Short Term Control of Blood Pressure Flashcards
Why does the mean arterial pressure need to be regulated?
It is the driving force pushing blood through the circulation, if it falls too low it causes syncope and if it becomes too high it causes hypertension
What is the function of the arterial baroreflex?
Acts rapidly to minimise fluctuations in the MABP
What are baroreceptors?
Afferent nerve endings in the walls of the carotid sinuses and aortic arch
What is the function of the baroreceptors?
Sense alterations in wall-stretch caused by pressure changes, and respond to these by modifying the frequency at which they fire action potentials
What effect do pressure elevations have?
Increase wall stretch and increase AP impulse frequency from the baroreceptors (vice versa for pressure decreases)
When MAP falls, what does the reduction in baroreceptor impulse frequency cause the brain to do?
Reduce the firing of vagal efferents which supply the sinoatrial node, resulting in tachycardia
When MAP falls and the brain reduces the firing of vagal efferents, what happens to the activity of the sympathetic nerves innervating the heart and blood vessels?
Activity increases, resulting in increased cardiac contractility and the constriction of arteries and veins
What does stimulation of the renal sympathetic nerves cause?
Increase in renin release and consequently angiotensin II production and aldosterone secretion, resulting in fluid retention
Tachycardia, vasoconstriction and fluid retention act together to do what?
Raise MAP
What are the main differences between A-fibres and C-fibres?
A-fibres have large unmyelinated axons and are activated over lower levels of pressure
C-fibres have small unmyelinated axons and respond over higher levers of pressure
Baroreceptor A and C fibres provide input to what?
The brain
The brain is most sensitive to pressure changes between which range?
60 and 150mmHg
How does the brain allow increases in MAP to occur?
It is able to reset the baroreflex
What causes a decrease in arterial wall compliance?
Ageing
Hypertension
Atherosclerosis
When are alterations in frequency of baroreceptor firing greatest? When do these alterations tend to moderate?
While pressure is changing
Tend to moderate when a new steady-state pressure is established
What happens if the baroreflex is unable to prevent a change in MAP?
It will reset within several hours to the new level of pressure
What are the other inputs to the medullary cardiovascular centres?
Cardiopulmonary baroreceptors Central chemoreceptors Chemoreceptors in muscles Joint receptors Higher centres
Regulation of blood pressure in the long term can’t be done by the arterial baroreceptors, what does it revolve around instead?
Blood volume
What are the main sensors involved in the long-term regulation of blood pressure?
Cardiopulmonary baroreceptors - effects tend to be hormonal, act on blood vessels and kidneys
What does angiotensin II cause?
Arteriolar construction, and increased TPR
What does aldosterone cause?
Increase Na+ reabsorption and increased plasma volume
What does vasopressin cause?
Arteriolar constriction and increased TPR
Increased water permeability of the collecting duct and increased plasma volume
What do ANP and BNP cause?
Arteriolar dilation and decreased TPR
Increased Na+ excretion and decreased blood volume
What effect does standing have on hydrostatic pressure?
Increases hydrostatic pressure causing pooling of blood in the veins and venules of the feet and legs
What effect does standing have on venous return, EDV, preload, SV, CO, MAP and baroreceptor firing rate?
Decreases these
What effect does decreased vagal tone have on HR and CO?
Increases these
What does increased sympathetic tone cause?
Increased; HR CO Contractility SV Venoconstriction Venous return EDV Arteriolar constriction TPR
What is the Valsalva manoeuvre?
Forced expiration against a closed glottis
The Valsalva manoeuvre causes increased thoracic pressure, this decreases what?
Venous return EDV CO SV MAP
What detects decreased MAP?
Baroreceptors - these initiate the reflex to increase CO and TPR
