Control of Blood Pressure Flashcards
1
Q
Why is blood pressure tightly controlled?
A
- 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
2
Q
What are Ohm’s and Darcy’s Laws?
A
- Relationship between pressure gradient and flow
- Maintain pressure to maintain perfusion
- Flow varies proportionally with pressure
3
Q
Describe blood pressure
A
- 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.
4
Q
Describe acute blood pressure control
A
- Continuous
- Baroreceptor reflex (sensory afferents, central relays, effector, efferents)
- Negative feedback
5
Q
Describe the baroreceptor reflex
A
- 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
6
Q
What is the function of baroreceptors?
A
- Detecting stretch in aortic arch and carotid artery
- Fibres go to vagus nerve from aortic arch
7
Q
Describe A fibres
A
- Large diameter, fast conducting, myelinated nerves
- Low threshold- at normal pressures they are activated (30-90 mmHg)
- Rate of firing may vary
8
Q
Describe C fibres
A
- Narrow, sow-conducting, unmyelinated
- High threshold (70-140 mmHg)
- More abundant, ~25% activated at normal pressure
- Number activates increases as pressure rises
9
Q
What do baroreceptors sense?
A
- 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
10
Q
What is the cardiovascular control centre?
A
- 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
11
Q
Describe the response to falling blood pressure
A
- 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
12
Q
Describe response to rising blood pressure
A
- 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
13
Q
What other reflexes control blood pressure?
A
- Chemoreceptors in carotid bodies can also induce changes in pressure (pH and PaO2 sensors)
- Cardiopulmonary reflexes
14
Q
Describe peripheral chemoreceptors
A
- 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
15
Q
Describe cardiopulmonary reflexes
A
- 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
