Control Of Blood Flow

Question 1

What does darcys law describe

Answer 1

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.

Question 2

What is darcys law as an equation

Answer 2

The law would therefore be
CO = (arterial pressure – venous pressure) x G. Total peripheral resistance is what really controls
blood flow.

Question 3

What governs tpr

Answer 3

TPR is governed by Darcy’s law, Poiseuille’s law, the myogenic response and blood
viscosity.

Question 4

What does tpr affect

Answer 4

TPR affects both blood flow and pressure.

Question 5

Describe the pressure drop in normal blood flow

Answer 5

During normal blood pressure, there is a pressure drop

between the arteries and arterioles that allows blood to flow down its pressure gradient.

Question 6

What happens if the blood pressure is low

Answer 6

If blood

pressure is low, the arterioles vasodilate to decrease TPR and maintain the pressure gradient.

Question 7

What happens when blood pressure is high

Answer 7

If blood
pressure is high, the arterioles vasoconstrict that increases blood pressure upstream but reduces
blood flow. This is due to increased TPR.

Question 8

What is hypertension

Answer 8

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.

Question 9

What happens to the body in sedentary state

Answer 9

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.

Question 10

What happens to the body when exercising

Answer 10

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.

Question 11

What organs are always perfused

Answer 11

There are some organs like the kidneys and the brain that are always perfused.

Question 12

What does poiseullie law describe

Answer 12

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.

Question 13

What does poiseuilles law suggest about radius - state equation

Answer 13

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.

Question 14

What does the nature of the p equation of radius suggest

Answer 14

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.

Question 15

How can the p and d law be combined

Answer 15

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).

Question 16

What blood vessel has the largest drop in pressure

Answer 16

During blood flow across the CVS, the arterioles are the blood vessels where the largest pressure
drop takes place (around 40 to 50 mmHg).

Question 17

What controls arteriol radius

Answer 17

Arteriole radius is controlled by sympathetic nerves and

changes in radius have a significant effect on resistance (as seen above).

Question 18

How do arterioles provide great resistance

Answer 18

These vessels are also quite

long meaning they provide greater resistance that way.

Question 19

What blood vessel doesn’t contribute to TPR

Answer 19

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.

Question 20

Why do capillaries have less resistance

Answer 20

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.

Question 21

What are the two ways blood flow can be controlled

Answer 21

Blood flow can be controlled locally or centrally.

Question 22

How is local blood controlled

Answer 22

Local blood
flow through individual organs is mainly controlled by
changes to the radius of arterioles supplying a given organ.

Question 23

What are the two ways arterioles are controlled

Answer 23

Arteriole radius can be controlled intrinsically or extrinsically.

Question 24

What can dilate arterioles

Answer 24

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.

Question 25

Extrinsic factors that effect arterioles

Answer 25

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.

Question 26

What is myogenic response

Answer 26

The myogenic response is a local response that affects arterial radius.

Question 27

What would happen is blood vessels could not vary in diameter

Answer 27

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.

Question 28

Outline the mechanism of constriction

Answer 28

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).

Question 29

What does viscosity of blood cause

Answer 29

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.

Question 30

What is the normal viscosity of blood

Answer 30

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.

Question 31

What factors affect heamatocrit ( no of red blood cells )

Answer 31

Factors affecting the haematocrit include how hydrated a

person is and how many RBCs a person has (training at altitude or taking EPO).

Question 32

What is polycytyaemia

Answer 32

Clinically
polycythaemia is when there are too many RBCs in the blood whilst anaemia is when there are not
enough RBCs in the blood.

Question 33

What does tube diameter affect

Answer 33

The tube diameter also affects viscosity.

Question 34

Where does the farhaeus- lindqvist effect happen

Answer 34

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.

Question 35

Compare viscosity in sickle cell

Answer 35

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.

Question 36

What can slow venous flow cause

Answer 36

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.

Question 37

What happens to veins during exercise

Answer 37

Blood can be diverted from

these vessels in times of need like exercise or haemorrhage (loss of blood from closed system).

Question 38

Describe the structure of veins

Answer 38

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.

Question 39

What happens when veins contract - effects

Answer 39

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.

Question 40

How is central venous pressure returned

Answer 40

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).

Question 41

Why is pressure high in the legs

Answer 41

Pressure is quite high in the legs due to gravity.

Question 42

What is the effect of high and low pressure in veins

Answer 42

Low
pressure in veins causes them to collapse whilst high
pressure causes them to expand.

Question 43

What can the sympathetic nerve do to veins

Answer 43

The sympathetic nerves

can stimulate expanded veins to constrict slightly to send more blood back to the heart.

Question 44

What does Bernoulli’s law explain

Answer 44

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.

Question 45

What law explains blood flow from the heart to feet

Answer 45

The flow of blood from the

heart to the feet can be explained by Bernoulli’s law and not Darcy’s.

Question 46

Describe the energies that blood possesses

Answer 46

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.