The Microcirculation (31)

Question 1

What is the passage of microcirculation?

Answer 1

arteriole–>capillaries–>venule

Question 2

What is the overall aim of the cardiovascular system?

Answer 2

ensuring adequate blood flow through the capillaries

Question 3

What is blood flow rate?

Answer 3

• volume of blood passing through a vessel per unit time

- Q (flow rate) = Pressure gradient / Resistance

Question 4

What is the pressure gradient in terms of blood flow to the capillaries?

Answer 4

pressure at the start of the arteriole - pressure at the end of the arteriole
(pressure A- pressure B)
N.B. inc. pressure gradient–> inc. flow rate

Question 5

What is resistance in terms of blood flow?

Answer 5

hindrance to blood flow due to friction between moving fluid and stationary vascular walls
R= 8Ln/ pi x r^4

Question 6

What happens to blood flow rate when you increase resistance?

Answer 6

blood flow rate decreases

Question 7

What vessels have the biggest influence on resistance?

Answer 7

arterioles

bc biggest pressure gradient

Question 8

Why is blood flow determined only by resistance?

Answer 8

as pressure gradient is basically just mean arteriole pressure, as venule pressure is so low–>0
so flow= MAP/resistance

Question 9

Why is it important that vessels are normally partially constricted?

Answer 9

‘vascular tone’
so that it can contract further OR dilate
(if it was totally dilated or totally contracted, you would remove a method of controlling blood flow)

Question 10

What happens to resistance and blood flow in vasoconstriction?

Answer 10

dec. radius
inc. resistance
dec. blood flow

Question 11

What happens to resistance and blood flow in vasodilation?

Answer 11

inc. radius
dec. resistance
inc. blood flow

Question 12

What are the 2 distinct functions of the arterioles?

Answer 12

1. matching blood flow to the metabolic needs of specific tissues via local/intrinsic controls (momentary needs)
2. helping regulating systemic arterial blood pressure via extrinsic controls (nerves or blood), centrally coordinated

Question 13

How does active hyperaemia work?

Answer 13

• chemoreceptors in tissues detect inc. CO2 etc…–> directly causes smooth muscle contraction/relaxation
• vasodilation increases blood flow to active tissue
  e. g. in skeletal muscle

Question 14

How does myogenic auto regulation work?

Answer 14

if tissue doesn’t need inc. blood flow–> detects inc. stretch (due to inc. BP) —> causes vasoconstriction of arterioles ensuring that blood doesn’t overly flow into particular tissue

Question 15

What happens to arterioles when blood temperature decreases?

Answer 15

vasoconstriction to preserve temp.

Question 16

What is an equation for blood pressure across the whole circulation?

Answer 16

BP= Q X TPR

blood pressure= cardiac output X total peripheral resistance

Question 17

How does the brain regulate arterial blood pressure via neurones?

Answer 17

e. g. if there is blood loss detected
- -> cardiovascular control centre in medulla
- -> vasoconstriction
- -> dec. blood flow to specific organs (brain always takes priority, so blood flow preserved here)

Question 18

How does the brain regulate arterial blood pressure via hormones?

Answer 18

vasopressin/ADH, angiotensin 2, adrenaline/noradrenaline

–> all cause vasoconstriction

Question 19

What is the purpose of capillary exchange?

Answer 19

to deliver metabolic substrates to the cells of the organism
- highly branched (max. SA)
- thin cell width (min. diffusion distance
^ enhance diffusion - Fick’s Law

Question 20

Why is capillary density important?

Answer 20

• depends on how active tissue is
• dec. diffusion distance and inc. SA
• highly metabolically active tissues have denser capillary networks
  e. g. brain, skeletal muscle and lungs (to enable proximity to alveoli)

Question 21

What are the types of capillaries?

Answer 21

• continuous (small, water-filled gap junctions)
• fenestrated (larger holes)
• discontinuous (less common, but found in liver)

Question 22

What is bulk flow?

Answer 22

a certain volume of protein-free plasma filters out of the capillary (bc hydrostatic pushing force), mixes w/ surrounding interstitial fluid and is reabsorbed (bc oncotic pulling force of proteins)

Question 23

What is Starling’s hypothesis in terms of hydrostatic/oncotic forces?

Answer 23

there is a balance between hydrostatic pressure pushing it out and oncotic pressure drawing it back in

Question 24

How do the pressure changes in capillaries affect fluid movement?

Answer 24

• if pressure inside capillary > pressure in interstitial fluid–> ultrafiltration
• if inward driving pressure > outward pressures across the capillary–> reabsorption (bc oncotic pressure is constant but hydrostatic dec.)

Question 25

Why do we need the lymphatic system?

Answer 25

net loss of fluid out of capillaries bc ultrafiltration is more effective than reabsorption

Question 26

What type of capillary structure is the blood brain barrier?

Answer 26

continuous

allows brain to have tight control over what enters