May compress the heart externally, impairing relaxation and filling.
Cardiac Tamponade
Life-threatening condition caused by pericardial effusion.
The heart is unable to pump enough blood to the body.
Treatment involves aspirating the serous fluid from the pericardial space using a long needle.
Double Pump and Two Circulations
The heart functions as a double pump: the right heart and the left heart.
Right Heart
Receives unoxygenated blood from the superior and inferior venae cavae.
Pumps blood to the lungs for oxygenation.
Pulmonary Circulation
The path blood takes from the right heart to the lungs and back to the left heart.
Function: To oxygenate blood and remove carbon dioxide.
Oxygen diffuses from the lungs into the blood, while carbon dioxide diffuses from the blood into the lungs for excretion.
Left Heart
Receives oxygenated blood from the lungs.
Pumps blood to all organs of the body.
Systemic Circulation
The path blood takes from the left heart to the body's organs and back to the right heart.
The larger of the two circulations.
Heart Chambers and Great Vessels
The heart has four chambers: two atria and two ventricles.
Atria
The upper chambers, which receive blood into the heart.
Ventricles
The lower chambers, which pump blood out of the heart.
Septa
The right and left hearts are separated by septa.
Interatrial septum: Separates the two atria.
Interventricular septum: Separates the two ventricles.
Detailed Chamber Anatomy
Right Atrium
A thin-walled cavity that receives unoxygenated blood from the superior and inferior venae cavae.
Superior vena cava: Collects blood from the head and upper body.
Inferior vena cava: Receives blood from the lower body.
Right Ventricle
Receives unoxygenated blood from the right atrium.
Pumps blood through the pulmonary arteries to the lungs.
Left Atrium
A thin-walled cavity that receives oxygenated blood from the lungs through four pulmonary veins.
Has a small ear-shaped sac called the left atrial appendage, which is a site of thrombus formation in atrial fibrillation.
Left Ventricle
Receives oxygenated blood from the left atrium.
Pumps blood into the systemic circulation through the aorta, the largest artery in the body.
Myocardial Thickness
The myocardial layer of the ventricles is thicker than that of the atria.
The left ventricular myocardium is thicker than the right ventricular myocardium due to the greater force required to pump blood into the systemic circulation.
Ventricular hypertrophy: Enlargement of a ventricle due to overwork.
Example: Chronic hypertension (high blood pressure) can cause left ventricular hypertrophy.
Pulmonary artery hypertension can cause right ventricular hypertrophy and right heart failure.
Great Vessels of the Heart
The large blood vessels attached to the heart include:
Superior and inferior venae cavae
Pulmonary trunk
Four pulmonary veins
Aorta
Heart Valves
The heart has four valves that maintain unidirectional blood flow.
They are located at the entrances and exits of the ventricles.
Atrioventricular Valves (AV Valves)
Located between the atria and the ventricles.
Entrance valves that allow blood to enter the ventricles.
Semilunar Valves
Control the outflow of blood from the right and left ventricles.
Exit valves.
Atrioventricular Valve Mechanics
AV valves have cusps or leaflets.
When the ventricles are relaxed, the cusps hang loosely, allowing blood to flow from the atria into the ventricles.
AV Valve Closure
The AV valves close when the ventricles contract, increasing pressure and pushing blood against the cusps.
Chordae tendineae and papillary muscles prevent the cusps from being pushed into the atria.
Chordae Tendineae
Tough fibrous bands that attach the cusps to the ventricular walls.
Papillary Muscles
Located in the ventricular walls; they contract and pull on the chordae tendineae to hold the cusps in a closed position.
Tricuspid Valve
The right AV valve, located between the right atrium and right ventricle.
Has three cusps.
Prevents backflow of blood from the right ventricle to the right atrium.
Bicuspid (Mitral) Valve
The left AV valve, located between the left atrium and left ventricle.
Has two cusps.
Prevents backflow of blood from the left ventricle to the left atrium.
Semilunar Valves Mechanics
The two semilunar valves are the pulmonic and aortic semilunar valves.
Pulmonic Valve
Located between the right ventricle and the pulmonary trunk.
