Control of Blood Flow in Differing Vascular Beds

Question 1

What is the effect on coronary flow resistance and CO increases?

Answer 1

As CO increases with an increase in activity level (exercise) the coronary flow resistance increases.

Question 2

a-VO2 definition

Answer 2

Arteriovenous oxygen difference.

It presents the difference in oxygen content in the blood between the arteriole blood and the venous blood

Question 3

The effects of a very large difference in a-VO2

Answer 3

The cells extracts almost maximum amount of O2 possible at rest. Therefore increased demand for O2 must be met by large increases in flow.

Question 4

Where would a small a-VO2 difference be found in the body

Answer 4

Organs with a low metabolic need / low O2 need (e.g. skin, kidney)

Question 5

Why is flow to coronary muscle intermittent?

Answer 5

The arteriole pressure (Pa) needs to be greater than the venous pressure (Pv) for the vessel to be open AND the Pinside pressure (pressure in the aorta) needs to be greater than Poutside (pressure in the ventricle) for the vessel to be open.

Question 6

If the ventricular pressure is greater than the aortic pressure will the blood flow?

Answer 6

There will be no blood flow

Question 7

What does Pout mean and what happens during systole to the vessels in the left side of the hear?

Answer 7

Pout = Pressure in the ventricle. During systole the vessels are compressed by the high pressure in the ventricle.

Question 8

Where does most blood flow during diastole in the left side of the heart?

Answer 8

Most blood flows to the left myocardium during diastole (85%)

Question 9

What does Pin mean?

Answer 9

Pin means aortic pressure. During diastole this determines flow.

Question 10

Why is the right ventricular pressure much lower than the left?

Answer 10

The right side of the heart is only pushing against the pulmonary circulation, where as the left side of the heart is pushing against the systemic circulation.

Question 11

How is the coronary flow controlled?

Answer 11

Coronary arteries exhibit myogenic autoregulation in pressure range 60-100 mmHg. There is some sympathetic control present but is overridden by local control.
Metabolic hyperaemia is the dominant form of regulation. As the cardiac muscles contract this produces lots of metabolites e.g. NO, K+, etc. The higher the metabolic rate the more metabolites.

Question 12

What is angina?

Answer 12

Narrowing of the coronary artery, when the O2 demand goes up (exercise) it is harder to supply enough oxygen to the heart muscle. This causes anaerobic respiration and the build up of lactic acid causing pain known as angina.

Question 13

Cerebral circulation ensures what?

Answer 13

1. Maintains totally secure O2 supply to the brain tissue (myogenic autoregulation) if this stops = death.
2. After local flow according to activity functional hyperaemia (metabolic regulation)

Question 14

Structural adaptations to cerebral circulation ensure blood reaches all parts of the brain. What adaptations at there?

Answer 14

1. Short arterioles, dense capillary network
2. Relatively high vascular resistance
3. Cerebral perfusion maintained if a carotid artery obstructed, because of the circle of Willis. If an artery to the brain is blocked, because of the circle of Willis, cerebral perfusion is maintained.

Question 15

Endothelial cells in the blood brain barrier are continuous not fenestrated. What do tight junctions allow?

Answer 15

Tight junctions allow for regulation of what can diffuse/move across the blood brain barrier. This is very important. There is no bulk flow or vesicular transport across the blood barrier just tight junctions formed by the cerebral capillaries.

Question 16

What is the importance of regulating movement of the blood brain barrier?

Answer 16

Protects the neurones, maintains the environment.

Question 17

Why does heroine last so long in the brain?

Answer 17

The blood brain barrier allows diffusion of heroine across the cerebral endothelium. Once in heroine is metabolised into morphine. Morphine cannot diffuse back out, therefore it remains in the brain for long periods of time.

Question 18

Describe how regulation of other organs safeguards cerebral circulation

Answer 18

Peripheral vasoconstriction (except heart) can maintain arterial pressure. A decrease in blood flow to one organ is achieved to supply another with more O2.

Question 19

Describe how autoregulation is a functional adaptation in the brain to maintain perfusion

Answer 19

A change in blood pressure is met by a change in resistance to maintain perfusion.
Autoregulatory range is 60 - 170 mmHG

Question 20

Cerebral vessels are very responsive to arterial CO2 what are the effects.

Answer 20

Hypercapnia (high PaCO2 > 40 mmHg).
Hypocapnia (low PaCO2 < 40 mmHg).
Respiration is more tightly regulated to CO2 than O2

Question 21

What is hypercapnia and what is the response?

