Regulation of Cadiac Activity
Transcript of Regulation of Cadiac Activity
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Regulation of cardiac activity
Cardiac output
Blood flow
Blood pressure
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Cardiac output= stroke volume X cardiac rate
(ml/min) (ml/beat) (beats/min)
At 70 beats/min and 80 ml/beat, this comes toabout 5.5 liters per minute
Equivalent to the total blood volume
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Regulation of cardiac rate
Rhythm is set by the SA node
Sympathetic nervesepinephrine and norepinephrinestimulate opening of calcium and sodiumchannels; increase cardiac rate
Parasympathetic (vagus) nervesacetylcholine promotes opening of potassiumchannels; reduces cardiac rate
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Exercise reduces vagus inhibition and increases
sympathetic nerve activity
Cardiac control center in medulla oblongatacoordinates this activity
This in turn is regulated by higher brain activityand pressure (baroreceptors) in aorta and
carotid arteries
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Regulation of stroke volume
EDV: end-diastolic volume (blood left in ventriclesafter diastole)increase in EDV increase in stroke volume
Total peripheral resistance to arterial blood flowstroke volume is inversely proportionalto this (temporarily)
Strength of ventricular contractionincreased as EDV increases)
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Frank-Starling law of the heart
Intrinsic variation as EDV increases, so doesforce of contraction (increased stretch)
Increased peripheral resistance
Increased EDV
Increased stretchNext contraction is stronger
Process is highly adjustable
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Contractility
Innervation from sympathetic nervesRaises calcium levels (positive inotropic effect)
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Venous return
At rest, most of the blood is in the veinsveins can give more and hold moreblood than arteries; venous pressureis much lower (2 mm Hg vs. 90-100 mm Hg
mean arterial pressure)
Venous pressure determines rate of blood returnto the heart
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Blood volume
Extracellular fluid distributed betweenblood plasma and interstitial fluid
Affected by:forces acting at capillaries (to drawfluid out of or into them)
overall balance of water loss and gain
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Exchange of fluid between tissues and capillaries
lucose and various other solutes are passedto tissues as well; balance is achieved
Movement of plasma proteins is restricted (oncotic
pressure)osmotic pressure is higher in capillaries
tarling forces favor movement of water out of
capillaries and back into venulesexchange is continuoussome of the fluid is returned to lymph
(about 15%) and eventually to circulatio
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Edema- excessive fluid in tissues
Causes:high blood pressurevenous obstructionleakage of plasma proteins into tissue
fluid (as in inflammation)kidney or liver disease leading to protein
loss or lack of formationobstruction of lymphatic vessels
(filiarisis)myxedema- increased secretion of proteins
into extracellular matrix
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Regulation of blood volume by kidneys
Filtration of blood- almost all of filtrate isreabsorbed by the kidneys
(out of daily production of ca. 180L of filtrate,only about 1.5 L actually excreted)
Various hormones acting on, or produced by,
the kidneys
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ADH (antidiuretic hormone; vasopressin)
Increase in plasma osmolality- osmoreceptorsin hypothalamus
posterior pituitary releases ADH
Also triggers sensation of thirst (osmoreceptor)
Why does this happen?
