Properties Of Special Circulations

Question 1

Name the special circulations in the body

Answer 1

There are many special circulations in the body including cerebral (brain), pulmonary (lungs), skeletal
muscle, renal (kidneys) and gastrointestinal circulations. These all have unique characteristics and
special requirements that they meet.

To meet these requirements, they have special structural or
functional features and problems to these have many consequences.

Question 2

Where does coronary circulation originate from

Answer 2

Coronary circulation originates from two coronary arteries (not from chambers due to inefficiency as
distance of diffusion to some cells are very large). The left and right coronary arteries originate from
the aorta and they branch and travel round the heart into capillaries before the capillaries collect
into venules and then veins that collect in the coronary sinus.

Question 3

Where does the coronary move back into

Answer 3

The coronary sinus then moves back

into the superior vena cava.

Question 4

Why do we have coronary circulation

Answer 4

The reason for coronary circulation is because cardiac muscle needs 20
times more oxygen then skeletal muscle. This increase in demand is proportional to increase in
cardiac work.

Question 5

What are unique features of coronary circulation

Answer 5

The reason for coronary circulation is because cardiac muscle needs 20
times more oxygen then skeletal muscle. This increase in demand is proportional to increase in
cardiac work.

Question 6

What are unique features of coronary circulation

Answer 6

This means coronary circulation has some unique features including high capillary
density for large surface area for exchange of oxygen. This reduces the diffusion distance between
the capillary and the heart muscle resulting in quicker diffusion to cardiac muscle (time is
proportional to square of distance when discussing diffusion).

Question 7

Why does cardiac muscle fibres have fine fibres compared to skeletal muscle

Answer 7

The cardiac muscle fibres are very fine
and small compared to skeletal muscle meaning there are a far greater number of capillaries
supplying them.

Question 8

Diameter and density of cardiac fibre vs skeletal

Answer 8

The fibre diameter is around 18 micrometres (as opposed to 50 micrometres in
skeletal muscle) and the density of capillaries is 3000/mm2 (as opposed to 400/mm2 in skeletal
muscle).

Question 9

Describe the nerve innervation of the heart

Answer 9

The heart also has relatively sparse sympathetic innervation whilst there is
high NO released leading to vasodilation (increase blood flow).

The amount of oxygen extracted from
circulation is also high at 75% compared to an average of 25% around the rest of the body.

Question 10

What happen to coronary blood in high demand

Answer 10

During
increased demand, coronary blood flow increases in proportion to demand. Vasodilators like
adenosine, potassium and acidosis are produced in the heart that outcompete the relatively low
sympathetic vasoconstriction. An increase in circulating adrenaline interacts with the abundant beta-
2 adrenoreceptors found in the heart further increasing vasodilation.

Question 11

How is the Bohr shift important for myocardium

Answer 11

The Bohr shift plays an important role in oxygen unloading onto myocardium.

Oxyhaemoglobin
dissociation curves can shift right or left from normal depending on the conditions they are in.

Question 12

When does more oxygen bind to heamoghlobin - factors

Answer 12

More
oxygen will bind to haemoglobin in conditions of high pH, decreased 2,3-DPG and decreased
temperature whilst an increase in these factors decreases the affinity of oxygen to haemoglobin.

Question 13

Why does coronary sinus blood jabs a high co2 conc

Answer 13

Coronary sinus blood that returns to the right atrium has a greater carbon dioxide content due to
high capillary density, surface area and small diffusion difference in the heart.

Question 14

What happens to the curve when high co2 and low ph

Answer 14

The high CO2 and low
pH shifts the curve to the right and more O2 is given up to the myocardium allowing to it extract 75%
of oxygen in haemoglobin rather than 25% in other tissues.

Question 15

How does the myocardium increase oxygen supply when efficiency is high

Answer 15

Because the efficiency of oxygen
extraction from haemoglobin is already very high, the way of increasing oxygen supply to the
myocardium is by increasing blood flow.

Question 16

How is the myocardium able to increase blood flow - name

Answer 16

This is done through myocardium metabolism that produces
vasodilators as one of the metabolites which then cause an increase in blood flow. This process is
known as metabolic hyperaemia.

Question 17

What is the name of problems associated with the coronary circulation

Answer 17

Problems associated with coronary circulation include ischaemic heart disease. Decreased perfusion
of the coronary arteries that are functional end arteries (not many cross links between arteries)
causes major problems.

