L42: Generation And Control Of Arterial Pressure

Question 1

Mean arterial blood pressure

Answer 1

MABP = Cardiac output (入: HR x SV) x Total peripheral resistance (出)

Estimation: MABP = diastolic BP + 1/3 pulse pressure

↑ SV —> ↑ flow from heart to artery
↑ TPR —> ↓ flow from artery to capillary
↑ HR —> ↓ time available for blood to drain out from arterial system
—> ALL ↑ volume in artery
—> ↑ stretch of wall
—> more PE transferred to wall
—> ↑ pressure

Question 2

Grade of BP and Target BP

Answer 2

Normal: 120/80
Prehypertension: >130/85
Grade 1 Hypertension: >140/90
Grade 2 Hypertension: >160/100

Target BP:
Healthy:
<60 years: <140/90
>=60 years: <150/90
Diabetes/CKD: <140/90

Question 3

Pressure-volume in artery curve

Answer 3

Compliance = 1/Slope (slope ↑, compliance ↓)

Normal arterial volume —> only elastin fibres stretched —> high compliance —> small slope
Large arterial volume —> collagen fibres (less distensible) also stretched —> low compliance —> large slope

Aging —> elastin fibres broken into shorter lengths —> collagen fibres are stretched at a low volume —> arterial compliance ↓ —> larger pulse pressure

Question 4

Increase in HR / no change in SV

Answer 4

↑ HR —> ↓ time for run off —> ↑ volume in artery —> ↑ MABP

↑ systolic volume = ↑ diastolic volume

↑ systolic BP > ↑ diastolic BP (since upper part of curve is steeper)

Question 5

Increase in SV / no change in HR

Answer 5

↑ SV —> ↑ systolic volume mainly (diastolic volume also ↑ due to less time for large systolic volume to run off)

↑ systolic volume > ↑ diastolic volume

↑ systolic BP&raquo_space; ↑ diastolic volume —> ↑ pulse pressure

Question 6

Increased total peripheral resistance

Answer 6

↑ TPR —> ↓ time for blood to run off from artery to capillary —> ↑ diastolic arterial volume —> ↑ diastolic pressure

venous return ↓ due to less run off from artery —> ↓ SV —> very little ↑ systolic volume —> unchanged systolic pressure

unchanged systolic pressure + ↑ diastolic pressure —> ↓ pulse pressure

Question 7

Systolic and Diastolic pressure depends on

Answer 7

Systolic: CO (SV and HR)
Diastolic: TPR

Question 8

Baroreflex (Baroreceptor)

Answer 8

Baroreceptor at Carotid sinus + Aortic arch

• responsible for REFLEX correction to ACUTE changes in BP
• long term regulation —> Volume receptors

↑ BP —> ↑ transmural pressure —> ↑stretch of wall
—> stimulate baroreceptor —> depolarisation and firing of nerve impulses
—> carried by vagus nerve (aortic arch) + glossopharyngeal nerve (carotid sinus)
—> nucleus tractus solitaries (medulla) —> CV centre (medulla):

1. ↓ sympathetic tone, ↑ parasympathetic tone on heart —> ↓ HR, ↓ force —> ↓ CO —> ↓ BP
2. ↓ sympathetic tone on arterioles (no parasympathetic nerve) —> Vasodilation —> ↓ TPR —> ↓ BP
3. ↓ sympathetic tones on veins —> ↑ compliance —> ↓ venous pressure —> ↓ venous return —> ↓ SV —> ↓ CO —> ↓ BP

Baroreceptor firing in proportion with BP within sensitive range:
Aortic arch: 80-200 mmHg
Carotid sinus baroreceptor: 50-180 mmHg
Below sensitive range: Chemoreceptors take over to detect low O2 level

Dynamic response vs Static response by baroreceptor:
Dynamic: burst of firing during anacrotic limb of arterial pressure wave
Static: number of impulse proportional to mean pressure

Question 9

Physiological control of BP

Answer 9

Higher regions of brain (Limbic system, hypothalamus) can suppress normal reflexes of CVS —> allow elevation of BP beyond set point by baroreceptor (e.g. exercise)

Vasovagal syncope:
- sudden withdrawal of sympathetic NE tone on vessels (↓TPR)
- sudden increase in vagal tone on heart (↓CO, bradycardia)
—> ↓ BP
—> cerebral P drops < 40mmHg —> cerebral flow ↓ —> faint