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Cardiac Output, Blood Flow, and Blood Pressure

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  1. Cardiac Output, Blood Flow, and Blood Pressure Chapter 14

  2. Circulation Pulmonary circulation: path of blood from right ventricle through lungs and back to heart Systemic circulationis path of blood from left ventricle to body and back to heart Rate of flow through systemic circulation = flow rate thru pulmonary circuit

  3. Atrioventricular Valves • Opening and closing of valves results from pressure differences • High pressure of ventricular contraction is prevented from everting AV valves by contraction of papillary musclesconnected to AVs by chordae tendinea

  4. Semilunar Valves • During ventricular contraction blood is pumped through aortic and pulmonary semilunar valves • Close during relaxation

  5. Cardiac Cycle • Repeating pattern of contraction and relaxation of heart • Systole refers to contraction phase • Diastole refers to relaxation phase • Both atria contract simultaneously; ventricles follow 0.1-0.2 sec later Cardiac Rate: 75 beats/min Contraction of the atria occurs in last 0.1 sec of ventricular diastole Relaxation of atria occurs during ventricular systole

  6. Cardiac Cycle • End-diastolic volume – volume of blood in ventricles at end of diastole • Stroke volume – amount of blood ejected from ventricles during systole • End-systolic volume – amount of blood left in ventricles at end of systole

  7. Cardiac Cycle • As ventricles contract, pressure rises, closing AV valves • isovolumetriccontraction because all valves are closed • When pressure in ventricles exceeds that in aorta • semilunar valves open and ejection begins • As pressure in ventricle falls below that in aorta • back pressure closes semilunars • All valves are closed • ventricles undergo isovolumetricrelaxation • When pressure in ventricles falls below atria • AVs open and ventricles fill • Atrial systolesends its blood into ventricles

  8. Heart Sounds • Closing of AV and semilunar valves produces sounds that can be heard thru stethoscope • Lub (1st sound) produced by closing of AV valves • Dub (2nd sound) produced by closing of semilunars

  9. Vascular Resistance to Blood Flow • Determines how much blood flows through a tissue or organ – related to flow in small arteries and arterioles • Vasodilation decreases resistance, increases blood flow • Vasoconstriction does opposite

  10. Estimated Distribution of CO at Rest • Amount of blood heart pumps per min = rate of venous return • Thus = rate of blood flow thru entire circulation • CO of 5-6 L/min distributed unequally to different organs

  11. Physical Laws Describing Blood Flow • Blood flow thru vascular system depends in part on the pressure difference (P) at its two ends • Blood will flow from the region of higher to the region of lower pressure • Flow rate is directly proportional to difference (P = P1 - P2)

  12. Physical Laws Describing Blood Flow • Flow rate is inversely proportional to resistance • Flow = P/R • Resistance is directly proportional to length of vessel (L) and viscosity of blood () • Inversely proportional to 4th power of radius • Resistance § L / r4 • Diameter of vessel is very important for resistance • Poiseuille's Lawdescribes factors affecting blood flow • Blood flow = Pr4() L(8)

  13. The resistance and blood flow are equally divided between 2 branches of a vessel • Doubling of the radius of 1 branch and halving of the radius of the other produces a 16-fold increase in blood flow in the former and 16-flold decrease of blood flow in the latter

  14. Physical Laws Describing Blood Flow • Mean arterial pressure and vascular resistance – 2 major factors regulating blood flow • Blood is shunted from one organ to another by degree of constriction of their arterioles

  15. Total Peripheral Resistance • Total peripheral resistance– sum of all vascular resistances within systemic circulation • Arteries supply tissues and organs in parallel circuits • Changes in resistance in these circuits determines relative blood flow

  16. Extrinsic Regulation of Blood Flow • Sympathoadrenal activation – increased CO and resistance in periphery and viscera • Blood flow to skeletal muscles is increased • Their arterioles dilate in response to Epinephrine • Cholinergic sympathetic fibers release ACh which also dilates their arterioles • Thus blood is shunted away from visceral and skin to muscles

  17. Extrinsic Regulation of Blood Flow • Parasympathetic effects cause vasodilation • Parasympathetic innervatation of BVs limited to digestive tract, genitalia, and salivary glands • Thus parasympathetic division is not as important as sympathetic • Angiotensin II and ADH (at high levels) cause general vasoconstriction of vascular smooth muscle • increases resistance and BP

