Cardiology Quiz: Cardiac Muscle Physiology

Cardiac Physiology

• The heart is a muscular pump driving blood through blood vessels.
• It's slightly larger than a fist.
• Objectives for a 2nd-year student include explaining cardiac muscle properties, discussing cardiac output, and explaining heart sounds.

Structure of the Heart

• The heart has four chambers: two atria (receiving) and two ventricles (pumping).
• The right atrium receives deoxygenated blood from the body via the vena cava.
• The right ventricle pumps this blood to the lungs.
• The left atrium receives oxygenated blood from the lungs.
• The left ventricle pumps oxygenated blood to the body via the aorta.
• The heart has four valves: mitral, tricuspid, pulmonary, and aortic, ensuring one-way blood flow.

Structure of the Heart Wall

• The heart wall has three layers: epicardium, myocardium, and endocardium.
• The myocardium, the thickest layer, consists of cardiac muscle fibers arranged in a twisted, ring-like manner.
• It's the contractile layer.

Cardiac Muscle

• Cardiac myocytes are short, branched, striated muscle cells.
• They're connected by gap junctions, allowing for a synchronized contraction as a single functional unit (syncytium).

Valves of the Heart

• Atrioventricular (AV) valves (mitral and tricuspid) separate atria from ventricles.
• Semilunar valves (aortic and pulmonary) separate ventricles from major arteries.
• Valves ensure one-way blood flow in the heart.

Cardiac Properties

• Rhythmicity (automaticity): The sinoatrial node (SAN) produces its own rhythm.
• Excitability: Cardiac cells initiate action potentials in response to depolarizing current.
• Conductivity: The impulse travels from the SA node to all cardiac muscle fibers.
• Contractility: Cardiac muscle's ability to develop force for contraction in response to the electrical impulse.

Auto-rhythmicity (Pacemaker Potential)

• Pacemaker cells have no voltage-gated Na+ channels.
• Depolarization is primarily due to Ca2+ influx.
• Repolarization is caused by K+ outflow.
• Spontaneous depolarization occurs due to a progressive fall in K+ permeability.

Conductivity of the Heart

• The heart acts as a syncytium, a coordinated unit, due to gap junctions.
• The sinoatrial node (SAN) acts as the heart's pacemaker.
• The atrioventricular node (AVN) delays the impulse to allow atria to empty completely.
• The atrioventricular bundle (bundle of His) transmits the impulse to the ventricles.
• Purkinje fibers spread the impulse rapidly throughout the ventricles.

Excitability (Cardiac Action Potential)

• Depolarization: Sodium influx causes rapid depolarization.
• Repolarization (Triphasic):
  • Phase 1: Rapid K+ efflux
  • Phase 2 (Plateau): Calcium influx
  • Phase 3: Rapid K+ efflux
• RMP (Resting Membrane Potential): Return to resting potential

Contractility (Frank Starling Law)

• The heart adjusts its contraction force based on the amount of blood received.
• Frank-Starling law states that the more the ventricles are filled, the stronger the contraction.
• Exercise increases contraction strength.

Cardiac Output (COP)

• Cardiac output is the volume of blood pumped by each ventricle per minute.
• COP = Stroke Volume (SV) × Heart Rate (HR)
• Stroke volume (SV) is the amount of blood ejected per heartbeat.
• End-diastolic volume (EDV) is the volume of blood in the ventricle before contraction.
• End-systolic volume (ESV) is the volume of blood in the ventricle after contraction.
• SV = EDV - ESV

Factors Determining COP

• Pumping ability of the heart: Increased by sympathetic stimulation during exercise.
• Venous return (VR): Increased VR results in increased COP.

Factors Affecting COP

• Increased: Excitement, eating, exercise, high environment temperature, epinephrine, and pregnancy.
• Decreased: Sitting/standing, rapid arrhythmia, and heart disease.

Heart Sounds

• First heart sound: Mitral and tricuspid valve closure during ventricular contraction.
• Second heart sound: Pulmonary and aortic valve closure during ventricular relaxation.

