Chapter 18: The Cardiovascular System – The Heart (BIO 221 Human Physiology Study Notes)

Chapter 18: The Cardiovascular System – The Heart

Learning Objectives

• Compare and contrast skeletal and cardiac muscle

• Describe the two types of myocytes within the heart: pacemaker cells and contractile cells

• Describe how pacemaker cells make up the electrical conduction system (SA Node, AV Node, Bundle of His, Purkinje fibers)

• Draw and explain pacemaker cell and contractile cell action potentials, including the plateau phase

• Explain how contractile cells generate the force of contraction to move blood through the heart and valves

• Describe the stages of the cardiac cycle: systole and diastole

• Explain factors that influence stroke volume, heart rate, and cardiac output

Skeletal Muscle vs. Cardiac Muscle

Similarities

• Both are striated: They have a banded appearance due to organized sarcomeres.

• Myofibrils contain typical sarcomeres: Both muscle types use the same contractile units.

• Sliding filament mechanism: Contraction occurs as actin and myosin filaments slide past each other.

Differences

Feature	Skeletal Muscle	Cardiac Muscle
Structure	Striated, long, cylindrical, multinucleate	Striated, short, branched, one or two nuclei per cell
Gap junctions between cells	No	Yes (intercalated discs)
Contracts as a unit	No, motor units must be stimulated individually	Yes, gap junctions create a functional syncytium
T tubules	Abundant	Fewer, wider
Sarcoplasmic reticulum	Elaborate; has terminal cisterns	Less elaborate; no terminal cisterns
Source of Ca2+ for contraction	Sarcoplasmic reticulum only	Sarcoplasmic reticulum and extracellular fluid
Ca2+ binds to troponin	Yes	Yes
Pacemaker cells present	No	Yes
Tetanus possible	Yes	No (long refractory period prevents tetanus)
Supply of ATP	Aerobic and anaerobic (fewer mitochondria)	Aerobic only (more mitochondria)

Key Points

• Intercalated discs in cardiac muscle contain gap junctions (for ion movement) and desmosomes (for mechanical strength).

• Functional syncytium: Cardiac muscle cells contract as a unit due to electrical coupling via gap junctions.

• Skeletal muscle requires neural input for contraction (voluntary), while cardiac muscle is autorhythmic (contracts without neural input).

Types of Cardiac Muscle Cells

Pacemaker Cells (Autorhythmic Cells)

• Comprise about 1% of cardiac muscle cells.

• Initiate and conduct action potentials responsible for contraction of working cells.

• Located in the SA node, AV node, Bundle of His, and Purkinje fibers.

• Do not have a true resting membrane potential; instead, they have a pacemaker potential (slow depolarization).

Contractile Cells

• Comprise about 99% of cardiac muscle cells.

• Responsible for the mechanical work of pumping blood.

• Have a stable resting membrane potential.

The Cardiac Conduction System

Spread of Depolarization

• Intrinsic depolarization triggers cardiac muscle contraction.

• Cardiac muscle cells are connected by gap junctions, but conduction via gap junctions alone is too slow for coordinated contraction.

• The cardiac conduction system (pacemaker cells) ensures rapid and coordinated spread of depolarization.

Major Components

1. Sinoatrial (SA) node: Sets the pace of the heart (fastest, ~75 bpm)

2. Atrioventricular (AV) node: Delays impulse (~0.1 sec), slower (~50 bpm)

3. AV bundle (Bundle of His)

4. Right and left bundle branches

5. Subendocardial conducting network (Purkinje fibers): Slowest (~30 bpm)

Sequence of Conduction

• Depolarization starts at the SA node, spreads through atria, delayed at AV node, then rapidly through the ventricles via the Bundle of His and Purkinje fibers.

• Depolarization immediately precedes contraction.

Membrane Potential and Action Potentials

Membrane Potential

• Membrane voltage (potential) is due to unequal ion distributions across the cell membrane.

• Resting membrane potential is typically negative inside the cell due to constant loss of K+ (potassium) ions.

• Maintained by Na+/K+ pumps and selective ion channels.

Pacemaker Cell Action Potential

• Pacemaker cells do not have a true resting membrane potential; instead, they have a pacemaker potential (slow depolarization).