  18. Paracrine Regulation of Blood Flow • The endothelium produces several paracrine regulators that promote relaxation: • Nitric oxide(NO), bradykinin, prostacyclin • NO is involved in setting resting “tone” of vessels • Levels increased by parasympathetic activity • Vasodilator drugs such as nitroglycerin or Viagra act thru NO • Endothelin 1 – vasoconstrictor produced by endothelium

  19. Intrinsic Regulation of Blood Flow (Autoregulation) • Maintains fairly constant blood flow despite BP variation • Myogenic control mechanismsoccur in some tissues • because vascular smooth muscle contracts when stretched and relaxes when not stretched • e.g. decreased arterial pressure causes cerebral vessels to dilate and vice versa

  20. Autoregulation • Metabolic control mechanism–matches blood flow to local tissue needs • Low O2 or pH or high CO2, adenosine, or K+ from high metabolism cause vasodilation • increases blood flow causing the tissue to appear red = reactivehyperemia • Similar increase in blood flow occurs in skeletal muscle and other organs • as a result of increase metabolism = active hyperemia

  21. Aerobic Requirements of the Heart • Heart (and brain) must receive adequate blood supply at all times • Heart – most aerobic tissue • each myocardial cell is within 10 µm of capillary compared to average distance of 70 µm • Contains lots of mitochondria and aerobic enzymes • During systole, coronary vessels are occluded • Heart gets around this by having lots of myoglobin • Myoglobin – O2 storage molecule  releases O2 to heart during systole

  22. Regulation of Blood Flow • Blood flow to heart affected sympathetic activity • Norepinephrine (alpha receptors) causes vasoconstriction • Epinephrine (beta receptors) causes vasodilation • Dilation accompanying exercise is due mostly to intrinsic regulation • At rest, flow through skeletal muscles is low • because of tonic sympathetic activity • Flow through skeletal muscles decreased during contraction because vessels are constricted

  23. Circulatory Changes During Exercise • At beginning of exercise - sympathetic activity causes vasodilation via epinephrine and local ACh release • Blood flow is shunted from periphery and visceral to active skeletal muscles • Blood flow to brain stays same • As exercise continues, intrinsic regulation is the major vasodilator • Sympathetic effects cause SV and CO to increase • HR and ejection fraction increases vascular resistance

  24. Distribution of CO during rest and heavy exercise

  25. Cardiovascular Adaptations to Exercise 1) Increase the CO, and thus total blood flow; and 2) Cause vasodilation in the exercising muscles • Diverting a higher proportion of blood flow to those muscles

  26. Cerebral Circulation • Gets about 15% of total resting CO • Held constant (750ml/min) over varying conditions • Because loss of consciousness occurs after few seconds of interrupted flow • Is not normally influenced by sympathetic activity

  27. Cerebral Circulation • Regulated almost exclusively by intrinsic mechanisms • Myogenic regulation– when BP increases, cerebral arterioles constrict when BP decreases, arterioles dilate • Arterioles dilate and constrict in response to changes in CO2 levels • Metabolic regulation– arterioles are very sensitive to increases in local neural activity • Areas of brain with high metabolic activity receive most blood

  28. Changing patterns of blood flow in the brain Listening to spoken words Follow moving object with eyes Moved opposite fingers Counted to 20

  29. Cutaneous Blood Flow • Thermoregulation – skin serves as a heat exchanger • Skin blood flow is adjusted to keep deep-body at 37oC • By arterial dilation or constriction and activity of arteriovenous anastomoseswhich control blood flow through surface capillaries • Sympathetic activity closes surface beds during cold and fight-or-flight, and opens them in heat and exercise

  30. Blood Pressure (BP) • Arterioles play role in blood distribution and control of BP • Blood flow to capillaries and BP is controlled by diameter of arterioles • Capillary BP is decreased because they are downstream of high resistance arterioles

