Coordinated responses of the CVS

Question 1

what is orthostasis?

Answer 1

standing up

Question 2

When you stand up what happens? (orthostasis)

Answer 2

1. Blood pressure falls at first (when you stand up brain gets further away from the feet)
   - Postural hypotension, lack of blood flow to the brain –> can make you feel faint
2. Quickly recovers
   - Due to homeostatic mechanisms such as the baroreflex- carotid arteries BP drops and unload the baroreflex- also unloads in the heart.

Baroreflex integrates three smaller changes:

• Heart rate increases
• Heart contractility increases
• Total peripheral resistance increases

Within a heartbeat or 2, BP and perfusion in the brain is back to normal- if there are problems with this you can faint

Question 3

explain the effects of gravity on blood pressures during orthostasis:

Answer 3

• Lying down, pressure at the head and feet is the same, obviously the arterial pressure is higher than venous pressure but there is no real pressure gradient. When you stand up however, you get blood pooling in the legs. How will blood get from the heart to the feet and back to the heart?
• If there is a higher pressure in the feet then blood will flow from the feet to the heart, when in fact you want the heart to pump blood to the feet- this occurs due to Bernoulli’s Law:

Question 4

what is Bernoulli’s Law?

Answer 4

Blood flow = pressure energy + potential energy + kinetic energy

• Total energies means blood flows from the heart to the feet
• Blood at the heart has a much higher kinetic energy and potential energy than blood at the feet, although it has a slightly lower pressure. But overall, its total energy is just slightly higher than at the feet.
• This is why people with problems with their cardiac output can have poor perfusion of their feet (e.g. in diabetes).

Question 5

explain what is meant by ‘gravity induced high venous blood pressures’

Answer 5

high pressure in the venous system at the feet is due to hydrostatic pressure

Question 6

how can orthostasis make you dizzy? what is the cure

Answer 6

• You have venous pooling of 500ml in legs, which reduces blood returning to the heart
• Orthostasis causes a fall in CVP, so a decrease in EDV (amount of blood entering the heart, RV)
  o Leads to decreased stroke volume (Starling’s Law, RV gets stretched a bit less so contracts more softly)
  o Decreased cardiac output
  o Decreased BP
  o Poorer perfusion of the brain (less blood reaches lung so less blood reaching LV so less blood to brain)
  o Dizziness and fainting

cure the problem of blood pooling in the legs is to get head at the same level as the feet (lying down), extra blood going to the heart means RV stretched

Question 7

explain why we have a reflex response to orthostasis

Answer 7

we want a mechanism to avoid fainting when we stand up - increase CO even though the filling pressure of the heart is decreased

Question 8

explain the reflex response to orthostasis

Answer 8

1) Less stimulation/firing (unloading) of baroreceptors. Unloaded indicates that they are less stretched, and this is due to the drop in BP as there is less blood in the heart because it is pooling in the legs
2) Lower afferent fibre activity- Always some afferent fibre activity because there is always some stretch going on- when they are unstretched they fire less
3) Firing stimulates neurons in the NTS, which stimulate the CVLM which will inhibit the RVLM which usually sends efferent signals to SAN (parasympathetic), increasing HR and arteriole vasoconstriction. So, firing will reduce HR and vasoconstriction. Less firing means more sympathetic drive.
4) Switches off inhibitory nerves that go from the CVLM to the RVLM

5) Results in RVLM being more active
a. Sends efferent signals to heart and arterioles

6) Many different effects (diff flashcard)

Question 9

explain the diff effects involved in the reflex response to orthostasis

Answer 9

Many different effects

a. Increased sympathetic drive to SAN and increased HR
b. Myocardium has increased contractility
c. Vasoconstriction (arterioles, veins) which increases TPR
d. Less vagal parasympathetic activity to SAN
i. Overall increase in BP, blood is driven to the brain as vessels squeeze together

Question 10

what is postural hypotension?

Answer 10

condition in which a person’s blood pressure drops abnormally when they stand up after sitting or lying down

Question 11

what makes postural hypotension worse?

Answer 11

• α-adrenergic blockade, generalised sympathetic blockade or other drugs that reduce vascular tone, e.g. side effect with voltage gated calcium channel blockers used to treat hypertension, angina
• Varicose veins
  o Impairs venous return
• Lack of skeletal muscle activity
  o Due to paralysis or forced inactivity, e.g. long-term bed rest, soldiers on guard
• Reduced circulating blood volume, e.g. Haemorrhage
• Increased core temperature
  o Peripheral vasodilation, less blood volume available, e.g. standing up after the bath

Question 12

initial effect of microgravity (space) on cardiovascular system (1)

Answer 12

• Initially
  o Blood is not pooling in the feet and returning to the heart easily, not fighting against gravity- Starling’s Law kicks in.
  o Increases atria/ventricle volume and so increased preload and cardiac output -this is sensed by cardiac mechanoreceptors leading to a reduction in sympathetic activity
  o This reduces ADH and increases atrial natriuretic peptide ANP
  o There is increased glomerular filtration rate (GFR)
  o Reduced RAAS
  o An overall reduction in blood volume (BV) by 20% because reduction in ADH and increase in ANP causes the kidneys to get rid of more sodium, so less water is recovered

Question 13

long term effect of microgravity (space) on cardiovascular system (2)

Answer 13

• Long term

o Less BV, reduces stress on heart, heart reduces in muscle mass, general drop in BP

Question 14

effect of microgravity (space) on cardiovascular system - on return to gravity (3)

