Cardiac Cycle and Heart Function

Phases of the Cardiac Cycle

The cardiac cycle describes the sequence of events in one heartbeat, including contraction (systole) and relaxation (diastole) of the heart chambers.

• Systole: Period of heart muscle contraction, resulting in blood ejection from the chambers.

• Diastole: Period of relaxation, allowing chambers to fill with blood.

• At rest, the heart spends more time in diastole than systole.

• During exercise, the time spent in diastole decreases more than systole.

• Ischemia: Reduced blood flow, often due to blocked coronary arteries, can cause chest pain (angina).

Key Terms:

• Preload: The degree of stretch of cardiac muscle fibers at the end of diastole.

• Afterload: The resistance the heart must overcome to eject blood.

• Contractility: The strength of heart muscle contraction.

Left vs. Right Side of the Heart

The heart is divided into left and right sides, each with distinct roles in circulation.

• Left Side:

  • Receives oxygenated blood from the lungs via the pulmonary veins.

  • Blood flows: Left atrium → Left ventricle → Aorta → Systemic circulation.

• Right Side:

  • Receives deoxygenated blood from the body via the vena cava.

  • Blood flows: Right atrium → Right ventricle → Pulmonary artery → Lungs.

Example: The left ventricle has a thicker wall than the right, as it must generate higher pressure to pump blood throughout the body.

Electrical Conduction System of the Heart

Pathway of Electrical Signals

The heart's rhythmic contractions are coordinated by its intrinsic conduction system.

1. Sinoatrial (SA) Node: Pacemaker of the heart; initiates electrical impulses.

2. Atrioventricular (AV) Node: Delays the impulse, allowing atria to contract before ventricles.

3. Bundle of His: Conducts impulses from AV node to ventricles.

4. Left and Right Bundle Branches: Carry impulses through the interventricular septum.

5. Purkinje Fibers: Distribute the impulse throughout the ventricles, causing contraction.

Electrocardiogram (ECG/EKG): Records the electrical activity of the heart. Key waves include:

• P wave: Atrial depolarization

• QRS complex: Ventricular depolarization

• T wave: Ventricular repolarization

Additional info: Atrial repolarization is not seen on the ECG as it is masked by the QRS complex.

Cardiac Output and Regulation

Cardiac Output (CO)

Cardiac output is the volume of blood pumped by the heart per minute.

• Formula:

• HR: Heart rate (beats per minute)

• SV: Stroke volume (volume of blood ejected per beat)

Normal values at rest: ~5 L/min; during exercise: up to 25 L/min.

Factors Affecting Stroke Volume

• Preload: Increased by greater venous return (muscle and respiratory pumps).

• Afterload: Increased resistance decreases stroke volume.

• Contractility: Increased by sympathetic stimulation and certain hormones.

Example: Endurance training increases plasma volume and red blood cell count, enhancing stroke volume.

Mechanisms to Increase Venous Return

1. Muscle Pump: Skeletal muscle contractions compress veins, pushing blood toward the heart.

2. Respiratory Pump: Changes in thoracic pressure during breathing help draw blood back to the heart.

Blood Pressure and Its Regulation

Blood Pressure (BP)

Blood pressure is the force exerted by circulating blood on the walls of blood vessels.

• Systolic BP: Pressure during ventricular contraction.

• Diastolic BP: Pressure during ventricular relaxation.

• Measured in the brachial artery.

Determinants of Blood Pressure

• Cardiac Output (CO)

• Peripheral Resistance (PR): Resistance to blood flow in the vessels.

Formula:

Poiseuille's Law

Describes the relationship between blood flow, vessel radius, viscosity, and pressure difference.

• Q: Flow rate

• P1 - P2: Pressure difference

• r: Vessel radius

• η: Blood viscosity

• l: Vessel length

Key Point: Small changes in vessel radius have a large effect on flow.

Blood Pressure Regulation

• Baroreceptor Reflex: Stretch receptors in carotid sinus and aortic arch detect changes in BP and send signals to the brain to adjust heart rate and vessel diameter.

• Autonomic Nervous System:

  • Sympathetic: Increases HR and contractility, constricts vessels.

  • Parasympathetic: Decreases HR.

• Hormonal Control: Includes the renin-angiotensin-aldosterone system (RAAS), which increases BP by vasoconstriction and increasing blood volume.

Heart Rate Variability (HRV)

HRV refers to the variation in time between heartbeats, reflecting autonomic nervous system balance.

• High HRV: Indicates good parasympathetic (rest-and-digest) activity and cardiovascular health.

• Low HRV: Associated with stress, fatigue, or disease.

• HRV is influenced by breathing, exercise, and emotional state.

Special Topics

Hypertrophic Cardiomyopathy (HCM)

A condition where the heart muscle becomes abnormally thick, potentially disrupting electrical conduction and increasing the risk of sudden cardiac death, especially in athletes.

Cardiovascular Drift

During prolonged exercise, heart rate gradually increases at a given intensity, while stroke volume decreases, often due to fluid loss and increased body temperature.

Summary Table: Key Cardiovascular Parameters

Additional info:

• Endurance training increases plasma volume and red blood cell count, improving oxygen delivery and cardiac efficiency.

• Peripheral runoff during exercise helps maintain diastolic pressure by moving blood from arteries to veins.