A&P II: The Cardiovascular System (chapter 18)

Cardiac Output (CO)

Volume of blood pumped by each ventricle in 1 minute CO = heart rate (HR) × stroke volume (SV) - HR = number of beats per minute - SV = volume of blood pumped out by one ventricle with each beat Normal: 5.25 L/min

circumflex artery

The circumflex artery supplies blood to the left atrium and the posterior walls of the left ventricle.

normal sinus rhythm (NSR)

regular rhythm of the heart cycle stimulated by the SA node (average rate of 60-100 beats/minute)

a shunt

A shunt is a bypass, or shortcut, e.g., the foramen ovale connecting the two atria in the fetal heart.

inferior vena cava

A vein that is the largest vein in the human body and returns blood to the right atrium of the heart from bodily parts below the diaphragm. The inferior vena cava brings blood from the lower regions of the body and empties into the right atrium.

Second-degree heart block

AV block in which only some atrial electrical impulses are conducted to the ventricles

Afterload: back pressure exerted by arterial blood

Afterload is pressure that ventricles must overcome to eject blood - Back pressure from arterial blood pushing on SL valves is major pressure - Aortic pressure is around 80 mm Hg - Pulmonary trunk pressure is around 10 mm Hg Hypertension increases afterload, resulting in increased ESV and reduced SV

Other factors that influence heart rate

Age - Fetus has fastest HR; declines with age Gender - Females have faster HR than males Exercise - Increases HR - Trained atheles can have slow HR Body temperature - HR increases with increased body temperature

Homeostatic Imbalances of the Blood Flow/Coronary Circulation

Angina pectoris - Thoracic pain caused by fleeting deficiency in blood delivery to myocardium - Cells are weakened Myocardial infarction (heart attack) - Prolonged coronary blockage - Areas of cell death are repaired with noncontractile scar tissue

Ventricular systole

Atria relax; ventricles begin to contract Rising ventricular pressure causes closing of AV valves Two phases 2a: Isovolumetric contraction phase: all valves are closed 2b: Ejection phase: ventricular pressure exceeds pressure in large arteries, forcing SL valves open - Pressure in aorta around 120 mm Hg End systolic volume (ESV): volume of blood remaining in each ventricle after systole

Homeostatic Imbalances of the ECG

Changes in patterns or timing of ECG may reveal diseased or damaged heart, or problems with heart's conduction system Problems that can be detected: - Enlarged R waves may indicate enlarged ventricles - Elevated or depressed S-T segment indicates cardiac ischemia - Prolonged Q-T interval reveals a repolarization abnormality that increases risk of ventricular arrhythmias - Junctional blocks, blocks, flutters, and fibrillations are also detected on ECG

Homeostatic Imbalances: Heart Defects

Congenital heart defects are most common birth defects (40,000 per year) - Corrected with surgery - Most defects are one of two types: - Mixing of oxygen-poor and oxygen-rich blood, as in septal defects, patent ductus arteriosus - Narrowed valves or vessels that cause increased workload on heart, as in coarctation of aorta - Tetralogy of Fallot - Both types of disorders present

Homeostatic Imbalance of Cardiac Output

Congestive heart failure (CHF) - Progressive condition; CO is so low that blood circulation is inadequate to meet tissue needs - Reflects weakened myocardium caused by: - Coronary atherosclerosis: clogged arteries caused by fat buildup; impairs oxygen delivery to cardiac cells - Heart becomes hypoxic, contracts inefficiently - Persistent high blood pressure: aortic pressure 90 mmHg causes myocardium to exert more force - Chronic increased ESV causes myocardium hypertrophy and weakness - Multiple myocardial infarcts: heart becomes weak as contractile cells are replaced with scar tissue - Dilated cardiomyopathy (DCM): ventricles stretch and become flabby, and myocardium deteriorates - Drug toxicity or chronic inflammation may play a role Either side of heart can be affected: - Left-sided failure results in pulmonary congestion - Blood backs up in lungs - Right-sided failure results in peripheral congestion - Blood pools in body organs, causing edema Failure of either side ultimately weakens other side - Leads to decompensated, seriously weakened heart - Treatment: removal of fluid, drugs to reduce afterload and increase contractility

Contractility

Contractile strength at given muscle length - Independent of muscle stretch and EDV Increased contractility lowers ESV; caused by: - Sympathetic epinephrine release stimulates increased Ca2+ influx, leading to more cross bridge formations - Positive inotropic agents increase contractility - Thyroxine, glucagon, epinephrine, digitalis, high extracellular Ca2+ Decreased by negative inotropic agents - Acidosis (excess H+), increased extracellular K+, calcium channel blockers

Homeostatic Imbalances of the Intrinsic Conduction System

Defects in intrinsic conduction system may cause: - Arrhythmias: irregular heart rhythms - Uncoordinated atrial and ventricular contractions - Fibrillation: rapid, irregular contractions - Heart becomes useless for pumping blood, causing circulation to cease; may result in brain death - Treatment: defibrillation interrupts chaotic twitching, giving heart "clean slate" to start regular, normal depolarizations To reach ventricles, impulse must pass through AV node If AV node is defective, may cause a heart block - Few impulses (partial block) or no impulses (total block) reach ventricles - Ventricles beat at their own intrinsic rate - Too slow to maintain adequate circulation - Treatment: artificial pacemaker, which recouples atria and ventricles

Diastole

Diastole refers to relaxation. The heart undergoes some dramatic writhing movements as it alternately contracts, forcing blood out of its chambers, and then relaxes, allowing its chambers to refill with blood. The term systole (sis′to-le) refers to these periods of contraction, and diastole (di-as′to-le) refers to those of relaxation. The cardiac cycle includes all events associated with the blood flow through the heart during one complete heartbeat—atrial systole and diastole followed by ventricular systole and diastole. These mechanical events always follow the electrical events seen in the ECG.

ventricular fibrillation

Disorganized, ineffective twitching of the ventricles, resulting in no blood flow and a state of cardiac arrest.

Electrocardiography

Electrocardiograph can detect electrical currents generated by heart Electrocardiogram (ECG or EKG) is a graphic recording of electrical activity - Composite of all action potentials at given time; not a tracing of a single AP - Electrodes are placed at various points on body to measure voltage differences - 12 lead ECG is most typical Main features: - P wave: depolarization of SA node and atria - QRS complex: ventricular depolarization and atrial repolarization - T wave: ventricular repolarization - P-R interval: beginning of atrial excitation to beginning of ventricular excitation - S-T segment: entire ventricular myocardium depolarized - Q-T interval: beginning of ventricular depolarization through ventricular repolarization

Isovolumetric relaxation: early diastole

Following ventricular repolarization (T wave), ventricles are relaxed; atria are relaxed and filling Backflow of blood in aorta and pulmonary trunk closes SL valves - Causes dicrotic notch (brief rise in aortic pressure as blood rebounds off closed valve) - Ventricles are totally closed chambers (isovolumetric) When atrial pressure exceeds ventricular pressure, AV valves open; cycle begins again

Homeostatic Imbalances of Heart Rhythm

Heart murmurs: abnormal heart sounds heard when blood hits obstructions Usually indicate valve problems - Incompetent (or insufficient) valve: fails to close completely, allowing backflow of blood - Causes swishing sound as blood regurgitates backward from ventricle into atria - Stenotic valve: fails to open completely, restricting blood flow through valve - Causes high-pitched sound or clicking as blood is forced through narrow valve

Modifying the Basic Rhythm: Extrinsic Innervation of the Heart

Heartbeat modified by ANS via cardiac centers in medulla oblongata - Cardioacceleratory center: sends signals through sympathetic trunk to increase both rate and force - Stimulates SA and AV nodes, heart muscle, and coronary arteries - Cardioinhibitory center: parasympathetic signals via vagus nerve to decrease rate - Inhibits SA and AV nodes via vagus nerves

