Physiology: Special Circulations

Question 1

what are the special adaptations of the coronary circulation?

Answer 1

• high capillary density (lots of blood goes to the heart)

- high oxygen extraction (75% , whereas the rest of the body is only 25%) under resting conditions

Question 2

how many mechanisms control the coronary blood flow?

Answer 2

2 (intrinsic, and extrinsic)

Question 3

Increasing coronary blood flow is the only way to supply extra O2 to the heart. Why can extra blood flow not be supplied to the heart just by extracting more oxygen?

Answer 3

because there is already really high O2 extraction from the coronary circulation (75% when the rest of the body is just 25%)

Question 4

what is the intrinsic mechanism of control of coronary blood flow?

Answer 4

active tissues use more O2 and produce more waste
reduced O2 and increased CO2, H, K, lactate and adenosine, cause vasodilatation of the coronary arteries. This is metabolic hyperaemia - and matches flow to demand.

Question 5

what is the extrinsic mechanism of control of coronary blood flow?

Answer 5

the coronary arteries are supplied by sympathetic vasoconstrictor nerves BUT:
this vasoconstriction is OVERRIDDEN by:
-metabolic hyperaemia (due to increased heart rate and SV - which are due to sympathetic stimulation)
-circulating adrenaline activated B2 receptors - causing vasodilatation
-

Question 6

when during the cardiac cycle does most coronary blood flow occur?

Answer 6

ventricular diastole (the relaxed ventricles mean the coronary arteries are not being squeezed - so can open and blood can flow through them)

Question 7

why can a very fast heart rate cause chest pain (even if the coronary arteries are not blocked by atherosclerosis)?

Answer 7

increased HR means diastole is shortened: so the time for coronary blood flow is reduced and metabolites build up and cause pain

Question 8

why are special adaptations of the cerebral circulation needed?

Answer 8

because the grey matter is very sensitive to hypoxia - consciousness lost after a few seconds of ischaemia, irreversible damage within 3 minutes

Question 9

which arteries supply the brain?

Answer 9

carotid and vertebral

Question 10

which arteries anastomose to form the circle of willis?

Answer 10

basilar (formed by the 2 vertebral arteries) and carotid arteries anastomose to form the circle of Willis. (There is further branching).
If one carotid artery is obstructed: cerebral perfusion should be maintained. However, obstruction of a smaller branch of a main artery would deprive region of the brain of its blood supply.

Question 11

by what regulatory process if the cerebral blood flow kept relatively constant?

Answer 11

autoregulation (which can function if MABP changes within a rang of 60-160mmHg)

Question 12

does sympathetic stimulation affect cerebral blood flow?

Answer 12

no

participation of the brain in baroreceptor reflexes is negligible

Question 13

how does autoregulation of cerebral blood flow work?

Answer 13

• MABP rises: resistance vessels automatically constrict to limit flow
• MABP falls: resistance vessels automatically dilate to maintain flow
• BUT: MABP below 50mmHg results in confusion, fainting and brain damage if not quickly corrected
• increased PO2 causes cerebral dilatation
• decreased PCO2 causes vasoconstriction. This is why hyperventilation could lead to fainting.
• blood flow increases to active parts of the brain (mechanism unknown). May be due to a rise in K as a result of K efflux from repetitively active neurones

Question 14

what is the effect of increasing CO2 on cerebral blood flow?

Answer 14

increasing PCO2 causes cerebral blood flow to increase

Question 15

what is the normal intracranial pressure range?

Answer 15

8-13mmHg

Question 16

What could cause the ICP to increase?

Answer 16

head injury (bleeding), or a brain tumour

Question 17

what effect does increasing the intracranial pressure have on cerebral perfusion pressure and cerebral blood flow?

Answer 17

it decreases CPP and cerebral blood flow

Question 18

what is the mathematical relationship between CPP, ICP, and MABP?

Answer 18

CPP = MABP - ICP

Question 19

what is special about the blood brain barrier (BBB)?

Answer 19

it has very tight intercellular junctions

Question 20

what is the BBB permeable to?

Answer 20

water, some gases, lipid-soluble molecules

also allows for the selective transport of molecules such as glucose and amino acids

Question 21

what is the BBB impermeable to?

Answer 21

hydrophilic substances e.g. ions, catecholamines, proteins…
this helps protect brain neurones from fluctuating levels of ions in the blood

Question 22

which arteries supply to metabolic needs of the lungs?

Answer 22

bronchial (NOT pulmonary - these are mainly for gas exchange)

Question 23

what is the typical range of the pulmonary artery BP?

Answer 23

20-25/6-12mmHg

very low

Question 24

what are the special adaptations of the pulmonary circulation?

Answer 24

• absorptive forces exceed filtration forces: protecting against pulmonary oedema!
• hypoxia causes vasoconstriction of pulmonary arterioles (opposite effect of hypoxia on systemic arterioles) –> purpose of this is to divert blood away from poorly ventilated areas (V/Q matching)

Question 25

why is the resting blood flow to muscles low?

Answer 25

due to sympathetic vasoconstrictor tone

Question 26

how does the skeletal muscle blood flow increase during exercise?

Answer 26

during exercise:

• local metabolic hyperaemia overcomes sympathetic vasoconstrictor activity
• circulating adrenaline causes vasodilatation (B2 receptors)
• increased CO during exercise increases blood flow to skeletal muscle

Question 27

explain the skeletal muscle pump?

Answer 27

large veins in limbs lie between skeletal muscles
contraction of the muscles aids venous return
one way venous valves allow blood to move towards heart

Question 28

blood pools in lower limb veins if venous valves become incompetent (varicose veins). But, despite this pooling, varicose veins don’t usually lead to reduction of CO. Why?

Answer 28

due to chronic compensatory increase in blood volume