cardiac output and blood pressure Flashcards
what is cardiac output?
- volume of blood ejected by the heart per minute
- measured in ml-min-3
how do you work out cardiac output?
cardiac output = stroke volume x heart rate
what is stroke volume?
- volume of blood ejected per beat
what is heart rate?
- beats per minute
describe parasympathetic nervous system
- vagus nerve releases acetylcholine which slows down HR
- stimulation concentrated to SA and AV nodes
- causes cell membranes to become more hyperpolarised causing SA node rhythm to slow
describe sympathetic nervous system
- releases norepinephrine which increase heart rate
- supplies SA and AV nodes so causes cell membrane to depolarise faster
- shortens AV node delay and speeds up relaxation
what does circulating epinephrine trigger?
- increase in heart rate but lags behind neural stimulation
what opposing factors is stroke volume controlled by?
- force of muscle cell contraction
- arterial pressure against which they eject blood
what is force of contraction regulated by?
- length- tension properties of cardiac muscle cells
- effects of hormonal influence on contractility of cardiac muscle
describe length tension properties
- relationship between end diastolic volume and stroke volume
what can length tension properties be known as?
- Starling’s law of the heart
- more blood returned= more blood pumped out
how can end diastolic volume be increased?
- by greater filling of the heart (venous return) so greater stroke volume because stretching muscle fibres cause them to contract more forcefully
how does training affect the equation?
increased stroke volume means increased cardiac output
- increased 02 delivery
describe cardiac hypertrophy
increased stroke volume = decreased resting heart rate
what is the effect of diastole?
- greater SV because the cardiac muscle fibres stretching cause them to contract more forcefully
what does sympathetic nerve activity and circulating epinephrine cause?
- increases force of contraction so cardiac muscle will contract more forcefully for a given amount of stretch
when does heart rate and stoke volume increase?
- when vagus nerve activity decreases (removal of parasympathetic)
- increased sympathetic nerve activity, increased epinephrine and end diastolic volume
what is blood pressure?
- pressure exerted by blood upon walls of blood vessels
on average, what is the normal blood pressure value?
120-180 mmHg
what three factors affect blood pressure?
- arteries become stiffer with age due to loss of elasticity
- pressure is higher in foot and lower in head due to hydrostatic pressure
- on average women have lower BP than ,men
where is blood pressure highest?
- close to heart
- declines as blood moves further away
what ensures that blood moves forwards even when heart is relaxed?
- elastic properties of the walls of major arteries
what is blood pressure for ventricles, arteries and arterioles? why?
- pulsatile due to rhythmic contraction of the heart
describe blood pressure in relation to cardiovascular system
- driving force for flow
- pressure gradient between arteries and veins
- must be maintained at all times
what is the pressure like in the veins?
- continuous to reflect loss of forward energy mainly due to friction
how is blood pressure measured?
- in systematic arteries
- varies between a max at peak of ventricular contraction (systole) and minumum at end of ventricular relaxation (diastole)
what happens to blood pressure during dynamic and static exercise?
-dynamic = mean blood pressure remains steady
- static= mean blood pressure rises dramatically
how is pressure written?
- millimetres of mercury
- systolic pressure over diastolic pressure
what is pulse pressure?
- difference between systolic pressure and diastolic pressure
why is MABP used over pulse pressure?
- pulse pressure gives the range within arteries whereas MABP gives a single value and accurate measure of perfusion pressure for tissues and organs
what is mean arterial blood pressure?
- average blood pressure within arteries over one cardiac cycle
- difference in pressure between arteries and veins
what is the normal mean arterial blood pressure range?
- 70 to 105mmHg
how do you work out mean arterial blood pressure?
DP (diastolic) + 1/3 x PP (pulse pressure)
why does the equation multiple pulse pressure by 1/3?
- because 2/3rds of cycle spent in diastole at rest so mean arterial blood pressure is closer to diastolic pressure than systolic
what lasts longer diastole or systole? what is MABP closer to?
