Makindo Medical Notes.com

Related Subjects: |Cardiac Anatomy and Physiology |Coronary Artery Anatomy and Physiology |Cardiac Electrophysiology |Cardiac Embryology

Overview of Cardiac Physiology

Cardiac physiology involves the study of the heart's function, including the mechanisms of cardiac muscle contraction, the electrical conduction system, and the regulation of cardiac output. The heart is a muscular organ responsible for pumping blood throughout the body, delivering oxygen and nutrients to tissues, and removing waste products.

Key Concepts

• Anatomy of the Heart:
  • Four chambers: Right atrium, right ventricle, left atrium, and left ventricle.
  • Valves: Tricuspid, pulmonary, mitral, and aortic valves ensure unidirectional blood flow.
  • Coronary circulation: Supplies blood to the heart muscle itself.
• Cardiac Muscle Contraction:
  • Excitation-Contraction Coupling:
    • Action potential triggers calcium influx.
    • Calcium binds to troponin, allowing actin-myosin interaction and contraction.
  • Sarcomere: The functional unit of muscle contraction.
  • Sliding Filament Theory: Describes how actin and myosin filaments slide past each other to produce contraction.
• Electrical Conduction System:
  • Sinoatrial (SA) Node: The natural pacemaker of the heart, initiates the heartbeat.
  • Atrioventricular (AV) Node: Delays the impulse to allow atrial contraction before ventricular contraction.
  • Bundle of His and Purkinje Fibers: Conduct the impulse rapidly through the ventricles.
• Cardiac Cycle:
  • Systole: Period of ventricular contraction and ejection of blood.
  • Diastole: Period of ventricular relaxation and filling with blood.
  • Heart Sounds: "Lub-dub" sounds caused by the closure of heart valves (S1 and S2).
• Regulation of Cardiac Output:
  • Heart Rate (HR): Number of beats per minute.
  • Stroke Volume (SV): Volume of blood pumped per beat.
  • Cardiac Output (CO) = HR x SV; the total volume of blood pumped by the heart per minute.
  • Factors affecting HR and SV: Autonomic nervous system, hormones, venous return, and afterload.

Detailed Processes

• Excitation-Contraction Coupling:
  • Action potential spreads across the sarcolemma and down T-tubules.
  • Calcium channels in the sarcoplasmic reticulum open, releasing calcium into the cytoplasm.
  • Calcium binds to troponin, causing a conformational change in tropomyosin, exposing binding sites on actin.
  • Myosin heads bind to actin, forming cross-bridges and generating force.
  • ATP is required for myosin head detachment and re-cocking for the next contraction cycle.
• Electrical Conduction System:
  • The SA node generates an action potential that spreads through the atria, causing atrial contraction.
  • The impulse reaches the AV node, where it is delayed to allow complete atrial emptying.
  • The impulse then travels through the Bundle of His, bundle branches, and Purkinje fibers, causing ventricular contraction.
• Cardiac Cycle:
  • Atrial Systole: Atria contract, pushing blood into the ventricles.
  • Ventricular Systole (isovolumetric contraction): Ventricles contract with no volume change, increasing pressure.
  • Ventricular Ejection: Ventricles continue to contract, ejecting blood into the aorta and pulmonary artery.
  • Isovolumetric Relaxation: Ventricles relax with no volume change, decreasing pressure.
  • Ventricular Filling: Ventricles fill passively with blood from the atria.

Clinical Relevance

• Arrhythmias:
  • Abnormal heart rhythms due to issues in the electrical conduction system.
  • Examples: Atrial fibrillation, ventricular tachycardia, bradycardia.
• Heart Failure:
  • Condition where the heart cannot pump sufficient blood to meet the body's needs.
  • Causes include ischaemic heart disease, hypertension, and cardiomyopathy.
• Myocardial Infarction (Heart Attack):
  • Occurs when blood flow to a part of the heart is blocked, causing tissue damage.
  • Typically due to coronary artery disease and atherosclerosis.
• Hypertension:
  • Chronic high blood pressure, increasing the risk of heart disease and stroke.
  • Can lead to hypertrophy of the heart muscle and heart failure.
• Valvular Heart Diseases:
  • Disorders affecting the function of the heart valves.
  • Examples: Mitral valve prolapse, aortic stenosis, regurgitation.

Summary

Cardiac physiology encompasses the mechanisms of heart function, including muscle contraction, electrical conduction, and regulation of cardiac output. Understanding these processes is crucial for diagnosing and treating cardiovascular diseases, which are a leading cause of morbidity and mortality worldwide.