Lecture 18: CV control 2

Question 1

What happens if isotonic saline is given?

Answer 1

It stays in the ECF because there is no osmolality difference AND Na/K ATPase will keep Na in the ECF

Question 2

What happens if hypotonic saline is given?

Answer 2

• Hypotonic saline will reduce the osmolality of that compartment and cause water to move into the ICF. At equilibrium the osmolality of both compartments will be reduced (compared to normal) and volume will be increased of both.

Question 3

Whats the big picture consequence of ECF volume change?

Answer 3

ECF volume changes will alter plasma volumes and thus overall blood volume changing cardiac performance.

Question 4

What determines cell volume?

Answer 4

• Cell volume is determined by Na homeostasis and water balance in the EC compartment

Question 5

What else does osmoreceptor activation lead to?

Answer 5

• Changes in ADH
• Stimulates brain thirst centres.

Question 6

Whats the effects of the cardiothoracic sensors?

Answer 6

Reduced filling =
- Increased SNS to heart and vessels
- Increased adrenal catecholamine release
- Increased SNS to kidney = increased RAAS
- Increased ADH release

Question 7

What stimulate ADH release?

Answer 7

• Osmoreceptors
• Cardiac filing receptors
• ANG2

Question 8

Describe what happens when theres blood loss:

Answer 8

• Decreased cardiac filling -. Inc SNS, reduced vagal activity, increased adrenal catech, increased renal SNS -> RAAS and increased ADH.
• If BP falls the baroreflex also kicks in.

Multiple lines of defence.

Question 9

Describe how the body can shift fluid from intersitital space to plasma:

Answer 9

• Reflexive pre-cap vessel contraction reduces capillary hydrostatic pressure and shifts the balance between cap hydrostatic pressure and oncotic pressure in favour of fluid reabsorption.

Skeletal muscle has lots of interstitial fluid and up to 800mls can be added to plasma from this pathway.

Question 10

How can ECF volume be restored?

Answer 10

THe increased ADH, ANG2, Aldo favour water and Na retention from the kidneys and thirst centres of brain. Then balance between retention and output determines time.

ADH happens quickly,RAAS takes a while. 12-72hrs to restore lost volume

Question 11

In an acute blood loss describe the time profile to restore its constituents:

Answer 11

• Water and electrolytes: hours to days
• RBC: 4-8 weeks
• Plasma proteins from liver: 3-4 days

Question 12

Does a blood loss of less than 10% impact BP?

Answer 12

• No, pulse pressure will drop though.
• Non-hypotensive heamorrhage
• Greater than 10% blood loss will decline art. pressure = hypotensive heamorrhage

Question 13

Describe the change in baroreceptor and cardiac filling receptor firing when blood loss is less than 10%:

Answer 13

• Baroreceptors dont change firing
• Cardiac filling receptors fire significantly less = Inc ALDO and ADH, HR and inotropy.

Question 14

Describe the change in baroreceptor and cardiac filling receptor firing when blood loss is GREATER than 10%:

Answer 14

• Baroreceptor firing rate declines gradually
  = Enhanced SNS, ADH, ALDO, Constriction, HR, Inotropy

Hemorrhagic shock can occur (pressure drops so low than vessels dilate because of ischeamia)

Question 15

Describe the CV response to altered posture:

Answer 15

• Similar adjustments to those seen in non-hypotensive heamorrhage

i.e prolonged lying down or going to space.

-> 300-500ml shifts from thoracic space to venous reserve when lying down. Reducing BV, so HR and inotropy increase to maintain CO. TPR is also increased (Vasocon)
-> These effects are abolished when a compression suit is worn. = These effects are largely caused by changes in BV.

BUT baroreceptors are very important in transient postural changes.

Question 16

What happens to CV regulation during exercise?

Answer 16

Fast mediated neural responses are vastly changed b/c of higher centre inputs.

-> Increase arterial pressure, HR, ventilation
-> Exercise pressor reflex

i.e central command and exercise pressor reflex facilitate changed CV responses in exercise associated with static and dynamic exercise

Question 17

How is the CV response modulated in dynamic exercise?

Answer 17

Dynamic i.e rhythmic exercise CV response is characterised by:
- Decreased TPR
- Increased CO
- Pulse pressure markedly increased
- MABP increases to some extent

Question 18

How is the CV response modulated in static exercise?

Answer 18

• i.e isometric contraction
• Does NOT ellicit decreased TPR
• CO does increase
• Substantial increase in MABP, HR and inotropy. i.e reflex control of arterial pressure is overridden.

-> HR and CO graded to intensity of isometric exercise and substantial increase in MAPB i.e baroreflex is reset significantly (less so in dynamic exercise)

Question 19

In summary what is the role of baroreceptors?

Answer 19

Buffer short term changes in BP

Question 20

In summary what is the role of cardiac filling receptors?

Answer 20

Powerful integrated response to changes in BV

Cant sense rapid fluctuations unlike baroreceptors.

Question 21

Describe the structural remodelling that can occur in changed of arterial pressure:

Answer 21

• Extensive cardiac and vessel remodelling in chronic hypertension to protect from high BP
  i.e Pre-cap SM hypertrophy, reducing BV lumen = Increased resistance and reduced distensibility. Same for cardiac chambers. = These actually lead to an decreased in baroreceptor and cardiac volume receptors. (structural resetting)

If renal system remodelled this will changed JGA sensitivity and actually this may cause hypertension itself.