circ sys physio notes
TRANSCRIPT
-
8/6/2019 Circ Sys Physio Notes
1/23
1
Circulatory System
BME 5010 - EngineeringPhysiology
Objectives
1. Describe components of circulatory system
2. Explain mechanical physiology of heart
3. Detail mechanisms of cardiac output control
4. Describe vascular system and the role of eachcomponent in blood and nutrient distribution
5. Describe mechanisms of vascular control
6. Define lymphatic system
7. Discuss major forms of cardiovascular disease
Circulatory System
Designed to move substances from one
region of the body to another
Oxygen - Carbon dioxide
Metabolites - Waste products
Hormones - Metabolic end products
Required in large, multicellular organismswhere diffusion is not sufficiently rapid to
meet the metabolic needs of cells
-
8/6/2019 Circ Sys Physio Notes
2/23
2
Blood
Composed of cells and plasma
Erythrocytes - red blood cellsn Transport oxygen
Leukocytes - white blood cellsn Immune system
Platelets - cell fragmentsn Blood clotting
Plasman Fluid containing proteins and dissolved molecules
ErythrocytesContain a large amount of hemoglobinn Iron in hemoglobin binds to oxygen to transport a greater
amount than could dissolve in plasma
High surface-volume ratio to allow oxygen to diffuserapidly to the center of the cell
Do not have nuclei or organellesn Cannot reproduce
Produced in the red bone marrown All bones in children
n Chest, base of skull, ends of long bones in adults
Destruction of old cells occurs in liver and spleen
Typical life span of 120 days
Cardiovascular System
Consists of heart, arteries, capillaries, andveins
Almost all cells are within a few cell diametersof a capillaryn Allows for movement of materials between the cell
and the capillary by diffusion and mediatedtransport
Two vascular systemsn Systemic circulation
n Pulmonary circulation
-
8/6/2019 Circ Sys Physio Notes
3/23
3
Pulmonary Circulation
Right atrium
Right ventricle
Pulmonary trunk
Pulmonary arteries
Capillaries within lungs
Pulmonary veins
Left Atrium
Blood picks up oxygen and deposits carbondioxide in the lungs
Systemic Circulation
Left atrium
Left ventricle
Aorta
Arteries
Arterioles
Capillaries
Venules
Veins
Superior/inferior venacava
Right atrium
Oxygenated blood carriedfrom heart to tissues
Blood returning from
heart contains low levelsof oxygen and high levels
of carbon dioxide
Blood also transportsmetabolites, waste
products, and hormones
to and from organs andtissues
Blood Pressure and Flow
Blood flows from regions of high pressure to regions oflow pressure
Blood pressure is originally created by the contractionforce of the heart
Blood flow through a region is related to:
n the pressure difference between the inlet and outlet
n the resistance of the blood vessels to fluid flow
F = P/R
F = P r4/8l
n r = radius
n = viscosity
-
8/6/2019 Circ Sys Physio Notes
4/23
4
Vascular Resistance
Resistance is dependent onn fluid viscosity
n vessel length vessel diameter
Viscosity depends on the concentration of redblood cells in the blood (hematocrit)n Not a physiologically controlled variable
n Pathologies can affect resistance
Vessel length is constant
Resistance is controlled through variations invessel diametern (R 1/r4)
n 2 fold decrease in radius --> 16 fold increase inresistance
The HeartFour chamber organ composed primarily of cardiacmuscle (myocardium) lined with epithelium cells
Valves exist betweenn Atrium and ventricle
n Ventricle and large arteries
Healthy valves offer little resistance to flown Diseased valves may become narrowed and cause high
resistance to flow
Receives substantial innervention from sympatheticand parasympathetic nervesn Regulate cardiac function (via messengers)
Receives blood supply through coronary arteries
coming off aorta
Atrioventricular Valves
Tricuspid valve is between the right atriumand ventricle
Mitral valve is between the left atrium andventricle
Valves open passively on contraction of theatria to allow blood to flow to the ventricles
Valves close passively on contraction of theventricles to prevent backflow into the atrian Chordae tendinae attach valve leaflets to
ventricular wall to prevent inversion of the valves
-
8/6/2019 Circ Sys Physio Notes
5/23
5
Semilunar Valves
