The Heart.docx

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Western University
Anatomy and Cell Biology
Anatomy and Cell Biology 3319
Kem Rogers

The Heart • Heart is a muscular double pump with two functions • Right side receives oxygen-poor blood from body tissues & pumps blood to lungs to pick up oxygen and dispel CO 2 • Blood vessels that carry blood to & from lungs form pulmonary circuit • Left side receives oxygenated blood returning from lungs & pumps blood throughout body to supply oxygen & nutrients to body tissues • Vessels that transport blood to & from all body tissues & back to heart form the systemic circuit • Heart has 2 receiving chambers, the right atrium & left atrium, that receive blood returning from systemic & pulmonary circuits • Heart has 2 pumping chambers: right ventricle & left ventricle – pump blood around two circuits Location & Orientation • Lies in thorax posterior to sternum & costal cartilages & rests on superior surface of diaphragm • Largest organ in mediastinum – region between lungs (& pleural cavity) • Assumes oblique position – apex lies to left of middle & anterior to rest of the heart • Press fingers between 5 & 6 ribs just inferior to left nipple – may feel heart beat where apex contacts thoracic wall • Cone-shaped objects have a base as well as apex • Heart’s base is its broad posterior surface • Has 4 corners defined by 4 points projected onto anterior thoracic wall • 2 rib is easily palpated just lateral to sternal angle o Superior right point: where costal cartilage of 3 rib joins sternum nd o Superior left point lies at costal cartilage of thrib, a finger breadth lateral to sternum o Inferior right point lies at costal cartilage of 6 rib – finger’s breadth lateral to sternum o Inferior left point (apex point) lies in 5 intercostal space at midclavicular line – line extending inferiorly from midpoint of left clavicle Structure of the Heart Coverings • Pericardium: triple-layered sac that encloses heart o Outer layer: fibrous pericardium  Strong layer of dense connective tissue  Adheres to diaphragm inferiorly & superiorly fused to roots of great vessels that leave & enter the heart  Acts as tough outer coat that holds heart in place & keeps it from overfilling with blood o Double layered: serous pericardium  Closed sac sandwiched between fibrous pericardium and the heart • Outer layer: reflects to be part of the parietal layer of serous pericardium – adheres to inner surface of the fibrous pericardium – continuous with next layer • Visceral layer of serous pericardium or epicardium – lies on the heart and is considered a part of the heart wall  Between parietal & visceral layers of serous pericardium is a space: pericardial cavity, a division of the embryonic coelom  Epithelial cells of serous pericardium that line pericardial cavity produce a lubricating film of serous fluid into pericardial cavity  Fluid reduces friction between the beating heart and the outer wall of the pericardial sac Layers of the Heart Wall • Wall of the heart has three layers: superficial epicardium, middle myocardium, & deep endocardium • Epicardium o Visceral layer of the serous pericardium o Infiltrated with fat, especially in older people • Myocardium o Consists of cardiac muscle tissue – bulk of heart o Layer that actually contracts o Surrounding cardiac muscle cells are connective tissues that bind these cells together into elongated, circularly & spirally arranged networks: bundles o Bundles function to squeeze blood through the heart in the proper directions o Connective tissues of myocardium form cardiac skeleton – reinforces myocardium internally and anchors cardiac muscle fibres • Endocardium o Located deep to myocardium o Sheet of simple squamous epithelium resting on a thin layer of connective tissue o Lines the heart chambers & covers the heart values Formation of Pericardial Cavity • Analogous to hand (heart) pushing into a balloon (serous body cavity) from behind o Part of the balloon that covers all the contours of fist: visceral o Outer surface: parietal o Space in between: two layered cavity  pericardial cavity Clinical Application: Cardiac Tamponade • Pericardial fluid accumulation (cavity is closed) makes a globular shaped heart • Generates excessive pressure • Heart cant fully expand with blood  cant fully contract Heart Chambers • Heart is divided longitudinally o Interatrial septum between atria o Interventricular septum between the ventricles • Externally, boundaries of the four chambers are marked by 2 grooves • Coronary sulcus extends horizontally, circling the boundary between atria & ventricles • Anterior interventricular sulcus extends vertically marking anterior position of interventricular septum • Posterior interventricular sulcus separates 2 ventricles on heart’s inferior surface • Posterior of the heart lies against the diaphragm and is thus its inferior surface Right Atrium • Forms entire right border of heart • Receiving oxygen-poor blood returning from the systemic circuit • Receives blood via 3 veins: superior vena cava, inferior vena cava & coronary sinus • Right auricle: small flap shaped (little ear) that projects anteriorly from superior corner of the atrium • Internally, right atrium has 2 parts: o Smooth-walled posterior part o Anterior part lined by horizontal ridges called pectinate muscles • Parts are separated by ridge: crista terminalis o Superior vena cava opens into atrium just posterior to the superior bend of the crista o Inferior vena cava opens into atrium just posterior to the inferior bend of the crista o Coronary sinus opens into atrium just anterior to the inferior end of the crista o Just