Control of Blood Vessels: Blood Flow Regulation

Question 1

Cerebral cardiac output

Answer 1

14% at rest

Question 2

Factors that may regulate blood flow in different vascular beds

Answer 2

neural
hormones
local
mechanical
special features

Question 3

Neural control of cerebral blood flow

Answer 3

relatively minor
(α vasoconstriction)

Question 4

Hormonal control of cerebral blood flow

Answer 4

minor

Question 5

Local control of cerebral blood flow

Answer 5

autoregulation over wide range of pressure
important metabolic control during mental activity (regional). H+, K+, adenosine, hypercapnia, hypoxia -vasodilatation
Endothelin may be important vasoconstrictor in pathological states e.g. subarachnoid haemorrhage

Question 6

Mechanical control of cerebral blood flow

Answer 6

constrained in rigid cranium; importantly influenced by CSF pressure e.g., space-occupying lesions increase ICP & reduce CBF

Question 7

Special features that control cerebral blood flow

Answer 7

medullary ischaemic reflex (Cushing) e.g., tumour-induced reduction in CBF causes medullary ischaemia which stimulates an increase in BP in an attempt to restore CBF

Question 8

Cardiac output for coronary

Answer 8

4%

Question 9

Neural control of coronary blood flow

Answer 9

minor direct influence (α vasoconstriction) but secondary effect on flow due to changes in cardiac function and hence metabolism
Sympathetic stimulation causes b-mediated increase in HR & StV which increases O2 consumption

Question 10

Local control of coronary blood flow

Answer 10

major influence of metabolites: hypoxia, hypercapnia, adenosine cause vasodilatation

Question 11

Hormonal control of coronary blood flow

Answer 11

adrenaline - vasodilator and stimulates metabolism

Question 12

Mechanical control of coronary blood flow

Answer 12

major influence on flow during the cardiac cycle;
peak flow in early diastole, zero or negative flow at onset of systole
compression at systole and relief of compression leads to increase of blood flow in diastole

Question 13

Special feature control of coronary blood flow

Answer 13

parallelism between metabolism and flow

Question 14

Skin cardiac output

Answer 14

4% at rest in thermoneutral environment

Question 15

Neural control of skin blood flow

Answer 15

arterioles have a relatively weak innervation (α vasoconstriction)
A-V anastomoses have a dense innervation (α vasoconstriction)
increase in core temperature causes AVAs to dilate, increasing skin blood flow and hence heat loss

Question 16

What do A-V anastomoses do?

Answer 16

capacity to deliver blood to the surface of the skin

Question 17

Local control of skin blood flow

Answer 17

arterioles have some degree of myogenic autoregulation
A-V anastomoses show no autoregulation and no reactive hyperaemia
Endothelin may be involved in pathological states (Raynauds)

Question 18

What is Raynauds?

Answer 18

resitriction of blood flow in digits

affects women more than men

Question 19

Hormonal control of skin blood flow

Answer 19

angiotensin, vasopressin, noradrenaline, adrenaline all cause vasoconstriction

Question 20

Mechanical control of skin blood flow

Answer 20

minimal

Question 21

Special features control of skin blood flow

Answer 21

primary function is thermoregulation
sweat glands have sympathetic cholinergic innervation (sudomotor) - vasodilatation via release of e.g. bradykinin - leaky capillaries, vasodilatation in arterioles

Question 22

Skeletal muscle cardiac output

Answer 22

15% at rest

Question 23

Neural control of skeletal muscle blood flow

Answer 23

rest : important α vasoconstriction, some β vasodilatation, maybe sympathetic cholinergic vasodilatation
exercise: very little neural influence, some β vasodilatation

Question 24

What is skeletal muscle involved in?

