Chapter 9 The Cardiovascular System

1. Chapter 9The Cardiovascular System

2. Learning Objectives • Describe the anatomy of the heart and vascular systems. • State the key characteristics of cardiac tissue. • Calculate systemic vascular resistance given mean arterial pressure, central venous pressure, and cardiac output. • Describe how local and central control mechanisms regulate the heart and vascular systems. • Describe how the cardiovascular system coordinates its functions under normal and abnormal conditions.

3. Learning Objectives (cont.) • Calculate cardiac output given stroke volume and heart rate. • Calculate ejection fraction given stroke volume and end-diastolic volume. • Identify how the electrical and mechanical events of the heart relate to a normal cardiac cycle.

4. Functional Anatomy • Heart is hollow, four chambered, muscular, roughly fist-sized • Lies just behind the sternum, two thirds lie to left, between the second through the sixth ribs • Heart apex at fifth intercostal space • Surface grooves (sulci) mark the boundaries of the heart chambers

5. Functional Anatomy (cont.) • Pericardium • Double walled sac enclosing heart • Pericarduim’s structure • Outer fibrous layer: Tough connective tissue, loose-fitting, inelastic sac surrounding heart • Inner serous layer: thinner, more delicate • Serous pericardium: Consisting of two layers: • Parietal layer: Inner lining of fibrous pericardium • Visceral layer (epicardium): covering outer surface of heart & great vessels

6. Functional Anatomy (cont.)

7. The Heart • Pericardial fluid • Thin layer of fluid separating parietal & visceral pericardium • Helps minimize friction during contraction & expansion • Pericardial effusion • Abnormal amount of accumulated fluid between layers

8. The Heart • Cardiac tamponade • Large pericardial effusion may affect pumping function • Can cause serious drop in blood flow to body • May ultimately lead to shock & death • Pericarditis • Inflammation of pericardium

9. What is the function of the pericardial fluid ? • helps minimize friction during heart contraction & expansion • provide protection against trauma • mark the boundaries of the chambers of the heart • prevents atrial backflow during ventricular contraction

10. The Heart (cont.) • Heart wall is composed of 3 layers: • Outer: epicardium • Middle: myocardium comprises bulk of heart & is composed of muscle tissue • Inner: Endocardium • Forms thin continuous tissue with blood vessels • Heart forms 4 muscular chambers • Upper chambers, right & left atria • Lower chambers are right & left ventricles • Responsible for forward movement of blood

11. The Heart (cont.)

12. Atrioventricular Valves • AV valves lie between atria & ventricles • Tricuspid valve is at right atrium exit • Mitral valve at left atrium exit • Ventricular contraction forces valves closed, preventing backflow of blood into atria • Lower ends of valves anchor to ventricular papillary muscles by chordae tendineae • Papillary contraction during systole pulls on chordae, preventing valve reversing into atria

13. Atrioventricular Valves (cont.)

14. What is the role of ventricular contraction ? • relaxes chordae tendineae, preventing valves reversing into atria • to rest the heart muscle • to forward movement of blood • prevents atrial blood backflow

15. Semilunar Valves • Consist of 3 half-moonshaped cusps • Separates ventricles from their arterial outflow tracts, pulmonary artery & Aorta • Situated at ventricle exits to outflow tracks (arterial trunks) • Pulmonary valve lies between right ventricle & pulmonary artery • Aortic valve lies between left ventricle & aorta

16. Semilunar Valves (cont.) • Systole: (cardiac contraction) valves open, allowing ventricular ejection into arteries (pulmonary artery and aorta) • Diastole: valves close preventing back flow of blood into ventricles

17. Coronary Circulation • Heart’s high metabolic demands require an extensive circulatory system • The heart requires more blood flow per gram of tissue weight than any other organ besides kidneys

18. Coronary Circulation (cont.) • Right & left coronary arteries arise under aortic valve cusps • Coronary artery pressure becomes higher than aortic pressure during systole • Prevents flow of blood into coronaries • Diastole is when coronary blood flow occurs thus, diastolic pressure is very important

