Coordinated responses of the CVS - exercise, gravity and haemorrhage Flashcards
What is orthostasis
Physiological response to counteract a fall in blood pressure when a person stands up from a sedentary position.
Describe what happens in orthostasis
- 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
What is Bernoulli’s law
Blood flow = pressure energy + potential energy + kinetic energy
How does blood move to the feet against the blood pressure gradient (higher at feet and lower at head when standing up)
Blood out from the heart has more potential energy due to gravity and kinetic energy due to its movement
How is hydrostatic pressure in the veins different to hydraulic pressure in the capillaries
Hydrostatic pressure accounts for density height and gravity. Hydraulic pressure involves the fact that fluid cannot be compressed, so fluid is pushed to interstitial spaces.
How does orthostasis cause hypotension
Venous pooling in the legs -> Less blood return to heart -> Reduction of preload, lower cardiac output -> LEads to decreased stroke volume, cardiac output and blood pressure, dizziness
Effect of orthostasis on baroreceptors
Baroreceptors unloaded, afferent fibre activity in vagus nerve and glossopharyngeal is decreased, both lead to nucleus solitarius in the medulla.
Where does the vagal parasympathetic pathway go from and to
NTS, to nucleus ambiguus (NA), before leading to SA node via vagus parasympathetic nerves. Unloading baroreceptors causes increase in heart rate and increase in blood pressure.
Describe vagal sympathetic sdystem apthway
NTS to caudal ventrolateral medulla (CVLM), which sends an inhibitory signal to the rostral ventrolateral medulla (RVLM). Decrease in the inhibitory signal from the CVLM leads to an increase in the signal from the RVLM, stimulating the heart.
Why is postural hypotension worse than normal hypotension
Risk of fainting and falling in the elderly.
What factors can contribute to postural hypotension
Calcium channel blocker drugs, varicose veins becoming too compliant, prolonged inactivity, reduced blood volume, and increased core temperature.
Describe what happens when a person enters a microgravity environment
Initially there is increased preload because the blood isn’;t pooling in the feet, and will gos traight back to the heart much easier. Increased stroke volume and cardiac output
Cardiac mechanoreceptors will detect stretching, which will lead to a reduction in sympathetic nerve activity, which leads to a reduction RAAS, ADH, and increased ANP< glomerular filtration rate and diuresis (increased urine production)