Chemical regulation of heart rate

Hormones - Epinephrine from adrenal medulla increases heart rate and contractility - Thyroxine increases heart rate; enhances effects of norepinephrine and epinephrine Ions - Intra- and extracellular ion concentrations (e.g. Ca2+ and K+) must be maintained for normal heart function - Imbalances are very dangerous to heart

Developmental Aspects of the Heart

Human heart is derived from mesoderm Begins as two endothelial chambers that fuse to form single actively pumping chamber by day 22 Tube develops four bulges that represent earliest chambers - Sinus venosus: gives rise to right atrium, coronary sinus, and SA node - Atrium: becomes pectinate muscles of atria - Ventricle: becomes left ventricle - Bulbus cordis: gives rise to aorta, pulmonary trunk, and right ventricle Heart tube contorts, and structural changes convert into a four-chambered heart by day 35 Two fetal heart structures bypass pulmonary circulation - Foramen ovale: opening that connects atria - Remnant is fossa ovalis in adult - Ductus arteriosus connects pulmonary trunk to aorta - Remnant: ligamentum arteriosum in adult - Close at or shortly after birth Age-Related Changes Affecting the Heart - Regular exercise can keep heart healthy - Age-related changes include: - Sclerosis and thickening of valve flaps: lead to heart murmurs - Decline in cardiac reserve: heart becomes less efficient - Fibrosis of cardiac muscle: leads to stiffened heart, arrhythmias caused by conduction system problems - Atherosclerosis: may be averted by good diet

Homeostatic Imbalances of Heart Rate

Hypocalcemia: depresses heart Hypercalcemia: increases HR and contractility Hyperkalemia: alters electrical activity, which can lead to heart block and cardiac arrest, death Hypokalemia: results in feeble heartbeat; arrhythmias Tachycardia (hurry heart): - abnormally fast heart rate (>100 beats/min) - If persistent, may lead to fibrillation Bradycardia (slow heart): - heart rate slower than 60 beats/min - May result in grossly inadequate blood circulation in nonathletes - May be desirable result of endurance training

Regulation of Heart Rate

If SV decreases as a result of decreased blood volume or weakened heart, CO can be maintained by increasing HR and contractility - Positive chronotropic factors increase heart rate - Negative chronotropic factors decrease heart rate Heart rate can be regulated by: - Autonomic nervous system - Chemicals - Other factors

Atrioventricular (AV) node

In inferior interatrial septum Delays impulses approximately 0.1 second - Because fibers are smaller in diameter, have fewer gap junctions - Allows atrial contraction prior to ventricular contraction Inherent rate of 50×/minute in absence of SA node input

Atrioventricular (AV) bundle (bundle of His)

In superior interventricular septum Only electrical connection between atria and ventricles - Atria and ventricles not connected via gap junctions

Regulation of Stroke Volume

Mathematically: SV = EDV − ESV - EDV is affected by length of ventricular diastole and venous pressure (~120 ml/beat) - ESV is affected by arterial BP and force of ventricular contraction (~50 ml/beat) - Normal SV = 120 ml − 50 ml = 70 ml/beat Three main factors that affect SV: - Preload - Contractility - Afterload

Regulation of Pumping

Maximal CO is 4-5 times resting CO in nonathletic people (20-25 L/min) Maximal CO may reach 35 L/min in trained athletes Cardiac reserve: difference between resting and maximal CO CO changes (increases/decreases) if either or both SV or HR is changed CO is affected by factors leading to: - Regulation of stroke volume - Regulation of heart rates

​​Sinoatrial (SA) node

Pacemaker of heart in right atrial wall - Depolarizes faster than rest of myocardium Generates impulses about 75×/minute (sinus rhythm) - Inherent rate of 100×/minute tempered by extrinsic factors Impulse spreads across atria, and to AV node

Pectinate muscles

Pectinate muscles are are muscle bundles found in the anterior portion of the right atrium.

Pericardial friction rub i

Pericardial friction rub is the creaking sound made by an inflamed pericardium (pericarditis).

Preload: degree of stretch of heart muscle

Preload: degree to which cardiac muscle cells are stretched just before they contract - Changes in preload cause changes in SV Affects EDV - Relationship between preload and SV called Frank-Starling law of the heart Cardiac muscle exhibits a length-tension relationship - At rest, cardiac muscle cells are shorter than optimal length; leads to dramatic increase in contractile force Most important factor in preload stretching of cardiac muscle is venous return—amount of blood returning to heart - Slow heartbeat and exercise increase venous return - Increased venous return distends (stretches) ventricles and increases contraction force

Ventricular filling: mid-to-late diastole

Pressure is low; 80% of blood passively flows from atria through open AV valves into ventricles from atria (SL valves closed) Atrial depolarization triggers atrial systole (P wave), atria contract, pushing remaining 20% of blood into ventricle - End diastolic volume (EDV): volume of blood in each ventricle at end of ventricular diastole Depolarization spreads to ventricles (QRS wave) Atria finish contracting and return to diastole while ventricles begin systole

stroke volume

Stroke volume is the volume of blood pumped by one ventricle with each heartbeat. Mathematically, stroke volume (SV) represents the difference between end diastolic volume (EDV), the amount of blood that collects in a ventricle during diastole, and end systolic volume (ESV), the volume of blood remaining in a ventricle after it has contracted. The EDV, determined by how long ventricular diastole lasts and by venous pressure, is normally about 120 ml. (An increase in either factor raises EDV.) The ESV, determined by arterial blood pressure and the force of ventricular contraction, is approximately 50 ml. (The higher the arterial blood pressure, the higher the ESV.) To figure normal stroke volume, simply plug these values into this equation: SV = EDV − ESV = 120 ml/beat − 50 ml/beat = 70 ml/beat As you can see, each ventricle pumps out about 70 ml of blood with each beat, which is about 60% of the blood in its chambers. So what is important here—how do we make sense out of this alphabet soup (SV, ESV, EDV)? Although many factors affect SV by altering EDV or ESV, the three most important are preload, contractility, and afterload. Preload affects EDV, whereas contractility and afterload affect the ESV.

current flow through the intrinsic conduction system of the heart

The correct sequence of current flow through the heart wall is SA node, AV node, AV bundle, right and left bundle branches, and Purkinje fibers.

Autonomic nervous system regulation of heart rate

Sympathetic nervous system can be activated by emotional or physical stressors Norepinephrine is released and binds to β1-adrenergic receptors on heart, causing: - Pacemaker to fire more rapidly, increasing HR - EDV decreased because of decreased fill time - Increased contractility - ESV decreased because of increased volume of ejected blood Because both EDV and ESV decrease, SV can remain unchanged Parasympathetic nervous system opposes sympathetic effects - Acetylcholine hyperpolarizes pacemaker cells by opening K+ channels, which slows HR - Has little to no effect on contractility Heart at rest exhibits vagal tone - Parasympathetic is dominant influence on heart rate - Decreases rate about 25 beats/min - Cutting vagal nerve leads to HR of 100 When sympathetic is activated, parasympathetic is inhibited, and vice-versa Atrial (Bainbridge) reflex: sympathetic reflex initiated by increased venous return, hence increased atrial filling - Atrial walls are stretched with increased volume - Stimulates SA node, which increases HR - Also stimulates atrial stretch receptors that activate sympathetic reflexes

Systole

Systole is the term for contraction. The heart undergoes some dramatic writhing movements as it alternately contracts, forcing blood out of its chambers, and then relaxes, allowing its chambers to refill with blood. The term systole (sis′to-le) refers to these periods of contraction, and diastole (di-as′to-le) refers to those of relaxation. The cardiac cycle includes all events associated with the blood flow through the heart during one complete heartbeat—atrial systole and diastole followed by ventricular systole and diastole. These mechanical events always follow the electrical events seen in the ECG.

