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Flashcards in Blood pressure and its control Deck (22):

Blood pressure measurement

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

MAP is a time weighted average of blood pressure


What causes Korotkoff sounds?

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


Reasons for variability of blood pressure

Time of day
White coat syndrome
Body weight


Physical factors affecting MAP

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


Influence of cardiac output on MAP

Increased flow rate through a vessel of fixed diameter increases pressure


Influence of resistance on MAP

Resistance to flow = viscosity x length / r^4
Radius of a blood vessel is the main determinant of flow/ resistance to flow


Short term control of blood pressure

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


Longer term control of blood pressure

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


Arterial baroreceptors

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


Baroreceptor sensitivity- central resetting

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


Baroreceptor sensitivity- peripheral resetting

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


Baroreceptor sensitivity- structural changes

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


Other types of sensor involved in control

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


Responses to hypotension

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


Long term control of BP- regulation of plasma volume

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



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


Atrial natriuretic peptide (ANP)

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



Osmosis out of cells into the circulation stimulated thirst
Increased water intake increases blood volume


Specific clinical example- shock

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


Hypovolemic shock

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


Physiological compensation for shock (short term)

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


Physiological compensation for shock (long term)

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