Microcirculation

Question 1

What are microcirculations?

Answer 1

The microvessels of the microvasculature present in organ tissues

Question 2

What is the pathway of blood through a microcirculatory unit? (5)

Answer 2

1. 1st order arteriole
2. Terminal arteriole —> precapillary sphincter
3. Capillaries
4. Pericytic venule
5. Venule

Question 3

What is Darcy’s Law?

Answer 3

Equation for a fluid circuit:
ΔP = Q x R
- ΔP —> pressure gradient
- Q —> flow rate
- R —> resistance

Question 4

What is flow rate and which 2 factors affect it?

Answer 4

Volume of blood passing through a vessel per unit time
1. Pressure gradient - direct
2. Resistance - inverse

—> Q = ΔP/R
- ΔP across organ —> 93 ∵ MAP = 93, MVP negligible
in organ —> 56 ∵ MAP = 93, capillary P = 37

Question 5

What is resistance and which 3 factors affect it?

Answer 5

Force against blood flow due to friction from vascular walls
1. Vessel length (L) —> constant
2. Vessel radius (r) —> main factor
- via vasoconstriction/dilation
3. Blood viscosity (η) —> usually constant

—> R = 8Lη / πr^4

Question 6

What are the 2 functions of arterioles?

Answer 6

Major resistance vessels —> control blood flow:
1. Match blood flow to specific tissue’s metabolic
needs
- intrinsically controlled by tissue
- active hyperaemia —> chemical
myogenic autoregulation —> physical

2. Regulate whole systemic arterial blood pressure
   
   • extrinsically controlled by brain
   • neural
     hormonal

Question 7

What is the equation for resistance?

Answer 7

R = 8Lη / πr^4
- L —> vessel length
- η —> blood viscosity
- r —> vessel radius

Question 8

What is vascular tone?

Answer 8

Arteriolar smooth muscle is always in a state of partial constriction
—> allows for both dilation and constriction to occur

Question 9

What is active hyperaemia?

Answer 9

Tissue’s chemical control of its blood flow:
- Inc metabolic activity of tissue —> inc metabolites
and O2 usage —> vasodilation of tissue’s arterioles

Question 10

What is myogenic autoregulation?

Answer 10

Tissue’s physical control of its blood flow:
- Dec temp —> restrict blood flow to regulate blood
temp —> vasoconstriction of tissue’s arterioles
- Inc blood pressure —> inc arteriole distension —>
some tissues don’t need more blood flow —>
vasoconstriction of tissue’s arterioles (to inc R and
counteract) eg. gut during exercise

Question 11

What is the equation for the blood flow of the whole cardiovascular system?

Answer 11

Q = MAP / TPR
- Q —> cardiac output
- MAP —> mean arterial pressure
- TPR —> total peripheral resistance

Question 12

What is the equation for the blood flow of an organ?

Answer 12

F(organ) = MAP / R
- MAP —> mean arterial pressure
- usually 93

Question 13

How is overall arterial blood pressure controlled by the brain? (2)

Answer 13

1. Neural
   
   • Baroreceptors sense inc BP —> CVS control centre
     in medulla senses —> sympathetic response —>
     vasoconstriction of arteries (eg. gut, skeletal
     muscle) —> doesn’t affect brain/heart/lungs
2. Hormonal
   
   • Pituitary gland —> ADH
   • Lungs —> angiotensin II
   • Adrenal glands —> adrenaline
     —> noradrenaline

Question 14

How are capillaries adapted to enhance Fick’s Law?

Answer 14

Inc conc grad —> inc diffusion
1. Dec diff distance —> 1µm width of wall
2. Inc SA —> 7µm diameter (tiny) —> squish RBCs
3. Extensive —> every cell close enough for exchange
- more metabolically active tissue —> inc
capillary density

Question 15

What is Fick’s law?

Answer 15

Rate of diffusion of a substance across unit area is proportional to the concentration gradient

Question 16

How is cardiac output distributed around the body at rest?

Answer 16

Overall = 5L/min
- Gut —> 1
- Heart —> 0.25
- Kidneys —> 1
- Brain —> 0.75
- Skin —> 0.25
- Bone —> 0.15
- Skeletal muscle —> 0.75

Question 17

How does exercise affect cardiac output?

Answer 17

Inc cardiac output - overall —> 25L/min
- Gut dec —> 1 to 0.75
- Heart inc —> 0.25 to 1.25
- Bone inc —> 0.15 to 0.25
- Skeletal muscle —> 0.75 to 20

Overall = 5L/min
- GI —> 0.75
- Heart —> 1.25
- Kidneys —> 1
- Brain —> 0.75
- Skin —> 0.25
- Bone —> 0.25
- Skeletal muscle —> 20

Question 18

What are the 3 types of capillaries?

Answer 18

1. Continuous —> only gap junctions between cells
   - eg. lungs
   - blood brain barrier —> tight junctions
2. Fenestrated —> fenestrae (80nm) between cells
   - eg. glomeruli
3. Discontinuous —> large gaps between cells
   - eg. bone marrow, liver

Question 19

What is bulk flow?

Answer 19

Process of plasma fluid leaking out of vessels and returning via Starling’s forces:
1. Ultrafiltration - hydrostatic pressure pushes out
—> mixes with interstitial fluid
2. Reabsorbed - oncotic pressure pulls in

• Overall - hydrostatic: 32 —> 15 mmHg
  - oncotic: 25 constant
  ∴ start —> Δ9
  end —> Δ8
  ultraflitration > reabsorption —> net loss
  (Δ1) —> lymphatic system returns fluid
  (right lymphatic duct —> right subclavian
  vein, thoracic duct —> left subclavian vein)