Coordinated CVS responses: Gravity & Exercise Flashcards
How are the everyday demands of life met through the CVS?
Many components of the CVS coordinate together to produce the everyday demands of life
adaptation by integration - small responses create a larger effect
What is orthostasis?
Standing up
How does CVS respond to orthostasis?
On standing up, CVS changes accordingly to the effect of gravity
- BP falls at first
- postural hypotension, lack of blood flow to brain = faint
- Quickly recovers
- due to homeostatic mechanism e.g. baroreflex
How does baroreflexes increase BP?
Increases BP by integrating 3 smaller changes by increasing:
- Heart rate
- Heart contractility
- Total peripheral resistance
What is the effect of gravity on BP when lying down?
Lying down, pressure is the same at the head and the feet
How does BP change due to the effect of gravity during orthostasis?
When standing up blood pools at the feet increasing pressure
Outline Bernoulli’s Law
Blood flow = pressure energy + potential energy + kinetic energy
Using Bernoulli’s Law, describe the pressure change due to gravity during orthostasis
Increased potential energy but slightly lower pressure at the heart level compared to the feet
Increased kinetic energy of ejected blood
What is meant by total energies?
Total energies means the blood flow from the heart to the feet
How is pressure calculated?
pressure = ｐhg
ｐ- fluid density
h - height
g - gravitational acceleration constant
What does magnitude of pressure depend upon?
Pressure is dependent on the height of the fluid column, the density of the fluid and gravity
What is the high pressure at the feet caused by?
The high pressure in the venous system at the feet is due to hydrostatic pressure
Why are veins described as the capacitance levels of the body?
Store most of the blood volume
Why do arteries have higher pressure than veins?
Arteries are muscular and under sympathetic tone
- have higher pressure which distends at the bottom –> blood pools at feet
What is hypotension caused by?
Hypotension (decreased CVP) caused by orthostasis
Describe how Hypotension occurs due to orthostasis
orthostasis causes a fall in CVP -> decreased EDV -> LV stretched less -> Contracts less -> less CO -> Decreased BP -> poorer perfusion of brain => dizziness and fainting
Describe the reflex response to orthostasis
- Less stimulation (unloading) of baroreceptors due to
less LV stretching - Lower afferent fibre activity to NTS
- Switches off inhibitory nerves from CVLM to RVLM
- Results in RVLM being more active, sending efferent
signals to the heart - Increased sympathetic drive to SA node and increased
HR.
Myocardium increased contractility, vasoconstriction
(arterioles, veins) increases TPR.
Less vagal parasympathetic activity to SA node
- overall BP increased
What can cause postural hypotension to be worsened?
- Generalised sympathetic or 𝛼 adrenergic blockade
- drugs that reduce vascular tone
e.g.
side effect with VGCC blockers used to treat
hypotension, angina etc.
What is a varicose vein?
enlarged / torturous vein
What are the consequences of varicose veins?
- impairs venous return
- valves become compromised
- blood pooling in legs worsen
- lack of skeletal muscle activity due to paralysis or forced
inactvity (e,g. long term bed rest, soldiers on guard)
What can cause a reduction in circulating blood volume?
Haemorrhage
How is core temperature increased?
Peripheral vasodilation, less blood volume available
e.g. standing up after a bath
What is a major difference in the effect of blood volume distribution between gravity and microgravity?
In microgravity, it doesn’t matter if you’re standing up or lying down
- less need of ANS, RAAS, ADH, ANP systems to control
BP
- redistribution of blood into chest region
What are the initial effects of microgravity?
Blood doesn’t pool at feet
Returns to heart easily - increased preload, increased AV volume sensed by cardiac mechanoreceptors (Starling’s law)
Decreased sympathetic nerve activity (RAAS, ADH, & increased GFR, ANP and diuresis) = 20% reduction in blood volume
= heart muscle becomes weaker
What are the long term effects on CVS of microgravity?
Less blood volume
Reduced stress on heart
heart reduces in muscle mass
general drop in BP
What is the effect on CVS on return to gravity after microgravity exposure?
Severe postural hypotension due to a much smaller heart, baroreceptor reflex can’t compensate
What is Dynamic exercise?
Constantly shortening and relaxing of different muscle groups
What is static exercise?
One specific muscle group being worked without constant movement
How are responses to different types of exercise initiated?
Integrated by central command in the brain
What is the response of the CVS to exercise?
Once exercise commences, feedback from muscles via mechanoreceptors and metaboreceptors will occur:
- Increase lung O₂ uptake, transport around body & supply
to exercising muscle - Control BP: despite huge changes in CO & TPR (protect
heart from excessive afterload)
Why is it important to selectively target where O₂ is delivered in the body during exercise?
