Co-ordinated Cardiovascular Responses - Gravity & Exercise Flashcards
Define Orthostasis
Standing up - the cardiovascular system according to the effect of gravity
What 2 things happen when we first stand up?
Blood pressure falls at first
Postural hypotension, lack of blood flow to the brain – faint
Quickly recovers
Due to homeostatic mechanisms such as baroreflex.
Baroreflex integrates three smaller changes
Increases:
heart rate
heart contractility
total peripheral resistance
What is the arterial pressure in the head, heart, and feet when lying down?
95, 100 and 95mmHg
What is the venous pressure when lying down?
10, 3-5 and 10mmHg
What is the arterial pressure in the heart, and feet when standing up?
60, 95 and 180
What is the venous pressure when standing up?
35mmHg, 0-5mmHg and 90mmHg
Describe gravity-induced high venous blood pressures
The high pressure in the venous system at the feet is really due to hydrostatic pressure.
Pressure (P) is higher at the bottom of the tube - Magnitude of pressure depends on the height of the fluid column, the density of the fluid, and gravity
Pressure = phg
We get distension of the veins, where blood volume is high and can be liberated in order to increase cardiac output according to Starlings Law
What happens when we stand up (venous pressure)
- Fall in central venous pressure
- Decreased end-diastolic pressure
- Decreased diastolic pressure
- Decreased stroke cardiac output
- Poor perfusion of brain - dizziness and fainting
Blood pooling of 500ml in legs reduces blood return to the heart
Increased transmural pressure of 90mmHg
What happens when we lie down (fainting)?
- Increased central venous pressure
- Increased end-diastolic pressure
3, Increased stroke volume - Increased Cardiac output
What is the reflex response to orthostasis?
- Less stimulation (unloading of baroreceptors)
- Lower afferent fibre activity
- Switches off inhibitory nerves that go from Caudal ventrolateral medulla (CVLM) to Rostral ventrolateral medulla (RVLM).
- Results in RVLM being more active sending efferent signals to heart and arterioles.
- 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 increase in blood pressure.
What makes postural hypotension worse?
α-adrenergic blockade, generalized sympathetic blockade or other drugs that reduce vascular tone - eg. Side effect with voltage gated calcium channel blockers used to treat hypertension, angina.
Varicose veins - Impairs venous return.
Lack of skeletal muscle activity - Due to paralysis or forced inactivity eg. Long term bed rest, soldiers on guard.
Reduced circulating blood volume - eg. Haemorrhage.
Increased core temperature - Peripheral vasodilatation, less blood volume available eg. standing up after bath.
Does microgravity matter whether you are standing up?
doesn’t matter whether you are standing or lying down in microgravity…
What initially happens in microgravity
Initially: Blood not pooling in feet and returning to the heart easily, increases atria/ventricle volume and so preload and cardiac output. Sensed by cardiac mechanoreceptors leading to a reduction in sympathetic activity.
This reduces ADH and increases atrial natriuretic peptide (ANP), there is increased glomerular filtration rate (GFR) and reduced RAAS. Overall reduction in blood volume (BV) by 20%.
What are the long term effects of microgravity?
Long-term: Less BV, reduced stress on heart, heart reduces in muscle mass, general drop in BP.
What happens when we return to gravity?
On return to gravity: Severe postural hypotension, due to much lower blood volume and smaller heart. Baroreceptor reflex can not compensate.
What is a dynamic exercise
Constantly shortening and relaxing with lots of different muscle groups involved – lower BP, lower sympathetic tone
What is static exercise
One specific muscle group is being worked without constant movement – higher BP, local metabolic hyperaemia
Describe the 4 general points about how the cardiovascular system responds to exercise
Integrated by central command in the brain.
Just anticipation of exercise will cause some of the changes to be initiated.
Once exercise commences there is feedback from the muscles via mechanoreceptors & metaboreceptors.
They are all going to increase sympathetic activity and reduce vagus inhibition.