Mechanical Events of Heart

Systole: period of heart contraction Diastole: period of heart relaxation Cardiac cycle: blood flow through heart during one complete heartbeat - Atrial systole and diastole are followed by ventricular systole and diastole - Cycle represents series of pressure and blood volume changes - Mechanical events follow electrical events seen on ECG Three phases of the cardiac cycle (following left side, starting with total relaxation)

P wave

The P wave represents atrial depolarization. A typical ECG has three almost immediately distinguishable waves or deflections: the P wave, the QRS complex, and the T wave. The first, the small P wave, lasts about 0.08 s and results from movement of the depolarization wave from the SA node through the atria.

Q-T Interval

The Q-T interval, lasting about 0.38 s, is the period from the beginning of ventricular depolarization through ventricular repolarization.

QRS Complex

The large QRS complex results from ventricular depolarization and precedes ventricular contraction. It has a complicated shape because the paths of the depolarization waves through the ventricular walls change continuously, producing corresponding changes in current direction. Additionally, the time required for each ventricle to depolarize depends on its size relative to the other ventricle. Average duration of the QRS complex is 0.08 s.

Right and left bundle branches

Two pathways in interventricular septum Carry impulses toward apex of heart

Heart Sounds

Two sounds (lub-dup) associated with closing of heart valves First sound is closing of AV valves at beginning of ventricular systole Second sound is closing of SL valves at beginning of ventricular diastole Pause between lub-dups indicates heart relaxation Mitral valve closes slightly before tricuspid, and aortic closes slightly before pulmonary valve - Differences allow auscultation of each valve when stethoscope is placed in four different regions

bicuspid (mitral) valve

The valve separating the left atrium and left ventricle is the bicuspid (mitral) valve. The aortic and pulmonary (semilunar, SL) valves guard the bases of the large arteries issuing from the ventricles (aorta and pulmonary trunk, respectively) and prevent backflow into the associated ventricles. Each SL valve is fashioned from three pocketlike cusps, each shaped roughly like a crescent moon (semilunar = half-moon). The tricuspid valve separates the right atrium and right ventricle. Failure of the mitral (bicuspid) valve would allow blood to move from the left ventricle to the left atrium. The two atrioventricular (AV) valves, one located at each atrial-ventricular junction, prevent backflow into the atria when the ventricles contract. - The right AV valve, the tricuspid valve (tri-kus'pid), has three flexible cusps (flaps of endocardium reinforced by connective tissue cores). - The left AV valve, with two cusps, is called the mitral valve (mi'tral) because it resembles the two-sided bishop's miter (tall, pointed hat). It is sometimes called the bicuspid valve. When the mitral valve closes, it prevents the backflow of blood from the left ventricle into the left atrium. The mitral, or bicuspid, valve is the valve that is most often faulty in the heart. Heart valves are simple devices, and the heart—like any mechanical pump—can function with "leaky" valves as long as the impairment is not too great. However, severe valve deformities can seriously hamper cardiac function. An incompetent, or insufficient, valve forces the heart to repump the same blood over and over because the valve does not close properly and blood backflows. In valvular stenosis ("narrowing"), the valve flaps become stiff and constrict the opening. [Stenosis is typically due to calcium salt deposits or scar tissue that forms following endocarditis (inflammation of the endocardium, which most often results from infection by bacteria that have entered the bloodstream and usually affects the heart valves).] The constricted opening compels the heart to contract more forcibly than normal. Both types of valve problems increase the heart's workload and may weaken the heart severely over time. The mitral and aortic valves are most often affected.The faulty valve can be replaced with a mechanical valve, a pig or cow heart valve chemically treated to prevent rejection,or cryopreserved valves from human cadavers. Heart valves tissue-engineered from a patient's own cells grown on a biodegradable scaffold are being developed. ~ Blood exits the left atrium through the bicuspid valve. Focus on Blood Flow through the Heart follows a single "spurt" of blood as it passes through all four chambers of the heart and both blood circuits in its ever-repeating journey. As you work your way through this figure, keep in mind that the left side of the heart is the systemic circuit pump and the right side is the pulmonary circuit pump. Notice how unique the pulmonary circuit is. Elsewhere in the body, veins carry relatively oxygen-poor blood to the heart, and arteries transport oxygen-rich blood from the heart. The opposite oxygenation conditions exist in veins and arteries of the pulmonary circuit.

left ventricle

The walls of the left ventricle are thicker than the walls of any other heart chamber. The pulmonary circuit, served by the right ventricle, is a short, low-pressure circulation. In contrast, the systemic circuit, associated with the left ventricle, takes along pathway through the entire body and encounters about five times as much friction, or resistance to blood flow. This functional difference is revealed in the anatomy of the two ventricles. The walls of the left ventricle are three times thicker than those of the right ventricle, and its cavity is nearly circular. The right ventricular cavity is flattened into a crescent shape that partially encloses the left ventricle, much the way a hand might loosely grasp a clenched fist. Consequently, the left ventricle can generate much more pressure than the right and is a far more powerful pump. The walls of both atria are thin compared to either ventricle. The left ventricle is considered the systemic circuit pump. The left side of the heart receives the oxygenated blood returning from the lungs and pumps this blood throughout the body to supply oxygen and nutrients to body tissues. The blood vessels that carry blood to and from all body tissues form the systemic circuit. The left ventricle receives blood oxygenated from the left atrium then pumps that blood throughout the rest of the body.

left atrium

receives blood from pulmonary veins (coming from the lungs), blood now oxy-rich

Junctional Rhythm

the SA node is nonfunctional, P waves are absent, and heart is paced by the AV node at 40-60 beats/min

Atrial repolarization

Atrial repolarization takes place during the period of ventricular excitation, and is normally obscured by the large QRS complex (ventricular depolarization).

Cardiac output

Cardiac output is the amount of blood pumped out of each ventricle in one minute. It is the product of heart rate (HR) in beats per minute and stroke volume (SV) in ml per beat. Stroke volume is defined as the volume of blood pumped out by one ventricle with each beat. In general, stroke volume correlates with the force of ventricular contraction. Using normal resting values for heart rate (75 beats/min) and stroke volume (70 ml/beat), the average adult cardiac output can be computed as follows: CO = HR × SV = 75 beats/min × 70 ml//beat = 5250 ml/min = 5.25 L min/min The normal adult blood volume is about 5 L (a little more than 1 gallon). As you can see, the entire blood supply passes through each side of the heart once each minute.

