Regulation of BP

Question 1

What is TPR?

How is it related to MAP?

Give the formula

Answer 1

Total Peripheral Resistance is the sum of the resistance of all peripheral vasculature in the systemic circulation

MAP and TPR are directly correlated

MAP = CO x TPR

MAP = SV x HR x TPR

Question 2

How much does each type of blood vessel contribute to TPR?

Answer 2

TPR proportions

• Arteries: 20%
• Arterioles: 50%
• Capillaries: 20%
• Veins 10%

Question 3

How is TPR regulated?

Answer 3

TPR is regulated by vascular smooth muscles

Contraction of vascular smooth muscles causes vasoconstriction and

• increases TPR
• increases MAP

Relaxation of vascular smooth muscles causes vasodilatation and

• decreases TPR
• decreases MAP

Question 4

What kind of innervation regulates the vascular smooth muscle? What are the exceptions?

Which neurotransmitter is used?

What are the receptors?

Answer 4

Vascular smooth muscles are supplied by sympathetic nerve fibers (ANS).

• There is no significant parasympathetic innervation of arterial smooth muscles - exceptions are penis and clitoris

The neurotransmitter is noradrenaline acting on α-adrenergic receptors

Question 5

What kind of neurotransmitter receptors are found in vascular smooth muscle vs cardiac muscle?

Answer 5

α-adrenergic receptors expressed in smooth muscle vs

β-adrenergic receptors expressed in cardiac muscle

• so beta blockers don’t have any impact on smooth muscle

Question 6

What is the state of vascular smooth muscles at rest? What is this called?

What causes it?

Answer 6

Vascular smooth muscles are partially constricted at rest. This is called the vasomotor tone

The vasomotor tone is caused by tonic discharge of sympathetic nerves resulting in continuous release of noradrenaline

Question 7

What is the upper limit of normal BP?

What is the normal MAP range?

What is the minimum MAP needed? Why must it be regulated within a narrow range?

Answer 7

Upper limits for normal BP

• 140 mm Hg: systolic
• 90 mm Hg: diastolic

“Normal” range of mean arterial blood pressure (MAP) is ~ 70-105 mmHg

MAP of at least 60 mmHg is needed to perfuse the

• coronary arteries,
• brain and
• kidneys

MAP must be regulated within a narrow rangeto ensure:

• Pressure is high enough to perfuse internal organs AND
• Pressure is not too high to
  
  • damage the blood vessels or
  • place an extra strain on the heart (afterload)

Question 8

What is a means of regulating MAP?

Name it and describe the kind of regulation involved

Answer 8

The Baroreceptor Reflex is a means of regulating MAP

It is important in moment-to-moment/immediate ST regulation of arterial blood pressure

• including changes in pressure due to postural changes

Question 9

Where are baroreceptors located?

How do they monitor BP?

What kind of BP is monitored at each these locations?

Answer 9

Baroreceptors are located in 2 sites -

• Carotid baroreceptors: just above the carotid sinus
• Aortic baroreceptors: at the arch of the aorta

They monitor BP by monitoring the amount of stretch in the blood vessel walls

Carotid baroreceptors monitor the pressure of blood going up towards CNS (the brain)

Aortic baroreceptors monitor the pressure of blood going to the systemic circulation

Question 10

What kind of feedback mechanism do barareceptors use?

What does this ensure?

List the components involved

Answer 10

The baroreceptor reflex uses a negative feedback mechanism

This acts to minimize any disturbance to the controlled variable (MAP)

Components:

1. Pressure sensors in the baroreceptors send messages about MAP to
2. The Medulla (lowest part of the brainstem), which is the Cardiovascular Integrating Centre. Medulla sends messages to the effectors:
3. the Heart (to influence HR & SV) the Blood Vessels (to influence TPR: vasodilation/vasoconstriction)

Question 11

List the baroreceptor reflex response to decreased ABP

Answer 11

Decreased ABP =

1. Baroreceptor discharge falls
2. CV Integrating Centre (Medulla) effects:
   
   1. Increase in Cardiac Sympathetic Activity
      
      1. HR increases
      2. SV increases
      3. = CO increases
   2. Increase in Vascular Sympathetic Constrictor Tone
      
      1. venoconstriction = SV increases
      2. vasoconstriction
      3. = TPR increases
   3. Decrease in Vagal Activity (parasympathetic)
      
      1. HR increases (no effect on SV)
      2. = CO increases
3. MAP (ABP) increases: MAP = CO x TPR

Question 12

List the barareceptor reflex response to increased BP

Answer 12

Increased ABP =

1. Baroreceptor discharge increases
2. CV Integrating Centre (Medulla) effects:
   
   1. Decrease in Cardiac Sympathetic Activity (NB. NOT increase in parasympathetics here, that is the vagus below)
      
      1. HR decreases
      2. SV decreases
      3. = CO decreases
3. Decrease in Vascular Sympathetic Constrictor Tone
   
   1. venodilation = SV decreases
   2. vasodilation
   3. = TPR decreases
4. Increase in Vagal Activity (parasympathetic)
   
   1. HR decreases (no effect on SV)
   2. = CO decreases
5. MAP (ABP) decreases: MAP = CO x TPR

Question 13

What kind of information about blood pressure can baroreceptors provide/not provide?

Why can’t they provide the latter kind of information about blood pressure?

Answer 13

Baroreceptors only provide information about acute changes in blood pressure e.g. change in posture from lying to standing - they cannot provide information about prevailing steady state blood pressure.

This is because baroreceptors “re-set” - their firing decreases if high blood pressure is sustained. They will fire again only if there is an acute change in MAP above the newhigher steady state level

Question 14

How is prevailing stready state/LT MAP controlled?

Answer 14

Control of MAP in the longer-termis largely by control of blood volume, though controlling Plasma volume in ECFV (Plasma Volume + Interstitial Fluid Volume)

If plasma volume falls, compensatory mechanisms shift fluid from the interstitial compartment to the plasma compartment

PV changes = changes in steady state blood Volume & MAP

Question 15

What % body weight is water for men/women?

Of this %, what % is ICF/ECF?

What are the components of ECF?

Answer 15

• 60% body weight of males is water (42 L for 70 kg male)
• 50% of body weight of females is water: more adipose tissue than males, which does not contain water
• 2/3rd of body water = ICF
• 1/3rd of body water = ECF

ECFV = IFV (insterstitial fluid volume) + PV (plasma volume)

• Plasma volume: contained within blood vessels
• Interstitial fluid volume: found outside blood vessels, surrounds the cells – the aqueous environment of cells in the body

Question 16

What are the factors affecting ECFV?

How is ECFV regulated (LT MAP regulation)?

Name the specific regulators involved

Answer 16

Two main factors affect ECFV:

1. Water excess or deficit
2. Na+ excess or deficit

ECFV (and hence blood volume and LT regulation of MAP) is regulated via the endocrine system, hormones, by regulating the water and salt (Na+) balance in our bodies.

• Aldosterone regulates Na+ level in the RAAS (Renin-Angiotensin-Aldoesterone) system
• Anti Diuretic Hormone/Vasopressin regulates water levels