The heart and circulation

Transportation- oxygen and carbon dioxidered blood cells

Regulation-hormones, temperature

Protection- against blood loss (clotting);infection (immune system)

Components of the circulatory system

Cardiovascular system- heart and blood vessels

Lymphatic system- lymph nodes, lymphaticvessels

Electrical activity of the heart

Myocardial cells beat automatically

Action potential is usually originated insinoatrial node

Spontaneous depolarization (pacemaker potential)diffusion of calcium through slow channels

threshold- fast calcium channels open, voltageregulated sodium channels open

repolarization produced through diffusion of potassium

Electrocardiogram

Conduction of electrical potentials through heart

P wave- atrial depolarization

QRS- ventricular depolarizationbeginning of systole

T wave- repolarization of the ventriclesbeginning of diastole

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

Heart muscle cannot sustain contraction

Long refractory periods- heart cannot be stimulated until it has relaxed fromprevious contraction

Arrhythmias- something affects the cardiaccycle; treatment depends on what it is

Fast Na channel blockersSlow Ca channel blockers-adrenergic receptor blockers

Arrhythmias

Bradycardia- slow rate (less than 60 bpm)Tachycardia- fast rate (more than 100 bpm)

Can occur normally; is abnormal if rateincreases during rest (ectopic pacemakers)

Flutters- extremely rapid contractions

Fibrillation- different groups of fibers are activatedso coordinated pumping of chambers is notpossible

Blood vessels- arteries and veins

Arteries, arterioles, capillaries

Veins and venules

Arteries are more muscular

Veins have valves

Veins

Veins can expand to accommodate increasingamounts of blood; arteries can’t

Venous pressure is low compared to arterial pressure

Blood flow through veins is facilitated by:contraction of skeletal musclesvalves that prevent backflow

Atherosclerosis

Damage to endothelium

“Fatty streaks” (macrophages and lymphocytes)

Fibrous plaques

High blood cholesterol, LDL contribute toatherosclerosis

HDL also help transport cholesterol, but donot contribute to atherosclerosis

Lymphatic system

Fluid transportfrom tissues

Fat transport from intestines

Immune response

Regulation of cardiac activity

Cardiac output

Blood flow

Blood pressure

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

Stroke volume regulated by

End-diastolic volumeamount of blood in ventricles before theybegin to contractincreases stroke volume

Total peripheral resistance to blood flow inthe arteriesthe higher the resistance, higher pressureheart compensates by beating morestrongly

Contraction strength of ventricle; proportionalto stroke volume

Exercise reduces vagus inhibition and increasessympathetic nerve activity

Cardiac control center in medulla oblongatacoordinates this activity

This in turn is regulated by higher brain activityand pressure (baroreceptors) in aorta andcarotid arteries

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 Hgmean arterial pressure)

Venous pressure determines rate of blood returnto the heart

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

(lymphatic system)

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

(to be discussed later, but think about: whatcauses fluid to be retained or lost?)

Resistance to blood flow

Related to pressure difference between the endsof the vessel

Inversely related to resistance of blood flow through vessel

In body, vasodilation in one organ system mightbe offset by vasoconstriction in another

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

Paracrine regulation, e.g., inflammationlocalized vasoconstriction, dilation

Intrinsic (autoregulation)myogenic- response to changes in bloodpressure (constrict to protect bloodvessels, dilate to improve blood flow)

metabolic-oxygen, carbon dioxide levelslocal vasodilation

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 cause

relaxation and dilation

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.

Total increased blood flowmore to muscles, less to skin and viscera

Overall flow to brain is about the same

Mainly due to increased cardiac rate

With conditioning, stroke volume alsoincreases

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 systemtemperature sensitive

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.

Pressure receptors (baroreceptors)

Action potentials will increase or decreaseas pressure rises or falls

Baroreceptor reflex activated when bloodpressure rises or falls. Activated when a person changes position

Vasomotor control centers- constriction/dilation

Cardiac control centers- cardiac rate

Blood pressure also regulated by:

Atrial stretch receptorsADH releaseRenin-angiotensin-aldosteroneANF

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 rateMean arterial pressure= diastolic + 1/3 pulse

pressure (indicator of peripheral resistance)

Pathophysiology of cardiovascular system

HypertensionSecondary- results from known diseases

(table 14.10)processes that affect blood flow; damageto tissue that results in release ofvasoactive chemicals; damage to sympa-thetic nervous system, etc.

Essential- accounts for most cases of hypertension

Increased total peripheral resistance

Low renin secretion?High salt intake?Stress?

Inability of kidneys to regulation salt and waterexcretion?

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

Shock due to loss of blood flowhypovolemic- blood LOSS

septic- blood-borne infection; nitric oxideformation might be the culprit(vasodilator)

anaphylactic- severe allergic reaction(histamine)

cardiogenic- infarction causes extensivedamage to heart muscle

Congestive heart failure- cardiac output isinadequatecauses: 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

Summary

Cardiac output can be regulated byautonomic nervous systemstroke volumevenous return (blood volume andefficiency of veins)

Blood flow to different parts of body isadjustable depending on need

Blood pressure is regulated by cardiacrate, blood volume and total peripheralresistance. High or low pressure isassociated with disease.