- diastole lasts almost twice as long as systole
- MABP closer to diastolic blood pressure value
what is a direct measurement of MABP?
- catheter into artery connected to a pressure gauge
- pressure measured as height of fluid it pushes up against gravity
what is the disadvantages of direct measurements?
- invasive
- inconvenient
- impractical
what are the two types of indirect measurements?
- stethoscope
- sphygmomanometer
describe systolic pressure using a cuff
- cuff placed on upper arm and inflated to stop blood flow from brachial artery to arm
- when pressure in cuff exceeds pressure in artery, blood flow is occluded and no pulse felt below cuff
what happens as you release the cuff? what sound is heard?
- pressure begins to decrease and when it reaches peak systolic pressure artery opens a little but flow is turbulent
- disorganised flow generates tapping sounds called Korotkoff sounds
describe using a cuff to measure diastolic blood pressure
- as cuff pressure continues to decrease, the artery regains its normal diameter so flow becomes laminar
what sound is heard in diastolic blood pressure?
- sound is more muffled and eventually disappears so cuff pressure recorded at point of muffling
why do we regulate blood pressure?
- tissues need nutrient and can’t accumulate by products
- flow rate proportional to pressure gradient along vessel
what is flow rate ?
- volume of blood which passes along a blood vessel per unit of time
what is flow velocity?
- distance travelled by an object relative to time (m/s)
how do you work out flow (Q)
Q= V x A
flow = velocity x area
what shows the flow in the cardiovascular system? why can this be used
- cardiac output as any blood that leaves the heart will be returned as system is closed
how do you work out cardiac output?
CO= HR x SV ; volume of blood leaving heart per minute
how do veins make up for slower velocity of blood flow compared to arteries?
- veins contain more blood volume
- net effect is the same volume of blood returns to heart as was ejected
what happens to blood pressure from aorta to vena cava?
- decreases due to resistance to blood flow in cardiovascular system
how do you work out resistance ?
resistance = pressure gradient/ flow rate
if streamlined non- turbulent flow, what factors can determine resistance?
- viscosity of blood
- radius of blood vessel
what’s the equation for resistance in streamlined object?
resistance= 8nl (liquid viscosity, length of vessel) / pie r (radius of vessel) ^4
what has the biggest influence on resistance and why?
- radius of vessel as when liquid is flowing through a tube, the outermost layer in contract with the wall moves slowly than layer inside so every layer is slower than layer inside it
where is the flow fastest?
- in the middle layer
what is blood flow sensitive to? what does this mean?
- sensitive to changes in blood vessel radius
- the greater the cross sectional area means the slower the blood will flow
what vessel has the slowest blood flow?
- capillaries due to the fact they have largest cross sectional area
- allows sufficient time for diffusion of nutrients and waste between blood and tissue
what is the total peripheral resistance?
- combined resistance of all vessels in systematic circulation
how do you work out total peripheral resistance?
total peripheral resistance = pressure gradient / flow rate
what is flow rate equal to and what is pressure gradient equal to?
- flow rate equal to cardiac output
- pressure gradient equal to mean arterial blood pressure
how do you work out mean arterial blood pressure linking other factors together?
MABP= cardiac output x total peripheral resistance
what vessel is most suited to regulate blood flow in systematic circulation ? why?
- arterioles due to layer of smooth muscle that allows them to alter their radius and hence resistance
what does contraction of smooth muscle cause?
- vasoconstriction
- contraction in arteriolar wall decreases radius which increases resistance and decreases flow
what does relaxation of smooth muscle cause?
- vasodilation
- increases radius which decreases resistance and increases flow
what do arterioles provide to capillaries?
- fine control of blood flow as can divert blood to and from capillary beds
is blood flow pulsatile in systemic circulation?
- no as it is non- pulsatile within capillary and venous vasculature
why is right ventricular wall thinner?
- pressure required within pulmonary circulation is not as great as that required within systemic circulation
what is autoregulation?
- some organs are intrinsically able to prevent and reduce changes in local blood flow (if pressure decreases) and revert it back to normal within minutes
what does autoregulation occur without and what does it link?