dehydrationexcessive salt intake
More water is reabsorbed, less is excreted
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Aldosterone
reabsorption of salt (Na) by kidneywater is reabsorbed toono dilution effect as with ADH because
both water and salt are retained
secreted by adrenal cortexactivated through renin-angiotensin-aldosterone system
salt intake tends to inhibit renin secretion
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ANF- atrial natriuretic factor
Produced by atria in response to highblood pressure
Promotes water and salt excretion
Also antagonizes effects of angiotensin II(therefore reduces aldosterone productionand promotes vasodilation)
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Resistance to blood flow
Related to pressure difference between the endsof the vessel
Inversely related to resistance of blood flowthrough vessel
In body, vasodilation in one organ system might
be offset by vasoconstriction in another
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Regulation of blood flow
Sympathetic nervous systemoverall, increase in cardiac output and inperipheral resistance
vasoconstriction in arterioles of visceraand skin
vasodilation in skeletal muscles
(depends on receptors)
Parasympathetic- vasodilation effect confinedto GI, genitalia, salivary glands
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Paracrine regulation, e.g., inflammation
Intrinsic (autoregulation)myogenic- response to changes in bloodpressure
metabolic-oxygen, carbon dioxide levelslocal vasodilation
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Regulation of blood flow to the heart
Alpha and beta adrenergic receptors(constriction and dilation; norepinephrineand epinephrine)
Also intrinsic regulationincreased metabolic rate- oxygen need,accumulation of carbon dioxide, etc.
smooth muscle stimulated to causerelaxation and dilation
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How are aerobic requirements of heart met?
Lots of capillariesMyoglobin releases oxygen during systole
(blood flow is reduced at that time)capacity for aerobic respiration:
extra mitochondria, enzymes
Blockages in blood supply are corrected byangioplasty, bypass, etc.
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Blood flow through skeletal muscles
High vascular resistance at rest
Blood flow decreases when muscle contracts
Intrinsic metabolic control promotes bloodflow through muscles during exercise
20-25% of total blood flow through musclesat rest
Up to 85% during exercise
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Blood flow to brain
Intrinsic mechanisms maintain constant flowmyogenic responses to changes in bloodpressuresensitive to CO2 levels in arterial blood
metabolic responses- local vasodilation
Blood flow to skin is highly sensitive to actionof sympathetic nervous system
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Blood pressure
Blood flow resistance highest in arteriolesFlow rate lowest in capillaries
Blood pressure can be raised by:
vasoconstriction of arteriolesincrease in cardiac output
(higher cardiac rate or stroke volume)
Various factors can affect these: kidneys,sympathetic nervous system, etc.
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Pressure receptors (baroreceptors)
Action potentials will increase or decreaseas pressure rises or falls
Baroreceptor reflex activated when bloodpressure rises or falls. Activated when aperson changes position
Vasomotor control centers- constriction/dilation
Cardiac control centers- cardiac rate
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Blood pressure also regulated by:
Atrial stretch receptorsADH releaseRenin-angiotensin-aldosteroneANF
M f bl d
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Measurement of blood pressuresphygmomanometer
Systolic/diastolic pressure, e.g., 120/80exercise tends to raise systolic morechanging position tends to affect diastolic
Pulse pressure: systolic- diastolicreflects stroke volumedrops in dehydration or blood loss
Pulse rate reflects cardiac rate
Mean arterial pressure= diastolic + 1/3 pulse
pressure
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Pathophysiology of cardiovascular system
HypertensionSecondary- results from known diseases
(table 14.10)processes that affect blood flow; damage
to tissue that results in release ofvasoactive chemicals; damage to sympa-thetic nervous system, etc.
Essential- accounts for most cases of hypertension
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Increased total peripheral resistance
Low renin secretion?High salt intake?Stress?
Inability of kidneys to regulation salt and waterexcretion?
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Consequences of high blood pressure
Can damage cerebral blood vessels and leadto stroke
Causes heart to work harder (harder to eject
blood if peripheral resistance is high)
Contributes to atherosclerosis
Treatments are many and varieddiet, diuretics, various receptor blockers
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Shock due to loss of blood flowhypovolemic- blood LOSS
septic- blood-borne infection; nitric oxideformation might be the culprit
anaphylactic- severe allergic reaction(histamine)
cardiogenic- infarction causes extensivedamage to heart muscle
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Congestive heart failure- cardiac output isinadequate
causes: heart disease, hypertension,electrolyte imbalance
Digitalis increases contractility of heart muscle
Diuretics lower blood volume
Nitroglycerin is a vasodilator
Make heart work more efficiently; reduce stresson heart