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Summary co-ordinated cardiovascular responses to changes in posture and exercise
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Summary of the co-ordinated cardiovascular responses to changes in posture and exercise
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Co-ordinated Cardiovascular Responses to Changes in Postureand Exercise
Key Points:
Description of orthostasis or ‘standing up’ on the CVS due to effect of gravity on
venous and arterial blood pressure
Changes in cardiac output, central venous filling, heart rate and arterial pressure in
response to changes in posture
Cardiovascular changes in response to exercise
Description of mechanisms leading to increased blood supply to exercising muscle
How blood pressure is controlled during exercise
Medical implications of different types of exercise
The cardiovascular system must be able to respond to important changes in
physiological adapations. Many components of the CVS respond and co-ordinate
together to produce the everyday demands of life.
There is the principle of adaptation by integration, which is where major responses
are caused by integration of lots of small changes.
Changing Posture – Orthostasis
On standing up (orthostasis), the CVS changes according to the effect of gravity, blood
pressure falls at first (postural hypotension), this is because gravity pulls blood down
to the feet leading to less CVP, less Starling’s law and less CO. Less CO drops the BP.
Because there is a drop in BP it means there is a lack of drive of flow to the brain
(which is above the heart), this can cause fainting.
We need to be able to quickly recover from this, because we are constantly getting up
and down, this is due to homeostatic mechanisms. We have integration of three
smaller changes which modulate this:
We have an increase in HR
An increase in ventricular contractility
^(tries to maintain SV/CO despite less blood returning to heart)
Increase in total peripheral resistance (+constriction of veins)
The reason we get this is because the blood vessels in our leg are like tubes with fluid
in it. These tubes have different pressures at different depths, depending on the height
of the fluid column. This is due to the equation:
Pressure = rhg
r = fluid density
h = height (of fluid column)
g = gravitational acceleration constant (gravity)
So because of this equation, the fluid at the bottom of the tube will be under a greater
pressure than the fluid at the top of the tube.
, This is why you get increased pressure in your feet, due to the effect of gravity pulling
down on your blood.
We can apply this to real life, when lying down in the supine position our BPs
throughout our body (in head, around heart and feet) are relatively constant. This is
because the tube of blood is horizontal.
In venous system pressures are also relatively equal in head and feet also, except it is
slightly lower in around heart where there are the great veins. This is because they are
larger with less resistance, lowering pressure (important as helps blood flow back to
heart).
However when we stand up, we get the effect of gravity. In arterial system, the mean
BP stays relatively similar but in the head, which is above the heart, has a BP much
lower whereas in feet it is much higher.
In venous system, in the feet it is also high and then decreases back to the heart till it is
low.
So in the feet:
Arterial pressure has increased from 95 to 185 mmHg now standing
Venous pressure has increased from 10 to 90 mmHg
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