Regulation of BP

Question 1

Heart and BVs integrated and controlled - required to maintain tissue perfusion across whole of the body - this is to…

Answer 1

• To keep a relatively constant arterial BP
  Too low -> blood flow to organs would fail
  Too high -> damage to vessels and organs
• To control distribution of total CO
  5L/min not sufficient to perfuse entire body
  Needs to respond to tissue demands
  Satisfied by local control mechanisms

Question 2

To what extent can nervous control influence arterial pressure?

Answer 2

Can increase to 2x normal within 5-10sec

Can decrease to 50% normal within 10-40s

Question 3

Reflex control of BP - describe fundamental components of a reflex control system

Answer 3

Fundamental components of a reflex control system:

• Internal variable to be maintained
• Receptors sensitive to change in variable
• Afferent pathways form receptors
• An integrating centre for afferent inputs
• Efferent pathways from integrated centre
• Target effectors that alter their activities

mean art BP = CO x TPR

Question 4

What are baroreceptors and their function?

Answer 4

Found in carotid artery (afferent fibres follow glossopharyngeal nerve) and the aortic arch (afferent fibres follow vagus nerve)
They are stretch receptors and respond to changes in arterial BP by sensing change in tension of arterial wall - fast response

Firing rate increases when BP inc and decreases when BP dec
Sensitive around a set point
Primary purpose is to reduce the minute to minute variations or arterial pulse

Question 5

What are cardiopulmonary baroreceptors?

Answer 5

Low pressure receptors that sense central blood volume - atria, ventricles, veins and pulmonary vessels
If rate of baroreceptor firing decreases (signalling decreased blood volume) then symp nerve activity to heart and BVs increase and parasymp activity to heart decreases

Question 6

What is medullary cardiovascular control (MCVC) ‘vasomotor’ area?

Answer 6

• Sensory area - input from baroreceptors
• Lateral portion - efferent symp nerves
• Medial portion - efferent parasymp (vagal) nerves

Question 7

Describe symp and parasymp effects on heart and BVs

Answer 7

Symp and parasymp effect the heart - both control HR and normally function simultaneously - at rest parasymp is predominate tone and symp can significantly effect SV and rate

Symp effects on BVs - continuous low level tone affects TPR - sympathetic vasoconstrictor tone exerts vasomotor tone on vessels, therefore kept partially constricted
Veins also innervated by symp - decreases capacitance -> increased venous return -> increased SV -> inc CO

Question 8

What is CNS ischaemic response?

Answer 8

Emergency pressure control system - last ditch stand
When blood flow to CVCC is decreased - increased peripheral vasoconstriction (almost completely occludes some peripheral vessels), increased symp stimulation of heart, increased systemic arterial pressure

Question 9

Fine control of blood flow

Answer 9

Local control superimposed on organ distribution of CO - tissues auto-regulate blood flow - build up of local factors (i.e. adenosine) and independent of innervation/hormonal control

Intrinsic ability to maintain blood flow safely across capillaries if Bp increases - myogenic theory (acute auto-regulation) - stretch induced vascular depolarisation of SM due to increased arterial pressure

Remember not all capillaries in an organ perfuse simultaneously

Question 10

Summarise TPR - influencing factors

Answer 10

Influenced by:

• Arteriolar radius - neural controls, hormonal controls, local controls
• Blood viscosity - hematocrit

Question 11

How is BP regulated long-term?

Answer 11

Control of body fluid volume by kidneys - when arterial pressure increases, urine production increases and vice versa

The two primary determinants are:

• Renal output curve for salt and water
• The level of salt and water intake

Question 12

What is ADH released in response to?

Answer 12

Released by the pituitary gland in response to:

• Increased osmotic pressure - hypothalamic osmoreceptors
• Hypovolemia - atrial baroreceptors normally inhibit ADH release, decreased volume leads to decreased firing rate and increased ADH release
• Hypotension - decreased areyrial baroreceptor firing, increased symp activity and increased ADH release
• Angiotensin II

Question 13

How does ADH work?

Answer 13

Increases blood volume by increasing water permeability in renal collecting ducts - decreased urine production

In severe hypovolemic shock ADH release is high, causes vasoconstriction, increases TPR

Question 14

What is renin and what is it released in response to?

Answer 14

Enzyme released from kidneys in response to:

• Symp nerve activation - mediated by baroreceptor feedback
• Renal artery hypotension - independent of baroreceptor feedback
• Decreased sodium in kidney distal tubules

Question 15

What are the effects of Angiotensin II (product of renin)?

Answer 15

• Acts on resistance vessels - increases TPR
• Acts directly on kidneys - constricts renal arteries and therefore decreases blood flow via kidneys
• Causes release of aldosterone from adrenal glands - increased sodium and water reabsorption
• Stimulates release of ADH from pituitary

Question 16

What is ANP and what is it released in response to?

Answer 16

Atrial natruiretic peptide/hormone is a peptide stored in muscle cells of atria

• Released in response to stretch of atria
• helps oppose effects of RAAS system

May help counteract volume overload

Question 17

What are other factors affecting BP control?

Answer 17

• Cortex - conscious effects on emotions - nerves from cortex to medullary CVC centre
• Time of day - diurnal variations due to hormones and cortical input
• Respiration - via mechanical movements, via chemoreceptors (aortic and carotid bodies detect changes in po2, if decreased po2 then rate of firing increases)