Coronary veins

Cardiac veins collect blood from capillary beds Coronary sinus empties into right atrium; formed by merging cardiac veins - Great cardiac vein of anterior interventricular sulcus - Middle cardiac vein in posterior interventricular sulcus - Small cardiac vein from inferior margin Several anterior cardiac veins empty directly into right atrium anteriorly

Setting the Basic Rhythm: The Intrinsic Conduction System

Coordinated heartbeat is a function of: 1. Presence of gap junctions 2. Intrinsic cardiac conduction system - Network of noncontractile (autorhythmic) cells - Initiate and distribute impulses to coordinate depolarization and contraction of heart Action potential initiation by pacemaker cells - Cardiac pacemaker cells have unstable resting membrane potentials called pacemaker potentials or prepotentials - Three parts of action potential ​​1. Pacemaker potential: K+ channels are closed, but slow Na+ channels are open, causing interior to become more positive 2. ​​Depolarization: Ca2+ channels open (around −40 mV), allowing huge influx of Ca2+ leading to rising phase of action potential 3. ​Repolarization: K+ channels open, allowing efflux of K+ and cell becomes more negative Sequence of excitation - Cardiac pacemaker cells pass impulses, in following order, across heart in ~0.22 seconds ​​1. Sinoatrial node → ​​2. Atrioventricular node → ​​3. Atrioventricular bundle → ​​4. Right and left bundle branches → ​​5. Subendocardial conducting network (Purkinje fibers)

Coverings of the heart

Pericardium: double-walled sac that surrounds heart; made up of two layers 1. Superficial fibrous pericardium: functions to protect, anchor heart to surrounding structures, and prevent overfilling 2. Deep two-layered serous (always have these 2 layers) pericardium -Parietal layer lines internal surface of fibrous pericardium. -Visceral (organ) layer (epicardium) on external surface of heart - Two layers separated by fluid-filled (serous fluid) pericardial cavity - Makes things slippery so the heart can beat and slide around with friction. YUP this is a serous membrane from chapter 4!

sinoatrial node (SA node)

The normal pacemaker of the heart is the SA node. The various cardiac pacemaker cells have different rates of spontaneous depolarization. The SA node normally drives the heart at a rate of 75 beats per minute. Without SA node input, the AV node would depolarize only about 50 times per minute. Without input from the AV node, the atypical pacemakers of the AV bundle and the subendocardial conducting network would depolarize only about 30 times per minute. Note that these slower pacemakers cannot dominate the heart unless faster pacemakers stop functioning.

Angina pectoris

Angina pectoris ("choked chest") is thoracic pain caused by a fleeting deficiency in blood delivery to the myocardium.

Size, location, and orientation of heart

Approximately the size of a fist - Weighs less than 1 pound Location - In mediastinum (thoracic cavity) between second rib and fifth intercostal space - Above diaphragm - Two-thirds of heart to left of midsternal line (not right in the middle, tips left) - Anterior to vertebral column, posterior to sternum - Base (posterior surface) leans toward right shoulder - Apex points toward left hip - Apical impulse palpated between fifth and sixth ribs , just below left nipple (we can hear the impulse there)

Electrical Events of the Heart

Heart depolarizes and contracts without nervous system stimulation, although rhythm can be altered by autonomic nervous system - Sympathetic and parasympathetic

SA node

The SA node contains faster depolarizing pacemaker cells. The various cardiac pacemaker cells have different rates of spontaneous depolarization. The SA node normally drives the heart at a rate of 75 beats per minute. Without SA node input, the AV node would depolarize only about 50 times per minute. Without input from the AV node, the atypical pacemakers of the AV bundle and the subendocardial conducting network would depolarize only about 30 times per minute. Note that these slower pacemakers cannot dominate the heart unless faster pacemakers stop functioning.

functional syncytium

The cells of the myocardium behave as a single, coordinated unit called a functional syncytium. Because gap junctions electrically couple cardiac cells, the myocardium behaves as a single coordinated unit, or functional syncytium (sin-sit′e-um; syn = together, cyt = cell).

sequence of layers in heart wall

The correct sequence of layers in the heart wall, starting with the outer layer is epicardium, myocardium, endocardium. The heart wall, richly supplied with blood vessels, is composed of three layers: the epicardium, myocardium, and endocardium.

Fibrous Pericardium

The fibrous pericardium is a loosely fitting superficial part of the sac enclosing the heart. The heart is enclosed in a double-walled sac called the pericardium (per"ĭ-kar'de-um; peri = around, cardi = heart). The loosely fitting superficial part of this sac is the fibrous pericardium. This tough, dense connective tissue layer (1) protects the heart, (2) anchors it to surrounding structures, and (3) prevents overfilling of the heart with blood.

ECG

electrocardiogram The P wave represents atrial depolarization. A typical ECG has three almost immediately distinguishable waves or deflections: the P wave, the QRS complex, and the T wave. The first, the small P wave, lasts about 0.08 s and results from movement of the depolarization wave from the SA node through the atria. The Q-T interval, lasting about 0.38 s, is the period from the beginning of ventricular depolarization through ventricular repolarization. Atrial repolarization takes place during the period of ventricular excitation, and is normally obscured by the large QRS complex (ventricular depolarization). The T wave, caused by ventricular repolarization, typically lasts about 0.16 s. Repolarization is slower than depolarization, so the T wave is more spread out and has a lower amplitude (height) than the QRS complex. Because atrial repolarization takes place during the period of ventricular excitation, the wave representing atrial repolarization is normally obscured by the large QRS complex being recorded at the same time.

The layers of the pericardium and of the heart wall

fibrous - connective tissue, strong pericardial cavity - serous fluid, slippery to prevent friction parietal layer - attached to pericardium epicardium - visceral layer, outside of the heart myocardium endocardium

Heart Valves

Ensure unidirectional blood flow through heart Open and close in response to pressure changes Two major types of valves: - Atrioventricular valves located between atria and ventricles - Semilunar valves located between ventricles and major arteries No valves are found between major veins and atria; not a problem because: - Inertia of incoming blood prevents backflow - Heart contractions compress venous openings

Pathway of Blood Through Heart

Equal volumes of blood are pumped to pulmonary and systemic circuits Pulmonary circuit is short, low-pressure circulation Systemic circuit is long, high-friction circulation Anatomy of ventricles reflects differences - Left ventricle walls are 3× thicker than right - Pumps with greater pressure

Coronary Circulation

Functional blood supply to heart muscle itself Shortest circulation in body Delivered when heart is relaxed Left ventricle receives most of coronary blood supply

posterior interventricular artery

The posterior interventricular artery runs to the apex of the heart. The right coronary artery courses to the right side of the heart, where it also gives rise to two branches: - The right marginal artery serves the myocardium of the lateral right side of the heart. - The posterior interventricular artery runs to the heart apex and supplies the posterior ventricular walls. Near the apex of the heart, this artery merges (anastomoses) with the anterior interventricular artery.

right atrium

The right atrium receives blood from the vena cavae. Blood enters the right atrium via three veins: - The superior vena cava returns blood from body regions superior to the diaphragm. - The inferior vena cava returns blood from body areas below the diaphragm. - The coronary sinus collects blood draining from the myocardium.

vagus nerve

The vagus nerve carries parasympathetic fibers to the sinoatrial (SA) node. The cardioinhibitory center sends impulses to the parasympathetic dorsal vagus nucleus in the medulla, which in turn sends inhibitory impulses to the heart via branches of the vagus nerves. Most parasympathetic postganglionic motor neurons lie in ganglia in the heart wall and their fibers project most heavily to the SA and AV nodes.

stenosis

A condition in which the valve flaps of the heart become stiff and constricts the opening is called stenosis ("narrowing"). Heart valves are simple devices, and the heart—like any mechanical pump—can function with "leaky" valves as long as the impairment is not too great. However, severe valve deformities can seriously hamper cardiac function. An incompetent, or insufficient, valve forces the heart to repump the same blood over and over because the valve does not close properly and blood backflows. In valvular stenosis ("narrowing"), the valve flaps become stiff (typically due to calcium salt deposits or scar tissue that forms following endocarditis) and constrict the opening. This stiffness compels the heart to contract more forcibly than normal. Both conditions increase the heart's workload and may weaken the heart severely over time.

myocardium high resistance to fatigue

A large number of mitochondria in the cytoplasm give the myocardium its high resistance to fatigue. Mitochondria account for 25-35% of the volume of cardiac cells (compared with only 2% in skeletal muscle), a characteristic that makes cardiac cells highly resistant to fatigue. Most of the remaining volume is occupied by myofibrils composed of fairly typical sarcomeres. The sarcomeres have Z discs, A bands, and I bands that reflect the arrangement of the thick (myosin) and thin (actin) filaments composing them. However, in contrast to skeletal muscle, the myofibrils of cardiac muscle cells vary greatly in diameter and branch extensively, accommodating the abundant mitochondria between them. This difference produces a banding pattern less dramatic than that seen in skeletal muscle.

