Review of Cardiac Structure and Function

1. Review of Cardiac Structure and Function

2. Location • Heart lies in the mediastinum behind the sternum • 1/3 of bulk of the heart lies to the right of the midline; 2/3 lies to the left • Apex lies between the 4th & 5th ribs when lying down and between the 5th and 6th ribs when sitting or standing

4. Pericardium • The heart lies within the pericardium • Outer layer of the pericardium is fibrous connective tissue – fibrous pericardium • Anchors the pericardium to the great vessels, the sternum and diaphragm

5. Two inner layers of serous membranes- parietal pericardium and visceral pericardium (epicardium) • Between the serous membranes is the pericardial space, filled with 15- 50 ml of fluid to reduce friction • Fibrous pericardium and parietal pericardium are called the pericardial sac • Visceral pericardium is the epicardium

7. Heart Wall • Epicardium • Myocardium – muscular layer • Thickest layer • Attached to fibrous skeleton of the heart • Bands of muscle arranged longitudinally • Fibers from one side enter other side • Better integration of contractions • Pathology of one ventricle can affect the other

9. Endocardium • Internal lining of squamous epithelium • Continuous throughout cardiovascular system

11. Four chambers (two pumps) • Right heart acts as a volume pump – through low-resistance vessels of the pulmonary system • Left heart acts as a pressure pump – through high-resistance vessels of the systemic circulation

13. Atria • Two upper, thin walled chambers that collect blood returning to the heart • Their contraction aids in filling the ventricles • Right atrium receives blood from superior and inferior vena cavae and coronary sinus • Left atrium receives blood from pulmonary veins

16. Ventricles • Thick walled pumping chambers of the heart • Make up about 60 % of the mass of the heart and receive most of coronary blood flow • Blood from right ventricle enters the pulmonary trunk • Blood from left ventricle enters the aorta

17. Path of Blood Flow Vena cavae → RA→ RV→ Pulmonary trunk→ Lungs→ Pulmonary veins→ LA→ LV→ Aorta → Systemic circulation→ Vena cavae

19. Valves • Made of connective tissue covered by endothelium • Prevent back flow of blood • Open and close passively in response to ventricular contraction • Anchored to the annuli fibrosi cordis to prevent dilation during contraction of the heart

21. Atrioventricular valves • Lie between the atria and ventricles • Tricuspid valve on the right • Bicuspid or mitral valve on left • Anchored against high pressure by the chordae tendineae and papillary muscles

23. Semilunar Valves • Lie between ventricles and great vessels • Pulmonary on right side • Aortic on left side

27. Cardiac Cycle • Sequence of events that compose the repeating pumping action of the heart • Typically, systole refers to ventricular contraction and diastole refers to ventricular relaxation • If referring to atria, specify atrial systole, etc.

28. Review • Coronary arteries and cardiac veins – note that it is the right coronary artery that supplies blood to the SA and AV nodes

31. Review • Pressures in chambers: look over the next figure - note especially the pressures in the right atrium – central venous pressure, and the pressures in the aorta – systemic blood pressure

33. Cardiac muscle - Myocytes • Contractile fibers • Fibers are branched • Intercalated discs - contain desmosomes and gap junctions • Gap junctions connect cytoplasm of adjoining cells so that the action potentials pass from one cell to the next • Cells contract as if one cell - syncytium

36. Muscle contraction • Striated muscle – contains actin and myosin • Action potential stimulates release of Ca++ • Ca++ binds to troponin • Troponin moves the tropomyosin exposing myosin binding sites on the actin • Myosin pulls on the actin, filaments slide past each other and muscle shortens

42. Remember, that unlike skeletal muscle, much of the Ca++ used in cardiac muscle contraction comes from the extracellular fluid.

43. Conduction System • Conduction system cells are specialized myocardial fibers • Heart has autorhythmicity- beats spontaneously at about 100 beats/min • Impulse begins at sinoatrial (SA) node or pacemaker – sinus rhythm • Spreads through atria via conducting myofibers to atrioventricular (AV) node in fibrous skeleton of heart

44. Signal pauses briefly, for ventricular filling by atrial contraction. • Signal continues into ventricle through conduction pathways: • AV bundle ( bundle of His) →right and left bundle branches→ Purkinje fibers→ ventricular contraction (bottom up)

46. Regulation of Heart Rate • Sympathetic N.S. increases heart rate and force of contraction – secrete norepinephrine –accelerator nerves • Parasympathetic N.S. decrease heart rate and force of contraction through the vagus nerve. Sends continuous impulses. Secretes acetylcholine

48. Electrocardiogram ECG • Measures electrical activity of the heart • P wave – represents atrial contraction (depolarization) • QRS complex – represents ventricular contraction (depolarization) • T wave – represents ventricular relaxation (repolarization)