Heart Problems and Smooth Muscle

Overview

This study guide covers key concepts related to heart problems (with a focus on EKGs), mechanical heart issues, stroke volume and cardiac output, and the organization and physiology of smooth muscle. These topics are essential for understanding cardiovascular and muscular function in human anatomy and physiology.

Heart Problems (EKGs)

Normal and Abnormal EKG Tracings

An electrocardiogram (EKG or ECG) records the electrical activity of the heart and is used to diagnose various cardiac conditions.

• Normal Sinus Rhythm: Regular pattern of P, QRS, and T waves. The P-P or R-R interval is used to calculate heart rate.

• AV Node Dysfunction: If the SA node is nonfunctional, the AV node takes over as the pacemaker, pacing the heart at 40-60 beats/min.

• Abnormal EKGs:

  • Missing P Waves: Indicates SA node failure; AV node becomes pacemaker.

  • Heart Block (1st, 2nd, 3rd degree):

    • 1st Degree: P-R interval longer than 200 ms.

    • 2nd Degree: Some P waves are not conducted through the AV node; more P waves than QRS complexes.

    • 3rd Degree: Complete block; atria and ventricles beat independently.

  • Ventricular Fibrillation: Irregular, chaotic EKG deflections; seen in acute heart attack or electrical shock.

Key Terms:

• P wave: Atrial depolarization

• QRS complex: Ventricular depolarization

• T wave: Ventricular repolarization

Example: In a patient with a missing P wave, the AV node paces the heart at a slower rate.

Heart Problems (Mechanical)

Valve Function and Heart Sounds

The heart contains atrioventricular (AV) and semilunar (SL) valves that regulate blood flow and prevent backflow.

• AV Valves: Open when atrial pressure exceeds ventricular pressure; close during ventricular contraction to prevent backflow.

• SL Valves: Open when ventricular pressure exceeds arterial pressure; close when blood attempts to flow back into the ventricles.

• Heart Sounds:

  • "Lub": Closure of AV valves (beginning of ventricular systole)

  • "Dup": Closure of SL valves (beginning of ventricular diastole)

Valve Disorders

• Incompetent Valve: Leaky valve causes blood to backflow, requiring the heart to re-pump the same blood.

• Stenosis: Stiff, thickened, or fused valves restrict blood flow; heart must work harder to pump blood.

• Treatment: Mechanical, animal, or cadaver valves may be used for replacement.

Problems Outside the Heart

• Pericarditis: Inflammation of the pericardium; causes friction rub (creaking sound).

• Cardiac Tamponade: Excess fluid in pericardial sac compresses the heart, limiting pumping ability and decreasing ventricular volume.

Stroke Volume and Cardiac Output

Definitions and Formulas

Stroke volume and cardiac output are key measures of heart function.

• Stroke Volume (SV): Amount of blood pumped by each ventricle per beat.

• Formula:

• Cardiac Output (CO): Volume of blood pumped by each ventricle per minute.

• Formula:

Example Calculation:

If HR = 75 beats/min and SV = 70 mL/beat:

Regulation of Stroke Volume

• Preload: Degree of stretch of cardiac muscle before contraction (Starling's Law).

• Contractility: Strength of contraction at a given muscle length; increased by sympathetic stimulation and positive inotropic agents (e.g., epinephrine, digitalis).

• Afterload: Pressure ventricles must overcome to eject blood; increased by high arterial blood pressure.

Factors Affecting SV:

• Preload (EDV)

• Contractility

• Afterload (ESV)

Additional info: Well-conditioned athletes have lower resting heart rates but higher stroke volumes, resulting in efficient cardiac output.

Smooth Muscle

Organization and Structure

Smooth muscle is found in the walls of most hollow organs (except the heart) and is responsible for involuntary movements such as peristalsis.

• Layers: Typically arranged in longitudinal and circular layers.

• Single-unit (visceral) Smooth Muscle: Electrically coupled by gap junctions; contracts as a unit; found in hollow organs.

• Multi-unit Smooth Muscle: Functions more like skeletal muscle; found in large airways, arteries, arrector pili muscles, and the iris.

Microscopic Structure

• Spindle-shaped fibers: Short, thin, single nucleus.

• No sarcomeres, myofibrils, or T-tubules.

• Dense bodies: Anchor thin filaments to sarcolemma; correspond to Z discs in skeletal muscle.

• Myofilament arrangement: Thick (myosin) filaments have heads along their entire length; thin (actin) filaments are spirally arranged.

• Calmodulin: Protein that binds Ca2+ (instead of troponin).

Excitation-Contraction Coupling

• Ca2+ influx from extracellular fluid and sarcoplasmic reticulum.

• Ca2+ binds to calmodulin, activating myosin light chain kinase.

• Myosin is phosphorylated, allowing cross-bridge formation with actin.

• Relaxation occurs when Ca2+ is removed and myosin is dephosphorylated by myosin light chain phosphatase.

Regulation of Contraction

• Contraction can be triggered by neurotransmitters, hormones, or local chemical changes.

• Can contract in response to stretch or chemical stimuli, even without nerve supply.

• Maintains contraction for prolonged periods with low energy cost (slow ATPases).

Comparison Table: Skeletal, Cardiac, and Smooth Muscle

Additional info: Smooth muscle can contract and relax slowly but maintain tension for long periods, which is important for functions such as maintaining blood vessel tone and moving food through the digestive tract.