Untitled Quiz

Cardiovascular System: The Heart - Anatomy

• The heart is approximately the size of a fist.
• Location: superior surface of diaphragm, left of midline, anterior to vertebral column, posterior to sternum.
•  The heart has coverings called pericardium which is a double-walled sac composed of superficial fibrous pericardium and a deep two-layer serous pericardium.
• The parietal layer of serous pericardium lines the internal surface of the fibrous pericardium.
• The visceral layer (epicardium) lines the surface of the heart.
• They are separated by the fluid-filled pericardial cavity.

Heart Wall

• Epicardium: visceral layer of the serous pericardium.
• Myocardium: cardiac muscle layer forming the bulk of the heart.
• Fibrous skeleton of the heart: crisscrossing, interlacing layer of connective tissue.
• Endocardium: endothelial layer of the inner myocardial surface.

External Heart: Major Vessels

• Vessels returning blood to the heart include: superior and inferior vena cavae, right and left pulmonary veins.
• Vessels conveying blood away from the heart include: pulmonary trunk, which splits into right and left pulmonary arteries; ascending aorta (three branches): brachiocephalic, left common carotid, and subclavian arteries.
• Arteries: right and left coronary (in atrioventricular groove), marginal, circumflex, and anterior interventricular arteries.
• Veins: small cardiac, anterior cardiac, and great cardiac veins.

Atria of the Heart

• Atria are the receiving chambers of the heart.
• Each atrium has a protruding auricle.
• Pectinate muscles mark atrial walls.
• Blood enters the right atria from the superior and inferior vena cavae and coronary sinus.
• Blood enters the left atria from pulmonary veins.

Ventricles of the Heart

• Ventricles are the discharging chambers of the heart.
• Papillary muscles and trabeculae carneae muscles mark ventricular walls.
• Right ventricle pumps blood into the pulmonary trunk.
• Left ventricle pumps blood into the aorta.

Myocardial Thickness and Function

• Thickness of myocardium varies according to the function of the chamber.
• Atria are thin-walled, delivering blood to adjacent ventricles.
• Ventricle walls are much thicker and stronger.
• The right ventricle supplies blood to the lungs (little flow resistance). The left ventricle wall is the thickest to supply systemic circulation.

Heart Valves

• Heart valves ensure unidirectional blood flow through the heart.
• Atrioventricular (AV) valves lie between the atria and the ventricles.
• AV valves prevent backflow into the atria when ventricles contract.
• Chordae tendineae anchor AV valves to papillary muscles.
• Semilunar valves prevent backflow of blood into the ventricles.
• Aortic semilunar valve lies between the left ventricle and the aorta.
• Pulmonary semilunar valve lies between the right ventricle and pulmonary trunk.

Atrioventricular Valve Function

• Blood returning to the heart fills the atria, putting pressure against atrioventricular valves; atrioventricular valves are forced open.
• As ventricles fill, atrioventricular valve flaps hang limply into ventricles.
• Atria contract, forcing additional blood into ventricles.
• Ventricles contract, forcing blood against atrioventricular valve cusps.
• Atrioventricular valves close.
• Papillary muscles contract, and chordae tendineae tighten, preventing valve flaps from everting into atria.

Semilunar Valve Function

• As ventricles contract and intraventricular pressure rises, blood is pushed up against semilunar valves, forcing them open.
• As ventricles relax and intraventricular pressure falls, blood flows back from arteries, filling the cusps of semilunar valves and forcing them to close.

Cardiac Muscle

• Cardiac muscle is striated, short, fat, branched, and interconnected.
• The connective tissue endomysium acts as both tendon and insertion.
• Intercalated discs anchor cardiac cells together and allow free passage of ions.
• Heart muscle behaves as a functional syncytium.

Heart Physiology

• Autorhythmic cells initiate action potentials.
• Autorhythmic cells have unstable resting potentials called pacemaker potentials.
• Autorhythmic cells use calcium influx (rather than sodium) for the rising phase of the action potential.
• The sinoatrial (SA) node generates impulses about 75 times/minute.
• The atrioventricular (AV) node delays the impulse approximately 0.1 second.
• Impulse passes from atria to ventricles via the atrioventricular bundle (Bundle of His).
• AV bundle splits into two pathways in the interventricular septum (bundle branches).
• Bundle branches carry the impulse toward the apex of the heart.
• Purkinje fibers carry the impulse to the heart apex and ventricular walls.

Heart Excitation Related to ECG

• SA node generates impulse; atrial excitation begins.
• Impulse delayed at AV node.
• Impulse passes to heart apex; ventricular excitation begins.

Extrinsic Innervation of the Heart

• Heart is stimulated by the sympathetic cardioacceleratory center.
• Heart is inhibited by the parasympathetic cardioinhibitory center.

Electrocardiography

• Electrical activity is recorded by electrocardiogram (ECG).
• P wave corresponds to depolarization of SA node.
• QRS complex corresponds to ventricular depolarization.
• T wave corresponds to ventricular repolarization.
• Atrial repolarization record is masked by the larger QRS complex.

Heart Sounds

• Heart sounds (lub-dup) are associated with closing of heart valves.
• First sound occurs as AV valves close and signifies beginning of systole (contraction).
• Second sound occurs when SL valves close at the beginning of ventricular diastole (relaxation).

Cardiac Cycle

• Cardiac cycle refers to all events associated with blood flow through the heart.
• Systole: contraction of heart muscle.
• Diastole: relaxation of heart muscle.

