Control of Blood Pressure

Question 1

Why is blood pressure tightly controlled?

Answer 1

• Arterial bp driving force of tissue perfusion, without adequate pressure, no flow
• If bp inadequate, brain blood flow falls (brain intrinsic and auto-regulation), kidney filtration falls (pre-renal failure)
• If bp too high, pathology damage occurs in tissues, aneurysm formation- damage walls, damage to capillaries- small structures

Question 2

What are Ohm’s and Darcy’s Laws?

Answer 2

• Relationship between pressure gradient and flow
• Maintain pressure to maintain perfusion
• Flow varies proportionally with pressure

Question 3

Describe blood pressure

Answer 3

• Systolic- arterial pressure systole, peak pressure, ~120mmHg
• Diastolic- arterial pressure systole, drop, ~80mmHg
• Pulse pressure- difference between systolic and diastolic, ~40mmHg
• Mean arterial pressure- across entire cardiac cycle, ~93mmHg, longer in diastole than systole.

Question 4

Describe acute blood pressure control

Answer 4

• Continuous
• Baroreceptor reflex (sensory afferents, central relays, effector, efferents)
• Negative feedback

Question 5

Describe the baroreceptor reflex

Answer 5

• Sensory afferents- mechanoreceptors in walls of aortic arch + carotid sinus, changing in firing of sensory afferents
• Central relays- CV centres in brain stem- medulla oblongata
• Effector efferents- innervating heart and blood vessels

Question 6

What is the function of baroreceptors?

Answer 6

• Detecting stretch in aortic arch and carotid artery

- Fibres go to vagus nerve from aortic arch

Question 7

Describe A fibres

Answer 7

• Large diameter, fast conducting, myelinated nerves
• Low threshold- at normal pressures they are activated (30-90 mmHg)
• Rate of firing may vary

Question 8

Describe C fibres

Answer 8

• Narrow, sow-conducting, unmyelinated
• High threshold (70-140 mmHg)
• More abundant, ~25% activated at normal pressure
• Number activates increases as pressure rises

Question 9

What do baroreceptors sense?

Answer 9

• Sense rate of rise in pressure and magnitude of pressure
• Dynamic sensitivity
• Change in firing based on rate of change of pressure
• And magnitude of pressure (static sensitivity)
• As arterial pressure increases as does firing rate

Question 10

What is the cardiovascular control centre?

Answer 10

• Medulla- primary
• Regulates SNS and PNS (vagal) outflow to heart and vessels
• Nucleus tractus solitairius receives sensory input from different receptors (e.g. baro/chemoreceptors)
• Hypothalamus and higher centres modify activity of medullary centres
• More firing receptor afferents–> impacts NTS–> increases vagal response
• • Increase PNS outputs, decrease heart rate

Question 11

Describe the response to falling blood pressure

Answer 11

• Decreased baroreceptor firing
• Increase in SNS output decrease in PNS activity
• SNS-induced vaso/veno-constriction (increases venous return and so CO, increases TPR)
• Rapid increase in HR and force of contraction- increase in CO and resistance- increases arterial BP
• Sympathetic ionotropic- improves Ca efficiency (separate mechanism by which force of contraction also increases)
• Sympathetic Adr chonotropic- rate of contraction

Question 12

Describe response to rising blood pressure

Answer 12

• Increased firing of baroreceptors
• Decrease in SNS output, increase in PNS (vagal) activity
• Decrease in SNS- induced tone- leads to reduction in TPR
• Rapid decrease in HR and reduction in cardiac contractility, reducing CO
• • Reduction in CO and TPR leads to fall in MABP

Question 13

What other reflexes control blood pressure?

Answer 13

• Chemoreceptors in carotid bodies can also induce changes in pressure (pH and PaO2 sensors)
• Cardiopulmonary reflexes

Question 14

Describe peripheral chemoreceptors

Answer 14

• Carotid and aortic bodies primarily involved in control of breathing
• Elicit sympathetic activity (vasoconstriction and increased HR)
• Physiological role protects brain perfusion when arterial BP falls
• Minor role under normal circumstances, but important in severe HT

Question 15

Describe cardiopulmonary reflexes

Answer 15

• Sensory inputs from heart and lungs contribute to overall circulatory regulation
• Diverse group of receptors located mainly on low pressure side of circulation
• Elicit tone (continuous) reduction in HR and constriction
• Role in humans poorly understood
• Important in minimising BP changes in response to changes in blood vol

Question 16

Describe chronic BP control

Answer 16

• Baroreceptor control short-term

- Requires fluid regulation and RAS

Question 17

Describe hypertension

Answer 17

• BP is a continuous variable

- It varies with age, sex, race, birth weight

Question 18

What are the risk factors for hypertension?

Answer 18

• Age
• Sex
• Low birth weight
• African ancestry
• Low socio-economic/ educational background
• High salt diet
• Obesity
• Excess alcohol
• Stress/ anxiety