coordinated cardiovascular responses - gravity and exercise Flashcards
what is orthostasis ?
when standing up
how does the CVS change according to the effect of gravity ?
- blood pressure drops at first
- postural hypotension - lack of blood flow to brain - fainting
- quick recovery
- due to homeostatic mechanisms eg baroreflex
- baroreflex integrates 3 smaller changes by increasing heart rate , force of contraction , TPR
why is pressure in the venous system higher at the feet ?
Hydrostatic Pressure:
When standing, gravity pulls blood downward, increasing the pressure in veins below the heart.
Column of Blood:
Veins are filled with blood, and the weight of this column adds pressure to the veins in the legs. Since the heart is higher than the feet, the blood in the veins of the lower extremities must support the weight of all the blood above it.
venous Compliance
Veins are more compliant (stretchable) than arteries, meaning they expand more in response to pressure. This allows them to accommodate more blood, further increasing pressure in the lower limbs.
how does orthostasis cause hypotension ?
lying down:
* blood is evenly distributed in the veins
* increased central venous pressure
* increased EDV
* increased stroke volume
* increased cardiac output
orthostasis:
* fall in central venous pressure
* decreased EDV
* decreased stroke volume
* reduced cardiac output
* poor perfusion of brain - dizziness and fainting.
describe the reflex response to orthostasis
what makes postural hypotension worse ?
- 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.
what is the effect of microgravity on the CVS ?
- doesnt matter if standing up or lying down
- 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 atrial natriuretic peptide (ANP), there is increased glomerular filtration rate (GFR) 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 can not compensate.
describe the CVS response to exercise
- 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.
how does integration of small adaptations create a large response to exercise ?
- Oxygen uptake by pulmonary circulation can increase
- 10-15 times during strenuous exercise.
- This very big change brought about by the integration of three smaller changes:
describe increased O2 uptake during exercise
describe increased cardiac output during exercise
describe exercise induced tachycardia and stroke volume
Heart Rate (tachycardia):
* Brain central command
* Ready for exercise & muscle mechanoreceptors
* fast feedback to brain on exercise being carried out
* Decreased signal down vagus nerve to SA & AV nodes
* Increased sympathetic activity to SA & AV nodes
Stroke volume (ejection):
* Sympathetic activity increases stroke volume
* 70ml to 105ml for 30 year old male = 1.5x
* Increased end-diastolic volume (filling pressure)
* Increased sympathetic activity & calf muscle pump cause venoconstriction which increases venous return/CVP
- activates Starling law increasing preload.
- Faster ejection:
- Increased sympathetic activation of 1 receptors cause faster ejection (inotropic increase in Ca2+).
- Decreased end-systolic volume (increased ejection):
- Increased contractility by sympathetic activation of 1 receptors and also increased stretching eg. Starling’s law
what happens to the increase in cardiac output ?
what is the effect of increased cardiac output on blood pressure ?
- Increased cardiac output (CO) generally leads to an increase in blood pressure (BP), but the extent depends on vascular resistance and arterial compliance.
Effects on Blood Pressure:
* Increased Systolic Pressure (SBP) – More blood is pumped per minute, raising the force on arterial walls.
* Minimal Effect on Diastolic Pressure (DBP) – If vascular resistance remains unchanged, diastolic pressure may not rise significantly.
* If Resistance Also Increases – Blood pressure rises significantly (e.g., in hypertension).
* If Resistance Decreases (Vasodilation) – BP may stay stable or increase only slightly.
what do compensatory vasocontraction of non essential circulations do ?
- Compensatory vasoconstriction of non-essential circulations helps maintain blood pressure and perfusion to vital organs (brain, heart) during stress, shock, or blood loss.
Key Effects:
* Redirects Blood Flow – Reduces blood supply to less critical areas (skin, GI tract, kidneys) to prioritize vital organs.
* Maintains Blood Pressure – Increases systemic vascular resistance (SVR) to counteract drops in blood pressure.
* Supports Cardiac Output – Helps sustain adequate circulation despite reduced blood volume or cardiac function.
describe 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 ie. selective metabolic hyperaemia.
describe how static exercise raises BP more than dynamic exercise
give a summary of CVS response to exercise