Phases of the Cardiac Cycle

• Ventricular filling: mid-to-late diastole; heart blood pressure is low; blood passively flows into ventricles; AV valves are open; atrial systole occurs.
• Ventricular systole: contraction; atria relax; rising ventricular pressure results in closing of AV valves; isovolumetric contraction phase; ventricular ejection phase opens semilunar valves.
• Isovolumetric relaxation: early diastole; ventricles relax; backflow of blood in aorta and pulmonary trunk closes semilunar valves; dicrotic notch: brief rise in aortic pressure caused by backflow of blood rebounding off semilunar valves.

Cardiac Output (CO) and Reserve

• Cardiac Output is the amount of blood pumped by each ventricle in one minute.
• CO is the product of heart rate (HR) and stroke volume (SV).
• HR is the number of heart beats per minute.
• SV is the amount of blood pumped out by a ventricle with each beat.
• Cardiac reserve is the difference between resting and maximal CO.

Regulation of Stroke Volume

• Stroke Volume (SV) = end diastolic volume (EDV) minus end systolic volume (ESV).
• EDV = amount of blood collected in a ventricle during diastole.
• ESV = amount of blood remaining in a ventricle after contraction.

Factors Affecting Stroke Volume

• Preload: amount ventricles are stretched by contained blood.
• Contractility: cardiac cell contractile force due to factors other than EDV.
• Afterload: back pressure exerted by blood in the large arteries leaving the heart.

Frank-Starling Law of the Heart

• Preload (degree of stretch) of cardiac muscle cells before contraction is the critical factor controlling stroke volume.
• Slow heartbeat and exercise increase venous return to the heart, increasing SV.
• Blood loss and extremely rapid heartbeat decrease SV.

Extrinsic Factors Influencing Stroke Volume

• Contractility: increase in contractile strength, independent of stretch and EDV; increased sympathetic stimuli; certain hormones; Ca2+ and some drugs.
• Agents/factors that decrease contractility: acidosis, increased extracellular K+, calcium channel blockers.

Regulation of Heart Rate

• Positive chronotropic factors: increase heart rate (e.g., caffeine).
• Negative chronotropic factors: decrease heart rate (e.g., sedatives).

Regulation of Heart Rate: Autonomic Nervous System

• Sympathetic nervous system (SNS) stimulation activated by stress, anxiety, excitement, or exercise.
• Parasympathetic nervous system (PNS) stimulation mediated by acetylcholine; opposes SNS; dominates autonomic stimulation; slows heart rate and causes vagal tone.
• If the vagus nerve is cut, the heart would lose its tone, increasing the heart rate by 25 beats per minute.

Atrial (Bainbridge) Reflex

• A sympathetic reflex initiated by increased blood in the atria.
• Causes stimulation of the SA node.
• Stimulates baroreceptors in the atria, causing increased SNS stimulation.

Chemical Regulation of the Heart

• Hormones epinephrine and thyroxine increase heart rate.
• Intra- and extracellular ion concentrations must be maintained for normal heart function.

Factors Involved in Regulation of Cardiac Output

• Increased activity of muscular pump and respiratory pump; increased renal activity (conservation of Na+ and water).
• Crisis stressors (exercise, physical or emotional trauma, increased body temperature); low blood pressure; low blood volume (hemorrhage, excessive sweating).
• Increased blood volume; chemicals: bloodborne thyroxine, epinephrine, excess Ca2+ (short-term effects only).
• Venous return; increased contractility of cardiac muscle; sympathetic nervous system activity; crisis has passed; parasympathetic nervous system controls via cardioinhibitory center and vagus nerves.
• Stroke volume (SV); cardiac output (CO)
• Factors include increased activity of muscular pump, renal activity; increased blood volume, venous return, chemicals, etc.

Congestive Heart Failure (CHF)

• Caused by: coronary atherosclerosis, persistent high blood pressure, multiple myocardial infarcts, dilated cardiomyopathy (DCM).
• DCM: pumping chambers of the heart are dilated and contract poorly.
• Left Side Failure: less effective pump, more blood remains in ventricle; heart is overstretched; blood backs up into lungs as pulmonary edema; suffocation; lack of oxygen to the tissues.
• Right Side Failure: fluid builds up in tissues as peripheral edema.

Coronary Artery Disease

• Heart muscle receiving insufficient blood supply.
• Narrowing of vessels: atherosclerosis, artery spasm or clot; atherosclerosis: smooth muscle & fatty deposits in walls of arteries.
• Treatment: drugs, bypass graft, angioplasty, stent.

Clinical Problems

• MI (myocardial infarction): death of area of heart muscle from lack of O2; replaced with scar tissue; results depend on damage size & location.
• Blood clot: use clot dissolving drugs (streptokinase or t-PA & heparin) and balloon angioplasty.
• Angina pectoris: heart pain from ischemia (lack of blood flow and oxygen to cardiac muscle).

By-pass Graft

• Surgical procedure where vessels are grafted to reroute blood flow around an obstruction in the coronary arteries.

Percutaneous Transluminal Coronary Angioplasty

• Minimally invasive procedure to open narrowed coronary arteries.
• Using a balloon catheter to expand the narrowed area.

Artificial Heart

• Mechanical devices to replace a failing heart.

Developmental Aspects of the Heart

• Fetal heart structures bypass pulmonary circulation.
• Foramen ovale connects the two atria.
• Ductus arteriosus connects pulmonary trunk and aorta.

Examples of Congenital Heart Defects

• Ventricular septal defect (VSD).
• Coarctation of the aorta.
• Tetralogy of Fallot.

Age-Related Changes Affecting the Heart

• Sclerosis and thickening of valve flaps.
• Decline in cardiac reserve.
• Fibrosis of cardiac muscle.
• Atherosclerosis.