Coronary arteries

Both left and right coronary arteries arise from base of aorta and supply arterial blood to heart Both encircle heart in coronary sulcus Branching of coronary arteries varies among individuals Arteries contain many anastomoses (junctions) - Provide additional routes for blood delivery - Cannot compensate for coronary artery occlusion Heart receives 1/20th of body's blood supply Left coronary artery supplies interventricular septum, anterior ventricular walls, left atrium, and posterior wall of left ventricle; has two branches: - Anterior interventricular artery - Circumflex artery Right coronary artery supplies right atrium and most of right ventricle; has two branches: - Right marginal artery - Posterior interventricular artery

Cardiac Muscle Fibers

Cardiac muscle cells: striated, short, branched, fat, interconnected - One central nucleus (at most, 2 nuclei) - Contain numerous large mitochondria (25-35% of cell volume) - Sarcomeres - Z discs, A bands, and I bands all present - T tubules are wider, but less numerous - Enter cell only once at Z disc - SR simpler than in skeletal muscle; no triads Intercalated discs are connecting junctions between cardiac cells that contain: - Desmosomes: hold cells together; prevent cells from separating during contraction - Gap junctions: allow ions to pass from cell to cell; electrically couple adjacent cells - Allows heart to be a functional syncytium, a single coordinated unit

pulmonary circulation

During pulmonary circulation, blood leaves the right ventricle and moves to the lungs. Focus on Blood Flow through the Heart follows a single "spurt" of blood as it passes through all four chambers of the heart and both blood circuits in its ever-repeating journey. As you work your way through this figure, keep in mind that the left side of the heart is the systemic circuit pump and the right side is the pulmonary circuit pump. Notice how unique the pulmonary circuit is. Elsewhere in the body, veins carry relatively oxygen-poor blood to the heart, and arteries transport oxygen-rich blood from the heart. The opposite oxygenation conditions exist in veins and arteries of the pulmonary circuit.

systemic circulation

During systemic circulation, blood leaves the left ventricle and goes directly to the aorta. Focus on Blood Flow through the Heart follows a single "spurt" of blood as it passes through all four chambers of the heart and both blood circuits in its ever-repeating journey. As you work your way through this figure, keep in mind that the left side of the heart is the systemic circuit pump and the right side is the pulmonary circuit pump. Notice how unique the pulmonary circuit is. Elsewhere in the body, veins carry relatively oxygen-poor blood to the heart, and arteries transport oxygen-rich blood from the heart. The opposite oxygenation conditions exist in veins and arteries of the pulmonary circuit.

EDV (end diastolic volume)

EDV is the amount of blood in the ventricle at the end of relaxation. Mathematically, stroke volume (SV) represents the difference between end diastolic volume (EDV), the amount of blood that collects in a ventricle during diastole, and end systolic volume (ESV), the volume of blood remaining in a ventricle after it has contracted. The EDV, determined by how long ventricular diastole lasts and by venous pressure, is normally about 120 ml. (An increase in either factor raises EDV.) The ESV, determined by arterial blood pressure and the force of ventricular contraction, is approximately 50 ml. (The higher the arterial blood pressure, the higher the ESV.) To figure normal stroke volume, simply plug these values into this equation: SV = EDV − ESV = 120 ml/beat − 50 ml/beat = 70 ml/beat As you can see, each ventricle pumps out about 70 ml of blood with each beat, which is about 60% of the blood in its chambers. So what is important here—how do we make sense out of this alphabet soup (SV, ESV, EDV)? Although many factors affect SV by altering EDV or ESV, the three most important are preload, contractility, and afterload. Preload affects EDV, whereas contractility and afterload affect the ESV.

ESV (end systolic volume)

ESV is the amount of blood in the ventricle at the end of contraction. Mathematically, stroke volume (SV) represents the difference between end diastolic volume (EDV), the amount of blood that collects in a ventricle during diastole, and end systolic volume (ESV), the volume of blood remaining in a ventricle after it has contracted. The EDV, determined by how long ventricular diastole lasts and by venous pressure, is normally about 120 ml. (An increase in either factor raises EDV.) The ESV, determined by arterial blood pressure and the force of ventricular contraction, is approximately 50 ml. (The higher the arterial blood pressure, the higher the ESV.) To figure normal stroke volume, simply plug these values into this equation: SV = EDV − ESV = 120 ml/beat − 50 ml/beat = 70 ml/beat As you can see, each ventricle pumps out about 70 ml of blood with each beat, which is about 60% of the blood in its chambers. So what is important here—how do we make sense out of this alphabet soup (SV, ESV, EDV)? Although many factors affect SV by altering EDV or ESV, the three most important are preload, contractility, and afterload. Preload affects EDV, whereas contractility and afterload affect the ESV.

mesoderm

Guided by powerful signaling molecules, the human heart develops from mesoderm. The human heart, derived from mesoderm and guided by powerful signaling molecules, begins as two simple endothelial tubes. They quickly fuse to form a single chamber or heart tube that is busily pumping blood by the 22nd day of gestation (figure below).

HR

HR is the the frequency of heart beats. Cardiac output (CO) is the amount of blood pumped out by each ventricle in 1 minute. It is the product of heart rate (HR) in beats per minute and stroke volume (SV) in ml per beat. Stroke volume is defined as the volume of blood pumped out by one ventricle with each beat. In general, stroke volume correlates with the force of ventricular contraction. Using normal resting values for heart rate (75 beats/min) and stroke volume (70 ml/beat), the average adult cardiac output can be computed as follows: CO = HR × SV = 75 beats/min × 70 ml//beat = 5250 ml/min = 5.25 L min/min The normal adult blood volume is about 5 L (a little more than 1 gallon). As you can see, the entire blood supply passes through each side of the heart once each minute.

blood pressure

Heart valves open and close in response to differences in blood pressure on their two sides. Blood flows through the heart in one direction: from atria to ventricles and out the great arteries leaving the superior aspect of the heart. Four valves enforce this one-way traffic. They open and close in response to differences in blood pressure on their two sides.

Chambers and Associated Great Vessels

Internal features - Four chambers - Two superior atria - Two inferior ventricles - Interatrial septum: separates atria - Fossa ovalis: remnant of foramen ovale of fetal heart - Interventricular septum: separates ventricles Surface features - Coronary sulcus (atrioventricular groove) - Encircles junction of atria and ventricles - Anterior interventricular sulcus - Anterior position of interventricular septum - Posterior interventricular sulcus - Landmark on posteroinferior surface Atria: the receiving chambers - Small, thin-walled chambers; contribute little to propulsion of blood - Auricles: appendages that increase atrial volume - Right atrium: receives deoxygenated blood from body - Anterior portion is smooth-walled - Posterior portion contains ridges formed by pectinate muscles - Posterior and anterior regions are separated by crista terminalis (imaginary line) Atria: the receiving chambers - Three veins empty into right atrium: - Superior vena cava: returns blood from body regions above the diaphragm - Inferior vena cava: returns blood from body regions below the diaphragm - Coronary sinus: returns blood from coronary veins - Left atrium: receives oxygenated blood from lungs - Pectinate muscles found only in auricles - Four pulmonary veins return blood from lungs Ventricles: the discharging chambers - Thicker walls than atria - Actual pumps of heart - Make up most of the volume of heart - Right ventricle: most of anterior surface - Pumps blood into pulmonary trunk - Left ventricle: posteroinferior surface - Pumps blood into aorta (largest artery in body) - Trabeculae carneae: irregular ridges of muscle on ventricular walls - Papillary muscles: project into ventricular cavity - Anchor chordae tendineae that are attached to heart valves

Decrease/Increase of heart rate

Parasympathetic stimulation would decrease heart rate. The parasympathetic division opposes sympathetic effects and effectively reduces heart rate when a stressful situation has passed. Parasympathetic-initiated cardiac responses are mediated by acetylcholine, which hyperpolarizes the membranes of its effector cells by opening K+ channels. Because vagal innervation of the ventricles is sparse, parasympathetic activity has little effect on cardiac contractility. Exercise and norepinephrine would directly increase heart rate. Sharply decreased blood volume would increase heart rate in order to compensate for the reduced stroke volume caused by the decreased blood volume (and decreased EDV).