Question 18

What is an example of sudden or elongated event which decreases perfusion of the coronary arteries - give eg

Answer 18

Any blockage of these arteries leads to damage to heart tissue. This can
happen in two ways; a sudden event or slow event. An example of a sudden event is acute
thrombosis and this results in myocardial infarction (heart attack). This is because an area of the
heart is starved and it dies. An example of a slow event is atheroma.

Question 19

What is atheroma

Answer 19

This is where sub-endothelium
lipid plaques build up along the walls of the heart arteries. This causes narrowing of the lumens of
these arteries and the result is an area of the heart is gradually starved. Symptoms of this include
pains like angina.

Question 20

Describe the structure of coronary arteries

Answer 20

Coronary arteries are functional end arteries. There are low numbers of cross branching collateral
vessels whereas these cross branches are present in most other vessels in the body.

Question 21

What happens if one of the vessels of the coronary arteries are blocked

Answer 21

A block in one

vessel leads to tissue that is supplied by that vessel to be starved.

Question 22

What happens to the body in high demand when coronary arteries are blocked

Answer 22

When demands of the body
increase (e.g. exercise or high blood pressure), this under perfused area either forms a zone of
necrosis that results in infarction if it is completely cut off or forms a peripheral rim of partial
ischaemia if it is partly supplied by an unblocked artery and partly by a blocked one. An example of a
blockage is one in the left anterior descending artery. This could be caused by atherosclerosis or
caused by a clot. The whole area of tissue it supplies becomes ischaemic due to the blockage. The
result of this is the stimulation of nociceptive C-fibres that cause the sensation of pain. There is also
impaired contractility of this region of the heart.

Question 23

What can cause arrhythmias

Answer 23

Arrhythmias can be caused by inefficient conduction

across the ischaemic region of the heart. There is eventually cell death (necrosis).

Question 24

What happens in an angiography

Answer 24

In angiography, an X-ray opaque dye is injected into the vessels around the heart and these can then
be tracked around the heart. Regions where the die can’t access or flow is restricted due to occlusion
or partial-occlusion.

Question 25

What can be inserted into arteries to open them

Answer 25

Arteries that are in this situation can have a stent placed in them to open them.

Question 26

Compare angina to normal heart and blood flow

Answer 26

Angina problems are aggravated during increased activity. In a normal heart, resistance is low in the
larger coronary arteries with the majority of the resistance provided by the arterioles. During
exercise, the arterioles vasodilate to reduce pressure and as a result increase blood flow to meet
increased oxygen demands. In angina, a large coronary artery that has stenosis due to atheroma has
a much higher pressure and metabolic hyperaemia occurs to vasodilate arterioles and reduce their
resistance. This allows blood flow to meet the needs of the cardiac cells. The heart is taking nearly all
the oxygen it can get at rest. During exercise, arterioles further dilate to reduce resistance however,
total resistance is still too high to meet the heart’s demands due to stenosis. The result of this is that
oxygen demand cannot be met and the pain sensation is sent down the nociceptive C-fibres that is
characteristic with angina.

Question 27

When can coronary blood flow only occur

Answer 27

Coronary blood flow can only occur in the heart during diastole because in systole, all the blood
vessels are squeezed due to the contraction of cardiac muscle around them.

Question 28

What happens when heart rate is increased

Answer 28

Increasing heart rate
means there is less time spent in diastole and therefore less time for blood to pass through the
heart.

Question 29

What effects coronary circulation

Answer 29

Increased end-diastolic pressure caused by heart failure (e.g. aortic stenosis or stiffness of the
ventricle caused by previous myocardial infarction for example) also affects coronary circulation
(heart is less relaxed). Low blood pressure caused by aortic regurgitation when there is problems
with the aortic valves means that coronary circulation occurs at a lower pressure and therefore blood
flow can be reduced unless there is some compensatory measure.

Question 30

Describe cutaneous circulation

Answer 30

These are some of the factors that
can affect coronary circulation.
Another special circulatory system of the body is cutaneous circulation (skin). Impaired microvascular
blood flow through the skin causes skin ulcers. The skin is a defence against the external
environment. It is used in temperature regulation through blood flow that delivers heat from body
core and radiation that is proportional to skin temperature. Sweating, as well a large temperature
tolerance range (the skin can range from without damage (0 to 40 oC)) also allow for temperature
regulation.

Question 31

What is skin temperature dependant on

Answer 31

The skin also conducts and facilitates convection. Skin temperature is dependent upon
blood flow and ambient temperature (room temperature).

