Control of Arterial Blood Pressure Flashcards
define blood pressure
the outwards (hydrostatic) pressure exerted by blood on blood vessel walls
define systolic blood pressure
the pressure exerted by the blood on the walls of the aorta and systemic arteries when the heart contracts
define diastolic blood pressure
the pressure exerted by the blood on the walls of the aorta and systemic arteries when the heart relaxes
define pulse pressure
difference between systolic and diastolic blood pressures
normal = 30-50 mm Hg
how is mean arterial blood pressure (MAP) measured
the average arterial blood pressure during a single cardiac cycle, which involves contraction and relaxation
[(2 x diastolic pressure) + systolic pressure]/3
what constitutes normal systolic arterial blood pressure
should not reach or exceed 140 mmHg under resting conditions
what constitutes normal diastolic arterial blood pressure
should not reach or exceed 90 mmHg under resting conditions
what constitutes normal mean arterial blood pressure
70-105 mm Hg
MAP of at least 60 mmHg is needed to perfuse coronary arteries, brain and kidneys
describe the physiologic basis of indirect blood pressure measurement using cuff sphygmomanometer and stethoscope
blood flows in normal arteries in a laminar fashion
such laminar flow is not audible through stethoscope
if external pressure (cuff pressure - sphygmomanometer) exceeding the systolic pressure is applied to an artery, the flow in that artery would be blocked and no sound is heard through stethoscope
if external pressure is kept between systolic and diastolic pressure, the flow becomes turbulent whenever blood pressure exceeds cuff pressure and can be heard through a stethoscope - korotkoff sounds;
1. first sound is heard at peak systolic pressure => systolic pressure recorded
2,3 . intermittent sounds are heard as blood pressure due to turbulent spurts of flow cyclically exceeds cuff pressure
4. last sound is heard at minimum/diastolic pressure
5. blood returns to laminar flow - no sounds => diastolic pressure recorded
explain the need for regulation of mean arterial blood pressure
a pressure gradient between the aorta and right atrium drives blood around systemic circulation - the right atrium pressure is close to 0 and so the main driving force for blood flow is mean arterial blood pressure => ensure pressure is high enough to perfuse internal organs (brain, heart, kidneys)
also makes sure pressure is not too high to damage the blood vessels or place extra strain on the heart
what is the prevailing mean arterial blood pressure the product of
MAP = cardiac output x systemic vascular resistance (total peripheral resistance)
cardiac output = stroke volume x heart rate
systemic vascular resistance - sum of resistance of all vasculature in the systemic circulation
describe the main resistance vessels
arterioles
describe the baroreceptor reflex
negative feedback system - minimises any disturbance to controlled variable - ONLY short term regulation of MAP (baroreceptor firing decreases if high blood pressure sustained) including prevention of postural changes
baroreceptors (carotid - IX CN and aortic - X CN baroreceptors) = pressure sensors
medulla = control centre
heart and blood vessels = effectors
low arterial blood pressure = decrease baroreceptor discharge and sympathetic responses (decrease vagal activity and veno/vasocontriction)
high arterial blood pressure = increase baroreceptor and parasympathetic responses (increase vagal activity and veno/vasodilaiton)
describe the normal heart rate response to suddenly standing up from lying down position
the venous return to the heart decreases - effect of gravity
MAP very transiently decreases and reduces the rate of firing baroreceptors
vagal tone to the heart decreases and sympathetic tone to heart increases –> increasing heart rate and stroke volume
sympathetic constrictor tone increases - increasing systemic vascular resistance (arterioles main site for this) (this also results in slight increase in diastolic blood pressure)
sympathetic constrictor tone to veins increases the venous return to the heart and stoke volume
results in rapid correction of transient fall in MAP - heart rate increases, stroke volume increases and systemic vascular resistance increases
define postural hypotension
results from failure of baroreceptor responses to gravitational shifts in blood, when moving from horizontal to vertical position
indicated by a drop within 3 minutes of standing from lying position;
systolic blood pressure of at least 20 mmHg
diastolic blood pressure of at least 10 mmHg
symptoms;
cerebral hypo-perfusion - lightheadedness, dizziness, blurred vision, faintness and falls