Opens when the right ventricle contracts, allowing blood to flow into the pulmonary trunk.
Closes when the right ventricle relaxes, preventing backflow of blood from the pulmonary trunk into the right ventricle.
Aortic Valve
Located between the left ventricle and the aorta.
Opens when the left ventricle contracts, allowing blood to flow into the aorta.
Closes when the left ventricle relaxes, preventing backflow of blood from the aorta into the left ventricle.
Semilunar Valve Closure
Semilunar valves close when the pressure in the pulmonary trunk and aorta exceeds the pressure in the relaxed ventricles.
Valve Malfunctions
Stenosis
Narrowing of a valve, making it difficult for the heart to pump blood through it.
Increases the workload of the pumping chamber and may cause ventricular hypertrophy and heart failure.
Incompetent (Leaky) Valve
Allows blood to regurgitate back into the chamber from which it was pumped.
Increases the workload of the heart and may lead to heart damage.
Heart Sounds
Heart sounds ("lubb-dupp") are caused by the vibrations of the heart valves closing.
Murmurs
Abnormal heart sounds caused by faulty valves.
S1 (lubb)
The first heart sound, caused by the closure of the AV valves at the beginning of ventricular contraction.
Best heard over the apex of the heart.
S2 (dupp)
The second heart sound, caused by the closure of the semilunar valves at the beginning of ventricular relaxation.
Best heard at the base of the heart.
Extra Heart Sounds (S3 and S4)
Caused by rapid blood flow into the ventricles.
When both S3 and S4 are heard, it creates a "gallop rhythm."
Pathway of Blood Flow Through the Heart
Unoxygenated blood enters the right atrium from the superior and inferior venae cavae.
Blood flows through the tricuspid valve into the right ventricle.
From the right ventricle, blood flows through the pulmonic valve into the pulmonary trunk.
The pulmonary trunk branches into the right and left pulmonary arteries, which carry blood to the lungs for gas exchange.
Oxygenated blood flows through four pulmonary veins from the lungs into the left atrium.
From the left atrium, blood flows through the bicuspid (mitral) valve into the left ventricle.
Left ventricular contraction forces blood through the aortic valve into the aorta for systemic circulation.
Blood Supply to the Myocardium
The myocardium is nourished by the coronary arteries, not the blood flowing through the heart chambers.
Coronary arteries arise from the base of the ascending aorta, distal to the aortic semilunar valve.
Coronary Arteries
The two main coronary arteries are the left and right coronary arteries.
Right Coronary Artery
Nourishes the right side of the heart, especially the right ventricle.
Supplies blood to the SA node and AV node.
Left Coronary Artery
Branches into the left anterior descending (LAD) artery and the circumflex artery.
Supplies blood to the left side of the heart, especially the left ventricular wall and interventricular septum.
Coronary Veins
Collect blood from the myocardium and carry it to the coronary sinus, which empties into the right atrium.
Characteristics of Coronary Blood Flow
Coronary blood flow can increase to meet the heart's oxygen demands.
Coronary arteries can dilate to increase blood flow during exertion.
Anastomoses
Coronary arteries can form anastomoses (multiple connections) to provide alternative routes for blood flow.
Collateral blood vessels develop in response to chronic diminished coronary blood flow.
Coronary blood flow is greatest during myocardial relaxation because contraction compresses the coronary arteries.
During a "racing heart," shortened relaxation can decrease coronary blood flow and cause angina.
What-Ifs of Poor Coronary Artery Blood Flow
Ischemia
Diminished coronary blood flow leads to oxygen deprivation (ischemia).
Angina: Chest pain that often radiates to the left shoulder and arm.
Relieved by rest and drugs like nitroglycerin and beta-adrenergic blockers.
Myocardial Infarction (MI or Heart Attack)
Complete blockage of coronary blood flow leads to the death of myocardial cells.
Symptoms: Nausea, vomiting, diaphoresis, and severe crushing chest pain.
Occlusion of the LAD artery is called the "widow maker."
Atypical symptoms (fatigue, digestive issues) may occur in older people and women.
Time to treatment is crucial to minimize myocardial damage.
Cardiac Enzymes and Leaky Cells
Dead myocardial cells release enzymes into the blood.
