Disturbances of Blood Pressure and Blood Volume Flashcards
Describe orthostasis hypotension
Low blood pressure that is caused by standing up from sitting or supine position
What is pressure in the arterioles and veins like when supine?
Arteriole pressure is 100 mmHg at the heart and 95 mmHg at the head and feet. There is very little pressure going through the arterioles in your legs because the blood is equally distributed around the body and is at the same level of the heart.
Venous pressure is 3 mmHg at the heart and 5 mmHg at the head and feet. Similar pressure gradient from heart to legs and heart to head.
What happens to your ABP when you stand up?
A decrease in the output of the left and right side of the heart.
An increase in venous pooling causing an decrease in CVP, decreasing right arterial pressure decreasing EDV, decreasing R. SV, left EDV, L. SV therefore a decrease in CO (CO = HR x SV) therefore a decrease in ABP (ABP = CO x TPR)
What non reflex mechanisms help to maintain ABP and CVP when upright?
- Skeletal muscle pump:
Tonic contraction for posture helps limit venous pooling. Muscle contraction compresses vessels decreasing the volume pushing the blood back towards the heart. - Respiratory Pump:
Deeper breathing when standing promotes venous return and limits venous pooling.
Both pumps help to increase CVP and therefore increases EDV, SV, CO, and ABP towards supine values. This is very important to prevent oedema in feet and ankles.
Exercise accentuates the effects of both pumps
What are the effects of removal of the baroreceptor input into the CNS
- There is a much greater variability in ABP
2. Increase in resting mean ABP (increase in HR and increase in TPR) by around 10 mmHg
What can regulate ABP if the baroreceptors are lost?
Atrial stretch receptors that regulate blood volume by acting on the kidney via:
Renin-angiotensin system
ADH (antidiuretic hormone)
This occurs if the resting ABP is > 140/90 mmHg
What does MSNA mean and what happens during hypertension?
MSNA = Muscle sympathetic nerve activity.
MSNA increases progressively as the hypertension develops.
Congestive heart failure arises from what?
Myocardial infarction or chronic hypertension (raised afterload).
Systolic contraction of ventricular muscle is weak meaning ejection fraction is low and ventricles, atria and venous vessels are over filled/distended.
Therefore there is a large EDV, but a low SV.
Therefore CO and ABP would be low, if no compensation (ABP = CO x TPR)
In congestive heart failure (CHF) what is the response to a decrease in ABP?
Decrease in ABP triggers the baroreceptor reflex.
Increase sympathetic activity, decreasing vagal activity to increase HR
Increase symp to ventricular muscle = increase ventricular contractility (however this is ineffective)
Increase symp causes venous and arteriolar vasoconstriction = increase CVP and TPR
Increase in renin-Angiotensin II = retention of water and Na+ by the kidneys
However, baroreceptor reflex responses tend to worsen primary conditions, the heart is damaged and cannot cope with increase filling.
By injecting a vasoconstrictor or vasodilator how can the baroreceptor reflex be monitored? What are the disadvantages?
An intravenous injection of vasoconstrictor e.g. phenylephrine (alpha agonist), by measuring the mean ABP it will increase and baroreflex bradycardia should occur.
An intravenous injection of vasodilator e.g. sodium nitroprusside (NO donor) we can see if there is a decrease in ABP and baroreflex tachycardia should occur.
Disadvantages are it is impossible to test the effect of baroreceptor reflex on vascular resistances because the agonists directly affects the arterioles.
By putting a cannula into the carotid sinus how can this test cardiac and vascular components of baroreceptor reflex be tested?
In an anaesthetised animal the carotid sinus is made into a “blind sac”, carotid artery is cannulated and connected to a pressure system. A change in sinus pressure can be applied and the sinus nerve afferent activity recorded (ABP, HR, contractility, vascular resistance in muscle).
How does a neck cuff allow a method of testing cardiac and vascular components of baroreceptor reflex?
Allows suction (mimics an increase in ABP), or pressure application (mimics a decrease in ABP).
Is the baroreceptor reflex changed in CHF?
There is a set point of baroreceptor reflex (resting ABP) is not changed, but the baroreflex sensitivity is reduced - less able to buffer changes in ABP.
Note: The volume receptor reflex is also depressed so is unable to decrease the CVP.
Is the baroreceptor reflex changed in essential hypertension?
There is a set point of the baroreceptor reflex (resting ABP) is increased giving a gain (sensitivity) is unchanged. The output of Sympathetic nerve activity from the CNS is chronically raised (CNA may modulate cardiovascular reflexes)