Between the ventricles and the large arteriesto prevent backflow from the arteries
Pulmonary valve is between the rightventricle and the pulmonary trunk
Aortic valve is between the left ventricle andthe aorta
Open passively on contraction of theventricles
Close passively on relaxation of the ventriclesn Do not have chordae tendinae
Mechanics of CardiacContraction
Cardiac contraction can be divided into two major andadditional minor phases
Systole - ventricular contraction and blood ejection (0.3seconds)
Step 1- Isovolumetric contraction:
n Increase in pressure closes tricuspid and mitral valves
n Pressure increases but not enough to open semilunarvalves
n Ventricular blood volume remains constant
Step 2- Ventricular ejection:
n Ventricular pressure exceeds vascular pressure,semilunar valves open
n Blood is forced from the ventricles
Mechanics of CardiacContraction
Diastole - ventricular relaxation and blood filling (0.5seconds)
Part 1- Isovolumetric relaxation:n Ventricular pressure drops below vascular pressure, semilunar
valves close
n Ventricular pressure remains above atrial pressure, mitral andtricuspind valves remain closed
n Relaxation reduces ventricular pressure but no blood flows in toincrease volume
Part 2 - Ventricular filling:n Ventricular pressure drops below atrial pressure, mitral and
tricuspid valves open
n Ventricles begin filling (about 80% of volume)
n Atial contraction finishes filling (remaining 20% of volume)
-
8/6/2019 Circ Sys Physio Notes
6/23
6
Pressure-Volume Curves
Volume
Pressure
Isovolumetric
Contraction
Isovolumetric
Relaxation
Cardiac Volumes
Stroke Volume (SV): amount of blood ejectedfrom each ventricle during a singlecontraction
End Diastolic Volume (EDV): amount of bloodin the ventricle at the end of diastole
End Systolic Volume (ESV): amount of bloodremaining in the ventricle at the end ofsystole
SV = EDV - ESV
Pressure-Volume Curves
Volume
Pressure
EDV ESV
SV
-
8/6/2019 Circ Sys Physio Notes
7/23
7
Ventricular Pressure-VolumeCurve
Describesrelationshipbetween pressureand volumethroughout cardiaccycle
Two curves:n Passive filling
curve
n Active contractilitycurve
Ventricular
Volume
Ventricular
Pressure
Passive
Filling
Active
Contractility
ESVEDV
Mechanics of CardiacContraction
Pressures and volumes change within the heartschambers and the vascular system during the cardiaccyclen Aorta and pulmonary artery
n Left and right atria
n Left and right ventricles
See Figures 14-25 and 14-26
Systemic arterial pressures typically vary between 120and 70 mmHg
Pulmonary arterial pressures typically vary between 24and 8 mmHg
Right ventricle pumps the same amount of blood over a
given time period as the left ventricle
Mechanics of CardiacContraction
Important Notes:
Most ventricular filling occurs before atrial contractionoccurs
Heart rates of 200 beats/minute or higher do not allowadequate time for full ventricular filling
Ventricular contraction does not completely enter theventricles
As ventricular pressure increases during isovolumiccontraction, vascular pressure is also decreasing asblood flows further into the system
The myocardium has some components of springbehavior and recoils on relaxationn Creates a slight negative pressure which draws blood into the
ventricle during filling
-
8/6/2019 Circ Sys Physio Notes
8/23
8
Cardiac Output
Volume of blood pumped by each ventricle per minuten Flow through either the systemic or pulmonary circuit
per minute
A function of the heart rate and the stroke volume
CO = HR x SV
Example:n HR = 72 beats/minute SV = 0.07 liter/beat
n CO = 5.0 liter/beat
Normal total blood volume = 5 liters
n Total blood volume pumped through one circuitevery minute
Control of Heart Rate
Controlled by natural pacemaker of heart (SA
Node)
Modified by sympathetic and parasympatheticinnervation
n Details to be discussed during cardiacelectrophysiology
Control of Heart Rate
Hormonal influences:
n Epinephrine speeds heart rate
wActs at same receptors as sympathetic
neurotransmitter norepinepherineOther minor influences:
n Body temperature
n Plasma electrolyte concentrations
n Additional hormones
-
8/6/2019 Circ Sys Physio Notes
9/23
9
Control of Stroke Volume
A more forceful contraction can cause anincrease in stroke volume
