regulation of stroke volume and heart rate

Question 1

what 2 things regulate heart rate

Answer 1

symp and parasymp NS

Question 2

how does the sympathetic NS regulate HR

Answer 2

symp nerves release NA
adrenal medulla releases adrenaline 
these act on beta 1 receptors on SAN 
increased slope of pacemaker potential 
increased HR

Question 3

how does the parasympathetic NS regulate HR

Answer 3

vagus nerve releases Ach which acts on muscarinic receptors on SAN
cells are hyperpolarised
decreased slope of pacemaker potential - cells start further from threshold and take longer to depolarise to threshold
HR decreases

Question 4

what nerve innervates SAN most heavily

Answer 4

vagus cranial nerve
at rest it is constantly working to slow down HR and keep it at 60-70bpm
HR is said to be under vagal restraint

Question 5

equation for CO

Answer 5

CO = HR X SV

Question 6

Effect of increasing HR with an electronic pacemaker

Answer 6

small increase in CO then SV decreases
shortened cardiac interval cuts into the rapid filling phase
reduced EDV, reduced preload, reduced SV

Question 7

physiological effects to offset reduced SV

Answer 7

• HR increases via decreased vagal tone and increased symp tone
• contractility increases via increased symp tone, alters inotropic state and shortens systole. Stronger but shorter contraction, increased SV
• VR increases via venoconstriction and skeletal/resp pumps, maintaining preload
• TPR falls (arteriolar dilation), reduced afterload
• CO increases 4-6x

Question 8

physiological regulation of SV

Answer 8

preload
afterload
neural

Question 9

starling’s law

Answer 9

the energy of contraction is proportional to the initial length of the cardiac muscle fibre
stronger contraction in cardiac muscle = bigger SV

Question 10

define preload

Answer 10

Preload is the initial stretching of the cardiac myocytes (muscle cells) prior to contraction. It is related to ventricular filling.

Question 11

what is preload affected by

Answer 11

EDV
increased venous return = increased EDV and SV (vice versa)
this ensures self-regulation; matches SV of RV and LV so they work in series

Question 12

define afterload

Answer 12

the load against which the muscle tries to contract i.e. how easy it is to get the blood out through the arterioles - TPR

Question 13

define total peripheral resistance

Answer 13

TPR
how contracted/dilated all the arterioles are when added together
increased TPR = increased afterload, decreased SV

Question 14

what happens when TPR increases

Answer 14

aortic pressure will increase

ventricle has to work harder to push open the aortic valve and has less energy left to eject the blood

Question 15

what vessels set the preload

Answer 15

capacitance vessels set the preload

venoconstriction = increased preload, increased SV, helps the heart

Question 16

what vessels set the afterload

Answer 16

resistance vessels set the afterload
arteriolar constriction = harder for the blood to get out, increased pressure and afterload
heart has to work harder, reduced SV

Question 17

neural regulation of SV

Answer 17

symp and parasymp NS

Question 18

sympathetic NS and SV

Answer 18

symp nerves release NA
adrenal medulla releases adrenaline
these act on beta 1 receptors on myocytes
increased contractility - increased amount of calcium coming into the cell from outside, more released from internal stores
contraction is stronger but shorter as Ca is taken up quicker

Question 19

parasympathetic NS and SV

Answer 19

little effect on contraction
vagus nerve doesnt innervate ventricular muscle
muscarinic receptors present aren’t activated

Question 20

pathological regulation of SV

Answer 20

blood Ca levels
ischaemia
barbituates
Starling’s law

Question 21

hypercalcemia and SV

Answer 21

larger conc grad
more Ca enters when channels are open
stronger contraction
shifts Starling curve up and left

Question 22

hypocalcemia and SV

Answer 22

smaller conc grad
fewer X bridges form
less steep curve - shifts down and right

Question 23

ischaemia and SV

Answer 23

obstruction of blood supply to part of the heart 
can't contract properly 
smaller overall strength of contraction 
reduced SV 
curve shifts down and right

Question 24

barbiturates and SV

Answer 24

shifts curve down and right

reduced SV

Question 25

Starling’s law and the pumping ability of the heart

Answer 25

starling’s law lets the heart compensate for a reduced pumping ability by working around a larger EDV (less blood pumped out of the heart so EDV increases)
resting SV is achieved at a higher EDV
lower ejection fraction
reduced exercise capacity