posterior to crista is fossa ovalis  Depression in interatrial septum that marks spot where opening existed in fetal heart: formamen ovale • Bypass of pulmonary trunk – fetal blood does not enters the lung • When baby takes its first breath – shunt closes and blood then enters lungs • When it doesn’t close: blue baby (does not have enough oxygenated blood) – surgery closes hole  Ductus arteriosus – connection between aorta & pulmonary artery that also prevents any blood in right ventricle from entering circulation of developing lungs • Remnant: Ligamentum arteriosum • Inferiorly & anteriorly right atrium opens into right ventricle through a tricuspid valve (right AV valve) Right Ventricle • Forms most of the anterior surface of the heart • Receives blood from right atrium & pumps it into the pulmonary circuit via artery: pulmonary trunk • Internally, ventricular walls are marked by irregular ridges of muscles: trabeculae carneae • Cone-shaped papillary muscles project from the walls into the ventricular cavity • Thin, strong bands: chordae tendineae (heart strings) – project superiorly from papillary muscles to the flaps (cusps) of the tricuspid valve • Superiorly the opening between the right ventricle & pulmonary trunk contains the pulmonary semilunar valve (pulmonary valve) Left Atrium • Makes up most of the heart’s posterior surface or base • Crescent shaped chamber • Receives oxygen-rich blood returning from the lungs through two right & two left pulmonary veins • Only part of the left atrium visible anteriorly is its triangular left auricle • Internally, most of the atrial wall is smooth, with pectinate muscles lining the auricles only • Left atrium opens into the left ventricle through mitral valve (left atrioventricular valve) Left Ventricle • Forms the apex of the heart & dominates the heart’s inferior surface • Pumps blood into systemic circuit • Like right ventricle, it contains traveulae carneae, papillary muscles, chordae tendineae & cusps of an atrioventricular (mitral) valve • Superiorly, left ventricle opens into stem artery of systemic circulation (aorta) through aortic semilunar valve (aortic valve) • Wall is thicker than right atrium • Chamber is circular shaped Heart Valves Valve Structures • Enforce one-way flow of blood • Each valve has 2 or 3 cusps: flaps of endocardium reinforced by cores of dense connective tissue o Located at junctions of atria & their respective ventricles are the atrioventricular valves:  Right atrioventricular (tricuspid) valve  Left atrioventricular (bicuspid) valve – mitral valve o Located at junctions of ventricles & great arteries are aortic & pulmonary (semilunar) valves • Cardiac skeleton o Lies in the plane between atria & ventricles & surround all four heart valves rather like handcuffs o Composed of dense connective tissues o Functions  Anchor the valve cusps  Prevents overdilation of the valve openings as blood pulses through them  Point of attachment for the bundles of cardiac muscle in the atria & ventricles  Blocks direct spread of electrical impulses from the atria to the ventricles (function is critical for the proper conduction of atrial & ventricular contractions) Valve Functions • Open (allow blood flow) & close (prevent backflow of blood) in response to differences in blood pressure on each side of the valves • 2 atrioventricular valves prevent backflow of blood into atria during contraction of ventricles • When ventricles are relaxed, cusps of AV valves hang limply into ventricular chambers while blood flows into atria & down through open AV valves into the ventricles • When ventricles start to contract, pressure within them rises & forces blood superiorly against the valve cusps  pushes the edges of the cusps together & closing the AV valves • Chordea tendineae and papillary muscles that attach to these valves look like the cords of an open parachute, limiting the closed cusps so they cannot fly and allow reflux of ventricular blood into the atria • Papillary muscles begin to contract slightly before the rest of the ventricle contracts, pulling on the chordae tendineae and preventing the AV valves from everting • If the cusps were not anchored in this manner, they would be forced superiorly into the atria • Two semilunar valves prevent backflow from the great arteries into the ventricles • When ventricles contract & rise intraventricular pressure, semilunar valves are forced open, & their cusps are flattened against the arterial walls as the blood rushes past them • When ventricles relax, blood that tends to flow back toward heart fills cusps of semilunar valves & forces them shut Heart Sounds • Closing of valves causes vibrations in adjacent blood & heart walls that allow for “lupdup” of heartbeat: o Lub sound is produced by the closing of AV valves at the start of ventricular contraction o Dub sound is produced by the closing of semilunar valves at the end of ventricular contraction • Mitral valve closes slightly before tricuspid closes & aortic valve generally closes before tricuspid • All 4 valve sounds are discernible when you listen through stethoscope placed on anterior chest wall • Don’t listen over valves, because sounds take oblique paths through chambers to reach chest wall o Don’t listen at ribs but in intercostal space (soft tissue) • Each valve is best heard near a different heart corner: o Pulmonary valve near superior left point o Aortic valves near superior right point, o Mitral valve at the apex point o Tricuspid valve near inferior right point Clinical Application: • Stenosis: will not let enough blood through • Incompetence: incomplete closure – leaky • Hypertrophy: heart muscle has to work harder – therefore walls increase in size Pray - pulmonary At – aortic Mid – mitral Pathway of Blood Through Heart • Blood coming from body superior to diaphragm (excluding heart wall) enters via superior vena cava • Blood coming from body inferior to diaphragm (excluding heart wall) enters via inferior vena cava • Blood draining from the heart wall is collected by and enters the right atrium through coronary sinus • Blood passes from right atrium through tricuspid valve to right ventricle by gravity & contraction • Then right ventricle contracts, propelling blood through pulmonary semilunar valve into pulmonary trunk and to the lungs through the pulmonary circuit • Freshly oxygenated blood returns via 4 pulmonary veins to left atrium & passes through mitral valve to left ventricle, propelled by gravity and the contraction of left atrium • Left ventricle then contracts & propels blood through aortic semilunar valve into aorta and its branches • After delivering oxygen & nutrients to body tissues through systemic capillaries, oxygen-poor blood returns through the systemic veins to the right atrium – and the whole cycle repeats continuously • Although blood passes through chambers sequentially, 4 chambers do not contract in this order o 2 atria always contract together, followed by simultaneous contraction of 2 ventricles o Single sequence of atrial contraction followed by ventricular contraction heartbeat • Heart of an average person at rest beats 70-80 times per minute • Both atria & ventricles experience systole & diastole o Term that describes the contraction of a heart chamber is systole o Time during which a heart chamber is relaxing and filling with blood is diastole • In common medial usage, diastole and systole most often refer to ventricular filling & contraction o Ex. blood pressure reading measures ventricular systole pressure over ventricular diastolic pressure (systolic/diastolic) in the systemic circuit that is the left ventricle o Because ventricle is relaxed during diastole, elevation of pressure during it is a symptom of hypertension • Walls of atria are much thinner because much of ventricular filling is done by gravity  atria exert little effort to propel blood inferiorly into the ventricles • Wall of left ventricle (systemic pump) is at least 3x as thick as right ventricle wall (pulmonary pump) o Left ventricle can generate much more force than right & pumps blood at much higher pressure o Higher pressure in systemic circuit reflects fact that systemic circuit is much longer than pulmonary circuit and offers greater resistance to flow o Thick wall of left ventricle gives chamber circular shape & flattens cavity of adjacent right ventricle into the shape of a crescent Conducting System & Innervation Conducting System • Cardiac muscle cells have an intrinsic ability to generate and conduct electrical impulses that stimulate these same cells to contract rhythmically • Properties are intrinsic to the heart muscle itself and do not depend on extrinsic nerve impulses • Even if all nerve connections to the heart are severed, the heart continues to beat rhythmically • Conducting system o Series of specialized cardiac muscle cells that carries impulses throughout heart musculature, o Signaling the heart chambers to contract in the proper sequence o Initiates each contraction sequence, thereby setting basic heart rate • Sinoatrial (SA) node o Crescent-shaped mass of muscle cells o Lies in the wall of the right atrium, just inferior to the entrance of the superior vena cava o Sets basic heart rate by generating 70-80 electrical impulses per minute - heart’s pacemaker • Signal initiated by SA node spreads throughout myocardium through gap junctions in intercalated discs • From SA node, impulses spread in a wave along atrial cardiac muscle fibers, signaling atria to contract • Some of these impulses travel along an intermodal pathway to atrioventricular (AV) node in inferior part of the interatrial septum, where they are delayed for a fraction of a second • After this delay, impulses race through atrioventricular (AV) node (Bundle of His) which enters the interventricular septum and divides into right & left bundle branches or crura • About halfway down septum, bundle branches terminate in the subendocardial conducting network (Purkinje fibres), which approach apex of the heart & then turn superiorly into ventricular walls • Arrangement ensures that the contraction of the ventricles begins at the apex of the heart & travels superiorly, so that ventricular blood is ejected superiorly into the great arteries • Brief delay of contraction-signaling impulses at AV node enables ventricles to fill completely before they start to contract • Because the fibrous cardiac skeleton between atria & ventricles is nonconducting, it prevents impulses in the atrial wall from proceeding directly to the ventricular wall • As a result, only those signals that go through AV node can continue on • Cells of the nodes & AV bundle are small, but otherwise typical cardiac muscle cells • Subendocardial conducting network is a long row of special, large-diameter, barrel-shaped cells • Muscle cells contain relatively few myofilaments because they are adapted more for conduction than for contraction • Large diameter maximizes the speed of impulse conduction • Network of conducting cells is located in deepest part of ventricular endocardium, between endocardium & myocardium layers Innervation • Although inherent rate of contraction is set by SA node, rate can be altered by extrinsic neural controls • Nerves to the heart consists of visceral sensory fibers, pa
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