Cardiac Physiology Summary

Cardiac Conduction System

  • Functional Syncytium: Atrial and ventricular syncytium connected by cardiac conduction fibers.

  • Intercalated Discs: Connect cardiac muscle fibers, contain gap junctions (ion passage) and desmosomes (anchors).

  • Autorhythmic Cells: Found in SA node, AV node, bundle branches, Purkinje fibers. Self-depolarize due to automatic changes in ext{Na}^+ / ext{K}^+ permeability.

  • \text{SA} node is the pacemaker.

  • Controlling Systems: Intrinsic conduction system (autorhythmic cells) and autonomic nervous system (alters rhythm).

Cardiomyocytes

  • Striated, involuntary, more mitochondria than skeletal muscle.

  • Don't contract in response to a motor neuron; atria and ventricles function as single syncytia with all fibers contracting together.

  • Possess a longer refractory period than skeletal muscle.

Action Potentials of Cardiac Muscle Cells

  • Autorhythmic Fibers: Slow \text{Na}^+ influx, then \text{Ca}^{2+} influx (L-type), followed by \text{K}^+ efflux for repolarization.

  • Contractile Fibers: Rapid \text{Na}^+ influx, then a plateau phase with \text{Ca}^{2+} influx (L-type) and some \text{K}^+ efflux, followed by rapid \text{K}^+ efflux for repolarization.

Cardiac Cycle

  • Systole: Contraction phase of atria or ventricles.

  • Diastole: Relaxation phase of atria or ventricles.

  • Both atria contract simultaneously, then both ventricles.

  • Phases include ventricular filling (mid-to-late diastole, passive, then atrial contraction), isovolumetric contraction (all valves closed), ventricular ejection (semilunar valves open), and isovolumetric relaxation (all valves closed).

Heart Sounds

  • Lub (1^{st} sound): Closing of atrioventricular valves.

  • Dup (2^{nd} sound): Closing of aortic and pulmonary (semilunar) valves.

  • Murmurs: Indicate incompetent valves (backflow) or stenosis (narrowing).

Electrocardiography (ECG/EKG)

  • Detects and records the heart's electrical current using ext{12} leads.

  • P wave: Atrial depolarization (SA node fires).

  • PR segment: AV nodal delay.

  • QRS complex: Ventricular depolarization (atria repolarize simultaneously).

  • ST segment: Time ventricles are contracting.

  • T wave: Ventricular repolarization.

  • TP interval: Time ventricles are relaxing and filling.

Heart Rate Regulation

  • **Autonomic Nervous System:

    • Parasympathetic (vagus nerve):** Slows heart rate.

    • Sympathetic (accelerator nerves): Increases heart rate and force of contraction.

  • Cardiac Control Center: In medulla oblongata, balances sympathetic and parasympathetic influences.

  • Other Regulators: Cerebrum/hypothalamus (emotions), ions (\text{Ca}^{2+}, \text{K}^+ imbalances can be critical), age, sex, exercise, body temperature.

Cardiac Variations and Conditions

  • Tachycardia: >\text{100} bpm; can be caused by stress, drugs, heart disease; promotes fibrillation.

  • Bradycardia: <\text{60} bpm; caused by low body temperature, certain drugs, or exercise-induced heart efficiency/hypertrophy.

  • Arrhythmias/Fibrillation: Rapid, irregular contractions where \text{SA} node loses rhythm control; fibrillating ventricles are ineffective pumps.

  • Heart Block: Impeded electrical conduction.

  • Wolff-Parkinson-White Syndrome: Extra electrical pathway (e.g., bundle of Kent) causes pre-excitation of ventricles; diagnosed by \text{ECG}.

  • Congestive Heart Failure (CHF): Heart unable to pump enough blood.

    • Causes: Coronary atherosclerosis, persistent high blood pressure, multiple myocardial infarcts, dilated cardiomyopathy.

    • Symptoms: Pulmonary congestion (left-side failure, leads to pulmonary edema), peripheral congestion (right-side failure, leads to systemic edema).

    • Treatment: Diuretics, blood pressure-lowering drugs, digitalis, transplant/mechanical fixes.

  • Congenital Heart Defects: Most common birth defect, often environmental.

    • Types: Mixing of oxygen-poor and oxygenated blood (septal defects, patent ductus arteriosus) or narrowed valves/vessels (stenosis).