CVS 15 - Responses to CVS Stress Flashcards
Blood pressure in arterioles above the heart is higher/lower?
Lower
Blood pressure below the heart is higher
Veins are more distensible than arteries (as veins are less muscular). What implication does this have?
More volume in lower veins (as gravity is forcing the blood back down). More blood in venous system = less volume in arteries = lower blood pressure
When standing, why is there potential for transient hypotension?
- Increase in hydrostatic pressure in blood vessels of the legs.
- Gravity adds to hydrostatic pressure - more fluid leaves capillary and enters tissue.
- Reduction in effective circulating volume (blood in interstitial space will eventually return via lymphatics but this takes a while).
- Starlings law = EDV determines SV.
- Because less venous volume reaching the heart, ventricular filling decreases so less blood ejected in systolic contraction.
- Transient hypotension = what happens if you stand up too quickly
Out of CO and TPR, which does venoconstriction increase?
CO, because more venous return to the heart, so more SV
Fainting is a mechanism for what?
To combat transient hypotension, lack of skeletal pumping, loss of fluid, etc
What are the compensatory mechanisms for transient hypotension and also haemorrhage?
- Increased baroreceptor firing, increased HR, increased contractility, increased vasoconstriction (organ specific).
- Venoconstriction
What is auto transfusion?
If there is a significant decrease in BP (i.e. through arterioles), there is significant reabsorption of fluid back into the capillary to preserve blood pressure
What are the additional compensatory mechanisms that haemorrhages have?
- Autotransfusion
Which 3 hormones are useful when haemorrhage occurs?
END RESULT OF THE 3 HORMONES IS TO REDUCE URINARY OUTPUT
- ADH/Vasopressin - increases H2O retention
- Aldosterone - increases Na reabsorption, H2O retention.
- Angiotensin 2 - powerful vasoconstrictor, reduces blood flow to kidney. Less blood to kidney = reduces amount of urine produced- retain more fluid.
Describe the stages of blood volume loss and how severe they are.
Losing 10% blood volume - mechanisms can manage it
Lose upto 30% blood volume - mechanisms can manage but slight drop in blood pressure
Losing over 30% blood volume = mechanisms cannot compensate and you will experience shock
What is active hyperemia?
When a tissue uses up a lot of oxygen and nutrients (e.g. glucose), for example in exercise, surrounding tissues will vasodilate to allow more oxygen and nutrients to reach the tissues
What are the control mechanisms when exercising?
- Local response (active hyperemia)
- Muscle chemoreceptors - respond to changing environment - sends signal to medullary cardiovascular centre.
- Preprogrammed pattern - autonomic activation of medullary cardiovascular centre in anticipation of exercise.
How does the medullary cardiovascular system regulate TPR in response to exercise?
- Profound vasoconstriction in Kidneys and GIT. Increases TPR.
- Sympathetic nervous system has a negative response on skin. Medullary cardiovascular system decreases sympathetic activity on skin and promotes vasodilation of skin.
- CO increased by increasing sympathetic activity and decreasing parasympathetic activity.
- Increases venous return by venoconstriction and also muscle pumping in exercise.
Increased SV outweighs decreased TPR, so BP increases.
What are the negative effects of changes due to exercise?
- Increased capillary pressure across muscle walls. More blood flows through skeletal muscle - more fluid lost to tissues.
- Sweating - decreases plasma volume which opposes venous return.
However, sympathetic effects outweigh these small changes and there is a huge increase in CO during exercise.