Force of contraction is influenced by:n End-diastolic volume (pre-load)
n Sympathetic innervention of ventricles
n Arterial pressure
wF = P/R
w Lower pressure differential (due to increasedarterial pressure) reduces flow from ventricleand thus stroke volume
Frank-Starling LawDefines how contraction stroke volume relates to
EDV
Ventricle contracts more forcefully when it is filled to
a greater extent before contraction
Relationship is defined by a ventricular function curve
0 100 200 300 400
200
100
0StrokeVolume(ml)
EDV (ml)
Normal Resting Value
Frank-Starling LawBased on the length-tension relationship ofcardiac musclen Stretch of cardiac muscle results in increased
muscular contraction force up to a maximal limit
n Normal resting length for cardiac muscle is not at
optimal length for contraction, but on rising curveIncreased flow of blood from the veins(venous return) results in an automaticincrease in end diastolic volume and strokevolumen If right heart begins to pump more than left,
increased venous return to left ventricle brings leftheart CO to correct level
-
8/6/2019 Circ Sys Physio Notes
10/23
10
Neural Control of Stroke Volume
Norepinephrine from sympatheticnerves acts to ventricular contractility
n Strength of contraction at any givenEDV
Shifts ventricular function curve up
Contractility can be measured throughejection fraction (EF)EF = SV/EDV
Normal ejection fraction is about 67%
Increases rate of cross-bridge cyclingas well as cytosolic calciumconcentration
SV
EDV
Vascular System
Arteries
Arterioles
Capillaries
Venules
Veins
Composed of various amounts ofn Smooth muscle
n Elastin
n Collagen
Lined with endothelial cells
Arteries
Large radii
Low resistance tubes to conduct blood flow
Arterial pressure depends on:n Volume of blood within arteries
n Compliance (stretchability) of vessel walls
Compliance defined as the amount of volumechange per unit pressure change
C = V/P
-
8/6/2019 Circ Sys Physio Notes
11/23
11
Arteries
Blood in the amount of SV flows into the arteriesduring systole
Only 1/3SV leaves arteries during systole
Remainder remains in arteries, resulting in arterialdistension
n Increases arterial pressure (systolic pressure)
During diastole, recoil in arterial walls causessecondary pumping of arterial blood to moveadditional portion of SV
n Arterial pressure gradually declines to its minimumlevel (diastolic pressure)
Some blood remains in arteries at all times, sodiastolic pressure is not zero
Arterial Pulse Pressure
Pulse pressure is defined as the differencebetween systolic and diastolic pressures
The magnitude of the pulse pressure isdetermined by:n Stroke volume - determines systolic pressure
n Speed of stroke volume ejection - influencestransition between systolic and diastolic pressure
n Arterial compliance - determines systolic anddiastolic pressure
wDecreased compliance due to atherosclerosis
increases pulse pressure
Mean Arterial Pressure
Average arterial pressure over the length ofone cardiac cyclen Due to asymmetric pressure curve, not the value
halfway between the systolic and diastolicpressure
n Approximated by:MAP = DP + 1/3(SP - DP)
Pressure driving blood into tissues over entirecardiac cycle
Arterial tree has minimal resistance (due tolarge diameter), so acts as single pressurereservoir with pressure equal to MAP
-
8/6/2019 Circ Sys Physio Notes
12/23
12
Arterioles
Distribute blood to vicinity of tissues and organsBlood pressure drops from mean value of 90 mmHgto 35 mmHg between beginning and end ofarterioles
Determine relative blood flow to the organs andtissues
n Resistance of arterioles locally controlled bycontrolling diameter
n Regions of higher resistance have less blood flow
n Contain smooth muscle which acts to constrict ordilute vessels
wVaried from natural state of myogenic tonewhich resutls from spontaneous contraction ofsmooth muscle
Local Control of ArterioleResistance
Control mechanisms independent of nervesor hormones which allow organs to controlown blood flow
Active Hyperemia: increased blood flow inresponse to increased metabolic activityn Results from local chemical changes in
extracellular fluid around arterioles
n Most highly developed in skeletal muscle, cardiacmuscles, and glands
Local Control of ArterioleResistance
