Coordinated cardiovascular responses- Gravity and exercise Flashcards
(16 cards)
Explain orthostasis
On standing up (orthostasis), the cardiovascular system changes according to the effect of gravity:
- Blood pressure falls at first
Postural hypotension, lack of blood flow to brain – faint - Quickly recovers, Due to homeostatic mechanisms such as baroreflex.
- Baroreflex integrates three smaller changes by increasing:
● heart rate
● force of contraction
● total peripheral resistance
Explain effects of gravity on blood pressures 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 flows from the heart to the feet
Explain 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
tube - Magnitude of pressure depends on
the height of the fluid column, the
density of the fluid, and gravity.
-Gravitational (hydrostatic)
pressure of a fluid in solid tube
Tube with compliant wall
eg. veins
How does orthostasis cause hypotension
Gravity Pulls Blood Down: When you stand, gravity pulls about 500 ml (a bit more than a pint) of your blood down into your legs. The diagram calls this “venous pooling of 500 ml in legs”.
Less Blood Returns to the Heart: Because blood is pooling in your legs, less blood is returning to your heart. This causes a “fall in central venous pressure” (the pressure of blood returning to the heart).
Heart Has Less Blood to Pump: With less blood returning, your heart has less blood to fill up with before it pumps. This leads to a “decreased end-diastolic volume” (the amount of blood in the heart at the end of filling).
Less Blood Pumped Out: If the heart has less blood to start with, it pumps out less blood with each beat. This results in a “decreased stroke volume”.
Lower Cardiac Output: Since the heart is pumping less blood per beat, the total amount of blood pumped by the heart per minute (“cardiac output”) also decreases.
Less Blood to the Brain: With less blood being pumped out by the heart, less blood gets to your brain (“poor perfusion of brain”).
Dizziness and Fainting: This reduced blood flow to the brain can cause symptoms like dizziness and even fainting.
Explain reflex response to orthostasis
Standing Up Leads to Less Stretch: When you stand, blood pools in your legs, and the blood vessels in your chest and neck (where baroreceptors are) experience “less stretch” because there’s slightly less blood volume there momentarily. Think of a balloon with less air – it’s less stretched.
Baroreceptors Notice: These baroreceptors sense this “unloading” (less stretch).
Signal to the Brain: They send signals to a part of your brain in the “medulla oblongata” called the “Nucleus Tractus Solitarius (NTS)”.
Brain Adjusts Signals: The NTS then does a couple of things:
It reduces the signals that normally slow down your heart and relax your blood vessels. It does this by affecting other areas in the medulla (CVLM and RVLM).
It increases signals that speed up your heart and constrict your blood vessels. This is done through the “sympathetic” nervous system.
What the Sympathetic System Does: This increased sympathetic activity leads to:
Your heart beating faster (“increased HR”).
Your heart squeezing more forcefully (“Myocardium increased contractility”).
Your blood vessels getting narrower (“Vasoconstriction increases total peripheral resistance”).
The Goal: All of these actions (faster heart, stronger squeeze, narrower vessels) help to increase your blood pressure, counteracting the drop that happens when you stand up and blood pools in your legs.
What makes postural hypotension
α-adrenergic blockade, generalized sympathetic blockade or other drugs that reduce vascular tone - eg. Side effect with
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.
Explain the effect of microgravity on the cardiovascular system
Initially: blood not pooling in feet —> it returns 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 ANP —> there is increased glomerular filtration rate and reduced RAAS. Overall reduction in blood volume by 20%
Long term: less blood volume, reduced stress on heart —-> heart reduces in muscle mass , general drop in BP
On return to gravity: severe postural hypotension, due to much lower Blood volume and smaller heart —-> baroreceptor reflex cannot compensate
Give a summary of the CVS effects of microgravity
-Less Blood Pooling Down: On Earth, gravity pulls blood to your legs. In space, this doesn’t happen as much, so blood tends to redistribute upwards. This leads to:
-Smaller Heart: Your heart doesn’t have to work as hard to pump blood upwards against gravity, so over time, it can become a bit smaller (“ventricular size ↓”).
