Responses to CVS Stress Flashcards
Movement from supine to standing position is a severe challenge to human circulation
Standing is a problem because you add gravity to the equation
When you’re lying down, the effect of gravity is the same across the body
When you stand up, gravity pushes a column of blood from your head to your toes
If the blood is above the heart, gravity is going to force it down to the heart so the pressure in the arteries above the heart will be lower
Blood pressure below the heart is going to be higher
That’s why you take blood pressure from the arm, because it is level with the heart so gravity isn’t an issue
If you add the effects of gravity on top of the blood pressure generated by the heart, you add around 80 mmHg
So there is greater pressure lower down than higher up
Why does gravity have an impact on veins and not arteries?
What does having more bloof in the venous resevior induce a hypotensive effect?
Effects of change in posture on blood vessels
Arteries are very muscular and so gravity has little impact on blood in the arteries - the thick musculature maintains the blood pressure in the arteries
Veins are less muscular and so it is easier to cause venous distention
The blood in the veins lower down is trying to return to the heart but you have gravity forcing the blood back down
The veins then stretch and the blood begins to pool in the veins in the lower leg
There is only a finite amount of blood in the cardiovascular system so having more blood in the venous reservoir means that there is less blood in the arterial system and hence lower blood pressure
In other words, more volume in veins - less volume in arteries - lower blood pressure
Effects of change in posture on fluid movement
There is an increase in hydrostatic pressure in the blood vessels of the legs
You have hydrostatic pressure AND gravity adding to the pressure
As a result, MORE fluid leaves the capillary and enters the tissue
In other words, there is more fluid loss to the interstitial compartment
This means that there is a reduction in effective circulating blood volume - when considering Starling’s law, this could induce a hypotensive state
Change in posture and Starling’s Law
There is more blood in the veins and less blood is re-entering the veins because you lose more fluid to the tissues
The lost fluid will return to the circulation later on via the lymphatics but that takes a long time
Starling’s Law suggests that end-diastolic volume (ventricular filling) determines the stroke volume and as less of the blood in the veins is returning to the heart (as gravity is pushing the blood down) so ventricular filling decreases so the volume of blood ejected during systolic contraction decreases as well
Transient Hypotension - what happens if you stand up too quickly
What is the normal physioloigcal response which prevents orthostatic hypotension?
Where does the drop in BP when standing up get sensed?
REMEMBER
Define Haemorrhage ?
Haemorrhage
REDUCTION in actual circulating blood volume
Haemorrhage
REDUCTION in actual circulating blood volume
All compensatory mechanisms for transient hypotension also happens for haemorrhage but there are extra mechanisms that also take place
So a decrease in baroreceptor firing stimulates:
Increase in heart rate
Increase in heart contractility
Organ specific vasoconstriction
Describe the additional compensatory mechanism for a haemorrhage?
Hint: capillaries
Haemorrhage- because the hydrostatic pressure drops so much, osmotic pressure now becomes the dominant force (a reversal).
Instead of a net loss of fluid into the tissues, now what you have is a net movement of fluid into the blood.
This process is called autotransufusion- helps maintain pressure by bulking up the blood with fluid.
Extra compensatory mechanisms for dealing with haemorrhage
There is higher hydrostatic pressure at the arteriolar end of the capillary than the venous end - so pressure decreases across the capillary but colloid osmotic pressure REMAINS THE SAME
So you get fluid loss at the arteriolar end and fluid retention at the venous end
When you lose large amounts of blood, the hydrostatic pressure at the capillaries is much lower so you still lose some fluid at the arteriolar end but you’ll retain a lot more at the venous end
AUTOTRANSFUSION = you get significant reabsorption of fluid back into the capillaries from the tissue to try and preserve the blood pressure
This is the body’s attempt to preserve blood pressure - you’re basically diluting the blood with fluid and increasing the blood volume
Which hormones might be useful for a haemorrhage and what effects do they have which will help the body during a haemorrhage?
Name THREE main hormones which are useful when haemorrhage occurs?
What is the end result of all of those hormones and why is that important?
Aldosterone and Vasopressin work together in the …………….. ………….
Aldosterone stimulated …………….. ………….
Vasopressin stimulates …………….. ………….