• Three main phases:

  1. Pacemaker potential: Slow depolarization due to opening of Na+ (funny) channels and closing of K+ channels.

  2. Depolarization: When threshold is reached, Ca2+ channels open, causing rapid depolarization.

  3. Repolarization: Ca2+ channels close, K+ channels open, and K+ leaves the cell, returning the membrane potential to its most negative value.

Summary Table: Pacemaker Cell Action Potential

Phase	Main Ion Movement	Channels Involved
Pacemaker potential	Na+ in, K+ out (decreasing)	Funny Na+ channels open, K+ channels close
Depolarization	Ca2+ in	Ca2+ channels open
Repolarization	K+ out	K+ channels open

Key Equations

• Resting membrane potential (Nernst equation): $E_{ion} = \frac{RT}{zF} \ln \left( \frac{[ion]_{out}}{[ion]_{in}} \right)$

Electrocardiography (ECG)

Overview

• An ECG is a graph of the overall electrical activity generated by the whole heart, not a single action potential.

• It is used to detect abnormal heart rates, rhythms, and heart muscle damage.

Key Components of the ECG

• P wave: Atria depolarizing

• QRS complex: Ventricles depolarizing (and atria repolarizing)

• T wave: Ventricles repolarizing

Cardiac Muscle Contraction and the Cardiac Cycle

Contractile Cells and Force Generation

• Contractile cells provide the force for pumping blood through the heart and into the circulatory system.

• Blood travels through two separate vascular loops:

  • Pulmonary circulation: Between the heart and lungs (right side)

  • Systemic circulation: Between the heart and all body systems (left side)

• Both sides of the heart pump equal volumes of blood, but the left ventricle has a much thicker wall to generate higher pressure for systemic circulation.

Heart Valves

• Maintain unidirectional flow of blood.

• Open and close in response to pressure differences.

• Types: Atrioventricular (AV) valves and semilunar valves.

The Cardiac Cycle

• Consists of alternating periods of systole (contraction and emptying) and diastole (relaxing and filling).

• Key volumes:

  • End-diastolic volume (EDV): Volume of blood in ventricle at end of filling

  • End-systolic volume (ESV): Volume of blood remaining after contraction

  • Stroke volume (SV): Amount of blood pumped per beat $SV = EDV - ESV$

Cardiac Output

• Volume of blood pumped by each ventricle per minute.

• Calculated as: $CO = SV \times HR$ where CO is cardiac output, SV is stroke volume, and HR is heart rate.

Regulation of Cardiac Output

• Both stroke volume and heart rate can be modified to increase or decrease cardiac output.

• Neural control:

  • Sympathetic nervous system increases heart rate and contractility.

  • Parasympathetic nervous system decreases heart rate.

• Factors affecting stroke volume:

  1. Preload: Degree of stretch of heart muscle before contraction (increased by venous return)

  2. Contractility: Strength of contraction at a given muscle length (increased by sympathetic stimulation and Ca2+ availability)

  3. Afterload: Back pressure exerted by arterial blood (increased by hypertension)

Summary Table: Factors Affecting Stroke Volume

Factor	Effect
Preload	Increased preload increases EDV and SV
Contractility	Increased contractility decreases ESV and increases SV
Afterload	Increased afterload increases ESV and decreases SV

Clinical Application: High Blood Pressure and Heart Failure

• High blood pressure increases afterload, making the heart work harder to eject blood.

• Chronic high afterload can lead to pathological changes and heart failure (inability of cardiac output to meet the body's needs).

Key Terms and Concepts

• Autorhythmicity: The ability of cardiac muscle cells to generate their own action potentials without external stimulation.

• Functional syncytium: Cardiac muscle cells contract as a coordinated unit due to electrical coupling via gap junctions.

• Pacemaker potential: The slow, spontaneous depolarization of pacemaker cells that leads to action potential generation.

• Plateau phase: A sustained depolarization in contractile cell action potentials due to Ca2+ influx, preventing tetanus.

Summary

• The heart is a dual pump composed of specialized muscle cells that contract rhythmically and in coordination to maintain blood flow throughout the body.

• Pacemaker cells set the rhythm, while contractile cells provide the force for circulation.

• Electrical and mechanical events in the heart are tightly regulated to ensure efficient and effective pumping.