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Chapter 14

Chapter 14. Cardiac Output, Blood Flow, and Blood Pressure. Cardiac Output. Pumping ability of the heart is a function of the beats per minute and the volume of blood ejected per beat. CO = SV x HR Total blood volume averages about 5.5 liters.

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Chapter 14

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

  2. Cardiac Output • Pumping ability of the heart is a function of the beats per minute and the volume of blood ejected per beat. • CO = SV x HR • Total blood volume averages about 5.5 liters. • Each ventricle pumps the equivalent amount of blood.

  3. Regulation of Cardiac Rate • In absence of neuronal influences, the heart beats according to the rhythm of SA node. • Regulation of HR (chronotropic effect): • Positive or negative chronotropic effect. • Autonomic control: • Major means by which cardiac rate is regulated. • Cardiac Control Center (medulla): • Coordinates activity of autonomic innervation.

  4. Regulation of SV • Stroke volume is regulated by 3 variables: • EDV: • Volume of blood in the ventricles at the end of diastole. • TPR: • Frictional resistance to blood flow in the arteries. • Contractility: • Strength of ventricular contraction.

  5. EDV • Workload on the heart prior to contraction (preload). • SV directly proportional to preload. • Increase in EDV results in increase in SV. • Strength of contraction varies directly with EDV.

  6. TPR • Total Peripheral Resistance: • Impedance to the ejection of blood from ventricle. • Afterload. • In order to eject blood, pressure generated in the ventricle must be greater than pressure in the arteries. • SV inversely proportional to TPR. • Greater the TPR, the lower the SV.

  7. Frank-Starling Law of the Heart • Relationship between EDV, contraction strength and SV. • Intrinsic mechanism: • Varying degree of stretching of myocardium by EDV. • As EDV increases, myocardium is increasingly stretched, and contracts more forcefully.

  8. Frank-Starling Law of the Heart • As the ventricles fill, the myocardium stretches so that the actin filaments overlap with the myosin at the edges of the A band. • Allows more force to develop. • Explains how the heart can adjust to rise in TPR.

  9. Extrinsic Contractility • Strength of contraction at any given fiber length. • Depends upon sympathoadrenal system: • Norepinephrine and epinephrine produce and increase in contractile strength. • + inotropic effect: • More Ca++ available to sarcomeres.

  10. Venous Return • Return of blood to the heart via veins. • Venous pressure is driving force for return of blood to the heart. • Veins have thinner walls, thus higher compliance. • Capacitance vessels. • 2/3 blood volume in veins.

  11. BP TPR CO ANS Hormones Chemicals Viscosity Blood vessel length Blood vessel diameter Local factors HR SV EDV ANS Hormones Lytes Body temp Brain Venous Return Kidney Respiratory pump Skeletal muscle pump Angiotensin Aldosterone ADH

  12. Total Body Water • Distribution of H20 within the body: • Intracellular compartment: • 2/3 of total body H20 within the cells. • Extracellular compartment: • 80% interstitial fluid. • 20% blood plasma. • Maintained by constant balance between H20 loss and gain.

  13. Fluid Equilibrium • Distribution of ECF between plasma and interstitial compartments is in state of dynamic equilibrium. • Balance between tissue fluid and blood plasma. • Hydrostatic pressure: • Exerted against the inner capillary wall. • Promotes formation of tissue fluid. • Net filtration pressure • Colloid osmotic pressure: • Exerted by plasma proteins. • Promotes fluid reabsorption into circulatory system.

  14. Filtration Pressure • Net filtration pressure: • Hydrostatic pressure of blood minus the hydrostatic pressure in the interstitial fluid. • Blood hydrostatic pressure (arteriolar pressure) = 37 mm Hg. • Blood hydrostatic pressure (venular end) = 17 mm Hg. • Interstitial hydrostatic pressure = 1 mm Hg.

  15. Colliod Osmotic Pressure • Pressure exerted by plasma proteins or interstitial proteins. • Difference between plasma osmotic pressure and interstitial osmotic pressure is called oncotic pressure. • Plasma osmotic pressure = 25 mm Hg. • Interstitial osmotic pressure = 0 mm Hg.

  16. Fluid Movement • Pc + Pi = Pi + Pp • Pc = • Hydrostatic pressure of capillary. • Pi = • Colloid osmotic pressure of the interstitial fluid. • Pi = • Hydrostatic pressure of the interstitial fluid. • Pp = • Colloid osmotic pressure of the blood plasma.