Flow Autoregulation: results when a tissue or organsuffers a change in its blood supply as a result of achange in blood pressure
n Change in resistance acts to maintain blood flow
n Decreased resistance triggered by:
wReduction of oxygen concentration, increased CO 2,increased H+, increased metabolites
wSame triggers as active hyperemia
n Acts in cases of decreased or increased bloodpressure
n Can also be triggered by stretch-response of smoothmuscle
w Increased pressure --> stretch --> vasoconstriction
-
8/6/2019 Circ Sys Physio Notes
13/23
13
Local Controls of ArterioleResistance
Reactive Hyperemia: results after a tissue ororgan has had its blood supply completelyoccludedn Blood supply increased substantially as soon as
occlusion removed
n During occlusion, arterioles in region dilatecompletely due to autoregulation triggers
n Arterioles are wide open when occlusion isremoved
Response to Injury: injured cells and tjissuesrelease various substancesn Trigger vasodilation to increase blood supply to
injured site
Extrinsic Control of ArterioleResistance
Sympathetic Nerves:
n Rich supply to arterioles
n Release norepinepherine
n Increased activity causes vasoconstriction
n Decreased activity causes vasodilation
wBased on steady stimulation of vessels
n Control global blood flow to serve wholebody needs
Extrinsic Control of ArterioleResistance
Noncholinergic, Nonadrenergic AutonomicNeuronsn Release nitric oxide (not acetylcholine or
norepinephrine)
n Plays a major role in control of blood supply to GItract
n Mediate penile erection
Hormonesn Epinephrine
n Angiotensin II
n Vasopressin
-
8/6/2019 Circ Sys Physio Notes
14/23
14
Hormonal Control of Arteriole
Resistance
Epinephrinen Binds to alpha-adrenergic receptors on smooth
muscle to cause vasoconstriction
n Binds to beta-adrenergic receptors on smoothmuscle to cause vasodilation
n Alpha receptors generally outnumber betareceptors, with the exception of skeletal muscle
Angiotensin IIn Constricts arterioles
n Increases sympathetic nervous activity
Hormonal Control of Arteriole
Resistance
Vasopressin
n Plasma borne hormone
n Released by posterior pituitary gland
n Causes constriction of arterioles
Paracrine Control of ArterioleResistance
Endothelial cells lining arterioles respond to hormonaland neurological stimulation, releasing paracrine agentsthat affect nearby smooth muscle cells
Nitric oxide
n Contributes to basal levels of vasodilation
n Increased levels released in response to chemicalmediators, result in increased dilationw example: inflammation processes
Prostacyclin (PGI2)
n Minimal basal secretion
n Increased secretion in response to chemical inputresults in vasodilation
n Participates in blood clotting
-
8/6/2019 Circ Sys Physio Notes
15/23
15
Paracrine Control of ArterioleResistance
Paracrine agents can also act asvasoconstrictorsn Endothelin-1 (ET-1)
In arteries, shear stress in endothelial cellsdue to blood flow also causes release ofparacrine agentsn Increased stress -->
w Increased PGI 2w Increased NO
w Decreased endothelin-1
n Flow-induced arterial vasodilation
Capillaries
At any moment, 5% of total circulating bloodis flowing through the capillaries
Blood in capillaries performs ultimate functionof exchanging gases, nutrients, andmetabolic end products
Approximately 25,000 miles of capillaries inan adultn 5 m in diameter - one cell
n Each is about 1 mm long
Capillary AnatomyThin-walled tube of endothelial cells
n No smooth muscle
Endothelial cells separated by intercellular clefts
n No firm attachment between cells
n Form channels from capillary to extracellular fluid
Fused-vessical channels within cells also formchannels from capillary to extracellular fluid
Blood flow generally controlled by arteriole resistance
Capillaries branch off from metarterioles
n Connect arterioles to venules
n Site of capillary exit is surrounded by precapillarysphincter (smooth muscle) to control flow
-
8/6/2019 Circ Sys Physio Notes
16/23
16
Capillary Blood Flow
Velocity of blood flow is inversely proportionalto total cross-sectional area of vessel type
n Arteries and arterioles have lower total cross-
sectional area, and thus higher blood flow rate
Aorta Arteries VeinsCapillaries
Total X-section
Area (cm2)