Heart Rate

• Heart rate is the number of heartbeats per minute.
• Normal resting heart rate is 60-100 bpm.
• Tachycardia: Heart rate above 100 bpm.
• Bradycardia: Heart rate below 60 bpm.
• Arrhythmia: Irregular heart rhythm.

Factors Affecting Heart Rate

• Autonomic nervous system: Parasympathetic decreases, sympathetic increases heart rate.
• Hormones: Epinephrine increases, thyroid hormones increases.
• Age: Infants have higher heart rates.
• Gender: Slight difference in heart rates between genders.
• Emotions and stress increase heart rate.

Heart Failure

• Heart failure is the heart's inability to pump enough blood.
• Causes include coronary artery disease, heart attacks, cardiomyopathy, congenital heart defects, and hypertension.
• Symptoms include chest pain, shortness of breath, fatigue, and swelling.

Vascular Physiology

• Objectives for a 2nd-year student include explaining blood flow regulation, discussing blood pressure regulation, explaining hypertension, and discussing edema.

Functional Organization of the Vascular System

• Aorta and arteries: Low resistance to blood flow. Important for preventing marked pressure elevation during ejection and for ensuring blood spread to the periphery.
• Arterioles: High resistance. Plays a role in regulating arterial blood pressure.
• Capillaries: Exchange of gases, hormones, electrolytes, fluids, and nutrients. Walls are very thin and permeable.
• Venules and veins: Carry deoxygenated blood back to the heart. Veins have a thin wall and act as blood reservoirs.

Blood Flow

• Blood flow is the volume of blood that passes through a vessel per unit time.
• F = ΔP/R (Blood flow = Pressure difference/Resistance)

Types of Blood Flow

• Laminar flow: Blood flows in layers.
• Turbulent flow: Blood flows in all directions.

Blood Flow Regulation

• Extrinsic regulation:
  • Sympathetic/adrenal activation: Increase heart rate/peripheral resistance
  • Parasympathetic: Decrease heart rate.
  • Hormones: Increase/decrease heart rate depending on hormone.
• Paracrine regulation:
  • Endothelium releases NO (vasodilator) and endothelin 1 (vasoconstrictor).
• Intrinsic regulation (Autoregulation):
  • Myogenic response: Vessels constrict/dilate depending on pressure.
  • Metabolic factors: Decreased O2/increased CO2/pH causes vasodilation.
  • Ions: K+, Mg++, H+ causes vasodilation. Anions (e.g., acetate citrate) cause vasodilation.

Arterial Blood Pressure (ABP)

• ABP: The force exerted by blood against the vessel wall.
• Systolic pressure: Maximum pressure during systole (contraction).
• Diastolic pressure: Minimum pressure during diastole (relaxation).
• Pulse pressure: Difference between systolic and diastolic pressure.

Physiological Factors Affecting ABP

• Sex, age, emotions, exercise, hormones, gravity, race, sleep, and pregnancy.

Hypertension (HTN)

• Hypertension: High blood pressure (above 140/90 mmHg).
• Primary (essential) HTN: Unknown cause.
• Secondary HTN: Caused by a known disease process.
• Complications include stroke, kidney damage, and heart attack.

Edema

• Edema: Swelling due to fluid accumulation in interstitial spaces.
• Causes include increased vascular pressure, reduced colloidal osmotic pressure of plasma proteins, increased capillary permeability, and lymphatic obstruction.
• Types of edema: Soft ("pitting") vs. hard ("non-pitting") and localized vs. generalized.

Treatment of Hypertension

• Lifestyle modification: Smoking cessation, moderate alcohol, weight reduction, exercise, dietary changes (Na+ and K+).
• Medications: Diuretics, beta-blockers, calcium channel blockers, ACE inhibitors, and angiotensin II receptor blockers.

Test your knowledge on the cardiac muscle action potential and its phases, including depolarization and the plateau due to calcium influx. Explore concepts like cardiac output, stroke volume, and the Frank-Starling law, as well as common symptoms and causes of heart conditions. This quiz is perfect for students of cardiac physiology and anatomy.