Answer 14

• On return to gravity
  o Severe postural hypotension, due to much smaller heart and lower BV
  o Baroreceptor reflex cannot compensate
  -takes a while for BV and everything else to return to normal

Question 15

different types of exercise and the cardiovascular response

Answer 15

Dynamic
- Constantly shortening and relaxing with lots of different muscle groups involved

Static
- One specific muscle group is being worked without constant movement

Coordinated response to exercise

• Integrated by central command in the brain
• Just anticipation of exercise will cause some of the changes to be initiated
• Once exercise commences, there is feedback from the muscles via mechanoreceptors and metaboreceptors
• They are all going to increase sympathetic activity and reduce vagus inhibition

Question 16

main effects of exercise

Answer 16

• Increase lung oxygen uptake
  o Transport around the body and supply to exercising muscle
• Control BP
  o Despite huge changes in CO and resistance (protect heart from excessive afterload which will reduce CO)
• Need to selectively target areas where the oxygen is delivered so co-ordinated dilation/constriction of vascular beds (some vessels will constrict and some will dilate)

Question 17

what is the importance of integrating several small adaptations? give an example

Answer 17

it can create an overall large response to exercise

for example, O2 uptake by pulmonary circulation can increase 10-15x during strenuous exercise

this very big change is brought about by the integration of three smaller changes:
HR increases 3x (60-180 bpm)
SV increases 1.5x (70-120 ml)
Arteriovenous O2 difference 3x (gradient + Bohr effect)

Question 18

during mild exercise does blood flow change?

Answer 18

not that much

Question 19

as exercise becomes more extreme what happens

Answer 19

there is a limit to much o2 we can give and take at the tissues so the arteriovenous oxygen difference reaches a plateau

Question 20

why is there a point in exercise when an extreme HR doesn’t help SV?

Answer 20

eventually as HR continues to increase, SV decreases

partly due to decrease in Starling’s Law, because when the HR is very high, there is not as much time for filling of the heart

also because overfilling of the heart can lead to a decrease in CO

Question 21

explain how exercise induced tachycardia comes about

Answer 21

• Brain central command (ready for exercise) & muscle mechanoreceptors which feedback to brain causing vasodilation of vessels in muscles to increase blood flow
• Decrease in vagal tone and increase in sympathetic activity, both of which increase HR and contraction

Question 22

how is SV effected during exercise?

Answer 22

• Increased sympathetic activity
• Increased End-Diastolic Volume
  o Increase in venous return/CVP through veno-constriction
  o Increase in sympathetic activity & calf muscle pump
  o Increasing the EDV meaning constricting the sympathetic tone in legs so more blood is sent back to the heart : activates Starling law which increases preload
• Faster ejection
  o Increase in contractility by sympathetic activation of β1 receptors (inotropic increase in Ca2+)
• Decreased end-systolic volume
  o (increase in ejection fraction)
  o Accounts for increase in SV
  o Increase in contractility by sympathetic activation of β1 receptors & Starling’s law

Question 23

what happens in the leg muscle during leg exercise?

Answer 23

• fall in local resistance due to local metabolic hyperaemia vasodilation - local dilation because this vasodilation affects the vessels involved in exercise because the low pH and high CO2 levels are detected
• how does the vasodilation occur? local sympathetic response and β2-mediated vasodilation via circulating adrenaline

    o	β2 receptor expression is high in skeletal muscle and coronary artery

Question 24

what is the effect of increased CO on BP?

Answer 24

BP = CO x TPR
relatively small increase in mean BP due to dilated skeletal muscle arterioles decreasing TPR
(large increase in CO, large decrease in TPR)

Question 25

explain the importance of compensatory vasoconstriction of non-essential circulations

Answer 25

• prevents hypotension due to exercise-induced decreased TPR
• occurs in inactive/unrequired tissues in order to prevent BP from falling
• controlled by RVLM, which controls specific pre-ganglionic sympathetic nerves in the spinal cord which sends out post-ganglionic nerves to specific tissues
• STABILISES BP

Question 26

does static exercise or dynamic exercise raise BP more?

Answer 26

static

Question 27

does static exercise or dynamic exercise raise HR more?

Answer 27

dynamic

Question 28

explain why static exercise raises BP so much?

Answer 28

There is a constant contraction of a small number of muscles (high load, eg. in weight lifting)

Less of a decrease in HR (because there is a large afterload)

Huge increase in BP- muscles are contracted but static- they aren’t loading and unloading

In those muscles metabolism is happening, local low pH and therefore local vasodilation, but the rest of the circulation wants to keep a high BP to keep blood pushing through those contracting muscles

Question 29

explain why dynamic exercise raises HR so much and has less of an effect on BP?

Answer 29

o Shortening of many muscles
o Low load
o E.g. running
o not much BP change but a big change in HR- all about reducing peripheral resistance.

Question 30

what is selective metabolic hyperaemia?

Answer 30

when local factors in the muscle dilate vessels

Question 31

what are metaboreceptors?

Answer 31

small diameter sensory fibres in skeletal muscle, and they are chemosensitive ie. stimulated by K+, H+, which increase in exercising muscle

they have reflex effects, including: tachycardia (via increased sympathetic activity), increased BP, “Pressor Response” to exercise

o Especially important during isometric exercise (increased muscle load)
o Raised BP maintains blood flow to contracted muscle to try to force blood into the contracted muscle
o Contracted muscle supplied by dilated resistance vessels due to metabolism