Blood pressure: role of cardiac output & peripheral resistance L8

Question 1

what is needed for blood to flow

Answer 1

need a pressure gradient

Question 2

what is the more precise version of blood pressure

Answer 2

mean arteriole blood pressure
- needs to be maintained in order to get blood to flow

Question 3

what is mean arteriole blood pressure the product of

Answer 3

1. Product of the input to the arterial system
   Cardiac output
2. total peripheral resistance
   - Sum of individual vessels resistance to flow

Question 4

what is the equation for mean arteriole blood pressure

Answer 4

cardiac output X total peripheral resistance

Question 5

what is the typical values of SV, HR and CO at rest

Answer 5

SV: 70mL
HR: 70 beats per min
CO: 4.9l/min

Question 6

during exercise, CO can increase
why is this

Answer 6

due to the autonomic neural control of the heart

Question 7

describe the systems that decrease heart rate
- what do the nerves release and what do they act on
- what type of receptor does the neurotransmitter bind to

Answer 7

parasympathetic nerves (vagus)
- realease ACH on to the SA and AV node
- ACh binds to muscarinic receptors which decreases heart rate

Question 8

describe the systems that increase heart rate and contractility
- what do the nerves release and what do they act on
- what type of receptor does the neurotransmitter bind to

Answer 8

sympathetic nerves
- releases noradrenaline on to SA, AV node and cardiac muscle
- binds to beta 1 adrenergic receptors
- increases heart rate and contractility

Question 9

if an antagonist is added to parasympathetic nerves

Answer 9

heart rate would increase
- small amount of ACh is being released which dampens down heart rate
therefore, if this is blocked, heart rate would increase
- increases dynamic range

Question 10

on a graph that shows the sympathetic stimulation with SA node pacemaker activity,
what does the curve look like compared to the curve of membrane potential at rest

Answer 10

there is more pacemaker potentials when sympathetically stimulated
- there is a steeper slope of pacemaker potentials and the resting membrane potential is less negative so threshold value is reached quicker
- increases heart rate

Question 11

what is increased heart rate called

Answer 11

tachycardia

Question 12

how does the sympathetic supply increase heart rate

Answer 12

noradrenaline causes sodium channels activity to increase so more Na+ entering allowing depolarisation

Question 13

on a graph that shows the parasympathetic stimulation with SA node pacemaker activity,
what does the curve look like compared to the curve of membrane potential at rest

Answer 13

there is less pacemaker potentials when parasympathetically stimulated
- there is a shallow slope and the resting membrane potential is more negative to being with making it harder to reach threshold
- decrease heart rate

Question 14

what is decrease in heart rate called

Answer 14

bradycardia

Question 15

why does parasympathetic supply decrease heart rate

Answer 15

ACH binds to K+ channels activating them
- increasing K permeability, more K leave then Na in
- MORE neg so harder to depolarise

Question 16

what are the two mechanisms of controlling stoke volume

Answer 16

1. intrinsic
   - Frank-Starling Mechanism or Starling’s Law of the heart
2. extrinsic
   - Increase in sympathetic activity

Question 17

describe Starling’s Law of the heart

Answer 17

the force of contraction is proportional to the initial muscle fibre length in diastole
(Initial means just before contraction commences)

Question 18

explain how Starling’s Law of the heart increases stroke volume

Answer 18

1. an increase in blood returning to the heart (venous return) will increase the end diastolic volume which stretches the cardiac muscle
2. this stretching increases the force of the subsequent contraction causing stoke volume to increase
   - The heart will pump whatever volume of blood it receives
   - the more you will a ventricle, the harder it will contract

Question 19

describe the resting stroke volume and stoke volume after stretching on a Left ventricular function curve

Answer 19

If you increase venous return to left side of the heart, the stroke volume will be higher than when at rest due to the stretching and increased filling

Question 20

why does stretching increase stroke volume and force of contraction

Answer 20

when muscle fibres stretch, they become more sensitive to calcium
therefore, when given fibre at rest and one that has been stretched the same amount of calcium, stretched fibre will produce the bigger contraction
- intracellular [Ca2+] required to generate 50% maximum tension is lower when the muscle fibre is first stretched

Question 21

what is the importance of starling effects

Answer 21

1. helps to match output of right and left sides of the heart
2. enables heart to adapt its pumping capacity when either venous return or arterial blood pressure changes

Question 22

why is matching output important

Answer 22

if the right output is higher than left (even by as little as 1%),
more blood would be going to lungs than the rest of the body at every heart contraction
Pulmonary blood volume would go from 0.6 to 2.1 litres in 30 mins
This would cause severe oedema in the lungs and you would literally ‘drown’!!
This is prevented from happening because if RV output goes up then LV filling increases during diastole, producing more stretch, and thus SV, and therefore CO, goes up to match this increase in VR.

Question 23

explain how extrinsic mechanisms control stroke volume

Answer 23

there is an increase in sympathetic activity
- this doesnt change muscle fibre length but instead enhances contractility of cardiac muscle

Question 24

how does sympathetic activity enhances the contractility of cardiac muscle

Answer 24

increases adrenaline or noradrenaline binding to B1 adrenergic receptors
- Results in enhanced stroke volume
- Smaller ESV at the end of contraction
Known as a positive inotropic effect

Question 25

what type of hormones are adrenaline and noradrenaline

Answer 25

catecholamines

Question 26

How do catecholamines increase contractility

Answer 26

1. bind to B1 on myocardial contractile cell activating the cAMP pathway
2. causes the phosphorylation of Voltage gated calcium channels and phospholamban
3. when voltage gated sodium channels are phosphorylated, channels stay open for longer, so more trigger calcium allowed in, so more calcium released from SR causing more forceful contraction
4. the phosphorylation of phospholamban causes increase activity of Ca ATPase
   - this increase Ca in SR which can be released and
   - means Ca is removed from cytosol quicker so shorter time it is bound to troponin so shorter contraction duration

Question 27

what are Changes in the rate of muscle relaxation known as

Answer 27

lusitropic effects

Question 28

what needs to be maintained when cardiac output changes

Answer 28

venous return

Question 29

how is venous return maintained

Answer 29

1. Venous to atrial pressure difference
2. Venous valves
   prevent ‘back-flow’ of blood
3. skeletal muscle contraction (‘skeletal muscle pump’)
4. Venomotor tone
   some veins have smooth muscle and receive sympathetic supply
5. Respiration (‘respiratory pump’)
   inspiration aids venous return