The Heart Chapter 18

Figure 18.1 The Heart Size – grams Location – in mediastinum of thoracic cavity Function – generates pressure to pump blood through circulatory system Orientation – flat base is directed toward right shoulder, and pointed apex points to left hip

Figure 18.2 Heart Coverings Pericardium – the two layered, membranous sac in which the heart sits Fibrous pericardium – the outer, thick layer composed of dense connective tissue, for protecting, anchoring the heart in position, and preventing overfilling Serous pericardium – the inner thin layer composed of a serous membrane Visceral layer – membrane clinging to the outer heart surface Parietal layer – membrane clinging to the inside of the fibrous pericardium Pericardial cavity – serous fluid-filled space between the visceral and parietal layers

Figure 18.2 Heart Layers Epicardium – the epithelium clinging to the outer heart wall (is the visceral pericardium) Myocardium – the middle layer composed of cardiac muscle tissue Endocardium – the epithelium clinging to the inner surfaces of the heart chambers

Figure 18.4 a, b The Heart 4 chambers: 2 atria, 2 ventricles Atria : the superior chambers auricles – ear-like extensions of the atria receiving chambers, limited pumping means thin walls Ventricles: the inferior chambers, the majority of the heart volume pumping chambers, thick walls Sulci: The indentations on the outer heart surface, corresponding to borderlines between chambers contain fat and vessels Ex – interventricular sulcus

Figure 18.4e, f The Heart Septa: The internal walls that divide the chambers Correspond to the external sulci Ex’s- Interventricular septum and Interatrial septum

Figure 18.4e Right Atrium Entrances: Superior Vena Cava – blood returning from above the diaphragm Inferior Vena Cava – blood returning from below the diaphragm Coronary sinus – blood returning from the heart wall Left Atrium Entrances: 4 pulmonary veins - blood returning from lungs The Atria

Figure 18.4e Right Ventricle: Receives blood from the right atrium Blood exits into the pulmonary trunk – to lungs Left Ventricle: Receives blood from the left atrium Blood exits into the aorta – to the body The Ventricles

Figure 18.5 Circulation Blood only passes through ½ of the heart at a time, and therefore must pass through the heart twice to complete circulation Pulmonary circuit: the pathway from the heart to the lungs and back is pumped by the right half of the heart blood leaves –O 2 and returns +O 2 Systemic circuit: the pathway from the heart to the body’s tissues and back is pumped by the left half of the heart blood leaves +O 2 and returns –O 2 NOTE – arteries do NOT always carry oxygenated blood, and veins do NOT always carry deoxygenated blood

Figure 18.6 Circulation The Systemic circuit is a longer circuit than the Pulmonary circuit Greater pressure is needed to pump blood through the systemic circuit The left ventricle therefore has more cardiac muscle tissue than the right ventricle

Figure 18.7 Coronary Circulation Coronary circulation – the series of vessels that supply blood flow to the wall of the heart, beginning at the aorta and ending at the right atrium Anastomoses – the merging of blood vessels, providing more than one way to deliver blood to one location…Why? Myocardial Infarction – “heart attack” – due to a blockage of a coronary artery, causing the death of cardiac muscle cells

Figure 18.4b Coronary arteries – deliver oxygen and nutrient rich blood to the cardiac muscle Coronary Circulation

Figure 18.4d Coronary Circulation Cardiac veins – drain the oxygen and nutrient poor blood from the cardiac muscle Coronary sinus – large vein on posterior side that empties into the right atrium

Figure 18.8 a, b Heart Valves Function – ensure a unidirectional flow of blood through the heart Types – there are 2 atrioventricular valves and 2 semilunar valves

Figure 18.8c Heart Valves Atrioventricular valves: separate an atrium from a ventricle prevent backflow into the atrium Tricuspid (Right AV) valve – separates right atrium from right ventricle Bicuspid (Left AV) valve – separates left atrium from left ventricle. Also known as mitral valve Flaps (cusps) of these valves are supported by papillary muscles and chordae tendineae

Figure 18.9 Heart Valves Atrioventricular valves: Papillary muscles attach to valve flaps via chordae tendineae These muscles contract to prevent the valve flaps from inverting into the atria

Figure 18.8c Heart Valves Semilunar valves: separate a ventricle from a great vessel prevent backflow into the ventricle composed of three cup-like flaps Pulmonary semilunar valve – regulates movement of blood into pulmonary trunk Aortic semilunar valve – regulates movement of blood into the aorta Flaps (cusps) of these valves are NOT supported by papillary muscles and chordae tendineae

Figure Heart Valves Semilunar valves: during contraction, pressure forces these valves open, allowing blood into the large vessels when the ventricles relax, the blood falls toward the ventricles, fills the cup- shaped flaps, closing the valves