Homeostatic Imbalance

Pericarditis - Inflammation of pericardium - Roughens membrane surfaces, causing pericardial friction rub (creaking sound) heard with stethoscope - Cardiac tamponade - Excess fluid that leaks into pericardial space - Can compress heart's pumping ability - Treatment: fluid is drawn out of cavity (usually with syringe)

Preload

Preload is the degree to which cardiac muscle cells are stretched just before they contract. Preload controls stroke volume. In a normal heart, the higher the preload, the higher the stroke volume. This relationship between preload and stroke volume is called the Frank-Starling law of the heart. The events associated with blood flow through the heart during one complete heartbeat are called the cardiac cycle. Cardiac output (CO) is the amount of blood pumped out by each ventricle in 1 minute. It is the product of heart rate (HR) in beats per minute and stroke volume (SV) in ml per beat. Stroke volume is defined as the volume of blood pumped out by one ventricle with each beat. In general, stroke volume correlates with the force of ventricular contraction. Using normal resting values for heart rate (75 beats/min) and stroke volume (70 ml/beat), the average adult cardiac output can be computed as follows: CO = HR × SV = 75 beats/min × 70 ml//beat = 5250 ml/min = 5.25 L min/min The normal adult blood volume is about 5 L (a little more than 1 gallon). As you can see, the entire blood supply passes through each side of the heart once each minute. Back pressure exerted by arterial blood is called afterload. It's the pressure that the ventricles must overcome to eject blood.

Heart Anatomy

Pulmonary circuit (right side) - two side-by-side pumps - Pulmonary circuit (right side) - receives oxygen-poor blood from tissues - Pumps blood to lungs to get rid of CO2, pick up O2 - Pulmonary means lungs. It assists the body by reoxygenating blood Systemic Circuit (left side) - receives oxygenated blood from lungs (oxyhemoglobin is not in our RBC) - Pumps blood to body tissues (whole body) Receiving chambers of heart - Right atrium Receives blood returning from systemic circuit - Left atrium Receives blood returning from pulmonary circuit (lungs)term-1 - atria is plural, atrium singular Pumping chambers of heart - Right ventricle Pumps blood through pulmonary circuit (the lungs are close to the heart, so it's a little guy) - Left ventricle Pumps blood through systemic circuit (has to push harder than the right because it requires blood for the whole body while the right is only to the lungs)

How Does the Physiology of Skeletal and Cardiac Muscle Differ?

Similarities with skeletal muscle - Muscle contraction is preceded by depolarizing action potential - Depolarization wave travels down T tubules; causes sarcoplasmic reticulum (SR) to release Ca2+ - Excitation-contraction coupling occurs - Ca2+ binds troponin causing filaments to slide Differences between cardiac and skeletal muscle - Some cardiac muscle cells are self-excitable - Two kinds of myocytes - Contractile cells: responsible for contraction - Pacemaker cells: noncontractile cells that spontaneously depolarize - Initiate depolarization of entire heart - Do not need nervous system stimulation, in contrast to skeletal muscle fibers Heart contracts as a unit - All cardiomyocytes contract as unit (functional syncytium), or none contract - Contraction of all cardiac myocytes ensures effective pumping action - Skeletal muscles contract independently Influx of Ca2+ from extracellular fluid triggers Ca2+ release from SR - Depolarization opens slow Ca2+ channels in sarcolemma, allowing Ca2+ to enter cell - Extracellular Ca2+ then causes SR to release its intracellular Ca2+ - Skeletal muscles do not use extracellular Ca2+ Tetanic contractions cannot occur in cardiac muscles - Cardiac muscle fibers have longer absolute refractory period than skeletal muscle fibers - Absolute refractory period is almost as long as contraction itself - Prevents tetanic contractions - Allows heart to relax and fill as needed to be an efficient pump The heart relies almost exclusively on aerobic respiration - Cardiac muscle has more mitochondria than skeletal muscle so has greater dependence on oxygen - Cannot function without oxygen - Skeletal muscle can go through fermentation when oxygen not present - Both types of tissues can use other fuel sources - Cardiac is more adaptable to other fuels, including lactic acid, but must have oxygen

absolute refractory period

The absolute refractory period refers to the time during which the muscle cell is not in a position to respond to a stimulus of any strength. The absolute refractory period is the period during an action potential when another action potential cannot be triggered. In skeletal muscle, the absolute refractory period is much shorter than the contraction, allowing multiple contractions to summate (tetanic contractions). If the heart were to contract tetanically, it would be unable to relax and fill, and so would be useless as a pump. To prevent tetanic contractions, the absolute refractory period in the heart is nearly as long as the contraction itself.

anterior interventricular artery

The anterior interventricular artery supplies blood to the interventricular septum and anterior walls of both ventricles. The left coronary artery runs toward the left side of the heart and then divides into two major branches: - The anterior interventricular artery (also known clinically as the left anterior descending artery) follows the anterior interventricular sulcus and supplies blood to the interventricular septum and anterior walls of both ventricles. - The circumflex artery supplies the left atrium and the posterior walls of the left ventricle.

aortic semilunar valve

The aortic semilunar valve prevents backflow of blood into the left ventricle. The aortic and pulmonary (semilunar, SL) valves guard the bases of the large arteries issuing from the ventricles (aorta and pulmonary trunk, respectively) and prevent backflow into the associated ventricles. Each SL valve is fashioned from three pocketlike cusps, each shaped roughly like a crescent moon (semilunar = half-moon). The valve separating the left ventricle and aorta is the aortic semilunar valve. The exit through which blood leaves the left ventricle is the aortic semilunar valve.

cardiac cycle

The cardiac cycle refers to all of the events during one heart beat. The heart undergoes some dramatic writhing movements as it alternately contracts, forcing blood out of its chambers, and then relaxes, allowing its chambers to refill with blood. The term systole (sis′to-le) refers to these periods of contraction, and diastole (di-as′to-le) refers to those of relaxation. The cardiac cycle includes all events associated with the blood flow through the heart during one complete heartbeat—atrial systole and diastole followed by ventricular systole and diastole. These mechanical events always follow the electrical events seen in the ECG.

chordae tendineae

The chordae tendineae are attached to the AV valve flaps. Attached to each AV valve flap are tiny white collagen cords called chordae tendineae (kor′de ten″dĭ′ne-e; "tendinous cords"), "heart strings" which anchor the cusps to the papillary muscles protruding from the ventricular walls. The chordae tendineae, attached only to the AV valves, act as tethers that anchor the valve flaps in their closed position. However, neither the chordae tendineae nor the papillary muscles attached to them, are responsible for opening and closing the AV valves. Heart valves open and close in response to differences in blood pressure on their two sides.

