4 The systemic arterial blood pressure Flashcards
Define systolic pressure
The BP in mmHg in an innate muscular artery during systole (brachial)
Define diastolic pressure
the BP in mmHg in an innate muscular artery during diastole (brachial)
Define pulse pressure
Difference in pressure
Systolic - Diastolic pressure
Describe mean arterial pressure
Mean arterial pressure (MAP)
- can be worked out using:
MAP = Diastolic pressure + [(SBP)/(DBP)]3
So, MAP is closer to the diastolic value
- as diastole lasts longer than systole at rest
- At high heart rates, however, MAP is closer to the arithmetic average of systolic and diastolic pressure (almost 100mmHg)
> This is due to the change in the shape of the arterial pressure pulse (narrower)
Describe the effects of decreased aortic distensibility on BP
A decrease in aortic distensibility leads to an increase in Systolic BP
- normally, the elastic aorta takes up KE from blood during systole + dampens the rise in pressure
- Inelastic aorta can cause systolic hypertension in the elderly
> Hard arteries reduce the baroceptor response
Describe the effects of increased heart rate on BP
- SVR = (MAP - CVP)/CO
or, rearranged MAP = (CO*SVR) + CVP
SO, an increase in HR will increase mean arterial pressure (due to lack of runoff)
- so, lower heart rate = increased runoff (lower BP)
- higher heart rate = decreased runoff (less time to go to diastolic pressure)
Describe the effects of systemic vascular resistance on BP
As SVR increases, MAP increases
- the resistance vessels are innervated by SNS nerve terminals, that cause vasoconstriction when active
- The SVR increase account for 80% of the compensatory response to a fall in the MAP under resting conditions
(so cardiac component contributes around 20%)
Describe ways in which the cardiovascular system monitors + maintains the arterial pressure
The C-V system includes 2 distinct pathways for monitoring + maintaining arterial pressure:
- Baroreceptor reflex - fast activating and helps compensate for short-term pressure changes
- Slow-activating system - manipulates mean arterial pressure through changes in circulating blood volume by modifying renal function
Describe the 2 baroreceptor groups
2 groups providing the ‘integrator’
- which is located in the medulla with information about pressure and flow in the C-V system
- High-pressure baroreceptors in carotid sinus + aortic arch
- Low-pressure cardiopulmonary receptors
Describe the carotid and aortic baroreceptors
They are the primary means of detecting changes in MAP
- They monitor pressure indirectly by responding to arterial wall stretch
- They are bare sensory receptors buried in the elastic layers of the aorta, carotid sinus
- Information from the sinus travels back to the brain (afferent limb)
o Via the glossopharyngeal nerve (CN IX)
- Information from the aortic arch travels back on CN X – the vagus nerve
Describe the function of baroreceptors (and how they carry this out)
In the absence of stretch, baroceptors are inactive
- As MAP increases, nerve endings are stretched – leading to a graded potential
> if deformation (stretch) is high = ACTION POTENTIAL triggers
The higher the pressure the higher the frequency.
(This level then drops during diastole when pressure is lower)
To enable a more nuanced response, stretch sensitivity varies from one nerve ending to the next, thereby allowing for responsiveness over a wide pressure range.
o The carotid baroreceptors have a response threshold of around 50mmHg and saturate at 180mmHg
o The aortic baroreceptors operate over a range of 100 – 300mmHg, meaning they are usually inactive under resting conditions.
SO, carotid baroreceptors bear the brunt of responses to moment-by-moment changes in BP
Describe cardiopulmonary baroreceptors
They are found in lower pressure regions of the cardiovascular system.
- They provide CNS with info about ‘fullness’ of the cardiovascular system (role is hence about modulating renal function)
- However, because fullness correlates with ventricular preload, they also have a role in maintaining MAP
o Preload is load applied to a myocyte + establishes muscle length before contraction.
Describe the structure of cardiopulmonary baroreceptors,
as well as where they are found,
and how information is conveyed back to the CNS
Anatomically they are similar to the high-pressure ones – also free nerve endings
- They are found in the vena cava, pulmonary arteries and veins + atria
o Info is conveyed back to CNS on Vagal afferents (X cranial nerve)
Describe the central integrator as an integration centre
in the medulla
These afferents converge (nerves from baroreceptors) converge on the medulla
- where the information is ‘compared’ to preset values
- and a decision is made about the nature of the compensatory response
The medulla is composed of a collection of nuclei that comprise the cardiovascular centre.
- Arterial pressure = CO x SVR
- and the control centres adjust both parameters simultaneously
Describe the 3 different control centres that are in the central integration centre in the medulla
3 control centres, which are interlinked extensively so as to generate a unified response to changes in arterial pressure:
- Vasomotor centre - cells in that area that cause vasoconstriction
- Cardioacceleratory centre - increases HR and myocardial inotropy (force of contraction)
- Cardioinhibitory centre - slows HR when active
Describe the other inputs to the Cardiovascular system from other regions of the brain and periphery (which help to monitor and regulate BP)
Hypothalmic control centres
- help coordinate vascular responses to changes in external and internal body temperatures
Cortical control centres
- account for changes in cardiovascular performance induced by emotions
- (e.g. fainting or anticipatory changes associated with exercise)
Pain centres
- can also precipitate profound changes in blood pressure by manipulating cardiovascular centre output
The brainstem also contains a respiratory centre that controls breathing
- the resp. centre monitors arterial O2 and CO2 using peripheral and central chemoreceptors
Describe what the effector pathway is
The cardiovascular centres adjust cardiac and vascular function via the Autonomic Nervous System (ANS)
The cardioinhibitory centre depresses HR.
- It acts via parasympathetic fibres travelling in the Vagus nerve
- that target the sinoatrial (SA) and atrioventricular (AV) nodes.
The cardioacceleratory and vasomotor centres
- act via sympathetic nerves.
o The cardioacceleratory centre increases HR by manipulating SA and AV node excitability + increasing myocardial contractility
o The vasomotor centre controls resistance vessels, veins and adrenal glands
Describe why the BP decreases when someone moves to stand from sitting/supine position
- When a person stands, 600-800mL of blood is forced down under the influence of gravity + begins to pool in the legs + feet
- Venous return and CVP fall as a consequence
More blood in lower limbs means:
- venous pressure and volume increases in lower limbs
SO
- volume of blood in the thorax and pressure is reduced in the thorax
- this decreases RV filling pressure + volume - Decrease in Stroke Volume
- This also decreases LV stroke volume due to reduced pulmonary return
HENCE
- BP falls
- MAP = (decreased CO x SVR) + decreased CVP
Describe what happens when BP falls > 20mmHg
when standing
This is known as orthostatic/postural hypotension
BP falls > 20mmHg
Describe the effector pathway mechanism that takes place when the BP falls due to standing up (postural hypotension)
Baroreceptors are activated
- Initially, as stroke volume decreases, there is an increase in Heart Rate
(baroreceptor-mediate tachycardia) to maintain CO and hence BP
- This increase in SNS tone also leads to an increase in SVR and an increase in venous tone (decreased compliance) - venoconstriction
- this also prevents pooling and oedema forming in lower limb
(though the pressure is raised in feet on standing) - and increased preload by increasing venous return (EDV increases)
Once this happens
- heart rate can reduce
- as can SVR
Describe other mechanisms that act as compensatory mechanisms to maintain BP (when it reduces), aside from baroreceptor reflexes - effector response
- Valves - prevent backflow
- Muscle pump
- Abdominothoracic pump
These work to increase venous return, increase preload (EDV) and increase the cardiac output once again
