CVS 16: Responses to cardiovascular stress Flashcards
What is the pressure gradient in the body?
- Blood above the heart is at a lower pressure
- Blood below the heart is at higher pressure
What is the problem with standing and why is lying down okay?
- standing: gravity pushes a column of blood from your head to your toes + pressure created by heart
- lying: Effect of gravity is the same across the body
Where is blood pressure normally taken from and why?
Taken from the arm
-> it is level with the heart so gravity isn’t an issue
What are the effects of standing on arteries?
- gravity has little impact
- very muscular so muscle maintains the bp in the arteries
Explain the effects of standing on veins?
- less muscular –> VENOUS DISTENSION
- blood in veins in lower limbs trying to return to heart
- gravity pushes it back down
- veins stretch-> blood pools in lower leg veins-> less blood in arteries-> lower bp
What happens to the hydrostatic pressure in the legs whilst standing?
- Increase in hydrostatic pressure
- because you have hydrostatic pressure generated by the heart and then the effect of gravity
So what happens to fluid movement in the legs whilst standing?
- More fluid loss to the interstitial compartment from the capillary which reduces the circulating blood volume
Why might standing induce a hypotensive effect?
- more blood in veins
- less blood re-entering the veins, more fluid lost to tissues but fluid returned slowly through lymphatics
- end-diastolic volume (ventricular filling) determines stroke volume
- less blood in veins returning to heart
- less ventricular filling (lower diastole) so volume of blood ejected during systole decreases as well
= TRANSIENT HYPOTENSION
What detects changes in blood pressure? Where are they located?
Baroreceptors
- carotid sinus
- aortic arch
What BP are baroreceptors most sensitive at? and why
100mmHg
This is similar to the mean arterial blood pressure
In terms of nervous stimulation, what happens when you have a fall in blood pressure?
- decreased firing of the baroreceptor
- less parasympathetic stimulation and less inhibition of the sympathetic
- more noradrenaline
- increased contractility and HR –> increases CO
- increased vasoconstriction of splanchnic/ renal which increases TPR
What happens if compensatory mechanisms fail?
- brain switches off
- faint and go to heart level
Which factors increase your risk of fainting whilst standing?
- if you are taller- increased gravity and less venous return
- dehydration/ sweating
What is the consequence of a haemorrhage
A reduction in the actual circulating blood volume
What are the compensatory mechanisms for a haemorrhage?
- same compensatory mechanisms for transient hypotension so:
- increase in hr-> increase in CO
- increase in heart contractility -> increase in CO
- organ specific vasoconstriction -> increase in TPR
- AUTOTRANSFUSION
Define auto transfusion?
The significant reabsorption of fluid back into the capillaries from the tissue to try and preserve the blood pressure
Explain the process of auto transfusion
- hydrostatic pressure at capillaries is much lower when you lose blood
- so you lose some fluid but retain a lot more at the venous end
- this is because osmotic pressure stays the same and pulls the fluid back in
- so you lose blood but replace it with fluid and dilute the blood to maintain its volume and you can replace the RBCs later
Which three hormones are useful when haemorrhage occurs?
- Angiotensin II
- aldosterone
- vasopressin ADH
What is the function of Angiotensin II?
A powerful vasoconstrictor
- reduces blood flow to the kidneys
- decreases urinary output
- retain this fluid
What is the function of aldosterone?
Sodium retention
What is the function of vasopressin?
Water retention
What volume of fluid can you lose for the effects to be felt?
- lose 10%, body can manage it
- lose up to 30% body can manage it but you will get a decrease in BP
- lose over 30% body can’t compensate and you will experience shock because of poor oxygenation of tissues
Which tissues need the most blood flow during exercise?
- heart
- lungs
- skeletal muscle
How do you increase blood flow to these tissues during exercise? What is the consequence of this?
- vasodilation in certain tissues in the body
- increased blood flow = decrease TPR = decreased BP because BP = CO x TPR
What is active hyperaemia?
- exercise –> increase in tissue metabolism
- > increase in O2 and glucose usage
- > local vasodilation
- > increase in supply of O2 and glucose
What does the afferent input from the medullary cardiovascular centre come from?
- pre-programmed pattern= autonomic activation in anticipation of exercise
- Muscle chemoreceptors= detect changing environment
- - send signal to the medullary cardiovascular centre–
What does the Medullary Cardiovascular system do in response to the afferent input to exercise?
- sympathetic and parasympathetic neurones are activated or inhibited
The sympathetic nervous system do to counteract the fall in TPR during exercise? What is the exception to this?
Vasoconstriction to the other vessels
- vasoconstriction of abdominal region
- decrease in vasoconstriction to the skin so that you can have heat loss by radiation
How is BP maintained during exercise?
- TPR falls because of vasodilation to skeletal muscle which would normally decrease BP but this effect is reduced by vasoconstriction to abdominal vessels which increases BP but fall in BP is still > rise in BP
- BUT CO is increased because of increased sympathetic activity to the heart so BP increases
- CO also increases because of skeletal muscle contraction increasing venous return
What are the negative effects caused by exercise?
- increased capillary pressure across the muscle walls= more blood flows through skeletal muscle and more fluid lost to the tissues
- lose fluid and salt through sweat
- DECREASE IN PLASMA VOLUME which opposes venous return
- BUT still positive overall effect on CO
What is the overall effect of exercise in terms of the BP = CO x TPR equation?
BP = CO x TPR 1. CO - RISE IN HR (+ve) - RISE IN CONTRACTILITY (+ve) - rise in venous return (-ve_ 2. TPR - RISE IN VASODILATION (-ve) - rise in vasoconstriction (+ve) increase in CO > decrease in TPR so overall increase in BP
