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Summary Haemodynamics Pt2
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Summary of Haemodynamics Pt2
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Haemodynamics IIFactors affecting volume, pressure and distribution of blood in veins
Description of CVP and major mechanisms of venous return
Vascular resistance – Poiseuille’s law
Explanation of how arteriolar radius regulates local blood flow and arterial blood
pressure
The main factors governing blood viscosity and their significance in anaemia and
polycythaemia
Factors involved in control of blood flow at the organ and tissue level
Venous Blood Pressure
-60% of our blood volume at rest is in the systemic veins and venules. Therefore our
venous system is acting as a blood reservoir. Blood can be diverted from it in times of
need e.g. exercise, haemorrhage, to bring more blood to the heart and
increase/maintain SV and BP.
Venous Pressures and Venous Return:
Veins are thin-walled and therefore collapsible, because of this they are voluminous
(can take up volume) vessels and due to this they contain 2/3ths of our blood volume.
They are also contractile (they can contract, radius can be decreased) because they
contain smooth muscle that is innervated by sympathetic nerves
When we contract veins, we increase venous return and increasing venous return to
the heart. This is different to when we contract arteries where flow is reduced.
So contraction of these vessels expels blood into the central vessels which will
increase end-diastolic volume and thus increase stroke volume (due to Starling’s law).
Typical venous pressures (mmHg) include:
Limb vein, heart level = 8-10
Central venous pressure (entering the heart) = 0 – 7
Foot vein, standing = 90
Pressure-volume curve of veins
As we increase local venous pressure our vessels expand and it only takes a small
increase in local venous pressure to increase the volume of blood in our veins.
However when we stimulate our veins through the sympathetic system, we are
causing venoconstriction this means they can no longer hold as much volume and
blood is shifted centrally (to the heart) this increases CVP (due to increased venous
return).
, How Does Blood Return to the Heart?
Firstly there is a pressure gradient (although small) that helps blood get back to the
heart from the feet. The pressure in venules is around 12-18mmHg, whereas in large
veins/RA it is below 5mmHg.
So Venous return = Venous Pressure – Pressure in RA/Venous resistance
(amount blood is pushing to go minus the resistance to it)
We also have a thoracic pump, when we inhale our thoracic cavity expands which
increases our abdominal pressure, this forces blood upwards towards the heart. This
means a greater RV SV. Through this ^ we know blood flows faster with inhalation.
Finally is the skeletal muscle pumps, contraction of leg muscles will return blood to
the RA and any retrograde (back) flow is prevented by our venous valves.
When in upright position we decrease the high local venous pressure, which helps
blood get back to the heart -> so increased CVP + SV in exercise (due to constriction of
vessels?). It also reduces swelling of the feet and ankles as lower venous pressures
means lower capillary pressure and hence less filtration.
In contrast, standing still (e.g. a soldier on parade) for a long time can lead to fainting
due to gravity preventing/reducing blood return to the heart, so we get less SV and
less perfusion of our brain. Heat induced venodilation will make this worse.
What Controls Vascular Resistance?
So Darcy’s law of flow states that
Blood flow (CO) = Pa – CVP
TPR
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