Figure Microscopic Anatomy Cardiac muscle tissue: Striated – a striped appearance Involuntary – no conscious control Cardiac Muscle Fibers (cells): shorter than skeletal muscle fibers, with central nuclei large amount of mitochondria for endurance branched – fibers divide and unite interconnected – fibers are linked and work in unison

Figure 18.11a Cardiac Muscle Fibers (cells): Intercalated discs- junctions where adjacent fibers connect desmosomes – hold fibers tightly together gap junctions – allow fibers to share cytoplasm and contract in unison Cardiac Muscle

Figure Cardiac Muscle Contraction Autorhythmicity The ability of cardiac muscle to trigger its own contraction, not needing a nervous system impulse Whole organ contraction There is no partial contraction of cardiac muscle, it is an all or none event

Figure Cardiac Muscle Contraction Action potentials in cardiac muscle involve 3 ions (Na+, K+ and Ca 2 +), which produces a plateau phase of the impulse This prolongs the contraction to ensure efficient blood ejection Also increases the time of the absolute refractory period, ensuring separate heart contractions (not prolonged muscle tetanus)

Figure Intrinsic Conduction System Intrinsic Conduction System: Stimuli that trigger cardiac muscle contraction come from within the heart itself Autorhythmic cells - specialized heart cells that generate action potentials which spread through the heart to trigger contraction These cells have unstable resting potentials, and therefore depolarize regularly These pacemaker potentials cause action potentials in the cardiac muscle fibers, triggering contraction

Figure Intrinsic Conduction System Sinoatrial node – in the right atrium, the “pacemaker” whose cells generate the sinus rhythm of 75 beats/min Atrioventricular node - causes a delay of.1sec to allow atrial contraction before ventricular contraction. Rhythm is beats/min Bundle of His – pathway into the interventricular septum

Figure Intrinsic Conduction System Bundle Branches – the right and left pathways through the interventricular septum Purkinje fibers – pathways to the walls of the ventricles The bundle of his, bundle branches and purkinje fibers would set a rhythm of only 30 beats/min

Figure Extrinsic Innervation Cardiac centers – gray matter areas in the medulla that can change heart rhythm Parasympathetic innervation – via the vagus nerve, slows heart rate Sympathetic innervation – via the sympathetic trunk, increases heart rate Heart rate disorders: Tachycardia – an abnormally fast heart rate 100+ beats/min Bradycardia – an abnormally low heart rate 60- beats /min Arrhythmias – irregular heart rhythms Fibrillation – rapid, irregular contractions that do not function to pump blood

Figure Electrocardiogram ECG ECG – a graphic representation of all of the action potentials in the heart in a given time P wave – shows the depolarization of the atria QRS complex – shows the depolarization of the ventricles T wave – shows the repolarization of the ventricles

Figure Electrocardiogram ECG Note: the waves of the ECG graph correspond to the spreading depolarization through the heart tissue. Muscle contraction follows this depolarization

Figure Electrocardiogram ECG

Figure Heart Sounds Heart sounds – the “lub dub” “Lub” – the sound produced by the closure of the AV valves “Dub” – the sound produced by the closure of the semilunar valves Murmurs – abnormal heart sounds, indicating valve problems

Figure Cardiac Cycle Cardiac cycle – the events of a single heart beat Systole – heart contraction Diastole – heart relaxation 1.Ventricular filling – blood flows through the atria into the ventricles. The AV valves are open, and the semilunar valves are closed. Then, the atria contract forcing the remaining blood into the ventricles 2.Isovolumetric contraction – ventricles contract, forcing the AV valves closed. The volume in the ventricles is now the end diastolic volume (EDV)

Figure Cardiac Cycle 3. Ventricular ejection – ventricular pressure forces the semilunar valves open and the blood enters the great vessels 4. Isovolumetric relaxation – ventricles relax, and the blood within them is the end systolic volume (ESV). The semilunar valves close and the atria begin to fill. 5. Ventricular filling – atrial pressure forces the AV valves open which restarts the cycle

Figure Cardiac Output Cardiac Output (CO) – the amount of blood pumped by each ventricle in one minute CO = heart rate x stroke volume Stroke volume (SV) = EDV - ESV Practice Question: If a person’s EDV is 125mL and their ESV is 50mL, what is their CO if their heart rate is 80 beats/min? Answer: SV = 125mL – 50 mL SV = 75mL CO = 75mL x 80 beats/min CO = 6000mL/min = 6.0L/min

Figure Fetal Heart Structures Foramen ovale – a hole in the interatrial septum allowing blood to pass from the right atrium to the left atrium. Its remnant is the fossa ovalis in adults Ductus arteriosus – a passage from the pulmonary trunk to the aorta. Its remnant is the ligamentum arteriosum in adults Both structures allow the fetal blood to bypass the pulmonary circuit….Why?