Endocrine Response to Haemorrhage
There are THREE main hormones which are useful when haemorrhage occurs:
Angiotensin II
Aldosterone
Vasopressin (ADH)
Angiotensin II is a powerful vasoconstrictor (particularly in the kidney) and is good at reducing blood flow to the kidney
The end result of all these hormones is to decrease urinary output
By reducing the amount of blood going to the kidneys, you reduce the amount of urine produced and as such retain more fluid
Aldosterone and Vasopressin work together in the collecting duct
Aldosterone stimulated sodium reabsorption
Vasopressin stimulates water retention
All three have the following effects:
Reduce blood flow to the kidneys thus reducing urine production
Promotes retention of fluid to preserve blood volume and pressur
You can lose 10% of your blood volume and your mechanisms can manage it
You can lose up to 30% of your blood and your mechanisms should be able to manage it though there will be a decrease in blood pressure
If you lose OVER 30% of your blood, your mechanisms can’t compensate for this loss and you will experience SHOCK
Exercise
With exercise you want to increase blood flow to certain tissues (heart, lungs and skeletal muscle)
To increase blood flow you need vasodilation so when you exercise you get massive vasodilation in certain tissues in the body
If you massively increase blood flow to certain tissues you massively decrease total peripheral resistance
As you start to exercise and utilise skeletal muscle, the metabolism in the tissues starts to INCREASE and there is an increase in oxygen and glucose usage
The increase in oxygen and glucose usage causes LOCAL VASODILATION
The tissue is basically telling the vasculature that we’re using lots of oxygen and nutrients so we need more oxygen and nutrients - to increase the supply of oxygen and nutrients they cause vasodilation
This process is called ACTIVE HYPEREMIA
Exercise
With exercise you want to increase blood flow to certain tissues (heart, lungs and skeletal muscle)
To increase blood flow you need vasodilation so when you exercise you get massive vasodilation in certain tissues in the body
If you massively increase blood flow to certain tissues you massively decrease total peripheral resistance
As you start to exercise and utilise skeletal muscle, the metabolism in the tissues starts to INCREASE and there is an increase in oxygen and glucose usage
The increase in oxygen and glucose usage causes LOCAL VASODILATION
The tissue is basically telling the vasculature that we’re using lots of oxygen and nutrients so we need more oxygen and nutrients - to increase the supply of oxygen and nutrients they cause vasodilation
This process is called ACTIVE HYPEREMIA
What is active hyperemia?
Exercise
With exercise you want to increase blood flow to certain tissues (heart, lungs and skeletal muscle)
To increase blood flow you need vasodilation so when you exercise you get massive vasodilation in certain tissues in the body
If you massively increase blood flow to certain tissues you massively decrease total peripheral resistance
As you start to exercise and utilise skeletal muscle, the metabolism in the tissues starts to INCREASE and there is an increase in oxygen and glucose usage
The increase in oxygen and glucose usage causes LOCAL VASODILATION
The tissue is basically telling the vasculature that we’re using lots of oxygen and nutrients so we need more oxygen and nutrients - to increase the supply of oxygen and nutrients they cause vasodilation
This process is called ACTIVE HYPEREMIA
What is a preprogrammed pattern in context of control mechanisms for dealing with exercise ?
What do Muscle Chemoreceptors do?
There is a …………………. ……………. which is an autonomic activation of the ……………… .. ……………….. ……………….. in anticipation of exercise - it’s the body’s way of preparing for exercise before it occurs
Also ……………….. in the muscle respond to the changing environment and send a signal to the ……………….. ……………….. ……………….. as well
From the cardiovascular centre the sympathetic and parasympathetic neurons are activated or inhibited
Outline of control mechanisms for dealing with exercise
There is a preprogrammed pattern which is an autonomic activation of the medullary cardiovascular centre in anticipation of exercise - it’s the body’s way of preparing for exercise before it occurs
Also chemoreceptors in the muscle respond to the changing environment and send a signal to the medullary cardiovascular centre as well
From the cardiovascular centre the sympathetic and parasympathetic neurons are activated or inhibited
So in summary, the afferent input to the medullary cardiovascular centre comes from:
Pre-programmed Pattern - autonomic response in anticipation of exercise
Muscle Chemoreceptors - detect a changing environment
During excercise how does the body change TPR and why does this happen?
What happens to the skin during excercise? Increase sympathetic or decrease
There is an overall fall in TPR during excercise- how is this lessened?
Cardiac output is ……………….. by the increased sympathetic activity and decreased parasympathetic activity which causes an increase in ……………. … …………. and ……………. …………….
How does excercise increase venous return?
Control Mechanisms: Changing Total Peripheral Resistance
The sympathetic nervous system tries to constrict any of the vessels it can to counteract the fall in TPR due to dilation of vessels to the heart, lungs and skeletal muscle
You get profound vasoconstriction in the abdominal region (e.g. to the kidneys and GIT) - this increases TPR
Sympathetic nervous system has a negative response on the SKIN
The preprogrammed pattern decreases the sympathetic response to the skin as vasodilation from skin allows radiation of heat from the skin
There is an OVERALL FALL IN TPR but the decrease is lessened by the decrease in blood flow to certain places (e.g. kidneys and GIT)
Cardiac output is increased by the increased sympathetic activity and decreased parasympathetic activity which causes an increase in stroke volume and heart rate
When you’re exercising, the skeletal muscle is very active so it squeezes the veins and forces blood up the body and has a profound on stroke volume by increasing venous return
Negative effects of changes due to exercise
There is increased capillary pressure across the muscle walls meaning that more blood flows through the skeletal muscle and more fluid is lost to the tissues
Sweating is another way of losing fluid
Therefore there is a decrease in plasma volume which opposes the venous return to a certain extent
Overall there is a positive effect on cardiac output
Why is there an increase in BP during excercise even though there is a decrease in TPR?
Cardiac output increases massively because of the increase sympathetic effects and increased venous return
Overall there is a fall in TPR
If we put the two together, the increase in cardiac output is greater than the fall in TPR so there is an INCREASE IN BLOOD PRESSURE