Describe changes in oxygen uptake, transport, heart rate, force of contraction. blood pressure
Increase lung oxygen uptake, transport around body & supply to exercising muscle. Increased HR and force of contraction.
Control BP – despite huge changes in CO and resistance (protect heart from excessive afterload which will reduce CO).
Selectively target areas where the oxygen is delivered so co-ordinated dilation/constriction of vascular beds.
Outline 3 small changes that occur in response to exercise
- Heart rate 3 x (60bpm to 180bpm)
- Stroke volume 1.5 x (70ml to 120ml)
- Arteriovenous O2 difference, (A-V)O2 3 x (gradient + Bohr effect)
3x1.5x3= 13.5
Describe the factors around the increased uptake of oxygen in response to exercise
- Increased blood flow & greater O2 gradient.
Increased lung uptake. From 5 -15 litres/ min - Arterio-venous oxygen difference reaches a plateau at high exercise levels. From 50 - 150 ml O2/ litre
Describe the factors around the increased cardiac output during exercise
Heart rate
Stroke
- Heart rate increase is main factor at high workloads. From 60 - 180 beats/min
- Increase in SV reaches max value.
Plateau phase on Starling’s curve & max contractility. From 80 - 120 ml and then decreases
Describe the effects of exercise-induced tachycardia
What is the maximum heart rate?
Tachycardia
Brain central command (ready for exercise)
& muscle mechanoreceptors (fast feedback on exercise being carried out).
Maximum HR = 220 – age, approximate increase 65 to 195 (3x).
- Vagal tone (SA & AV nodes)
- Sympathetic activity (SA & AV nodes)
Describe the effects of exercise-induced stroke volume
Increased end-diastolic volume
Faster ejection
decreased end-systolic volume
Stroke volume
sympathetic activity 70 ml to 105 ml for 30 year old male = 1.5x
Increased end-diastolic volume
Venous return/CVP through veno-constriction
Sympathetic activity & calf muscle pump - activates Starling law increasing preload.
Faster ejection
Contractility by sympathetic activation of 1 receptors (inotropic increase in Ca2+).
Decreased end-systolic volume ( ejection fraction)
Accounts for stroke volume
Contractility by sympathetic activation of 1 receptors & Starling’s law,
Describe what is happening to increase the cardiac output
Fall in local resistance due to metabolic hyperaemia vasodilatation.
Local sympathetic response and β2-mediated vasodilatation via circulating adrenaline.
β2 receptor expression high in skeletal muscle and coronary artery.
Describe the effect of increased cardiac output on blood pressure
Large increase in CO
Relatively small
increase in mean BP due to dilated skeletal muscle arterioles decreasing TPR
Large decrease in TPR
Describe compensatory vasoconstriction of non-essential circulations
Compensatory vasoconstriction of non-essential circulations prevents hypotension during exercise-induced
decreased TPR
Compensatory vasoconstriction in inactive and unrequited tissues, for example, GI tract, kidney, inactive muscle
Which raises the BP more: Static or Dynamic
Static
Describe the small-diameter sensory fibres and reflex effects (metaboreceptors)
Small diameter sensory fibres in skeletal muscle
Chemosensitive - Stimulated by K+, H+, lactate, which increase in exercising muscle.
Reflex effects
Tachycardia (via increased sympathetic activity)
Increased blood pressure
‘Pressor response’ to exercise
Especially important during isometric exercise (increased muscle load). Static exercise raises BP more than dynamic exercise.
Raised BP maintains blood flow to contracted muscle to try to force blood into the contracted muscle.
Contracted muscle supplied by dilated resistance vessels due to metabolism…selective metabolic hyperaemia.
Give a summary of cardiovascular responses in exercise
Increased oxygen lung uptake = Increase HR and SV
Increase oxygen transport around the body = Extraction of oxygen from the Bohr shift
Direct the increased oxygen supply to exercising muscles = Decrease in vascular resistance in exercising muscle: metabolic vasodilation
Stabilisation of BP = vasoconstriction in non-exercising and non-required tissue
