Part 2 - Chapter 19: Heart - COMPLETE Learning Objectives

define and distinguish between the pulmonary circuit and systemic circuit.

Pulmonary circuit: Right side of the heart.

- Sends blood to the lungs via pulmonary trunk.

- Oxygen-Poor Blood.

Systemic circuit: Left side of the heart.

- Newly oxygenated blood to every tissue cell in the body.

- Returned back to the heart via aorta.

- Thicker and more muscular than the pulmonary side.

describe the general location, size, and shape of the heart.

The heart is located in the thoracic cavity, in the mediastinum, between the lungs.

Base: wide, superior portion of the heart, all major/ large vessels attach here.

Apex: Tapered inferior end, tilts to the left.

At any age, the heart is the size of a fist.

Identify the four chambers of the heart and identify the four valves of the heart.

• Four chambers:

  • Right and left atria: "Two superior chambers, receive blood returning to heart."

  • Right and left ventricles: "Two inferior chambers, pump blood into arteries."

• Valves:

  • Atrioventricular (AV) valves:

    • Right AV valve: "Three cusps (tricuspid valve)."

    • Left AV valve: "Two cusps (mitral valve, formerly ‘bicuspid’)."

    • Chordae tendineae: "Cords connect AV valves to papillary muscles on floor of ventricles, prevent AV valves from flipping or bulging into atria when ventricles contract."

  • Semilunar valves:

    • Pulmonary semilunar valve: "In opening between right ventricle and pulmonary trunk."

    • Aortic semilunar valve: "In opening between left ventricle and aorta."

Describe the pericardium that encloses the heart and describe the three layers of the heart wall.

Pericardium: "Double-walled sac that encloses the heart... allows heart to beat without friction, provides room to expand, yet resists excessive expansion."

• Fibrous pericardium: "Outer wall, not attached to heart."

• Serous pericardium:

  • Parietal layer: "Lines fibrous pericardium."

  • Visceral layer (epicardium): "Covering heart surface."

• Pericardial cavity: "Space between parietal and visceral layers of serous pericardium, filled with pericardial fluid."

Trace the flow of blood through the four chambers and valves of the heart and adjacent blood vessels.

• Ventricles relax:

  • "Pressure drops inside the ventricles."

  • "Semilunar valves close as blood attempts to back up into the ventricles from the vessels."

  • "AV valves open."

  • "Blood flows from atria to ventricles."

• Ventricles contract:

  • "AV valves close as blood attempts to back up into the atria."

  • "Pressure rises inside of the ventricles."

  • "Semilunar valves open and blood flows into great vessels."

Blood flows from the body into the right atrium through the superior and inferior vena cavae, then through the tricuspid valve into the right ventricle. From there, it is pumped through the pulmonary valve into the pulmonary arteries to the lungs for oxygenation. Oxygenated blood returns to the left atrium via the pulmonary veins, passes through the mitral valve into the left ventricle, and is then pumped through the aortic valve into the aorta, supplying the body.

Describe the arteries that nourish the myocardium and the veins that drain it.

The coronary arteries supply oxygenated blood to the myocardium, while the cardiac veins drain deoxygenated blood from the heart muscle back to the right atrium.

The arteries that nourish the myocardium (heart muscle) are called coronary arteries which branch off from the aorta at the base of the heart, with the main two being the left coronary artery and the right coronary artery; these arteries further divide into smaller branches to supply blood to different parts of the heart muscle. The veins that drain the myocardium are called coronary veins which collect deoxygenated blood from the heart muscle and eventually empty into the right atrium via the coronary sinus. 

Describe the heart's pacemaker and internal electrical conduction system.

Conduction system: "Coordinates the heartbeat: Composed of an internal pacemaker and conduction pathways through myocardium."

• Sinoatrial (SA) node: "Modified cardiomyocytes, located in right atrium near base of superior vena cava. Pacemaker, initiates each heartbeat and determines heart rate."

• Atrioventricular (AV) node: "Located near the right AV valve at lower end of interatrial septum. Electrical gateway to the ventricles."