Question 32

What are the cutaneous circulation special structural features

Answer 32

Cutaneous circulation has some special structural features. The skin, like most other circulatory
systems, has arteries supplying it that split up into arterioles and then capillaries, and venules
collecting blood into veins. The skin, however, also has arteriovenous anastomosis that join venules
directly to arterioles. This allows blood flow to be stopped through the skin by vasoconstriction and
bypass it all through anastomosis. The vasoconstriction is facilitated by sympathetic vasoconstrictor
fibres that release noradrenaline.

Question 33

What so sudomotor vasodilator fibres do

Answer 33

Sudomotor vasodilator fibres release acetylcholine to act on the
endothelium to produce nitric oxide for vasodilation. These two fibres are driven by temperature
regulation nerves in the hypothalamus.

Question 34

What happens when the skin is first exposed to cold

Answer 34

When the skin is first exposed to cold, cold-induced
vasoconstriction occurs to conserve heat in the body and prevent its loss through the skin. The
sympathetic nerves react to local cold by releasing noradrenaline that bind to alpha-2 receptors on
vascular smooth muscle in the skin (alpha-2 bound to by noradrenaline at lower temperatures than
3
alpha-1).

Question 35

What halogens if the skin remains cold

Answer 35

However, if this continues, the skin can be damaged due to not enough oxygen supply. This
is why paradoxical cold vasodilation kicks in. The sympathetic nerves are paralysed due to extended
exposure to cold temperatures, and noradrenaline production stops. When the temperature in the
skin is high enough again, the sympathetic nerves begin producing noradrenaline again and the cycle
repeats (person turns bright red, then pale, then red….etc). Also, increased core temperature due to
a factor like exercise causes vasodilation. This is because the warmth receptors in the anterior
hypothalamus are stimulated and sympathetic sudomotor activity is increased to produce more
acetylcholine to act on the endothelium.

Question 36

What causes sweating

Answer 36

Sweating is also caused when core temperature increases

due to increased sympathetic activity at sweat glands.

Question 37

What is baroreflex

Answer 37

Other functional specialisations of cutaneous circulation include the baroreflex. This is where blood
flow is directed to the more important organs like the brain, kidneys and liver during loss of blood
pressure. This can be caused by haemorrhage, sepsis and acute cardiac failure.

Question 38

What does the baroreflex stimulate

Answer 38

The baroreflex
stimulates sympathetic vasoconstriction fibres, as well as the production of vasopressin, angiotensin
II and adrenaline. This effect is what is responsible for pale cold skin of patient in shock.

Question 39

How is warmimg someone who has suffered from, heamorrage potentially dangerous

Answer 39

Warming a
person who has suffered from haemorrhage can be potentially dangerous as cutaneous
vasoconstriction is reduced and blood flow to the skin is increased. This can be potentially dangerous
as blood flow to more important organs can be compromised. A different specialisation of cutaneous
circulation is the emotional centres in the brain linked to some sudomotor nerves (e.g. blushing).
The response to trauma of the skin can be described through the Lewis triple response.

Question 40

What fibres give the sensation of pain - describe how they function

Answer 40

Nociceptive
afferent C fibres are stimulated to give the sensation of pain.
There are however branches to this
main fibre such that if the C fibres are fired enough, the send an impulse down these branches as
well as one to the spinal cord.

Question 41

What are the c fibre branches and what they do

Answer 41

These branches are known as collateral axons that release substance P
that causes mast cell degranulation and histamine. This causes vasodilation of blood vessels. The
triple response is local redness, local swelling due to oedema caused by an increase in the
permeability of blood vessel walls and spreading flare as the surrounding blood vessels vasodilate.

Question 42

What are special problems associated with cutaneous circulation

Answer 42

There are however some special problems associated with cutaneous circulation. Prolonged
obstruction of blood flow to the skin by compression due to lack of movement can cause severe
tissue necrosis. This is associated with bed sores that occur at heels, buttocks and weight bearing
areas. This is because diffusion only occurs over a small distance and compressing one area of skin
for long periods of time means it will not receive oxygen or nutrients that keep it alive.

Question 43

How can sores of skin be avoided

Answer 43

This problem
can be avoided by shifting position and turning the patient. The removal of compression causes
reactive hyperaemia due to lack of oxygen supply. The skin has quite a high tolerance to ischaemia.

Question 44

What is a gravity related problem cutaneous circulation

Answer 44

Another problem associated with cutaneous circulation is postural hypotension that is caused by
gravity. Standing for long periods of time will decrease central venous pressure and increase capillary
permeability that causes oedema.