sequence of blood flow through the heart

The correct sequence of blood flow through the chambers of the heart is right atrium, right ventricle, lungs, left atrium, and left ventricle. Stripped of its romantic cloak, the heart is no more than the transport system pump, and the blood vessels are the delivery routes. In fact, the heart is actually two pumps side by side. - The right side of the heart receives oxygen-poor blood from body tissues and then pumps this blood to the lungs to pick up oxygen and dispel carbon dioxide. The blood vessels that carry blood to and from the lungs form the pulmonary circuit (pulmo = lung). - The left side of the heart receives the oxygenated blood returning from the lungs and pumps this blood throughout the body to supply oxygen and nutrients to body tissues. The blood vessels that carry blood to and from all body tissues form the systemic circuit. The heart has two receiving chambers, the right atrium and left atrium, that receive blood returning from the systemic and pulmonary circuits. The heart also has two main pumping chambers, the right ventricle and left ventricle, that pump blood around the two circuits.

endocardium

The endocardium is composed of simple squamous epithelium. The heart wall, richly supplied with blood vessels, is composed of three layers: the epicardium, myocardium, and endocardium. The third layer of the heart wall, the endocardium ("inside the heart"), is a glistening white sheet of endothelium (squamous epithelium) resting on a thin connective tissue layer. Located on the inner myocardial surface, it lines the heart chambers and covers the fibrous skeleton of the valves. The endocardium is continuous with the endothelial linings of the blood vessels leaving and entering the heart. The lining of the heart chambers is called the endocardium. The heart wall, richly supplied with blood vessels, is composed of three layers: the epicardium, myocardium, and endocardium. The third layer of the heart wall, the endocardium ("inside the heart"), is a glistening white sheet of endothelium (squamous epithelium) resting on a thin connective tissue layer. Located on the inner myocardial surface, it lines the heart chambers and covers the fibrous skeleton of the valves. The endocardium is continuous with the endothelial linings of the blood vessels leaving and entering the heart.

epicardium

The epicardium lies on the outside surface of the heart and is an integral part of the heart wall. The heart wall, richly supplied with blood vessels, is composed of three layers: the epicardium, myocardium, and endocardium. The superficial epicardium is the visceral layer of the serous pericardium. It is often infiltrated with fat, especially in older people. At the superior margin of the heart, the parietal layer attaches to the large arteries exiting the heart, and then turns inferiorly and continues over the external heart surface as the visceral layer, also called the epicardium ("upon the heart"), which is an integral part of the heart wall. The epicardium is the inner layer of pericardium (called the visceral layer).

shape, position, and location of the heart

The heart is enclosed in a double-walled sac called the pericardium (per"ĭ-kar'de-um; peri = around, cardi = heart). The loosely fitting superficial part of this sac is the fibrous pericardium. This tough, dense connective tissue layer (1) protects the heart, (2) anchors it to surrounding structures, and (3) prevents overfilling of the heart with blood. The modest size and weight of the heart belie its incredible strength and endurance. About the size of a fist, the hollow, cone-shaped heart has a mass of 250 to 350 grams—less than a pound. Snugly enclosed within the mediastinum (me"de-ahsti' num), the medial cavity of the thorax, the heart extends obliquely for 12 to 14 cm (about 5 inches) from the second rib to the fifth intercostal space. As it rests on the superior surface of the diaphragm, the heart lies anterior to the vertebral column and posterior to the sternum. Approximately two-thirds of its mass lies to the left of the midsternal line; the balance projects to the right. The lungs flank the heart laterally and partially obscure it. Its broad, flat base, or posterior surface, is about 9 cm (3.5 in) wide and directed toward the right shoulder. Its apex points inferiorly toward the left hip. If you press your fingers between the fifth and sixth ribs just below the left nipple, you can easily feel the apical impulse caused by your beating heart's apex where it touches the chest wall.

interventricular septum

The interventricular septum forms a dividing wall between the left and right ventricles. The heart has four chambers —two superior atria (a′tre-ah) and two inferior ventricles (ven′trĭklz). The internal partition that divides the heart longitudinally is called the interatrial septum where it separates the atria, and the interventricular septum where it separates the ventricles. The right ventricle forms most of the anterior surface of the heart. The left ventricle dominates the inferoposterior aspect of the heart and forms the heart apex.

Myocardium

The myocardium is the middle layer of the heart, composed mainly of cardiac muscle. The heart wall, richly supplied with blood vessels, is composed of three layers: the epicardium, myocardium, and endocardium. The middle layer, the myocardium ("muscle heart"), is composed mainly of cardiac muscle and forms the bulk of the heart. This is the layer that contracts. In the myocardium, the branching cardiac muscle cells are tethered to one another by crisscrossing connective tissue fibers and arranged in spiral or circular bundles. These interlacing bundles effectively link all parts of the heart together.

papillary muscles

The papillary muscles prevent the atrioventricular valves from everting during ventricular contraction. The chordae tendineae and the papillary muscles act as tethers that anchor the valve flaps in their closed position. If the cusps were not anchored, they would be blown upward (everted) into the atria, in the same way an umbrella is blown inside out by a gusty wind. The papillary muscles contract with the other ventricular musculature so that they take up the slack on the chordae tendineae as the full force of ventricular contraction hurls the blood against the AV valve flaps.

Homeostatic Imbalances of Heart Valves

Two conditions severely weaken heart: Incompetent valve - Blood backflows so heart repumps same blood over and over Valvular stenosis - Stiff flaps that constrict opening - Heart needs to exert more force to pump blood Defective valve can be replaced with mechanical, animal, or cadaver valve

right marginal artery

The right marginal artery supplies blood to the lateral right side of the myocardium. The right coronary artery courses to the right side of the heart, where it also gives rise to two branches: - The right marginal artery serves the myocardium of the lateral right side of the heart. - The posterior interventricular artery runs to the heart apex and supplies the posterior ventricular walls. Near the apex of the heart, this artery merges (anastomoses) with the anterior interventricular artery.

right ventricle

The right ventricle receives blood from the right atrium. The right ventricle sends blood into the pulmonary trunk.

atrioventricular node (AV node)

The role of the atrioventricular node (AV node) is to slow down impulses so that the atria can contract to fill the adjacent ventricles with blood. From the SA node, the depolarization wave spreads via gap junctions throughout the atria and via the internodal pathway to the atrioventricular node, located in the inferior portion of the interatrial septum immediately above the tricuspid valve. At the AV node, the impulse is delayed for about 0.1 second, allowing the atria to respond and complete their contraction before the ventricles contract. This delay reflects the smaller diameter of the fibers here and the fact that they have fewer gap junctions for current flow. Consequently, the AV node conducts impulses more slowly than other parts of the system, just as traffic slows when cars are forced to merge from four lanes into two. Once through the AV node, the signaling impulse passes rapidly through the rest of the system. The AV node is the part of the conduction system cells that delays the impulse for about 0.1 second. From the SA node, the depolarization wave spreads via gap junctions throughout the atria and via the internodal pathway to the atrioventricular node, located in the inferior portion of the interatrial septum immediately above the tricuspid valve. At the AV node, the impulse is delayed for about 0.1 second, allowing the atria to respond and complete their contraction before the ventricles contract. This delay reflects the smaller diameter of the fibers here and the fact that they have fewer gap junctions for current flow. Consequently, the AV node conducts impulses more slowly than other parts of the system, just as traffic slows when cars are forced to merge from four lanes into two. Once through the AV node, the signaling impulse passes rapidly through the rest of the system.

The second heart sound (the 'dup' of 'lub-dup')

The second heart sound (the 'dup' of 'lub-dup') is caused by the closure of the semilunar valves. Auscultating (listening to) the thorax with a stethoscope will reveal two sounds during each heartbeat. These heart sounds, often described as lub-dup, are associated with the heart valves closing.