  17. Edema • Excessive accumulation of tissue fluid. • Edema may result from: • High arterial blood pressure. • Venous obstruction. • Leakage of plasma proteins into interstitial fluid. • Myexedema. • Decreased plasma protein. • Obstruction of lymphatic drainage.

  18. Kidney Regulation of Blood Volume • Formation of urine begins by filtration of plasma through glomerular capillary pores. • Volume of urine excreted can be varied by changes in reabsorption of filtrate. • Adjusted according to needs of body by action of hormones.

  19. Kidney Regulation of Blood Volume • ADH: • Antidiuretic hormone. • Produced by neurons of hypothalamus. • Released by posterior pituitary when osmoreceptors detect an increase in plasma osmolality.

  20. Aldosterone • Mechanism to maintain blood volume and pressure through absorption and retention of Na+ and Cl-. • Stimulates reabsorption of NaCl. • Increases H20 reabsorption. • Does not dilute osmolality.

  21. Angiotension II • When blood pressure and flow are reduced in renal artery, juxtaglomerular apparatus secretes renin. • Renin converts angiotensinogen to angiotensin I. • Angiotensin I is converted to angiotensin II by ACE. • Powerful vasoconstrictor. • Stimulates production of aldosterone. • Stimulates thirst.

  22. ANF • Atrial natriuretic factor. • Stretch of atria stimulates production of ANF. • Antagonistic to aldosterone and angiotensin II. • Promotes Na+ and H20 excretion by the kidney.

  23. Resistance to Blood Flow • Amount of blood pumped is equal to venous return. • The flow is = to the difference in pressure at the two ends (DP). • Flow = DP/R • R = TPR (all vascular resistance within the systemic circulation).

  24. Resistance • Opposition to blood flow. • Resistance directly proportional to length of vessel and to the viscosity of the blood. • Inversely proportional to 4th power of the radius of the vessel. • R = _Ln_ r4 • L = length of the vessel • n = viscosity of blood • r = radius of the vessel

  25. Poiseulle’s Law • Blood flow = DPr4(p) Ln(8) • Vessel length and blood viscosity do not vary significantly. • Major regulators of blood flow through an organ are: • Mean arterial pressure. • Vascular resistance to flow.

  26. Extrinsic Regulation of Blood Flow • Controlled by ANS and endocrine system. • Sympathoadrenal: • Increase CO. • Increase TPR: • Alpha-adrenergic stimulation: • Vasoconstriction. • Cholinergic sympathetic fibers: • Vasodilate to skeletal muscles.

  27. Extrinsic Regulation of Blood Flow • Parasympathetic nervous system: • Parasympathetic promotes vasodilation to the digestive tract, external genitalia, and salivary glands.

  28. Extrinsic Regulation of Blood Flow • Paracrine: • NO: • Vasodilate: • ACh stimulates opening of Ca++ channels. • Ca++ binds to calmodulin. • Calmodulin activates an enzyme to produce NO. • Bradykinin, prostacyclin, and endothelin-1: • Vasodilate.

  29. Intrinsic Regulation of Blood Flow • Myogenic: • Occurs because of the stretch of the vascular smooth muscle. • A decrease in systemic arterial pressure causes cerebral vessels to dilate. • Maintains adequate flow.

  30. Intrinsic Regulation of Blood Flow • Metabolic: • Intrinsic receptors sense chemical changes in environment • Vasodilation: • Decreased 02: • Increased metabolic rate. • Increased C02: • Decreased ventilation. • Decreased pH: • Lactic acid. • Increased adenosine or increased K+: • From tissue cells.

  31. Blood Flow to Heart • Survival requires that the heart and brain receive adequate blood supply. • Systole contracts the coronary blood vessels. • Increase blood flow during diastole. • Myoglobin stores 02 during diastole to release during systole. • Heart muscle contains increased number of mitochondria and enzymes.

  32. Coronary Blood Flow • Sympathetic nervous system: • a receptors: • Vasoconstriction at rest. • b receptors: • Vasodilation. • Intrinsic: • Accumulations of C02, K+, adenosine, decreased 02. • Act directly on vascular smooth muscle to cause vasodilation.

  33. Skeletal Blood Flow • Decreased blood flow in contraction • Sympathetic: • a receptors: • Vasoconstriction at rest. • Cholinergic and b receptors: • Vasodilation . • During exercise have increased accumulations of C02, K+, adenosine, decreased 02. • Capillary recruitment increases.

