Coordinated Response Of CVS Flashcards
The cardiovascular system responds to different physiological conditions.
Name some physiological conditions
These include exercise,
diving, fight or flight responses, sleeping and gravity. Many components of the CVS co-ordinate
together to meet the demands of every life.
What are the principles of adaption
The principles of adaptation are that a number of small
responses are integrated to adapt to a physiological change.
What are the effects due to when in orthostasis
Orthostasis (or standing up) causes changes to the cardiovascular system according to the effects of
gravity.
What happens during orthostasis
The first thing that occurs is that the distance between the brain and feet changes and there
is a sudden drop in blood pressure to the brain. This is known as postural hypotension and results in
the faint feeling one can experience. However, the CVS quickly recovers
How is the cvs able to recover from orthostasis
due to homeostatic
mechanisms like the baroreflex that use baroreceptors in the carotid arteries to detect these
pressure changes. The body then increases heart rate, heart contractility and total peripheral
resistance such that blood pressure is restored within a couple of heart beats and perfusion to the
brain is restored.
Why does blood still flow to the feet
The reason why blood still flows down to the feet despite the pressure at the
feet being much higher than at the heart (95 mmHg) is due to Bernoulli’s law that considers potential
and kinetic energy as well as pressure energy (rather than simply pressure only as is the case in
Darcy’s law).
What two things cancel out the opposing pressure energy
Potential and kinetic energy cancel out the opposing pressure energy such that energy
at the heart is slightly higher than at the feet (only just however). This is why there can be problems
with perfusion at the feet in the elderly, especially people who are diabetic.
How can increased blood pressure due to gravity be explained
The increased blood pressures due to gravity can be explained using the formula pressure = density x
gravitational acceleration constant x height. If we consider pressure at the heart as opposed to
pressure at the feet, the only constant that changes in the equation is height. Distance from origin is
much greater at the feet than at the heart which explains the pressure difference. The lower
pressure in the veins is explained due to veins having compliant walls that can expand and pool
blood. There is still however pressure difference at the bottom compared to the top which is why
blood generally pools at the feet.
What happens when venous pooling occurs in the legs
Venous pooling in the legs can be around 500 ml and this reduces
blood returning to the heart as well as the returning blood pressure. This decreases end diastolic
volume which in turn decreases stroke volume. The decreased cardiac output causes poor perfusion
of the brain causing dizziness and fainting.
How can venous pooling be prevented
This pooling can be stopped if one lays down such that
blood is evenly distributed in the veins. This is because central venous pressure is increased that
increases end diastolic volume which increases stroke volume. There is now increased cardiac
output.
The effects discussed above are due to intrinsic properties of the heart (mainly Starling’s law).
What are the responses to orthostasis
There are however also reflex responses to orthostasis. These responses work to maintain blood pressure
despite there being less filling pressure in the heart.
What happens when unloading of the baroreceptors occurs
Unloading of the baroreceptors causes lower
afferent fibre activity to the brain. The result is the NTS does not activate the CVLM to inhibit the
RVLM as much. This allows the RVLM to be more effective at sending efferent sympathetic signals to
the heart and arterioles. This drives up heart rate and contractility, causes vasoconstriction to
increase TPR and inhibits the vagus from slowing down the heart. The overall effect is increased
blood pressure. This prevents under perfusion of the brain.
What worsens postural hypotension
Postural hypotension can be worsened by a number of factors. Alpha-adrenergic blockers and other
sympathetic blockers that reduce vascular tone are used to treat hypertension and angina. However,
a side effect can be postural hypotension. Varicose veins (valves compromised) impair central venous
return and therefore cardiac output and the result can be under perfusion of the brain. Lack of
skeletal muscle activity while standing up due to paralysis or forced inactivity can cause blood to pool
in legs and the baroreceptors are normalised to lower blood pressure and therefore no longer
compensate for the lack of venous return (e.g. long term bed rest or soldiers on guard).
This effect is
amplified in the sun (also increased core temperature) due to peripheral vasodilation and therefore
less volume available for cerebral circulation (e.g. standing up after a bath).
What does reduced circulating blood volume do - law
Reduced circulating
blood volume (e.g. haemorrhage) overpowers Starling’s law and the baroreflex and the result is
under perfusion of the brain.
What happens in microgravity
In microgravity, there is no difference between standing and lying down. There is in fact no pooling
around the body but blood instead is redistributed to the chest region. Blood returns to the heart
very easily and this causes high preload and the heart to therefore pump much faster. This causes
loading of the baroreceptors that reduces the sympathetic tone on the CVS. This lowers heart rate
and causes vasodilation to reduce blood pressure. ADH and the RAAS system is basically turned off
due to the lack of need of higher blood pressure. There is in fact an increase in GFR due to the body
not needing as much blood volume. ANP is increased to further reduce blood pressure. The overall
effect is blood volume is reduced by around 20% and the body adapts. The heart muscle reduces in
mass due to reduce stress on the heart. There is also a general drop in blood pressure.
