Anatomy of the Heart Chapter 18

Location and Structure of the Heart

• The heart is located in the thoracic cavity, in the mediastinum, and is approximately the same size as a closed fist.
• The base of the heart is the wider, superior portion, and the apex is the point.
• The heart is protected by the pericardium, a tough, inelastic sac of fibrous connective tissue.

Pericardium

• The pericardium consists of two layers: the fibrous pericardium and the serous pericardium.
• The fibrous pericardium is attached to the diaphragm and anchors the heart, preventing it from overfilling.
• The serous pericardium has two layers: the parietal layer and the visceral layer.
• The parietal layer produces a serous fluid that reduces friction as the heart contracts and twists.
• The visceral layer forms the epicardium, the outer layer of the heart wall.

Heart Wall

• The heart wall has three layers: the epicardium, the myocardium, and the endocardium.
• The epicardium is the outer layer, a thin, transparent layer.
• The myocardium is the middle layer, a mass of cardiac muscle.
• The endocardium is the inner layer, a smooth lining for the chambers and valves.

Cardiac Muscle

• Cardiac muscle fibers are connected by intercalated discs, which allow action potentials to pass from fiber to fiber.
• Gap junctions and desmosomes ("spot welds") help to coordinate contractions and prevent cardiac fibers from separating.

Surface of the Heart

• The heart has external landmarks, including the atrioventricular grooves and the anterior/posterior interventricular sulcus.
• The coronary vessels run in these grooves, supplying blood to the heart.

Chambers of the Heart

• The heart has four compartments: the right and left atria, and the right and left ventricles.
• The interatrial septum separates the atria, and the interventricular septum separates the ventricles.
• The left ventricular wall is thicker than the right ventricular wall, as it must pump blood throughout the body and against gravity.

Blood Flow through the Heart

• The right atrium receives deoxygenated blood from the superior vena cava, inferior vena cava, and coronary sinus.
• The blood flows from the right atrium to the right ventricle, and then to the pulmonary trunk, which carries it to the lungs for gas exchange.
• The oxygenated blood from the lungs returns to the left atrium, and then flows to the left ventricle, which pumps it to the body via the aorta.

Myocardial Blood Supply

• The myocardium has its own blood supply, which branches off the aorta.
• The coronary vessels supply blood to the cardiac muscle, and the coronary veins return deoxygenated blood to the coronary sinus.
• The coronary circulation is crucial for the heart's function, and problems with it can lead to ischemia and infarction.

Pathologies

• Pericarditis is an inflammation of the pericardium, which can cause pain and damage to the lining tissues.
• Cardiac tamponade is a buildup of fluid in the pericardial cavity, which can lead to cardiac failure.
• Coronary circulation pathologies, such as emboli, atherosclerosis, and smooth muscle spasms, can cause ischemia and infarction.
• Angina pectoris is a classic chest pain caused by temporary myocardial ischemia.
• Myocardial infarction (heart attack) occurs when a thrombus or embolus blocks a coronary artery, leading to tissue death and scarring.

Valve Structure and Function

• The heart has four valves: the tricuspid and bicuspid valves, which separate the atria and ventricles, and the aortic and pulmonary semilunar valves, which prevent backflow of blood from the arteries.
• The valves are made of dense connective tissue covered by endocardium, and they open and close passively in response to pressure changes.
• Chordae tendineae and papillary muscles help to prevent the valves from opening too widely.

Conduction System and Pacemakers

• The heart has a conduction system, which generates and propagates electrical signals that control the heartbeat.
• The sinoatrial (SA) node is the primary pacemaker, firing 60-100 times per minute.
• The atrioventricular (AV) node and the bundle of His are also part of the conduction system.
• The Purkinje fibers are specialized fibers that transmit the electrical signals to the ventricular muscle.
• Arrhythmias are irregular rhythms, which can be caused by problems with the conduction system or pacemakers.### Cardiac Electrophysiology
• Fibrillation: rapid, fluttering, out of phase contractions, no pumping, heart resembles a squirming bag of worms
• Ectopic pacemakers: abnormal pacemakers controlling the heart due to SA node damage, caffeine, nicotine, electrolyte imbalances, hypoxia, or toxic reactions to drugs
• Heart block: AV node damage, severity determines outcome, may slow conduction or block it

Conduction System and Pacemakers

• SA node damage: AV node can take over, firing at 40-50 beats/min
• If AV node is out, AV bundle, bundle branch, and conduction fibers fire at 20-40 beats/min
• Artificial pacemakers: can stimulate single or dual chambers, can be activity-dependent

Atrial, Ventricular Excitation Timing

• 0.05 sec from SA to AV node
• 0.1 sec to get through AV node, conduction slows
• Allows atria time to finish contraction and better fill ventricles
• Once action potentials reach AV bundle, conduction is rapid to rest of ventricles

Extrinsic Control of Heart Rate

• Basic rhythm of the heart is set by internal pacemaker system
• Central control from medulla is routed via ANS to pacemakers and myocardium
• Sympathetic input: norepinephrine, increases heart rate
• Parasympathetic input: acetylcholine, decreases heart rate

Electrocardiogram

• Measures the sum of all electro-chemical activity in myocardium at any moment
• P wave, QRS complex, T wave

Cardiac Cycle: Electrical & Mechanical Events

• Systole: heart muscle contracts, pushes blood out of body
• Diastole: heart relaxes, fills with blood
• Isovolumetric contraction: heart contracts but no blood is ejected
• Isovolumetric relaxation: heart relaxes, blood flows in

Cardiac Output

• Amount of blood pumped by each ventricle in 1 minute
• Cardiac Output (CO) = Heart Rate x Stroke Volume
• Example: HR = 70 beats/min, SV = 70 ml/beat, CO = 4.9 L/min

Cardiac Reserve

• Cardiac Output is variable
• Cardiac Reserve = maximal output (CO) – resting output (CO)
• Average individuals have a cardiac reserve of 4X or 5X CO
• Trained athletes may have a cardiac reserve of 7X CO

Regulation of Stroke Volume

• SV = EDV – ESV
• EDV: End Diastolic Volume, volume of blood in heart after it fills
• ESV: End Systolic Volume, volume of blood in heart after contraction
• Each beat ejects about 60% of blood in ventricle
• Preload, contractility, and afterload are the 3 most important factors in regulating SV

Regulation of Stroke Volume (continued)

• Preload: degree of stretching of cardiac muscle cells before contraction
• Contractility: increase in contractile strength separate from stretch and EDV
• Afterload: pressure that must be overcome for ventricles to eject blood from heart

Preload: Frank-Starling Law of the Heart

• Length-tension relationship of heart, where Length = EDV and Tension = SV
• Normally, muscle fibers are shorter than optimal length, increasing/decreasing fiber length increases/decreases force generation

Contractility

• The contractile strength at a given muscle length
• Sympathetic nervous stimulation opens Ca channels, increases contractility
• Positive inotropic effects: increase contractility, e.g., glucagon, thyroxin, epinephrine, digitalis
• Negative inotropic effects: reduce contractility, e.g., acidosis, high extracellular K, calcium channel blockers

Afterload

• If blood pressure is high, it is difficult for the heart to eject blood
• More blood remains in the chambers after each beat
• Heart has to work harder to eject blood, due to increased length/tension of cardiac muscle cells

Regulation of Heart Rate

• Normally, SV is constant
• Control of CO is exerted through changes in heart rate
• Intrinsic controls: Bainbridge effect, increase in EDV increases HR
• Extrinsic controls: autonomic nervous system, hormones, ions, body temperature, age/gender, body mass/blood volume, exercise, stress/illness