as the P wave (atrial depolarization), QRS complex (ventricular depolarization), and T wave (ventricular repolarization).

Heart Sounds

• First Heart Sound (S1, "lub"): Associated with closure of AV valves at the beginning of ventricular systole.

• Second Heart Sound (S2, "dup"): Associated with closure of semilunar valves at the beginning of ventricular diastole.

• Heart Murmurs: Abnormal heart sounds, often indicating incompetent or stenotic valves.

Cardiac Output (CO)

Definition and Formula

Cardiac Output (CO) is the volume of blood pumped by each ventricle per minute. It is a key indicator of heart function and tissue perfusion.

• Formula: $CO = HR \times SV$

• Heart Rate (HR): Number of beats per minute.

• Stroke Volume (SV): Volume of blood pumped out by one ventricle with each beat.

• Normal Value: Approximately 5.25 L/min at rest.

Example Calculation:

• If $HR = 75$ beats/min and $SV = 70$ ml/beat, then $CO = 75 \times 70 = 5250$ ml/min = 5.25 L/min.

Cardiac Reserve: The difference between resting and maximal cardiac output. Maximal CO can be 4-5 times resting CO in nonathletes and up to 35 L/min in trained athletes.

Regulation of Stroke Volume

Main Factors Affecting Stroke Volume

Stroke volume is regulated by three primary factors:

• Preload: Degree of stretch of cardiac muscle cells before contraction. Increased by higher venous return, slow heartbeat, and exercise.

• Contractility: Contractile strength at a given muscle length, independent of muscle stretch and end-diastolic volume (EDV). Increased by sympathetic stimulation, epinephrine, and high extracellular Ca2+; decreased by acidosis, high K+, and calcium channel blockers.

• Afterload: Pressure ventricles must overcome to eject blood. Increased afterload (e.g., hypertension) raises end-systolic volume (ESV) and reduces stroke volume.

Key Equations:

• $CO = SV \times HR$

• $SV = EDV - ESV$

Regulation of Heart Rate

Autonomic Nervous System (ANS) Regulation

The ANS plays a crucial role in regulating heart rate and contractility:

• Sympathetic Nervous System: Activated by emotional or physical stress; releases norepinephrine, which increases heart rate and contractility via cAMP-mediated pathways and Ca2+ influx.

• Parasympathetic Nervous System: Opposes sympathetic effects; releases acetylcholine, which hyperpolarizes pacemaker cells by opening K+ channels, resulting in slower heart rate. The heart at rest exhibits vagal tone (parasympathetic dominance).

Chemical Regulation of Heart Rate

• Hormones: Epinephrine (from adrenal medulla) and thyroxine increase heart rate and contractility.

• Ions: Intra- and extracellular concentrations of Ca2+ and K+ must be maintained for normal heart function.

Other Factors Influencing Heart Rate

• Age: Fetus has the fastest heart rate.

• Gender: Females generally have a faster heart rate than males.

• Exercise: Increases heart rate.

• Body Temperature: Heart rate increases with elevated temperature.

Homeostatic Imbalances

Electrolyte Imbalances

• Hypocalcemia: Depresses heart function.

• Hypercalcemia: Increases heart rate and contractility.

• Hyperkalemia: Alters electrical activity, may cause heart block and cardiac arrest.

• Hypokalemia: Results in feeble heartbeat and arrhythmias.

Abnormal Heart Rates

• Tachycardia: Abnormally fast heart rate (>100 beats/min); may lead to fibrillation if persistent.

• Bradycardia: Abnormally slow heart rate (<60 beats/min); may result in inadequate blood circulation in nonathletes, but can be a desirable adaptation in endurance athletes.

Congestive Heart Failure (CHF)

• Definition: Progressive condition where cardiac output is too low to meet tissue needs, reflecting a weakened myocardium.

• Causes:

  • Coronary atherosclerosis (clogged arteries)

  • Persistent high blood pressure

  • Multiple myocardial infarcts

  • Dilated cardiomyopathy (DCM)

Age-Related Changes Affecting the Heart

• Sclerosis and thickening of valve flaps

• Decline in cardiac reserve

• Fibrosis of cardiac muscle

• Atherosclerosis

Summary Table: Factors Affecting Cardiac Output

Factor	Effect on Cardiac Output	Mechanism
Preload	Increases CO	Greater stretch increases contraction force
Contractility	Increases CO	Stronger contractions eject more blood
Afterload	Decreases CO	Higher pressure opposes ejection
Heart Rate	Increases CO	More beats per minute pump more blood
Sympathetic Stimulation	Increases CO	Increases HR and contractility
Parasympathetic Stimulation	Decreases CO	Decreases HR
Hormones (Epinephrine, Thyroxine)	Increases CO	Increase HR and contractility
Electrolyte Imbalances	Variable	Alter electrical activity and contractility

Additional info: Academic context and expanded explanations have been added to ensure completeness and clarity for college-level study.