Mean Linear
Velocity (cm/s)
Capillary ExchangeThree mechanisms of exchange between capillariesand extracellular fluid
n Bulk flow
n Diffusion
n Vesicle transport
Diffusion is predominant transport mechanism for
n Nutrients
n Metabolic end products
n Oxygen
Exception is brain due to blood-brain barrier
n Requires carrier-mediated transport of water-
soluble molecules
Capillary Exchange - Diffusion
Lipid-soluble molecules diffuse throughmembrane of endothelial cells
Ions and polar molecules diffuse throughintracellular clefts and fused vesicle channels
n Water-filled channelsn Reasonably high permeability, but lower than that
of lipid-soluble molecules
n Only small amounts of proteins can di ffuse throughmost channels
-
8/6/2019 Circ Sys Physio Notes
17/23
17
Capillary Exchange -- Diffusion
Size of channels determines leakiness ofcapillaries in various tissues/organsn Brain - no intracellular clefts
n Liver - large clefts and plasma membranewindows
wAllows diffusion of even large proteins
Diffusion depends on tissue and bloodconcentrations of materialsn Increased cellular activity reduces tissue
concentration of O2 and nutrients, increasesconcentration of CO2- and metabiolic endproducts
n Change in concentration gradient increasesdiffusion
Capillary Exchange -- VesicleTransport
Additional mechanism for transport ofsmall amounts of protein
n Endocytosis of protein-containing plasmaat blood-side of endothelial cell
n Exocytosis of proteins into extracellar fluid
by resultant vesicle
Capillary Exchange - BulkFlow
Bulk flow of plasma acts to distribute extracellularfluid
If there exists a hydrostatic pressure differenceacross capillary wall, endothelial cells act as porousfilter allowing transport of protein-free plasma*ultrafiltrate) through water-filled channels
Normally, capillary blood pressure is higher thaninterstitial hydrostatic pressuren Capillaries --> extracellular fluid
Hydrostatic driving pressure offset by osmotic forcedue to protein concentration in plasma (high) vs.extracellular fluid (low) which drives water to flow intocapillaries
-
8/6/2019 Circ Sys Physio Notes
18/23
18
Capillary Exchange - BulkFlow
Starling forces are the four factors whichgovern bulk flow of fluid
n Capillary hydrostatic pressure (Pc)
n Interstitial hydrostatic pressure (Pt)
n Plasma protein concentration (pc)
n Interstitial fluid protein concentration (pt)
F = K[(Pc - Pt) - (pc - pt)]
Capillary Exchange -- BulkFlow
At the beginning of the capillary hydrostatic
pressure difference is 35 mmHg, osmoticpressure difference is 25 mmHg
n Fluid flows into tissue (filtration)
At end of capillary, hydrostatic pressuredifference is 15 mmHg, osmotic pressuredifference is 25 mmHg
n Fluid flows out of tissue (absorption)
Capillary Exchange -- BulkFlow
Filtration and absorption along length of capillary tend tocancel each other out
Net filtration in systemic circulation (not includingcapillaries in kidneys) of about 4 liters/day
n Fluid then transported by lymphatic system
Dilation of arterioles leading to a capillary bed increasescapillary pressure, increasing filtration (and vice versa)
Capillary filtration and absorption are not significantmechanisms of nutrient and waste product transport
In pulmonary circulation, low resistance means lowcapillary pressures
n Normal hydrostatic pressure of 15 mmHg means netabsorption
-
8/6/2019 Circ Sys Physio Notes
19/23
19
Veins
First blood flows into venulesn Exchange of materials can occur in venules
Peripheral veinsn Outside of the chest
n Have valves that permit flow only towards theheart
n Pressure is low as greatest resistance occurs inarterioles and capillary beds
n Act as low resistance conduits to heart
n Diameters altered through smooth muscle tomaintain peripheral venous blood pressure andblood return to the heart
Venous Pressure -Determinants
Total blood volume
n Most of blood in veins at any given moment due tohigh compliance
Constriction of veins
n Smooth muscle innervated by sympathetic neurons
n Sympathetic activation causes vasoconstriction toreturn more blood to right heart
Skeletal muscle pump and respiratory pump
n Veins through muscles are compressed with musclecontraction
n Increased abdominal pressure during inspiration
