Blood pressure and its control

Question 1

Blood pressure measurement

Answer 1

MAP = (P systolic - P diastolic)/3 + P diastolic

MAP is a time weighted average of blood pressure

Question 2

What causes Korotkoff sounds?

Answer 2

Laminar flow if efficient
Turbulence occurs when blood velocity too high for diameter of vessel e.g. atherosclerotic plaque
Turbulent flow is inefficient
Generates noise

Question 3

Reasons for variability of blood pressure

Answer 3

Age
Time of day
Gender
White coat syndrome
Fitness
Body weight

Question 4

Physical factors affecting MAP

Answer 4

Flow from the heart
Resistance to flow
Pressure in the veins
Therefore MAP= CO x TPR

Question 5

Influence of cardiac output on MAP

Answer 5

Increased flow rate through a vessel of fixed diameter increases pressure

Question 6

Influence of resistance on MAP

Answer 6

Resistance to flow = viscosity x length / r^4

Radius of a blood vessel is the main determinant of flow/ resistance to flow

Question 7

Short term control of blood pressure

Answer 7

Reflex control regulated by the autonomic nervous system
Heart rate (chronotropy)
Force of contraction (inotropy)
Contraction/ relaxation of blood vessels

Question 8

Longer term control of blood pressure

Answer 8

Endocrine control of fluid balance
Increased/decreased diuresis
Increased/decreased thirst

Question 9

Arterial baroreceptors

Answer 9

Pressure sensitive receptors that respond to stretch
Stretch increases the frequency of firing
Static sensitivity: respond to change in pressure
Dynamic sensitivity: respond to change in pressure

Question 10

Baroreceptor sensitivity- central resetting

Answer 10

Exercise- work sensors in skeletal muscle cause resetting of baroreflex to a higher pressure, allows pressure to rise without impairing mechanisms of increase cardiac output

Question 11

Baroreceptor sensitivity- peripheral resetting

Answer 11

Threshold for baroreceptors resets to higher pressure after a few days, ensures best sensitivity to changes in BP, downside is unreliable information about BP

Question 12

Baroreceptor sensitivity- structural changes

Answer 12

If arterial walls become less compliant intraluminal pressure causes less stretch therefore baroreceptors are not as stretchy anymore
Causes: old age, hypertension

Question 13

Other types of sensor involved in control

Answer 13

Myelinated veno-atrial mechanoreceptors- sense central blood volume, cause reflex tachycardia and diuresis, redistribution of blood from veins
Non-myelinated mechanoreceptrs- activity weak unless heart distended, reflex is bradycardia and peripheral vasodilation
Coronary artery baroreceptors- function like arterial baroreceptors
Chemosensors- respond to ischaemic metabolites, produce sympathetic activation and rise in BP

Question 14

Responses to hypotension

Answer 14

Acute hypotension leads to decreased baroreceptor traffic, CNS control in the medulla/hypothalamus decrease PNS and increase SNS
Vasoconstriction, venoconstriction, increased force of contraction and increased heart rate

Question 15

Long term control of BP- regulation of plasma volume

Answer 15

Renin angiotensin system
Atrial natriuretic peptide
Vasopressin (increases blood volume)
Thirst

Question 16

Vasopressin

Answer 16

Release controlled by osmoreceptors and baroreceptors
Stimulated by increase in osmolarity and fall in BP
Reduces water excretion from the kidneys
SUpports blood pressure during hypovolaemia

Question 17

Atrial natriuretic peptide (ANP)

Answer 17

Secreted from atria in response to stretch
Increases renal salt and water excretion
Causes a shift from plasma to interstitial compartment

Question 18

Thirst

Answer 18

Osmosis out of cells into the circulation stimulated thirst

Increased water intake increases blood volume

Question 19

Specific clinical example- shock

Answer 19

Definition- pathologic failure of tissue perfusion
Causes: hypovolemic shock, septic shock, cardiogenic shock, anaphylactic shock
Symptoms: pale, cold and sweaty skin, pulse rapid and weak, pulse pressure reduced, breathing rapid and shallow, urine output reduced, mental confusion/loss of consciousness

Question 20

Hypovolemic shock

Answer 20

0-20%: no change in MAP, no need for clinical intervention
20-30%: possible fall in MAP, treatment required
30-40%: 50-70mmHg fall in MAP, severe possible non-recoverable

Question 21

Physiological compensation for shock (short term)

Answer 21

Sympathetic nervous activation
Peripheral vasoconstriction, venoconstriction, bradycardia
Increase in vascular resistance, increase in circulating volume, increase in cardiac output
All leads to increased blood pressure

Question 22

Physiological compensation for shock (long term)

Answer 22

Increase in blood volume by increasing angiotensin/aldosterone, increasing ADH, decreasing ANP, increasing thirst, increasing erythropoetin