Serous Pericardium

The serous pericardium is a thin, slippery, two-layered membrane. Deep to the fibrous pericardium is the serous pericardium, a thin, slippery, two-layer serous membrane that forms a closed sac around the heart. Its parietal layer lines the internal surface of the fibrous pericardium. At the superior margin of the heart, the parietal layer attaches to the large arteries exiting the heart, and then turns inferiorly and continues over the external heart surface as the visceral layer, also called the epicardium ("upon the heart"), which is an integral part of the heart wall. The inner lining of the fibrous pericardium is formed by the parietal layer of serous pericardium. Deep to the fibrous pericardium is the serous pericardium, a thin, slippery, two-layer serous membrane that forms a closed sac around the heart (see figure below). Its parietal layer lines the internal surface of the fibrous pericardium. At the superior margin of the heart, the parietal layer attaches to the large arteries exiting the heart, and then turns inferiorly and continues over the external heart surface as the visceral layer, also called the epicardium ("upon the heart"), which is an integral part of the heart wall.

atria

The superior chambers of the heart are called the atria. The heart has four chambers —two superior atria (a′tre-ah) and two inferior ventricles (ven'trĭklz). The internal partition that divides the heart longitudinally is called the interatrial septum where it separates the atria, and the interventricular septum where it separates the ventricles. The right ventricle forms most of the anterior surface of the heart. The left ventricle dominates the inferoposterior aspect of the heart and forms the heart apex.

trabeculae carneae

The trabeculae carneae are located in the ventricles. Together the ventricles (ventr = underside) make up most of the volume of the heart. The right ventricle forms most of the heart's anterior surface and the left ventricle dominates its posteroinferior surface. Irregular ridges of muscle called trabeculae carneae (trah-bek′u-le kar′ne-e; "crossbars of flesh") mark the internal walls of the ventricular chambers. Other muscle bundles, the papillary muscles, which play a role in valve function, project into the ventricular cavity.

Semilunar (SL) valves

Two semilunar (SL) valves prevent backflow from major arteries back into ventricles - Open and close in response to pressure changes - Each valve consists of three cusps that roughly resemble a half moon - Pulmonary semilunar valve: located between right ventricle and pulmonary trunk - Aortic semilunar valve: located between left ventricle and aorta

tricuspid valve

The valve separating the right atrium and right ventricle is the tricuspid valve. The two atrioventricular (AV) valves, one located at each atrial-ventricular junction, prevent backflow into the atria when the ventricles contract. - The right AV valve, the tricuspid valve (tri-kus'pid), has three flexible cusps (flaps of endocardium reinforced by connective tissue cores). - The left AV valve, with two cusps, is called the mitral valve (mi'tral) because it resembles the two-sided bishop's miter (tall, pointed hat). It is sometimes called the bicuspid valve. The tricuspid valves is the valve between the right atrium and right ventricle. Blood exits the right atrium through the tricuspid valve. Focus on Blood Flow through the Heart follows a single "spurt" of blood as it passes through all four chambers of the heart and both blood circuits in its ever-repeating journey. As you work your way through this figure, keep in mind that the left side of the heart is the systemic circuit pump and the right side is the pulmonary circuit pump. Notice how unique the pulmonary circuit is. Elsewhere in the body, veins carry relatively oxygen-poor blood to the heart, and arteries transport oxygen-rich blood from the heart. The opposite oxygenation conditions exist in veins and arteries of the pulmonary circuit.

pulmonary semilunar valve

The valve separating the right ventricle and pulmonary trunk is the pulmonary semilunar valve. Focus on Blood Flow through the Heart follows a single "spurt" of blood as it passes through all four chambers of the heart and both blood circuits in its ever-repeating journey. As you work your way through this figure, keep in mind that the left side of the heart is the systemic circuit pump and the right side is the pulmonary circuit pump. Notice how unique the pulmonary circuit is. Elsewhere in the body, veins carry relatively oxygen-poor blood to the heart, and arteries transport oxygen-rich blood from the heart. The opposite oxygenation conditions exist in veins and arteries of the pulmonary circuit. The pulmonary semilunar valve prevents backflow of blood into the right ventricle. Blood exits the right ventricle through the pulmonary semilunar valve.

coronary arteries

The vessels that carry oxygen to the myocardium are called coronary arteries. Although the heart is continuously filled with various amounts of blood, this blood provides little nourishment to heart tissue. (The myocardium is too thick to make diffusion a practical means of delivering nutrients.) How, then, does the heart get nourishment? It does so through the coronary circulation, the functional blood supply of the heart, and the shortest circulation in the body. The left and right coronary arteries both arise from the base of the aorta and encircle the heart in the coronary sulcus. They provide the arterial supply of the coronary circulation. The coronary arteries receive blood from the aorta.

pulmonary veins

There are two pulmonary veins from each lung that carry blood into the left atrium. Focus on Blood Flow through the Heart follows a single "spurt" of blood as it passes through all four chambers of the heart and both blood circuits in its ever-repeating journey. As you work your way through this figure, keep in mind that the left side of the heart is the systemic circuit pump and the right side is the pulmonary circuit pump. Notice how unique the pulmonary circuit is. Elsewhere in the body, veins carry relatively oxygen-poor blood to the heart, and arteries transport oxygen-rich blood from the heart. The opposite oxygenation conditions exist in veins and arteries of the pulmonary circuit.

Layers of the heart wall

Three layers of heart wall ​​ 1. Epicardium: visceral layer of serous pericardium (makes serous fluid, slippery) ​​ 2. Myocardium: circular or spiral bundles of contractile cardiac muscle cells (muscle of the heart wall, arranged in circular motion from birth so that it can expand in every direction) 3. ​​Endocardium: innermost layer; is continuous with endothelial lining of blood vessels - Lines heart chambers and covers cardiac skeleton of valves. - The endothelial lining is very smooth and slick and any fibers sticking out would cause blood clots. That's why this layer is important. Also smooth so the blood flows right through

Atrioventricular (AV) Valves

Two atrioventricular (AV) valves prevent backflow into atria when ventricles contract Tricuspid valve (right AV valve): made up of three cusps and lies between right atria and ventricle Mitral valve (left AV valve, bicuspid valve): made up of two cusps and lies between left atria and ventricle Chordae tendineae: anchor cusps of AV valves to papillary muscles that function to: - Hold valve flaps in closed position - Prevent flaps from everting back into atria

skeletal vs. cardiac muscle

Unlike skeletal muscle cells, cardiac muscle have gap junctions between cells (that allow them to be autorhythmic). The heart contracts as a unit. Gap juctions tie cardiac muscle cells together to form a functional syncytium. This allows the wave of depolarization to travel from cell to cell across the heart. As a result, either all fibers in the heart contract as a unit or the heart doesn't contract at all. In skeletal muscle, on the other hand, impulses do not spread from cell to cell. Only skeletal muscle fibers that are individually stimulated by nerve fibers contract, and the strength of the contraction increases as more motor units are recruited. Such recruitment cannot happen in the heart because it acts as a single huge motor unit. Contraction of all of the cardiac myocytes ensures effective pumping by the heart—a halfhearted contraction would just not do.

incompetent cardiac valve

When a doctor puts his stethoscope on a patient's chest over the location of the heart and hears a swishing sound, the best diagnosis for the patient's condition is probably an incompetent cardiac valve. Blood flows silently as long as its flow is smooth and uninterrupted. If blood strikes obstructions, however, its flow becomes turbulent and generates abnormal heart sounds, called heart murmurs, that can be heard with a stethoscope. Heart murmurs are fairly common in young children (and some elderly people) with perfectly healthy hearts, probably because their heart walls are relatively thin and vibrate with rushing blood. Most often, however, murmurs indicate valve problems. An insufficient or incompetent valve fails to close completely. There is a swishing sound as blood backflows or regurgitates through the partially open valve after the valve has (supposedly) closed. A stenotic valve fails to open completely and its narrow opening restricts blood flow through the valve. In a stenotic aortic valve, for instance, a high-pitched sound or click can be detected when the valve should be wide open during ventricular contraction, but is not.