Control Of Blood Flow Flashcards
What does darcys law describe
There are a number of factors that affect blood flow affecting different blood vessels. Darcy’s law
describes the role pressure and resistance have on blood flow. In the formula, resistance can be
converted to conductance and this can be represented with the letter G.
What is darcys law as an equation
The law would therefore be
CO = (arterial pressure – venous pressure) x G. Total peripheral resistance is what really controls
blood flow.
What governs tpr
TPR is governed by Darcy’s law, Poiseuille’s law, the myogenic response and blood
viscosity.
What does tpr affect
TPR affects both blood flow and pressure.
Describe the pressure drop in normal blood flow
During normal blood pressure, there is a pressure drop
between the arteries and arterioles that allows blood to flow down its pressure gradient.
What happens if the blood pressure is low
If blood
pressure is low, the arterioles vasodilate to decrease TPR and maintain the pressure gradient.
What happens when blood pressure is high
If blood
pressure is high, the arterioles vasoconstrict that increases blood pressure upstream but reduces
blood flow. This is due to increased TPR.
What is hypertension
Hypertension is when there is over constriction of arterioles
that results in reduced capillary blood flow and therefore under perfusion.
Blood flow can change in response to changes in need.
What happens to the body in sedentary state
For example, when the body is in a sedentary
state (resting), the superior mesenteric (artery to intestines) is dilated and the common iliac (artery
to lower limbs) is constricted.
What happens to the body when exercising
When the body is exercising however, the superior mesenteric is
constricted and the common iliac is dilated. Blood supplying all the organs can be redirected and
controlled.
What organs are always perfused
There are some organs like the kidneys and the brain that are always perfused.
What does poiseullie law describe
Poiseuille’s law describes the factors that affect the total peripheral resistance. This law relates
length and viscosity to resistance. Both viscosity and length of the arteriole increase resistance.
What does poiseuilles law suggest about radius - state equation
The
law also states that the increase in radius decreases TPR. The equation used is; resistance = 8ηL/πr4
where η (eta) represents viscosity, L represents length and r represents radius.
What does the nature of the p equation of radius suggest
Since the radius is to
the power of 4, this means any small change in radius has a big effect on resistance. For example,
doubling the radius will decrease resistance by 16 times. If we consider this law in terms of
conductance, the equation is just flipped.
How can the p and d law be combined
Both Poiseuille’s and Darcy’s law can be combined by
substituting conductance with the equations to give CO = (Pa – CVP)πr4/8ηL (in this equation, Pa is
pressure arterial and CVP is central venous pressure).
What blood vessel has the largest drop in pressure
During blood flow across the CVS, the arterioles are the blood vessels where the largest pressure
drop takes place (around 40 to 50 mmHg).
What controls arteriol radius
Arteriole radius is controlled by sympathetic nerves and
changes in radius have a significant effect on resistance (as seen above).
How do arterioles provide great resistance
These vessels are also quite
long meaning they provide greater resistance that way.
What blood vessel doesn’t contribute to TPR
Capillaries however have an even smaller
radius than arterioles but they do not contribute much to TPR. The length of capillaries are quite
short (many in number but arranged in parallel compared to arterioles that are more in series) and
blood flow is bolus in them that has the effect of reducing viscosity.
Why do capillaries have less resistance
Capillaries have very little
nervous innervation or smooth muscle that can vary their radius and affect blood flow. This means
the capillaries have less resistance and therefore there is less of a pressure drop across capillaries.
According to the combined Darcy’s and Poiseuille’s law, this means blood flow is not changed very
much.
What are the two ways blood flow can be controlled
Blood flow can be controlled locally or centrally.
How is local blood controlled
Local blood
flow through individual organs is mainly controlled by
changes to the radius of arterioles supplying a given organ.
What are the two ways arterioles are controlled
Arteriole radius can be controlled intrinsically or extrinsically.
What can dilate arterioles
Tissue metabolites can dilate arterioles as the tissue is
becoming hypoxic due to the reactions taking place in it.
Endothelial factors (e.g. nitric oxide that is constantly being
produced that dilates vessels) or local hormones can also be
produced.