Elevated plasma levels of creatine phosphokinase (CPK), aspartate aminotransferase (AST), and lactic dehydrogenase (LDH) indicate MI.
Troponin, a myocardial protein, also leaks into the blood.
Cardiac Conduction System
The heart's conduction system initiates and coordinates electrical signals to control heart muscle contraction and relaxation.
First, both atria must contract, forcing blood into the relaxed ventricles.
Then, the ventricles contract, forcing blood out of the heart.
Components of the Cardiac Conduction System
SA node
Atrial conducting fibers
AV node
His-Purkinje system
Sinoatrial (SA) Node
Located in the upper posterior wall of the right atrium.
Initiates the cardiac impulse (action potential).
Fires 60 to 100 times per minute (average 72 times per minute).
The pacemaker of the heart.
Ectopic: Electrical signal originates outside of the SA node.
Atrial Conducting Fibers
Spread the cardiac impulse from the SA node throughout both atria.
Transmit the signal to the AV node.
Atrioventricular (AV) Node
Located in the floor of the right atrium, near the interatrial septum.
Functions:
Acts as a path for the cardiac impulse to travel from the atria to the ventricular bundle of His.
Slows down the cardiac impulse to allow the ventricles time to fill with blood during atrial contraction.
His-Purkinje System
Bundle of His: Specialized conduction tissue in the interventricular septum.
Divides into right and left bundle branches.
Purkinje fibers: Distribute the cardiac impulse rapidly throughout the ventricles, ensuring coordinated contraction.
Automaticity and Rhythmicity of the Heart
Automaticity: The ability of cardiac pacemaker cells to generate their own electrical signals without external nerve stimulation.
Rhythmicity: The regularity with which cardiac tissue fires a cardiac impulse.
Pacemaker Cells
SA node: Sets the heart rate between 60 and 100 beats per minute.
AV node: Can take over as pacemaker at a slower rate of 40 to 60 beats per minute if the SA node fails.
Ventricles: Can assume the pacemaker role at a rate of 30 to 40 beats per minute.
Impaired pacemaker activity often requires an artificial pacemaker.
What-Ifs of Electrical Malfunction
Dysrhythmias
Disturbances in the heart's rhythm.
Tachydysrhythmias: Caused by excess electrical activity.
Bradyarrhythmias: Characterized by diminished electrical activity.
Ventricular Fibrillation
A life-threatening dysrhythmia that prevents effective cardiac muscle contraction and pumping of blood.
Electrocardiogram (ECG)
Measures the electrical activity of the heart using electrodes on the chest.
Components:
P wave: Reflects atrial depolarization.
QRS complex: Reflects ventricular depolarization.
T wave: Reflects ventricular repolarization.
The P-R interval represents the time it takes for the cardiac impulse to travel from the atria to the ventricles.
Normal sinus rhythm (NSR) indicates that the ECG appears normal and the impulse originates in the SA node.
Pacemaker Cells
Resting membrane potential (RMP): The electrical charge across the membrane of a resting cell (negative).
Depolarization: When the cell is stimulated, it depolarizes to threshold potential.
Repolarization: The cell returns to its negative RMP after depolarization.
Spontaneous depolarization: Pacemaker cells spontaneously depolarize to threshold potential.
Pacemaker potential: The slope of spontaneous depolarization.
Factors Affecting Spontaneous Depolarization
Autonomic nerve stimulation:
Sympathetic stimulation increases the rate of spontaneous depolarization, increasing heart rate.
Parasympathetic (vagal) stimulation slows the rate of spontaneous depolarization, decreasing heart rate.
Hormones and drugs also affect depolarization rates.
Nodal Rhythm
If the SA node slows down or the AV node increases activity, the AV node can become the pacemaker.
A diseased SA node can cause the AV node to take over as pacemaker, generating a heart rate of 40 to 50 beats/min.
Ventricular Pacemaker Cells
In complete heart block (no electrical signal crosses the AV node), ventricular pacemaker cells take over.
Generate a very slow heart rate of 30 to 40 beats/min, decreasing cardiac output.
Requires an artificial pacemaker to increase the heart rate and cardiac output.