5 - The Microcirculation

Question 1

What is the arrangement of vessels in the microcirculation?

Answer 1

• 1st order arterioles
• terminal arterioles
• capillaries
• pericytic (post capillary) venules
• venule

Question 2

What is meant by ‘microcirculation’?

Answer 2

individual circulation for any individual tissue

Question 3

What is blood flow rate?

Answer 3

the volume of blood passing through a vessel per unit time

Question 4

Give the equation relating blood flow rate with pressure gradient and vascular resistance

Answer 4

F = ∆P/R

i.e. blood flow rate = pressure gradient / vascular resistance

Question 5

Define resistance

Answer 5

hindrance to blood flow due to friction between moving fluid and stationary vascular walls

Question 6

Which of the 3 factors impacting resistance has the major influence? Why is this?

Answer 6

vessel radius has the major influence because:

• blood viscosity and vessel length remain mostly constant (blood flow is predetermined by these factors_
• the resistance is indirectly proportional to the radius^4, so altering the radius has a huge effect on resistance

Question 7

What is meant by the pressure gradient?

Answer 7

the difference between the BP of the blood entering the vessel and that of the blood leaving the vessel

Question 8

F = ∆P/R ∆P= FxR

What would an increase in BP cause?

Answer 8

An increase in the pressure gradient —–> an increase in flow

Question 9

F = ∆P/R ∆P= FxR

What would arteriolar vasoconstriction cause?

Answer 9

Decrease in radius —–> increase in resistance —–> decrease in flow

Question 10

In which vessels is the biggest drop in pressure seen?

Why is the result of this?

Answer 10

the arterioles

its allows the blood to flow through the capillary bed passively/slowly, too allow exchange of nutrients

Question 11

What happens to the pressure of the blood in the arteries?

Answer 11

not much change from the pressure generated by the heart

Question 12

What is MAP?

Answer 12

Mean Arterial Pressure
(the average pressure in a patient’s arteries during one cardiac cycle)
NOTE: in any (1°) artery in the body, the blood pressure is around MAP

Question 13

Overall, what is the major determinant of blood flow?

Answer 13

the resistance of the arterioles in the organ/tissue

Question 14

Why is the arteriolar smooth muscle usually in a state of partial constriction?

Answer 14

it allows for the changing of blood flow up AND down by contracting or dilating

Question 15

What is active hyperaemia?

Answer 15

an increase in organ blood flow that is associated with increased metabolic activity of an organ or tissue

Question 16

Which receptors facilitate constriction and dilation?

Answer 16

adrenoreceptors

Question 17

Which 2 hormones are used to control the heart rate?

Answer 17

adrenaline and noradrenaline

Question 18

Which 2 hormones control Bp by acting on the arterioles?

Answer 18

Vasopressin and angiotensin II

Question 19

How are capillaries perfectly designed for exchange?

Which Law does this come under?

Answer 19

• minimise diffusion distance - very narrow walls (only 1 micrometer thick)
  (no capillary is more than a few micrometers away from a tissue cell)
• maximise SA - extensive branching

via Fick’s law

Question 20

Why does the myocardium and brain need a high capillary density?

Answer 20

They are particularly vulnerable to hypoxia

Question 21

What is the capillary density like in skeletal muscles?

What happens to blood flow at rest and during exercise?

Answer 21

has a high capillary density

• a large number of these capillaries are shut off at rest (by pre-capillary sphincters)
• during exercise and active hyperaemia, the sphincters relax, so a larger amount of the cardiac output travels to here

Question 22

What are the 3 main types of capillaries?

Which is the most common?

Answer 22

• continuous - most common
• fenestrated
• discontinuous

Question 23

Where are there fenestrated capillaries in the body?

Answer 23

in the kidney

Question 24

Where are there discontinuous capillaries in the body?

Answer 24

close to the bone marrow, where white cells have to get into the blood

Question 25

What does the term ‘bulk flow’ refer to?

Answer 25

a volume fo protein free plasma filters out of the capillary, mixes with the surround interstitial fluid and is reabsorbed

Question 26

What is oncotic pressure?

Answer 26

an osmotic force due to protein in the capillary drawing water back in
NOTE: it doesn’t really change down a capillary since the protein conc doesnt really change

Question 27

Describe some characteristics of the lymphatic system.

Answer 27

• NOT a closed loop
• Consists of blind-ended lymphatic capillaries (not once to induce flow-uses forces from the diaphragm as a pump)
• Valves - prevent backflow (once the fluid has drained into the lymph, it cannot drain back out)
• All but the right upper quadrant of the body drains via the thoracic duct into the left subclavian vein
• The right upper quadrant drains into the left subclavian vein