Answer 24

systemic BP regulation. Skeletal muscle ~ 40% of body mass, hence vasoconstriction has large influence on TPR

Question 25

Local control of skeletal muscle blood flow

Answer 25

rest: neural control (baroreflexes) over-ride autoregulatory mechanisms exercise: local metabolites have a major influence (K+, adenosine, lactate etc)

Question 26

Hormonal control of skeletal muscle blood flow

Answer 26

adrenaline at low concentrations will vasodilate (β)

Question 27

Mechanical control of skeletal muscle blood flow

Answer 27

muscle pumping

Question 28

Special feature control of skeletal muscle blood flow

Answer 28

capacity to increase flow in exercise (20-fold) - active hyperaemia. Large increase in flow post-occlusion - reactive hyperaemia (increased blood flow)

Question 29

Splanchnic cardiac output

Answer 29

superior mesenteric - 10% | hepatic - 25%

Question 30

Neural control of splanchnic blood flow

Answer 30

intestinal: moderate α vasoconstriction, hepatic: important α venoconstriction

Question 31

Local control of splanchnic blood flow

Answer 31

intestinal: poor autoregulation but importantly influenced by local peptides, hepatic: portal vein - no autoregulation, hepatic artery - good autoregulation

Question 32

Hormonal control of splanchnic blood flow

Answer 32

G-I hormones (gastrin, cholecystokinin) vasodilate; vasopressin, angiotensin constrict potently

Question 33

Why is hepatic venoconstriction important?

Answer 33

liver stores around 15% of blood volume | hepatic venoconstriction can expel around 50% hepatic blood volume into circulation

Question 34

Mechanical control of splanchnic blood flow

Answer 34

minimal

Question 35

Special feature control of splanchnic blood flow

Answer 35

intestinal circulation exhibits functional hyperaemia following feeding

Question 36

How is vasoconstriction in splanchnic vessels beneficial and detrimental?

Answer 36

+ baroreflex - haemorrhage/septic shock - intense vasoconstriction can lead to damage and release of toxins

Question 37

Renal cardiac output

Answer 37

25%

Question 38

Neural control of renal blood flow

Answer 38

important α vasoconstriction; some β vasodilatation | Renin secreting cells have a sympathetic innervation (β adrenoceptors)

Question 39

Local control of renal blood flow

Answer 39

good autoregulation of flow over wide pressure range

Question 40

Hormonal control of renal blood flow

Answer 40

noradrenaline, adrenaline, angiotensin can cause constriction vasopressin may cause vasodilatation via prostaglandin/NO release dopamine causes vasodilatation

Question 41

Mechanical control of renal blood flow

Answer 41

renal capsule may restrict flow due to compression of blood vessels in pathological states

Question 42

Special feature of renal blood flow

Answer 42

excretory function of the kidney depends on well-maintained flow (autoregulation) vascular connections provide for capacity to regulate afferent/efferent resistances

Question 43

Pulmonary cardiac output

Answer 43

100%

Question 44

Neural control of pulmonary blood flow

Answer 44

relatively minor influence | α vasoconstriction

Question 45

Local control of pulmonary blood flow

Answer 45

hypoxia causes vasoconstriction which is augmented by hypercapnia - possibly mediated by endothelin NO causes dilatation - may be used therapeutically

Question 46

Possible therapeutic strategies for pulmonary hypertension

Answer 46

endothelin receptor antagonism | NO inhalation

Question 47

Mechanical control of pulmonary blood flow

Answer 47

flow affected by changes in alveolar pressure and lung volume increase in flow (cardiac output) associated with recruitment and distension of microvessels and a decrease in vascular resistance

Question 48

If alveolar pressure is > intravascular pressure, what does this mean?

Answer 48

flow is reduced

Question 49

How does lung inflation affect resistance?

Answer 49

reduces resistance in extra-alveolar vessels (traction) | increases resistance in intra-alveolar vessels (compression)

Question 50

Special feature control of pulmonary blood flow

Answer 50

thin walled vessels with low resistance and low vasoconstrictor capacity hydrostatic pressure < colloid osmotic pressure - favours reabsorption

Question 51

Why is a low hydrostatic pressure wanted and higher colloid?

Answer 51

so no fluid goes to the alveoli, preventing gas exchange