preload, afterload and contractility

Question 1

what affects the amount of blood pumped by the heart for each cardiac cycle

Answer 1

stroke volume

Question 2

what affects the amount of blood delivered to the tissues

Answer 2

• stroke volume
• heart rate
• vasculare tone

Question 3

what is cardiac output

Answer 3

the amount of blood pumped in 1 minute
- usually close to blood volume (blood usually circulates every minute)
- at rest, tissue oxygen delivery exceeds oxygen consumption (2:1)

cardiac output = heart rate x stroke volume

Question 4

what happens if the cardiac output from the left and right ventricles arent matched

Answer 4

normal heart automatically pumps our whatever blooc volume is put into it
stroke volume must be matched (volume entering lungs = volume entering systemic circulation otherwise:
- increase in pressure on venous side
- leading to oedema (pulmonary or peripheral hence:
- tightly regulated by a range of mechanisms

Question 5

how is stroke volume regulated

Answer 5

by preload (end diastolic volume), afterload (resistance to ventricular ejection) and contractility (end systolic colume)
stroke volume = end diastolic volume - end systolic volume

Question 6

what is preload

Answer 6

dependent of venous return of blood
- if low: ventricular filling is reduced thus stroke volume is reduced (ex. hemorrahge)
- if high: ventricular filling is increased hence stroke volume increases (ex. exercise)

is stretch of cardiac muscle prior to contraction

Question 7

descrieb the frank-starling mechanism

Answer 7

• generally, a linear relationship between preload and stroke volume
• ventricular muscle stretching leads to stronger contractile force

Question 8

describe the length-tension relationship

Answer 8

increased preload leads to increased exposure of myosin to actin leading to increased cross bridge formation leading to increased force of contraction

Question 9

what are the limits to the frank-starling mechanism

Answer 9

1. excessive stretching causes a decrease in cross bridge formation
2. laplace’s law: in a large sphere more wall tension is required to generate the same internal as it does in a small sphere as governed by pressure=tension/radius

clinically: as the heart fills up with more blood, then the muscle will find itself at an increasing mechanical disadvantage. thus, the chambers will become more difficult to empty (=> DCM, because bigger has to work harder to maintain pressure)

Question 10

what factors influence preload

Answer 10

1. filling time of the heart (low heart rates -> longer period for ventricular filling = greater distension of the ventricle
2. venous return: i.e pressure difference between venous system and atrium

Question 11

how does the skeletal muscle pump increase venous return

Answer 11

contraction of skeletal muscle leads to compression of veins leading to the blood being forced toward heart. this increases preload (more blood to stretch cardiac muscles)

Question 12

describe how the respiratory pump increases venous return

Answer 12

inspiration: diaphragm moves caudally, increasing abdominal pressure and reducing thorax pressure increasing abdominal return of blood. this increases preload

Question 13

how does sympathetic control of venous return increase preload

Answer 13

• venous system acts as reservoir for blood
• sympathetic stimulation of the venous system causes venous vasoconstriction, increases venous pressure. this leads to increased preload

Question 14

how does the SNS influence stoke volume

Answer 14

increases contractility thus increases strength of contraction at any given preload

Question 15

how does contractility effect stroke volume

Answer 15

increased contractility = greater force of contraction = increases amount of blood being pushed out = increase stroke volume = increased cardiac output

Question 16

what does increased contractility enable the heart to do

Answer 16

1. empty more completely and thus there is reduced end systolic volume
2. handle a greater pre-load and thus creates a greater filling volume
3. deliver an increased stroke volume even when increased HR reduces time for ventricular filling
4. to do all this against, if afterload is increased

involves activation of beta 1 adrenoreceptors (when activated by adrenaline causes increased contractility

Question 17

how does the PSNS influence contractility

Answer 17

• decreased force of contraction
• inhibition of noradrenaline release from sympathetic NS
• also decreases HR
• this involves activation of M2 muscarinic receptors

Question 18

what is afterload

Answer 18

the amount of pressure that the heart needs to exert to eject the blood during ventricular contraction
limiting factor in stroke volume

Question 19

how does afterload effect stroke volume

Answer 19

• creates resistance against which ventricle pumps
• increased stroke volume leads to increased cardiac output and thus increased afterload
• also influenced by pressure of blood in circulation

Question 20

describe the effects on stroke volume if afterload is normal, increased and decreased

Answer 20

normal: the ejection pressure is greater than afterload hence blood is ejected out of the heart
reduced afterload: more blood can be ejected because the afterload is much less
increased afterload: reduced stroke volume, heart has to work harder to maintain cardiac output (because afterload pressure is too high)

Question 21

what are intrinsic mechanisms for stroke volume

Answer 21

mechanisms of maintenance that are turned on all the time regardless of ANS –> the normal heart pumps venous return each cycle

Question 22

what are extrinsic mechanisms of stroke volume

Answer 22

sympathetic nervous system activation which increases contractility (inotropy) of the heart and consequently there is an increased contraction at any given preload

Question 23

how is the heart rate (and thus cardiac output) controlled

Answer 23

• intrinsic activity from SA node (has external influences by PSNS and SNS)
• controlled by SNS and PSNS to enable rapid response (increase or decrease), and is tightly regulated to maintin blood pressure)

Question 24

how does blood pressure influence heart rate and thus cardiac output

Answer 24

if blood pressure is elevated: PSNS activated to slow HR, reduce CO and return BP to normal
if blood pressure low: activation of SNS to increase HR, increase vascular tone, increase CO and resotre BP

Question 25

how is blood pressure controlled

Answer 25

blood pressure (MAP) = cardiac output x total peripheral resistance (vasoconstriction)
1. caridac output = intrinsic control (starlings law;preload) and SNS (contractility)
2. TPR is in effect arterial vascular resistance (tone) and is influenced by:
- SNS (alpha and beta adrenoreceptors) in the vasculature
- renin-angiotensin aldosterone system
- local endothelial derived factors
- also by arterial blood volume and compliance

Question 26

how to you calculate blood flow

Answer 26

blood flow = pressure difference/resistance
resistance is proportional to length/radius^4
decreased radius - less blood flow

Question 27

what is pulse pressure

Answer 27

pulse pressure = Psystolic-Pdiastolic
influenced by:
- arterial compliance (ability to increase in pulse pressure, decreases with age)
- stroke volume
- ejection rate
- consistent high pulse pressure will age the heart quicker

Question 28

what will result in increased cardiac output

Answer 28

increased preload

Question 29

SNS stimulation will increase HR by acting on which receptor

Answer 29

beta 1 adrenoreceptor

Question 30

what is the effect of arterial vasodilation on BP

Answer 30

will decrease it