19. Left Coronary Artery (LCA) • Positioned underneath aortic semilunar valves • LCA branches into: • Left anterior descending (LAD): courses between left & right ventricles • Circumflex: courses around left side of heart between left atrium & left ventricle

20. LCA (cont.) • LCA provides blood to left atrium, left ventricle, majority of interventricular septum, half of interatrial septum, & part of right atrium • See Figure 9-4 & Table 9-1

21. LCA (cont.)

22. Right Coronary Artery (RCA) • RCA proceeds around right side of heart between right atrium & right ventricle • Many small branches as RCA moves around right ventricle • RCA ends in its posterior descending (RPD) branch, which courses between right & left ventricles. • Provides blood flow to most of right ventricle & right atrium, including sinus node • See Figure 9-4 & Table 9-1

23. Problems With Coronary Blood Flow • Myocardial Ischemia • Partial obstruction of coronary artery • Decreasing oxygen supply to tissue • a.k.a. Angina Pectoris • Myocardial Infarction (MI) • Sometimes called “infarct” • Complete obstruction of coronary artery • Causes death of heart tissue

24. Coronary Veins • Collect venous blood after passing through myocardial capillary bed • Veins closely parallel coronary arteries • Great cardiac vein follows LAD • Small cardiac vein follows RCA • Left posterior vein follows circumflex • Middle vein follows RPD • These all come together to form coronary sinus, which empties into right atrium • Thebesian veins drain some coronary venous blood into all heart chambers • Those draining into left atrium & left ventricle bypass lungs, creating an anatomic shunt • Normal anatomic shunt = 2-3% of total cardiac output

25. Properties of Heart Muscle • Heart’s ability to pump depends on: • Initiating & conducting electrical impulses • Synchronous myocardial contraction • Made possible by key properties of myocardial tissue • Excitability: ability to respond to stimuli • Inherent rhythmicity: initiation of spontaneous electrical impulse • Conductivity: spreads impulses quickly • Contractility: contraction in response to electrical impulse • Unique featurecannot go into tetany

26. Properties of Heart Muscle (cont.) • Refractory period • Time period myocardium cannot be stimulated • Lasts 250 milliseconds; nearly as long as systole

27. Properties of Heart Muscle (cont.)

28. The Vascular System • Composed of 2 major subdivisions: • Systemic vasculature • Pulmonary vasculature • Systemic vasculature begins with aorta on left ventricle & ends in right atrium • Pulmonary vasculature begins with pulmonary trunk out of right ventricle & ends in left atrium • Systemic venous blood returns to right atrium via: • Superior vena cava (SVC): drains upper extremities & head • Inferior vena cava (IVC): drains lower body • Blood flows through tricuspid valve into right ventricle

29. The Vascular System (cont.) • Pumped from right ventricle through pulmonary valve into pulmonary artery, which carries it to lungs (oxygenation) • Pulmonary arterial blood returns via pulmonary veins to left atrium • From left atrium oxygenated blood flows through mitral valve into left ventricle • Left ventricle pumps the blood out through aortic valve into systemic circulation • Blood passes through systemic capillary beds into systemic veins & back to SVC & IVC

30. The Vascular System (cont.)

31. Where does systemic vasculature begin & end? • begins in the right atrium & ends in the right ventricle • begins in the aorta on the left ventricle & ends in the right atrium. • begins in the pulmonary trunk out of the right ventricle & ends in the left atrium. • begins the left atrium & ends in the inferior vena cava

32. Systemic Vasculature • 3 components: • Arterial system (conductance vessels) • Large elastic low resistance arteries • Small muscular arterioles (resistance vessels) • Major role in distribution & regulation of blood pressure • Like faucets, control local blood flow into capillaries • Capillary system, microcirculation (exchange vessels) • Transfer of nutrients & waste products • Venous system (capacitance vessels) • Reservoir for circulatory system • Generally holds 3/4 of body’s blood volume • Conduct blood back to heart

33. Systemic Vasculature (cont.)

34. Vascular Resistance • Sum of all opposing forces to blood flow through systemic circulation is systemic vascular resistance (SVR) • SVR = Change (Δ) in pressure from beginning to end of system, divided by flow SVR = (MAP – RAP)/CO Where: MAP = mean aortic pressure RAP = right atrial pressure or CVP CO = cardiac output

