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.
Extrinsic factors that effect arterioles
Extrinsic factors include neural innervation
provided by the sympathetic nervous system. This produces around one pulse per minute of
noradrenaline that mainly constricts blood vessels. A second extrinsic factor is hormones that can
include adrenaline (extrinsic as it is released from the adrenal gland) that also causes constriction.
What is myogenic response
The myogenic response is a local response that affects arterial radius.
What would happen is blood vessels could not vary in diameter
If blood vessels could not vary
in diameter, the blood flow to organs would vary widely as in low pressure, TPR resists blood flow
whilst in high pressure, blood flow is increased greatly (see right). This means organs would receive
very large differences in blood quantities. This is unsustainable as flow would be too low or too high
in most situations.
Outline the mechanism of constriction
This is why blood vessels tend to resist blood flow when stretched by constricting.
The mechanism behind this is that when the smooth muscle surrounding blood vessels are stretched,
this causes ion channels to open which leads to depolarisation and therefore muscle contraction (see
red line). This property of these muscles is intrinsic (known as Bayliss myogenic response).
What does viscosity of blood cause
Viscosity of blood increasing will reduce blood flow. It is a measure of the internal friction of the
blood that opposes the separation of the lamina in lamina blood flow.
What is the normal viscosity of blood
Viscosity of blood increasing will reduce blood flow. It is a measure of the internal friction of the
blood that opposes the separation of the lamina in lamina blood flow.
What factors affect heamatocrit ( no of red blood cells )
Factors affecting the haematocrit include how hydrated a
person is and how many RBCs a person has (training at altitude or taking EPO).
What is polycytyaemia
Clinically
polycythaemia is when there are too many RBCs in the blood whilst anaemia is when there are not
enough RBCs in the blood.
What does tube diameter affect
The tube diameter also affects viscosity.
Where does the farhaeus- lindqvist effect happen
The Farhaeus-Lindqvist effect
takes effect in vessels that are less than 100 micrometres in diameter. In these vessels, the flow
through them increases as the RBCs move towards the centre (to reduce friction with the walls that
have solvation shells). This means blood flow in micro vessels like capillaries is high.
Compare viscosity in sickle cell
The ability of
RBCs to deform to fit through capillaries also affects viscosity. In sickle cell anaemia for example, the
viscosity is increased as the RBCs are not flexible. The velocity of blood can also affect viscosity.
What can slow venous flow cause
For
example, slow venous flow in immobile legs can increase viscosity due to partial clotting.
The veins are the reservoir vessels of blood that hold around 60% of it.
What happens to veins during exercise
Blood can be diverted from
these vessels in times of need like exercise or haemorrhage (loss of blood from closed system).
Describe the structure of veins
The
veins have thin walls and are collapsible. They are voluminous vessels that do contain some smooth
muscle that is innervated by sympathetic nerves meaning that they can contract. This muscle
however is more compliant to blood flow.
What happens when veins contract - effects
The contraction of these vessels expels blood into central
veins that serves the purpose of increasing central venous return and therefore end diastolic volume.
This increases stroke volume according to Starling’s law. Contraction of muscle around veins
squeezes the veins and as they are unidirectional (one-way valves), this moves more blood back to
the heart.
How is central venous pressure returned
Breathing moves the diaphragm which increases the pressure in the abdomen which
squeezes the veins in them and therefore central venous
return (typical venous pressures shown on the right).
Why is pressure high in the legs
Pressure is quite high in the legs due to gravity.
What is the effect of high and low pressure in veins
Low
pressure in veins causes them to collapse whilst high
pressure causes them to expand.
What can the sympathetic nerve do to veins
The sympathetic nerves
can stimulate expanded veins to constrict slightly to send more blood back to the heart.
What does Bernoulli’s law explain
Bernoulli’s law explains blood flow to the lower part of the body.
Pressure in arteries at the head is
around 60 mmHg, 95 mmHg near the heart, and 185 mmHg at the feet.
What law explains blood flow from the heart to feet
The flow of blood from the
heart to the feet can be explained by Bernoulli’s law and not Darcy’s.
Describe the energies that blood possesses
The blood has kinetic energy
and potential energy to oppose the blood pressure preventing flow.
This is however a vulnerable
system as a 5 mmHg difference can disrupt blood circulation at the feet.