  31. Blood Pressure (BP) • Capillary BP is also low because of large total cross-sectional area

  32. Blood Pressure (BP) • Controlled mainly by HR, SV, and peripheral resistance • An increase in any of these can result in increased BP • Sympathoadrenal activity raises BP via arteriole vasoconstriction and by increased CO • Kidney plays role in BP by regulating blood volume and thus stroke volume

  33. Baroreceptor Reflex • Activated by changes in BP • Detected by baroreceptors – located in aortic arch and carotid sinuses • Increase in BP causes walls of these regions to stretch, increasing frequency of action potentials • Baroreceptors send action potentials to vasomotor and cardiac control centers in medulla • Most sensitive to decrease and sudden changes in BP

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  35. Atrial Stretch Receptors • Activated by increased venous return and act to reduce BP and in response: 1) Stimulate reflex tachycardia, as a result of increased sympathetic activity 2) Inhibit ADH release – resulting in excretion of larger volumes of urine and lowering of blood volume and 3) Promote secretion of ANP – lowers blood volume by increasing urinary salt and water excretion

  36. Measurement of Blood Pressure • Via auscultation – to examine by listening • No sound is heard during laminar flow(normal, quiet, smooth blood flow) • Korotkoff soundscan be heard when sphygmomanometer cuff pressure is greater than diastolic but lower than systolic pressure • Cuff constricts artery creating turbulent flowand noise as blood passes constriction during systole and is blocked during diastole • 1st Korotkoff sound heard at pressure when blood is 1st able to pass thru cuff • Last sound occurs when one can no longer hear systole because cuff pressure = diastolic pressure

  37. Measurement of BP • BP cuff is inflated above systolic pressure, occluding artery • As cuff pressure is lowered, blood flows only when systolic pressure is above cuff pressure, producing Korotkoff sounds • Sounds are heard until cuff pressure equals diastolic pressure

  38. The indirect, or auscultatory, method of blood pressure measurement:

  39. Pulse Pressure • Pulse pressure= (systolic pressure) – (diastolic pressure) • Mean arterial pressure (MAP) represents average arterial pressure during cardiac cycle • Has to be approximated because period of diastole is longer than period of systole • MAP = diastolic pressure + 1/3 pulse pressure

  40. Hypertension • Blood pressure in excess of normal range for age and gender (> 140/90 mmHg) • Afflicts about 20% of adults • Primary or essentialhypertension– most common type • Caused by complex and poorly understood processes • Secondary hypertension – caused by known disease processes

  41. Essential Hypertension • Increase in peripheral resistance is universal • CO and HR are elevated in many • Secretion of renin, Angio II, and aldosterone is variable • Sustained high stress (increases sympathetic activity) and high salt intake act synergistically in development of hypertension • Prolonged high BP causes thickening of arterial walls, resulting in atherosclerosis • Kidneys appear to be unable to properly excrete Na+ and H2O

  42. Dangers of Hypertension • Patients are often asymptomatic until substantial vascular damage occurs • Contributes to atherosclerosis • Increases workload of the heart leading to ventricular hypertrophy and congestive heart failure • Often damages cerebral blood vessels leading to stroke • These are why it is called the "silent killer"

  43. Treatment of Hypertension • Includes lifestyle changes such as cessation of smoking, moderation in alcohol intake, weight reduction, exercise, reduced Na+ intake, increased K+ intake • Drug treatments include diuretics to reduce fluid volume, beta-blockers to decrease HR, calcium blockers, ACE inhibitors to inhibit formation of Angio II, and Angio II-receptor blockers

  44. Possible Causes of Secondary Hypertension • Kidney disease—kidney disease; renal artery disease • Endocrine disorder—excess catecholamines; excess aldosterone • Nervous system disorder—increase intracranial pressure; damage to vasomotor center • Cardiovascular disorder—complete heart block; arteriosclerosis of aorta

  45. Circulatory Shock • Occurs when there is inadequate blood flow to, and/or O2 usage by, tissues • Cardiovascular system undergoes compensatory changes • Sometimes shock becomes irreversible and death ensues

  46. Hypovolemic Shock • Circulatory shock caused by low blood volume • e.g. from hemorrhage, dehydration, or burns • Characterized by decreased CO and BP • Compensatory responses include sympathoadrenal activation via baroreceptor reflex • Results in low BP, rapid pulse, cold clammy skin, low urine output