• Atrioventricular (AV) bundle (bundle of His): "Branches pass through interventricular septum toward apex, forks into right and left bundle branches."

• Subendothelial conducting networks (Purkinje fibers): "Nerve-like processes spread throughout ventricular myocardium. Cardiomyocytes then pass signal from cell to cell through gap junctions."

The heart's pacemaker, known as the Sinoatrial (SA) node, is located in the right atrium near base of superior vena cava.

Generates electrical impulses that initiate each heartbeat + determines rate.

Signals travels throughout Atria.

*Atrioventricular (AV) node, bundle of His, and Subendothelial Conducting Networks (Purkinje fibers), coordinating the contraction of the heart chambers.*

Describe the unique structural and metabolic characteristics of cardiac muscle.**

Designed for endurance.

Cardiomyocytes: are smaller, striated, short, branched, one central nucleus, allowing for synchronized contractions.

Repair for damage of cardiac muscle is almost entirely by fibrosis (Scarring).

Intercalated Discs: Join Cardiomyocytes end to end.

Mechanical Junctions (Desmosomes): Tight mechanical linkages that prevent contracting cardiomyocytes from being pulled apart from each other. Zippers that hold the heart together.

Electrical Junctions (Gap Junction): Allows ions to flow between cells; can stimulate neighboring, entire myocardium of either two atria or two ventricles acts like, single, unified cell.

Interdigitating folds: folds interlock with each other, and increase surface area of contact.

Cardiac muscle depends almost exclusively on aerobic respiration to make ATP.

• Rich in myoglobin and glycogen

• Huge mitochondria

Explain why the SA node fires spontaneously and rhythmically and explain how the SA node excites the myocardium.

The SA node fires spontaneously because: The SA Node firing: "SA node does not have a stable resting membrane potential, starts at −60 mV and drifts upward due to slow Na+ inflow. Gradual depolarization is called pacemaker potential."

• "When it reaches threshold of −40 mV, voltage-gated fast Ca2+ and Na+ channels open."

• "Faster depolarization occurs peaking at 0 mV."

• "K+ channels then open and K+ leaves the cell, causing repolarization."

• "Once K+ channels close, pacemaker potential starts over."

• "When SA node fires, it sets off heartbeat."

SA node does not have a stable resting membrane potential; it starts at -60 mV and drifts upward due to slow Na+ inflow (leakage), leading to gradual depolarization known as the 'pacemaker potential'. Once it reaches a threshold of -40 mV, voltage-gated fast Ca2+ and Na+ channels open, causing a quick depolarization that initiates each heartbeat.

The SA node excites the myocardium: When the SA node fires, it sends electrical impulses that stimulate both atria to contract almost simultaneously, reaching the AV node in about 50 ms. This leads to the signal slowing down through the AV node (delaying for 100 ms to allow ventricles to fill), followed by rapid conduction through the AV bundle and the subendothelial conducting network (punjenke muscles?), causing the entire ventricular myocardium to depolarize and contract in near unison.

describe in detail one complete cycle of heart contraction and relaxation. (Wigger's Diagram, Figure 19.20)

A graphical representation of the cardiac cycle showing the relationship between the heart electrical activity, pressure changes, volume changes, and heart sounds during one complete heartbeat.

This would describe the progression through the four phases of the cardiac cycle:

ventricular filling

isovolumetric contraction

ventricular ejection

isovolumetric relaxation.

describe the unusual action potentials of cardiac muscle and relate them to the contractile behavior of the heart.

• Cardiac muscle action potentials consist of three phases:

  1. Depolarization: Rapid influx of Na+.

  2. Plateau: Slow influx of Ca2+ maintains depolarization and sustains contraction.

  3. Repolarization: Efflux of K+ restores resting potential.

Interpret a normal electrocardiogram.

Normal Electrocardiogram (ECG):

• P wave: Atrial Depolarization

  "SA node fires, atria depolarize and contract; atrial systole begins immediately after SA signal."

• QRS complex: Ventricular depolarization

  "Ventricular depolarization; complex shape of spike due to different thickness and shape of the two ventricles."

