Co-ordinated cardiovascular responses Flashcards
What does orthostasis mean?
means standing up
What happens in orthostasis?
CVS changes according to the effect of gravity
BP falls at 1st -> postural hypotension, lack of blood flow to brain – faint
quickly recovers due to homeostatic mechanisms such as baroreflex -> baroreflex integrates 3 smaller changes: increases HR, heart contractility and TPR
What is the effect of gravity on BP during orthostasis?
Bernoulli’s law
blood flow = pressure energy + potential energy + kinetic energy
increased potential energy at heart level vs feet + increased kinetic energy of ejected blood
total energies means blood flow from heart to feet
Gravity induced venous blood pressure
high pressure in venous system at feet is really due to hydrostatic pressure
consider tube with compliant wall e.g. veins
-> pressure is higher at bottom of tube - magnitude of pressure depends on height of fluid column, the density of fluid and gravity.
-> veins are capacitance vessels, their pressure is lower than arterial side. blood causes them to distend a little bit at bottom. reason why blood tends to pull towards our feet
How does orthostasis cause hypotension?
orthostasis causes a fall in CVP-> decreased SV -> decreased CO -> decreased BP -> poorer perfusion of brain -> dizzy and fainting
What is reflex response to orthostasis?
- less stimulation (unloading) of baroreceptors
- lower afferent fibre activity
- switches off inhibitory nerves that go from CVLM to RVLM
- results in RVLM being more active sending efferent signals to heart and arterioles
- increased sympathetic drive to SAN and increased HR. Myocardium increased contractility. vasoconstriction (arterioles, veins), increases TPR. Less vagal parasympathetic activity to SAN -> overall increase in BP
What makes postural hypotension worse?
- a-adrenergic blockade or generalised sympathetic blockade or other drugs that reduce vascular tone
- varicose veins- impairs venous return
- lack of skeletal muscle activity due to paralysis or forced inactivity
- reduced circulating blood volume
- increased core temp
What is the effect of microgravity (space) on CVS?
no gravity so initially blood is not pooling in feet and returning to heart easily, increases atria/ventricle volume, preload and CO. Sensed by cardiac mechanoreceptors leading to reduction in sympathetic activity
- this reduces ADH and increases ANP -> increased GFR and reduced RAAS -> reduction in blood volume overall
ADH + RAAS are turned off as they increase blood volume
Long term: less blood volume, reduced stress on heart, heart reduces in muscle mass and general drop in BP
What is coordintated response to exercise?
- integrated by central command in brain
- just anticipation of exercise will cause some of changes to be initiated
- once exercise commences, there is feedback from muscles via mechanoreceptors and metaboreceptors
- they are all going to increase sympathetic activity and reduce vagus inhibition
- increase lung O2 uptake, transport around body and supply to exercising muscle
- control BP- despite huge changes in CO and TPR
- need to selectivity target areas where O2 is delivered so coordinated dilation/constriction of vascular beds
increase in O2 uptake from lungs
- increased blood flow and greater O2 gradient, increased lung uptake
- arterio-venous O2 difference reaches a plateau at high exercise levels
Increase in cardiac output during exercise
- HR increase is main factor at high workloads
- increase in SV reaches max value- plateau phase on Starling’s curve and max contractility
exercise induced tachycardia
brain central command (ready for exercise) and muscle mechanoreceptors (fast feedback on exercise being carried out) -> causes vasodilation of vessels in muscles so you increase blood flow to the muscles
decrease in vagal tone (SAN +AVN) -> increases HR and contraction
increase in sympathetic activity (SAN +AVN) -> increases HR and contraction
Stroke volume and exercise
increase sympathetic activity
increased EDV
-> increase venous return/CVP through venoconstriction
-> increase sympathetic + calf muslce pump - activates Starling’s law increasing preload
faster ejection
-> increases contractility by sympathetic activation of B1 receptors (inotropic increase in Ca2+)
decreased ESV (increase ejection fraction)
-> accounts for increase SV
-> increase in contractility by sympathetic activation of B1 receptors and Starling’s law
What are 2 types of exercise? give examples
Static e.g. weightlifting and dynamic e.g. running
What is static exercise?
constant contraction of small number of muscles, high load
less increase in HR and huge increase in BP