35. Pulmonary Vascular Resistance (PVR) • PVR: sum of all opposing forces to blood flow through pulmonary circulation • PVR then calculated as is SVR (ΔP/flow) PVR = (MPAP – LAP)/CO Where: MPAP = mean pulmonary artery pressure LAP = left atrial pressure or wedge pressure CO = cardiac output • PVR: normally much lower than SVR as pulmonary system is low pressure, low resistance

36. Determinants of Blood Pressure (BP) • Normal CV function maintains blood flow throughout body • Under changing conditions, need constant BP MAP = CO × SVR And MAP = Volume/Capacity • To maintain BP, capacity must vary inversely with CO or volume

37. Determinants BP(cont.) • Normal adult: MAP values range: • 80 to 100 mm Hg • If MAP falls significantly below 60 mm Hg • Perfusion to brain & kidneys is severely compromised • Organ failure may occur in minutes

38. Control of Cardiovascular System • The heart works as a demand pump • CV system may alter capacity - how much blood it holds • Decreased capacity results in greater venous return & greater CO • CV system tells heart how much to pump • Accomplished by local & central control mechanisms • Heart plays secondary role in regulating blood flow • Blood flow through large veins can also be affected by abdominal & intrathoracic pressure changes

39. Cardiac Output (CO) & its Regulation • CO = total amount of blood pumped by heart per minute • CO = Heart rate (HR) × stroke volume (SV) • Normal CO = 5 L/min • End Diastolic Volume (EDV): blood left in atria

40. CO & its Regulation (cont.) • End Systolic Volume (ESV): blood left in ventricles • HR is primarily determined by CNS • CO is directly related to HR • HR > 160180 is exception; too little time for filling results in decreased EDV, EF, SV, &, thus, CO • SV is determined by • Preload • Afterload • Contractility • SV = EDV-ESV

41. CO & its Regulation (cont.)

42. Stroke Volume & Preload • Preload essentially equals venous return • Amount of volume & pressure at end diastole (EDV, EDP) stretches myocardium • Greater the stretch, the stronger the contraction • Frank-Starling Law • Normal EDV is ~110120 ml • Normal SV is ~70 ml • Ejection fraction (EF) = SV/EDV • Normal ~65% • If it falls to 30% range or below, patient’s exercise tolerance becomes severely limited

43. Stroke Volume & Preload (cont.)

44. Stroke Volume & Preload (cont.) • Afterload: resistance against which ventricles pump, so more afterload makes it harder for ventricles to eject SV • RV afterload = PVR • LV afterload = SVR • All else constant, increase in vascular resistance would decrease SV • Usually does not occur as contractility increases to maintain SV & thus CO

45. Stroke Volume & Preload (cont.) • Afterload represents sum of all external factors opposing ventricular ejection • Tension in ventricular wall • Peripheral resistance or impedance

46. Stroke Volume & Contractility • Contractility: amount of force myocardium produces at any EDV • Increased contractility results in greater EF for any EDV • Called positive inotropism • If afterload & contractility increase together, SV is maintained • Positive inotropes • Drugs that increase contractility of heart muscle • Negative inotropes • Drugs decreasing contractility of heart muscle

47. Stroke Volume and Contractility (cont.)

48. If a patient is given Dobutamine, a positive inotropic drug, how is contraction of the heart affected? • decrease the force of contractions • increase the force of contractions • not affect the force of contractions • intermittently increase & decrease the force of contractions

49. Cardiovascular Control Mechanisms • Integration of local & central mechanisms to ensure all tissues have enough blood flow • Normally, local control is primary determinant • With large changes in demand, central control becomes primary • Central control in medulla has areas for: • Vasoconstrictionincreases adrenergic output • Vasodepressorinhibits vasoconstrictor center • Cardioacceleratoryincreases heart rate • Cardioinhibitorydecrease heart rate (by increasing vagal stimulation to heart)

50. Cardiovascular Control Mechanisms (cont.)