Lower Blood Volume: You might lose some fluid, leading to “hypovolemia ↓”.
Changes in Blood Pressure:
-Initially, you might have a slightly higher amount of fluid in your chest area, which could temporarily increase blood pressure. However, overall, your body adapts, and your blood pressure regulation changes. The diagram shows a decrease in “MAP” (mean arterial pressure), “SBP” (systolic blood pressure), and “DBP” (diastolic blood pressure) with a downward arrow.
Heart Works Differently:
“CO” (cardiac output, the amount of blood pumped per minute) might initially increase due to the fluid shift, but then tends to decrease or stay the same. The diagram shows “CO ↓”.
“SV” (stroke volume, the amount of blood pumped per beat) also tends to decrease (“SV ↓”).
-Nervous System Changes: The balance between the parts of your nervous system that control heart rate and blood pressure shifts. The diagram shows “Sympathetic activity ↓” (the “fight or flight” response becomes less active) and “Parasympathetic activity ↑” (the “rest and digest” system becomes more active).
Other Effects:
“Anemia” (lower red blood cell count) can occur.
Blood vessels might become stiffer (“Arterial stiffness ↑”).
Trouble Standing on Return: When astronauts return to Earth, they can sometimes experience “Postflight OI” (orthostatic intolerance), meaning they might feel dizzy or faint when standing up because their body isn’t used to gravity pulling blood downwards anymore.
What are cardiovascular responses 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.
All changes are going to affect sympathetic activity and vagus inhibition.
Increase lung oxygen uptake, transport around body & supply to exercising
muscle. Increased HR and force of contraction.
Control of BP – despite huge changes in cardiac output and resistance (protect
heart from excessive afterload which will reduce cardiac output).
Co-ordinated dilation/constriction of vascular beds, selectively target areas where the oxygen is delivered.
explain exrcise induced techycardia and stroke volume
-brain central command: ready for exercise and muscle mechanoreceptors
-decreased signal down vafus nerve to SA and AV nodes
-increased sympathetic activity to SA and AV nodes
stroke volume:
-sympathetic activity increases stroke volume
-increased end diastolic volume: increased sympathetic activity and calf muscle pump cause venoconstriction which increases venous return
-activates Starling’s law increasing preload
-faster ejection: increased sympathetic activation of a1 receptors cause faster ejection
-decreased end systolic volume: increased contractility by sympathetic actvation of a1 recepors and also increased stretching
explain cardiac output and selective flow changes
-fall in local resistance due to metabolic hyperaemia vasodilation
-local sympathetic response and b2- mediated vasodilation via circulating adrenaline.
-B2 receptor expression high in skeletal muscle and coronary artery
explain the effect of increased cardiac output on blood pressure
-large increase in Cardiac output
-relatively small increase in mean BP due to dilated skeletal muscle arterioles decreasing TPR
-large decrease in TPR
Explain compensatory vasoconstriction of non essential circulations
Compensatory vasoconstrictions in inactive or unrequired tissues e.g. kidneys, GI tract, inactive muscle.
-prevents BP from falling.
-central control: RVLM controls specific pre ganglionic sympathetic nerves in spinal cord which send out post ganglionic nerves to specific tissues
explain the characteristics of metaboreceptors and reflex effects
-small diameter sensory fibres in skeletal muscle; chemosensitive- stimulated by K+, H+, lactate which increase in exercising muscle
reflex effects:
1) tachycardia via increased sympathetic activity
2) increased blood pressure
3) 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
explain how static exercise raises BP more than dynamic exercise
Dynamic:
-constantly shortening and relaxing with lots of different muscle groups involved- lower BP; lower sympathetic tone
Static:
-one specfic muscle group is being worked without constant movement- higher BP, lower metabolic hyperaemia
give a summary of cardiovascular responses in exercise
1) increase lung O2 uptake:
-increased heart rate and stroke volume
2) Increase O2 transport around body:
-increased extraction of O2 from blood Bohr shift
3) Direct the increased O2 supply to the exercising muscle:
-lower vascular resistance in exercising muscle: metabolic vasodilation
4) Stabilization of BP:
-vasoconstriction in non-exercising and non required tissuue