Answer 21

Hypercapnia is a high PaCO2 > 40 mmHg. 1 mmHg increase in PaCO2 causes 2-4% increase in flow by vasodilation.

Question 22

What is hypocapnia and what is the response?

Answer 22

Hypocapnia is a decrease PaCO2 < 40 mmHg. Vasoconstriction is required to decrease flow. Hyperventilation causes you to breath out a lot of CO2 this makes you dizzy.

Question 23

Cerebral vessels are less responsive to levels of arterial O2. What is hypoxia?

Answer 23

Hypoxia is low PaO2. 
Moderate hypoxia (100 down to 50 mmHg) evokes little change in cerebral flow. 
Sever hypoxia (100 down to 25 mmHg) leads to vasodilation to increase the flow by x2. Vasodilation is caused by adenosine, K+ or NO.

Question 24

What are the normal arterial values for PaO2 and PaCO2?

Answer 24

PaO2 = 13 kPa (100 mHg)
PaCO2 = 5 kPa (40 mmHg)

Question 25

Nervous control is important in determining cerebral flow. What are the effects of sympathetic stimulation?

Answer 25

The maximal sympathetic stimulation increases resistance by only 20-30%. Sympathetic stimulation shifts the autoregulatory curve (cerebral blood flow against mABP) to the right. Chronic hypertension does the same effect to the autoregulatory curve, you cannot get the blood pressure down to 90 mmHg therefore, the whole system shifts to suit a higher level.

Question 26

What problems are associated with cerebral vasculature?

Answer 26

Raised intracranial pressure (ICP) = a decrease in flow because the pressure outside is greater than inside.
ICP is increased by: tumour, intracranial bleeding, cerebral oedema.
Increased ICP causes: Collapses veins and decreases effective CPP reduces blood flow.

Postural syncope (fainting upon standing up) if baroreflex/autonomic activity is impaired e.g. with aging.
Cerebral ischemia - stroke (due to an area deprived of blood due to obstruction)
Haemorrhage stroke (area of bleeding de to weakened vessel wall ruptures causing bleeding in the brain.
Vascular dilation -headaches, migraine

Question 27

Cerebral Perfusion Pressure (CPP) equation

Answer 27

CPP = mean ABP - ICP

Question 28

What does cutaneous circulation do?

Answer 28

1. Regulate body temperature- skin is the major thermoregulator
2. Respond to trauma

Question 29

How do the arterioles of cutaneous circulation allow for heat loss?

Answer 29

An increase in core temperature causes a removal of alpha-adrenoceptor mediated sympathetic tone, this causes cutaneous vasodilation (arterioles under the skin) this allows heat loss.
Skin arterioles are under sympathetic control, rather than metabolic control.

Question 30

How do the AV anastomoses allow for heat loss

Answer 30

Increase in core temperature means a decrease in alpha1 sympathetic activity which causes dilation of arteriovenous (AV) anastomoses increasing blood flow into the venous plexus allowing for heat loss.

Question 31

What are the effects of sweating to decrease core temperature?

Answer 31

An increase in sweating to decrease core temperature, sweat contains an enzyme that causes the release of bradykinin (potent vasodilator). This increases TPR by the baroreceptor reflex and an increase in HR increases CO.

Question 32

How is core temperature increased?

Answer 32

Core temperature has decreased which stimulates the alpha-adrenergic receptors causing vasoconstriction and heat gain as heat is trapped near the core. Skin feels cold as there is less blood circulating there.

Question 33

How does the countercurrent exchange help increase cold blood returning to the heart?

Answer 33

Cold blood in the veins can be 13’ cooler than in the trunk. Radiation from warm arteriole to cold venous blood flowing in the opposite direction helps heat transfer and ‘traps’ heat near the trunk.

Question 34

What is Raynaud’s disease?

Answer 34

Skin vessels are ‘over reactive’.
The cold cases vasoconstriction (ischaemic attacks - low blood flow). Skin is white then blue (lack of O2) then red (when the blood flow returns) when warmer.
When blood flow returns it causes numbness, pain and a burning sensation.

Question 35

How can Raynaud’s be treated?

Answer 35

Could be treated by cutting the sympathetic fibres to the hands, this means however then hands are always warm and loosing heat. The best way is to prevent hands from getting cold.

Question 36

What is the triple response?

Answer 36

Pointed object drawn over skin, or small burn causes:
1. White reaction - blanching (stopped flow momentarily) due to mechanical stimuli.
2. Red reaction - local vasodilation, histamine?
3. Flare - wider intense vasodilation
4. Wheals / local oedema - skin is raised.
(2,3 and 4 are the triple response)