• T wave: Ventricular Repolarization

  "Ventricular repolarization and relaxation."

describe how changes in blood pressure operate the heart valves and explain what causes heartbeat sounds.

• Blood Pressure and Valves:

  • "Opening and closing of valves are governed by pressure changes."

  • Heart sounds:

    • S1 (first heart sound): "Louder and longer 'lubb,' occurs with closure of AV valves, turbulence in the bloodstream, and movements of the heart wall."

    • S2 (second heart sound): "Softer and sharper 'dupp,' occurs with closure of semilunar valves, turbulence in the bloodstream, and movements of the heart wall."

Relate the events of the cardiac cycle to the volume of blood entering and leaving the heart.

Ventricular filling (during ventricular diastole):

"Ventricles expand and their pressure drops below that of the atria."

"AV valves open and blood flows into the ventricles."

"Filling occurs in three phases: Rapid ventricular filling, Diastasis, Atrial systole."

"End-diastolic volume (EDV) achieved in each ventricle (about 130 mL of blood)."

Isovolumetric contraction (during ventricular systole):

• "Atria repolarize, relax, and remain in diastole for the rest of the cardiac cycle."

• "Ventricles depolarize, causing QRS complex, and begin to contract."

• "AV valves close as ventricular blood surges back against the cusps."

"Although ventricles contract, they do not eject blood because the aorta and pulmonary trunk pressures are still higher than those in the ventricles."

Ventricular ejection (during ventricular systole):

• "Begins when ventricular pressure exceeds arterial pressure and semilunar valves open."

• "First: rapid ejection: blood spurts out of ventricles quickly."

• "Then: reduced ejection: slower flow with lower pressure."

• "Stroke volume (SV) is ~70 mL."

"Remaining blood (~60 mL) is end-systolic volume (ESV) = EDV − SV."

Isovolumetric relaxation (during ventricular diastole):

• "T wave ends and ventricles begin to expand."

• "Blood from aorta and pulmonary trunk briefly flows backward filling cusps and closing semilunar valves."

• "Creates pressure rebound that appears as dicrotic notch in graph of artery pressure."

• "Heart sound S2 occurs.” "Ventricles are not taking in any blood until AV valves open again."

Trace the nerve routes of the sympathetic and parasympathetic nervous systems to their target cells in the heart.

• Sympathetic innervation:

  • Cardioacceleratory sends sympathetic innervation."

  • Sympathetic nerves increase heart rate and contraction strength: Fibers terminate in SA and AV nodes, in atrial and ventricular myocardium, and also in the aorta, pulmonary trunk, and coronary arteries."

• Parasympathetic innervation:

  • Cardioinhibitory center sends parasympathetic innervation (via the vagus nerve)."

  • Parasympathetic nerves slow heart rate: Fibers of vagus nerve lead to the SA node and AV node, little or no vagal stimulation of the myocardium."

Define cardiac output, explain its importance, and identify the factors that govern cardiac output.

Cardiac output (CO): "Amount ejected by each ventricle in 1 minute."

• "Cardiac output = heart rate × stroke volume."

• "About 4 to 6 L/min at rest (5 L/min)."

• "A RBC leaving the left ventricle will arrive back at the left ventricle in about 1 minute."

• "Vigorous exercise increases CO to 21 L/min for a fit person and up to more than 40 L/min for a world-class athlete."

Discuss some of the nervous and chemical factors that alter heart rate, stroke volume, and cardiac output.

Factors affecting heart rate and stroke volume:

• Sympathetic activity: Increases heart rate and stroke volume by releasing norepinephrine, which accelerates depolarization of the SA node and increases contractility.

• Parasympathetic activity: Decreases heart rate by releasing acetylcholine, which slows the depolarization of the SA node.

• Hormonal influences:

  • Epinephrine and thyroid hormone increase heart rate and contractility.

• Physical exercise: Increases heart rate and stroke volume during exercise to meet the body’s increased demands for oxygen.

• Blood pressure and blood volume: High blood pressure or increased blood volume can increase cardiac output by increasing stroke volume, but excessive pressure can lead to reduced cardiac output.