Path of Blood Flow

Pulmonary and Systemic Circuits

The heart pumps blood through two main circuits: the pulmonary circuit and the systemic circuit. These circuits ensure oxygenation of blood and delivery of nutrients throughout the body.

• Pulmonary circuit: Carries deoxygenated blood from the right ventricle to the lungs for gas exchange, then returns oxygenated blood to the left atrium.

• Systemic circuit: Distributes oxygenated blood from the left ventricle to the body tissues and returns deoxygenated blood to the right atrium.

Gas Exchange

Pulmonary and Systemic Capillaries

Gas exchange occurs at the capillary level, where oxygen and carbon dioxide are exchanged between blood and tissues.

• Pulmonary capillaries: Surround alveoli in the lungs; oxygen enters blood, carbon dioxide is expelled.

• Systemic capillaries: Oxygen leaves blood for tissues; carbon dioxide enters blood from tissues.

Coronary Circulation

Blood Supply to the Heart

The heart receives its own blood supply via the coronary arteries. Disruption can lead to myocardial infarction (heart attack).

• Myocardial infarction: Occurs when blood flow to part of the heart is blocked, causing tissue death.

Gross Anatomy of the Heart

Chambers and Valves

The heart consists of four chambers and four main valves that regulate blood flow.

• Chambers:

  • Right atrium

  • Left atrium

  • Right ventricle

  • Left ventricle

• Valves:

  • Atrioventricular (AV) valves:

    • Mitral (bicuspid) valve – between left atrium and ventricle

    • Tricuspid valve – between right atrium and ventricle

  • Semilunar (SL) valves:

    • Pulmonary valve – between right ventricle and pulmonary artery

    • Aortic valve – between left ventricle and aorta

Microscopic Anatomy of the Heart

Cardiac Muscle Cells

Cardiac muscle cells are specialized for continuous rhythmic contraction and are interconnected by intercalated discs.

• Intercalated discs: Specialized junctions between cardiac cells containing:

  • Desmosomes: Provide mechanical strength

  • Gap junctions: Allow electrical signals to pass rapidly between cells

Electrical Activity of the Heart

Intrinsic Conduction and Action Potentials

The heart's rhythm is controlled by pacemaker cells in the sinoatrial (SA) node, which initiate action potentials.

• Pacemaker potential: Gradual depolarization leading to threshold

• Depolarization: Rapid influx of ions causing cell activation

• Repolarization: Return to resting membrane potential

Sequence of Excitation

• Sinoatrial (SA) node → Atrioventricular (AV) node → AV bundle → Right and left bundle branches → Subendocardial conducting network (Purkinje fibers)

Extrinsic Innervation of the Heart

Autonomic Nervous System Regulation

The heart rate and force of contraction are regulated by the autonomic nervous system via cardiac centers in the medulla.

• Cardioacceleratory center: Sympathetic stimulation increases heart rate

• Cardioinhibitory center: Parasympathetic stimulation decreases heart rate

Action Potentials of Contractile Cardiac Cells

Phases of Action Potential

Contractile cardiac cells undergo a distinct action potential with three main phases:

• Phase 1: Depolarization opens fast voltage-gated Na+ channels

• Phase 2: Slow Ca2+ channels open, Ca2+ enters cell

• Phase 3: Ca2+ channels close, voltage-gated K+ channels open, rapid K+ efflux repolarizes cell

Ionic Basis of Electrical Activity in the Heart

Contraction and Relaxation

Cardiac contraction is triggered by depolarization and calcium influx, while relaxation involves removal of calcium.

• Contraction:

  • Depolarization via gap junctions

  • Opening of calcium channels

  • AP travels down T tubules

  • Calcium released from sarcoplasmic reticulum

  • Calcium-induced calcium release

  • Calcium binds to troponin, shifting tropomyosin

  • Myosin binding sites on actin exposed

  • Crossbridge cycle occurs

• Relaxation:

  • Calcium removed from cytosol

  • Ca2+ ATPase in sarcoplasmic reticulum and plasma membrane

  • Na+-Ca2+ exchanger in plasma membrane

Electrocardiogram (ECG/EKG)

Waves and Intervals

An ECG records the electrical activity of the heart, showing characteristic waves and intervals.

• P wave: Depolarization of SA node and atria

• QRS complex: Ventricular depolarization and atrial repolarization

• T wave: Ventricular repolarization

• P-R interval: Beginning of atrial excitation to beginning of ventricular excitation

• S-T segment: Entire ventricular myocardium depolarized

• Q-T interval: Beginning of ventricular depolarization through ventricular repolarization

Mechanical Events of the Heart

Systole, Diastole, and Cardiac Cycle

The cardiac cycle consists of alternating periods of contraction (systole) and relaxation (diastole).

• Systole: Period of heart contraction

• Diastole: Period of heart relaxation

• Cardiac cycle: Blood flow through heart during one complete heartbeat

  • Phase 1: Ventricular filling (mid-to-late diastole) – End diastolic volume (EDV)

  • Phase 2: Isovolumetric contraction

  • Phase 3: Isovolumetric relaxation (early diastole) – End systolic volume (ESV)

Heart Sounds

Valve Closure and Clinical Significance

Heart sounds are produced by the closing of heart valves and can indicate normal or abnormal function.

• First sound (lub): Closing of AV valves at beginning of ventricular systole

• Second sound (dup): Closing of SL valves at beginning of ventricular diastole

• Heart murmurs: Abnormal sounds due to:

  • Incompetent (insufficient) valve

  • Stenotic valve

Regulation of Pumping

Cardiac Output, Stroke Volume, and Heart Rate

Cardiac output is the volume of blood pumped by the heart per minute, regulated by stroke volume and heart rate.

• Cardiac output (CO):

• Regulation of Stroke Volume (SV):

  • Preload

  • Contractility

  • Afterload

• Regulation of Heart Rate (HR):

  • Autonomic nervous system

  • Chemicals (hormones, ions)

Clinical Considerations

• Hypocalcemia/hypercalcemia: Low/high calcium levels affect heart function

• Hyperkalemia/hypokalemia: High/low potassium levels can cause arrhythmias and feeble heartbeat

• Tachycardia: Abnormally fast heart rate

• Bradycardia: Abnormally slow heart rate

• Congestive Heart Failure (CHF): Right-sided vs left-sided failure

Summary Table: Heart Valves and Their Functions

Summary Table: ECG Waves and Their Significance

Example: Cardiac Output Calculation

• If stroke volume (SV) is 70 mL/beat and heart rate (HR) is 75 beats/min, then:

• This means the heart pumps 5.25 liters of blood per minute.