  34. Exercise • CO increases. • Blood flow to brain stays same. • HR increases to maximum of 190 beats/min. • SV increases. • Ejection fraction increases due to increased contractility. • Vascular resistance: • Decreases to skeletal muscle. • Increases to GI tract and skin.

  35. Cerebral Circulation • Cerebral blood flow is not normally influenced by sympathetic nerve activity. • Normal range of arterial pressures: • Cerebral blood flow regulated by intrinsic mechanisms. • Myogenic: • Maintain constant flow rate. • Metabolic: • Exquisitely sensitive to changes in metabolic activity.

  36. Cutaneous Blood Flow • Blood flow through the skin is adjusted to maintain deep-body temperatures about 37o C. • Occur due to: • Vasoconstriction/vasodilation arteries. • Arteriovenous anastomoses • Divert blood to deep venules. • Bradykinin: • Sweat glands secrete bradykinin which increases blood flow to skin and sweat glands.

  37. Blood Pressure • Pressure of arterial blood is regulated by blood volume, TPR, and cardiac rate. • Arteriole resistance is greatest because they have the smallest diameter. • Capillaries BP is reduced because of the total cross-sectional area. • 3 most important variables are HR, SV, and TPR.

  38. Baroreceptor Reflex • Stretch receptors located in the aortic arch and carotid sinuses. • An increase in pressure causes the walls of these regions to stretch, increasing frequency of APs. • Baroreceptors send APs to vasomotor control and cardiac control centers in the medulla.

  39. Atrial Stretch Reflexes • Located in the atria of the heart. • Receptors activated by increased venous return. • Stimulate reflex tachycardia. • Inhibit ADH release. • Promote secretion of ANF.

  40. Measurement of Blood Pressure • Auscultation: • Art of listening. • Laminar flow: • Normal blood flow. Blood in the central axial stream moves faster than blood flowing closer to the artery wall. • Turbulent flow and vibrations produced in the artery when cuff pressure is greater than diastolic pressure and lower than systolic pressure.

  41. Measurement of Blood Pressure • Blood pressure cuff is inflated above systolic pressure so the artery is silent. • As cuff pressure is lowered, the blood will flow only when systolic pressure is above cuff pressure, producing the sounds of Korotkoff. • Korotkoff sounds will be heard until cuff pressure equals diastolic pressure, causing the sounds to disappear.

  42. Measurement of Blood Pressure • Different phases in measurement of blood pressure are identified on the basis of the quality of the Korotkoff sounds. • Average arterial BP is 120/80. • Average pulmonary BP is 22/8.

  43. Pulse Pressure • The expansion of the artery in response to the volume of blood ejected. • Pulse pressure = systolic pressure – diastolic pressure • Mean arterial pressure: • Average arterial pressure during the cardiac cycle. • Is closer to diastolic pressure, as the period of diastole is longer than the period of systole. • Mean arterial pressure = diastolic pressure + 1/3 pulse pressure

  44. Hypertension • Blood pressure in excess of normal range for age and gender. • > 140/90 mm Hg. • Primary or essential hypertension: • Result of a complex or poorly understood process. • Secondary hypertension: • As a result of a know disease.

  45. Essential Hypertension • 95% of population with hypertension. • Increase in TPR is a universal characteristic. • Secretion of renin, angiotensin II, and aldosterone is variable. • Sustained high stress (via SNS) and high Na+ intake act synergistically in development of hypertension. • Adaptive response is thickening of arterial wall, resulting in atherosclerosis. • Kidneys may not be able to properly excrete Na+ and H20.

  46. Dangers of Hypertension • Silent killer: • Patients are asymptomatic until substantial vascular damage occurs. • Atherosclerosis. • Increases afterload. • Increases workload of the heart. • Congestive heart failure. • Damage cerebral blood vessels. • Stroke.

  47. Treatment of Hypertension • Modification of lifestyle: • Cessation of smoking. • Moderation in alcohol intake. • Weight reduction. • Reduction in Na+ intake. • Medications: • Diuretics. • Beta-blockers. • Calcium antagonists. • ACE inhibitors.

  48. Circulatory Shock • Hypovolemic shock: • Circulatory shock that is due to low blood volume. • Decreased CO and blood pressure. • Compensations: • Baroreceptor reflex: • Tachycardia. • Vasoconstriction to GI, skin, kidneys and muscles. • Kidneys stimulate production of renin-angiotensin-aldosterone system. • Increase in ADH.

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