Regulation of Cadiac Activity

8/14/2019 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

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Regulation of Cadiac Activity