compresses intrabdominal veins, decreased thoracicpressure assists in venous return
Venous Return
Must be identical to cardiac outputexcept for brief instances
Assisted by muscular and respiratory
pumpsValves prevent gravity or pumps fromdriving blood away from heart
-
8/6/2019 Circ Sys Physio Notes
20/23
20
Lymphatic System
Network of lymph nodes and lymphatic vessels thattransport a fluid derived from interstitial fluid (lymph)
Interstitial fluid enters lymphatic capillaries by bulkflow
n The four liters not reabsorbed into capillariesfollowing filtration
Fluid returned to vascular system via one-way valvesat the subclavian veins of the neck
Failure to return lymph through this system results inedema, fluid build-up in the tissues
Lymph is pumped through the lymphatic system byrhythmic contraction of the smooth muscle in thevessels
Regulation of Systemic ArterialPressure
Mean arterial pressure of the systemic circulation is themain controlled variable in the circulatory system
n Driving force for all blood flow except pulmonary
MAP = CO x TPR
n Function of cardiac output and total peripheral resistance
n Resistance sums like electrical resistors
wRT = R1 + R2 for vessels in series
w 1/RT = 1/R1 + 1/R2 for vessels in parallel
If resistance in one area decreases (ex: arterioles toskeletal muscles relax during exercise), TPR can bemaintained if resistance in another area is increased (ex:kidneys, GI)
n Brain arterioles maintain constant resistance
Regulation of Systemic ArterialPressure
Juggling of resistances can only work withina limited range
If resistance in one area drops drastically(such as in hemorrhage), constriction of other
vessels cannot maintain systemic bloodpressure
Short-term control is through baroreceptorreflexes
Long-term control is through blood volumechanges
-
8/6/2019 Circ Sys Physio Notes
21/23
21
Pulmonary Arterial Pressure
Mean Pulmonary Arterial Pressure =n CO x Total Pulmonary Vascular Resistance
CO is the same for pulmonary and systemiccirculations
Mean pulmonary arterial pressure is less thanmean systemic arterial pressure
THEREFORE, Total pulmonary vascularresistance is less than total systemic vascularresistance
Arterial Baroreceptors
1. Located in the carotid arteries of the neck
n Carotid sinus
n Numerous afferent nerve endings
2. Located in the aortic arch
Respond to stretch of arterial wall
n Rate of neural discharge is proportional to theMAP (for steady pressures) or the pulse pressure(for pulsatile flow)
Medullary CardiovascularCenter
Main integrating center for baroreceptor reflexes
Increase in baroreceptor discharge results in:
n Decrease in sympathetic stimulation to vasculature
wVasodilation (decreased vasoconstriction)
n Increase in parasympathetic stimulation to heartwDecreased heart rate
Also affect secretion of vasopressin and angiotensin II
Short-term effect
n Long term changes in MAP or pulse pressure willresult in resetting of baroreceptor threshold
-
8/6/2019 Circ Sys Physio Notes
22/23
-
8/6/2019 Circ Sys Physio Notes
23/23
Hemorrhage and Hypotension
Baroreceptor reflexes and absorption through capillariescan compensate for loss of up to 1.5 liters of blood (30%
total volume) with only slight reductions in MAP or CO
n Increased heart rate above normal
n Increased stroke volume towards normal
n Increased total peripheral resistance above normal
Any loss of fluid results in a decrease in circulatory
volume and reaction based on cardiovascular reflexes
n Hemorrhage
n Dehydration
n Diarrhea or vomiting
Shock
Denotes any situation where a decrease inblood flow results in damage to organs ortissues
Can result from:n Severe hemorrhage
n Loss of fluid
n Excessive release of vasodilators (allergy orinfection)
n Loss of sympathetic stimulation of cardiovascularsystem
n Severe bodily damage (general)
Hypertension
Chronically elevated systemic blood pressure
n Over 140/90 mmHg (systolic/diastolic)
Most common cause is increased TPR due toreduced arteriolar radius
Can result in left ventricular hypertrophyn Left ventricle has to pump against increased
resistance
n Changes in myocardium can result in heart failure
Increases risk of:
n Atherosclerosis and heart attacks
n Kidney damage
n Rupture of cerebral blood vessel (stroke)