Need to selectively target areas where O₂ is delivered so coordinated dilation/constriction of vascular beds
During exercise, is the CVS under sympathetic or parasympathetic control?
Due to brain central command - early phase: sympathetic
Muscle mechanoreceptors - vagal sympathetic
Muscle metaboreceptors - sympathetic
How do such small adaptations control the response to exercise?
Integration of small adaptations creates overall large response to exercise
How does strenuous exercise effect O₂ uptake?
O₂ uptake by pulmonary circulation increases by 10 - 15x during strenuous exercise
What small changes integrate to cause the big change in the CVS during exercise?
- Increased HR
- Increased SV
- Increased (AV)O₂
Describe the changes in pulmonary circulation during exercise
Increased blood flow and greater O₂ gradient - increased lung uptake
(AV)O₂ difference reaches a plateau at high exercise levels
What does O₂ uptake tell us about exercise done?
O₂ consumption tells us the amount of exercise undertaken
What effect does exercise have on CO?
During exercise SV reaches a maximum value
Plateau phase on Starling’s Curve and max contractility
Diastolic time reduces so less Starling’s Law & filling of the heart
- there’s a limit to CO
Describe the effect of exercise induced tachycardia on the CVS responses
Brain central command - ready for exercise
Muscle mechanoreceptors - fast feedback on exercise
being carried out
HR increases from 65 to 195
What causes exercise induced tachycardia?
- Decreased vagal tone (SA and AV nodes)
- Increased sympathetic activity (SA and AV nodes)
- CO increases = 22 L/min
- HR and SV increase
Why does stroke volume increase during exercise?
Increased Sympathetic activity - causes increased EDV
Increased Venous return / CVP through venoconstriction
Increased sympathetic and calf muscle pump
=> activates Starling’s Law; preload increases
What causes faster ejection during strenuous exercise?
Increased contractility by sympathetic activation of β1 receptors (inotropic increase in Ca2+)
Why does the End systolic volume decrease?
There’s increased ejection fraction due to an increase in SV
Increased contractility by sympathetic activation of β1 receptors and Starling’s Law
Why does vasodilation occur in arterioles during exercise?
Vasodilation of arterioles in active myocardium and skin during moderate exercise due to increased HR and CO
Why is there a fall in local resistance during leg exercise?
Fall in local resistance due to metabolic hyperaemia vasodilation
Local sympathetic response and β2 mediated vasodilation via circulating adrenaline
β2 adrenoceptor expression is high in skeletal muscle and coronary arteries
How does an increase in CO affect BP?
Large increase in CO = relatively small increase in BP due to dilated skeletal muscle arterioles, decreasing TPR
=> causes a large decrease in TPR
What is the purpose of compensatory vasoconstriction?
Compensatory vasoconstriction of non-essential circulations prevents hypotension due to exercise induced decreased TPR
-> prevents BP from falling
Where does compensatory vasoconstriction take place?
- kidneys
- GI tract
- inactive muscle
What is the significance of central control (medulla)?
RVLM controls specific preganglionic sympathetic nerves in the spinal cord which send out postganglionic nerves to specific tissues
Which type of exercise raises BP most?
Static exercise increases BP more than dynamic
What is static exercise?
Constant contraction of a small number of muscles, high load
e.g. weight lifting
How does static exercise effect the rest of the body?
Muscles contracting statically but rest of body needs higher BP to allow blood flow to static vessels
What is Dynamic exercise?
Shortening / lengthening of many muscles - low load
e.g. running
How does dynamic exercise affect blood flow?
Need to reduce TPR and allow blood flow to active muscles
What are muscle metaboreceptors?
Small diameter fibres in skeletal muscles
How are metaboreceptors stimulated?
They are chemosensitive
stimulated by k+, H+, lactate - all which increase in exercising muscle
What type of reflex response is produced during exercise?
Pressor response - especially significant in isometric exercise (incraesed muscle load)
produces tachycardia via increased sympathetic activity due to increased BP
How does increasing BP aid static exercising muscle?
Raised BP maintains blood flow to contracted muscle which contain dilated vessels (local) due to metabolism - selective metabolic hyperaemia
How is the requirement of an increased lung O₂ uptake managed by the CVS?
HR and SV is increased
How does the CVS increase O₂ transport in the body?
Increased extraction of O₂ from blood- Bohr effect
How is there a direct increase in O₂ supply to exercising muscle?
There’s decreased vascular resistance in exercising muscle; metabolic vasodilation
How is BP stabilised by the CVS?
Vasoconstriction in non-exercising / unrequired tissue