- occurs without neural or hormonal input
- links perfusion pressure, blood flow and resistance
what happens to resistance if pressure drops?
- resistance decreases to maintain blood flow
what are the three primary mechanisms of autoregulation ?
- endothelial, metabolic and myogenic
describe endothelial mechanism
- endothelium produces specific vasodilating factors like prostacyclin and nitric oxide
describe metabolic mechanism
- when blood flow is reduced, resultant hypoxia causes localised release of vasodilating factors from tissue surrounding blood vessels
describe myogenic mechanism
- blood vessels that contain smooth muscle are able to alter their diameter in response to stretch or compression of blood vessel
what does percentage of total blood volume in each group of vasculature depend on?
-combined cross- sectional area
- vessel length
how much blood found in aorta/ arteries, systematic arterioles, capillaries and venules?
-aorta/ arteries= 11%
- systematic arterioles= 3%
- capillaries = 6%
- veins= 64%
where is the remainder of blood volume found?
- heart= 7%
- pulmonary circulation= 9%
what is an important determinant of volume of blood?
- change in radius alters blood resistance
- radius changes during contraction or relaxation of vascular smooth muscle
what does compliance mean?
- how much a blood vessel’s volume changes with a given pressure change
what is the compliance of a vessel that easily expands at low pressure?
- high compliance
describe low compliance
- more pressure is required for the vessel to accommodate at given volume
why do veins store most blood?
- cross sectional area isn’t static as vessels can change shape
- described as distensible as swell due to pressure from inside
what name have veins been given?
- capacitance vessels as they have the ability to rapidly increase its volume at lower pressures
what happens as vessel expands?
- elastic tension generate which increases pressure inside vessel
what determines amount of tension generated?
- elastic properties of vessel wall
how much larger is compliance of veins compared to arteries at lower pressure? how is this shown?
- 10-20 times larger
- shown by steeper slope as vein’s cross sectional shape is elliptical at low volumes and circular at higher volumes
can vessels keep expanding?
- vessels can only expand to a certain size
- indicated by flat slope at top of curve
what happens to gravity when we lay down?
- gravity is evenly distributed across our blood
what is the expected thing to happen when we stand up? but why doesn’t this occur?
- gravity should act to pull blood towards fit
- but cardiovascular system is closed and effects of gravity on arterial and venous compartments will be the same
what is enough to distend the veins when we stand?
- hydrostatic pressure as veins are compliant
what happens to blood when we stand up? what does this do to venous return?
- blood pools in lower extremities
- venous return decreased initially as if more blood pools in one area then less is available to return to the heart
what are the 5 mechanisms that pump the pooled blood back to the heart?
- nerves
- valves
- skeletal muscle
- diaphragm
- heart
describe nerve mechanism
- regulate smooth muscle in walls of veins to constrict them and hence decrease their volume and increase pressure to facilitate return of blood
describe valves mechanism
- prevents blood flowing backwards
- half- moon shaped and close passively when blood flow reverses
describe skeletal muscle mechanism
- helps squeeze veins; becomes more pronounced during exercises that utilise lower limb e.g. cycling
describe diaphragm mechanism
- during expiration> diaphragm ascends which ^ intrathoracic pressure and lowers abdominal pressure to help blood to vena cava
- during inspiration> decrease in intrathoracic pressure but ^ abdominal pressure to help blood move to thorax
describe heart mechanism
- movements of heart during ventricular systole increases volume of right atrium thus decreasing right atrial pressure > suction effect so blood from VC to right atrium
what is stroke volume dependent on?
- volume of blood in ventricles at end of diastole= end diastolic pressure
- EDV dependent on adequate venous return
describe starling’s law of the heart
- states that up to a point the heart will pump whatever volume it receives out
what happens if blood flow to the brain is reduced?
- lose consciousness and fall
how is stroke volume increased and decreased?
- increased by vasoconstriction of veins
- decreased by decrease in circulating blood volume, decrease in venous return, dehydration and haemorrhage
