Regulation of cardiac 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 to about
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 nerves epinephrine and
norepinephrine stimulate opening of calcium and
sodium channels increase cardiac
rate Parasympathetic (vagus) nerves acetylcholin
e promotes opening of potassium channels
reduces cardiac rate
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Exercise reduces vagus inhibition and
increases sympathetic nerve activity Cardiac
control center in medulla oblongata coordinates
this activity This in turn is regulated by
higher brain activity and pressure
(baroreceptors) in aorta and carotid arteries
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Regulation of stroke volume EDV end-diastolic
volume (blood left in ventricles after
diastole) increase in EDV ?increase in stroke
volume Total peripheral resistance to arterial
blood flow stroke volume is inversely
proportional to this (temporarily) Strength of
ventricular contraction increased as EDV
increases)
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Frank-Starling law of the heart Intrinsic
variation as EDV increases, so does force of
contraction (increased stretch) Increased
peripheral resistance ? Increased
EDV ? Increased stretch ? Next contraction is
stronger Process is highly adjustable
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Contractility Innervation from sympathetic
nerves Raises calcium levels (positive inotropic
effect)
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Venous return At rest, most of the blood is in
the veins veins can give more and hold
more blood than arteries venous pressure is
much lower (2 mm Hg vs. 90-100 mm Hg mean
arterial pressure) Venous pressure determines
rate of blood return to the heart
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Blood volume Extracellular fluid distributed
between blood plasma and interstitial
fluid Affected by forces acting at capillaries
(to draw fluid out of or into them) overall
balance of water loss and gain
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Exchange of fluid between tissues and
capillaries Glucose and various other solutes
are passed to tissues as well balance is
achieved Movement of plasma proteins is
restricted (oncotic pressure) osmotic pressure
is higher in capillaries Starling forces favor
movement of water out of capillaries and back
into venules exchange is continuous some of the
fluid is returned to lymph (about 15) and
eventually to circulation
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Edema- excessive fluid in tissues Causes high
blood pressure venous obstruction leakage of
plasma proteins into tissue fluid (as in
inflammation) kidney or liver disease leading to
protein loss or lack of formation obstruction
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 is reabsorbed
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- osmoreceptors in
hypothalamus ? posterior pituitary releases
ADH Also triggers sensation of thirst
(osmoreceptor) Why does this happen? dehydration
excessive salt intake More water is
reabsorbed, less is excreted
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Aldosterone reabsorption of salt (Na) by
kidney water is reabsorbed too no dilution
effect as with ADH because both water and salt
are retained secreted by adrenal
cortex activated through renin-angiotensin- aldo
sterone system salt intake tends to inhibit
renin secretion
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ANF- atrial natriuretic factor Produced by atria
in response to high blood pressure Promotes
water and salt excretion Also antagonizes
effects of angiotensin II (therefore reduces
aldosterone production and promotes vasodilation)
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Resistance to blood flow Related to pressure
difference between the ends of the
vessel Inversely related to resistance of blood
flow through 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
system overall, increase in cardiac output and
in peripheral resistance vasoconstriction in
arterioles of viscera and skin vasodilation
in skeletal muscles (depends on
receptors) Parasympathetic- vasodilation effect
confined to GI, genitalia, salivary glands
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Paracrine regulation, e.g., inflammation Intrinsi
c (autoregulation) myogenic- response to changes
in blood pressure metabolic-oxygen, carbon
dioxide levels local vasodilation
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Regulation of blood flow to the heart Alpha and
beta adrenergic receptors (constriction and
dilation norepinephrine and epinephrine) Also
intrinsic regulation increased metabolic rate-
oxygen need, accumulation of carbon dioxide,
etc. smooth muscle stimulated to
cause relaxation and dilation
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How are aerobic requirements of heart met? Lots
of capillaries Myoglobin releases oxygen during
systole (blood flow is reduced at that
time) capacity for aerobic respiration extra
mitochondria, enzymes Blockages in blood supply
are corrected by angioplasty, 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 blood flow through muscles
during exercise 20-25 of total blood flow
through muscles at rest Up to 85 during
exercise
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Blood flow to brain Intrinsic mechanisms
maintain constant flow myogenic responses to
changes in blood pressure sensitive to CO2
levels in arterial blood metabolic responses-
local vasodilation Blood flow to skin is highly
sensitive to action of sympathetic nervous system
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Blood pressure Blood flow resistance highest in
arterioles Flow rate lowest in capillaries Blood
pressure can be raised by vasoconstriction of
arterioles increase 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 decrease as pressure
rises or falls Baroreceptor reflex activated
when blood pressure rises or falls. Activated
when a person changes position Vasomotor
control centers- constriction/dilation Cardiac
control centers- cardiac rate
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Blood pressure also regulated by Atrial stretch
receptors ADH release Renin-angiotensin-aldosteron
e ANF
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Measurement of blood pressure sphygmomanometer S
ystolic/diastolic pressure, e.g.,
120/80 exercise tends to raise systolic
more changing position tends to affect
diastolic Pulse pressure systolic-
diastolic reflects stroke volume drops 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 Hypertension Secondary- results from
known diseases (table 14.10) processes that
affect blood flow damage to tissue that results
in release of vasoactive 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
water excretion?
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Consequences of high blood pressure Can damage
cerebral blood vessels and lead to
stroke Causes heart to work harder (harder to
eject blood if peripheral resistance is
high) Contributes to atherosclerosis Treatments
are many and varied diet, diuretics, various
receptor blockers
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Shock due to loss of blood flow hypovolemic-
blood LOSS septic- blood-borne infection
nitric oxide formation might be the
culprit anaphylactic- severe allergic
reaction (histamine) cardiogenic- infarction
causes extensive damage to heart muscle
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Congestive heart failure- cardiac output
is inadequate causes heart disease,
hypertension, electrolyte imbalance Digitalis
increases contractility of heart
muscle Diuretics lower blood volume Nitroglyceri
n is a vasodilator Make heart work more
efficiently reduce stress on heart