Co-ordinated cardiovascular responses - Gravity & Exercise Flashcards
What is orthostasis?
→ Standing up
What happens to blood pressure in response to orthostasis?
→Blood pressure falls at first
→Postural hypotension - lack of blood flow to the brain
→Quickly recovers - due to homeostatic mechanisms such as baroreflex
What three changes does the baroreflex integrate?
→Increase in heart rate
→Increase in heart contractility
→Increase in total peripheral resistance
How do we know that blood flows from the heart to the feet?
→ Bernoullis law
→ Blood flow = pressure energy + potential energy + kinetic energy
→ potential energy at heart > feet + increased KE of ejected blood
→Total energy means blood flows from the heart to the feet
What is the equation for pressure?
→ ρhg
Where is pressure lower?
→ on the venous side
How can orthostasis cause fainting?
→ Orthostasis causes a fall in CVP → Decreased SV → Decreased CO → Decreased BP → Poor perfusion of the brain → dizziness + fainting
How does laying down cause increased pulse pressure?
→ Increased CVP
→ Increased EDV
→ Increased SV
→ Increased pulse pressure
What is the baroreceptor reflex response to orthostasis?
→ Baroreceptors are unloaded
→ Decrease in afferent fibre activity
→Signal goes to NTS
→ NTS switches off inhibitory nerves that go from the CVLM to the RVLM
→ Results in the RVLM being more active, sending efferent heart signals to the heart and arterioles.
What makes postural hypotension worse?
→α-ADRENERGIC BLOCKADE OR GENERALISED SYMPATHETIC BLOCKADE: drugs that reduce vascular tone
→VARICOSE VEINS: impairs venous return
→ LACK OF SKELETAL MUSCLE ACTIVITY DUE TO PARALYSIS: eg. long term bed rest, soldiers on guard, etc.
→ REDUCED CIRCULATING BLOOD VOLUME: eg. haemorrhage
→INCREASED CORE TEMPERATURE:
peripheral vasodilation, less blood volume available (eg. standing up after a bath)
What do you need less of to control blood pressure in microgravity?
→less need for ANS, RAAS, ADH, ANP systems
to control blood pressure.
What happens to the blood initially in microgravity?
INITIALLY: → increase in preload →increase in atria/ventricle volume. →sensed by mechanoreceptors. → Decreased sympathetic nerve activity, → reduction in RAAS, ADH → increased GFR, ANP and diuresis →all leading to a 20% reduction in blood volume.
What happens long-term to the blood in microgravity?
→Less BV
→ reduced stress on heart
→ heart reduces in muscle mass
→general drop in BP
What happens on return to normal gravity?
→Severe postural hypotension
→due to much smaller heart
→Baroreceptor reflex can not compensate
What is the difference between static and dynamic exercise?
→static exercise raises the blood pressure more than dynamic exercise.
→Static exercise is the constant contraction of a small number of muscles, so there is a higher load.
→dynamic exercise
→ there is a shortening/lengthening of many muscles, which is a low load
What are general CVS responses to exercise?
→ increased lung O2 uptake, which is transported around the body and supplied to exercising muscle
→controlled BP, despite huge changes in CO and TPR
(to protect the heart from excessive afterload)
→increase mechanoreceptor and metaboreceptor stimulation
How much can O2 uptake by pulmonary circulation increased by?
10-15 times
How can the integration of small adaptations create a big response to exercise?
→increased heart rate (3x)
→increased stroke volume (1.5x)
→increased arteriovenous O2 difference (3x)
→3 x 1.5 x 3 = 13.5 times
What happens during exercise induced tachycardia?
→increased stimulation of the brain central command
→increased stimulation of the muscle mechanoreceptors.
→decrease in parasympathetic simulation
→increase in sympathetic stimulation
→ decrease in vagal tone
What happens to the stroke volume during exercise?
→increased stroke volume
→ increase in sympathetic activity.
Why is there increased end diastolic volume during exercise?
→increased Venous return/CVP through venoconstriction
→increased sympathetic activity + calf muscle pump
→ activates Starling law increasing preload.
Why is there faster ejection during exercise?
Increased contractility by sympathetic activation of β1 receptors (inotropic increase in Ca2+).
Why is there decreased end-systolic volume during exercise?
(Increased ejection fraction)
Accounts for increase in stroke volume
Increased contractility by sympathetic activation of β1 receptors & Starling’s law,
Describe the changes in cardiac output and flow changes during exercise
→fall in local resistance due to metabolic hyperaemia (an excess of blood) vasodilation.
→ increased sympathetic activity
→ β2-mediated vasodilation via the circulating adrenaline.
→[there is a high β2-receptor expression in skeletal muscle and coronary arteries]
→This increases cardiac output and flow changes.
What does compensatory vasoconstriction of non-essential circulations during exercise prevent?
→compensatory vasoconstriction of non-essential circulations prevents hypotension due to an exercise-induced decrease in TPR.
What does compensatory vasoconstriction of inactive tissues during exercise prevent?
→The compensatory vasoconstrictions of inactive or unrequired tissues (such as in the kidneys, GI tract, inactive muscle) prevents the BP from falling.
What are metaboreceptors and what are they stimulated by?
→ small-diameter sensory fibres in skeletal muscle.
→They are chemosensitive
→stimulated by K+, H+ and lactate, which increase in exercising muscle.
What are the reflex effects of metaboreceptors?
→tachycardia (via increased sympathetic activity)
→increased blood pressure
When you stand up what happens to blood in the veins?
→ Venous pooling in the legs of about 500ml
What is the arterio-venous oxygen difference like during high exercise?
→ Reaches a plateau
Why does increasing HR eventually not increase SV?
→ Starling’s law
→ if you have a high HR there is not enough filling time
→ Less preload
→ you can also overfill the heart and get less CO
What are the effects of the baroreceptor reflex during orthostasis?
→ Increased Sympathetic drive SA node → Increased HR → Increased contractility →Vasoconstriction (arterioles, veins) →Increased TPR → Decrease in vagal parasympathetic activity to SA node →Together = Increase in BP
What is exercise also known as?
→ Extreme form of standing up!
