Microcirculation

Question 1

Precapillary Sphincter

Answer 1

Cuff of smooth muscle
Open: blood flows to true capillaries

Closed: blood flows through meta-arteriole thoroughfare channels

Therefore they determine the distribution of capillary blood flow

Question 2

How are the precapillary sphincters controlled??

Answer 2

AUTOREGULATION
-Decreased tissue O2 levels or increased CO2 levels

• generation of acid (lactic)
• Adenosine, prostaglandins and NO from endothelial cells
• Rising concentrations of K+ or H+ ions
• Elevated local temps

NOT NEURAL (like arterioles and metaarterioles)

Question 3

Capillaries

Answer 3

Where exchange occurs, important the flow and pressures are controlled. Very small, 8 micrometeres.

Question 4

3 Main ways of Transcapillary Exchange?

Answer 4

1) Diffusion (conc gradient)
2) Filtration (pressure gradient)
3) Large molecule movement (pinocytosis)

Question 5

Diffusion

Answer 5

Driving force is concentration gradient

-Dependant on ability to cross membrane

Described by ficks law, (area and thickness and the concentration gradient)

Question 6

Filtration

Answer 6

Bulk flow of fluid across membrane

Pressure dependent (hydrostatic and osmotic pressures)

Question 7

In terms of filtration, what does fluid movement depend on (starlings law)

Answer 7

1) K= permeability
2) Pc= capillary hydrostatic P
3) Pi interstitual hydrostatic pressure
4) Colloid osmotic pressure

Question 8

Arterial end vs Venous end

Answer 8

A.E NFP= 10mmHg (fluid moves out)

V.E NFP= -8mmHg (fluid moves in)

Question 9

What is hydrostatic Pressure?

Answer 9

Due to fluid pressing against a wall “pushes”

In capillary: pushes fluid out, 35 (art end) and 17 (venous end)

In interstitial fluid: Pushes fluid into capillary (ommHf in this

Question 10

What is Osmotic Pressure

Answer 10

Due to presence of ‘non-diffusible’ solute “sucks”

In capillary: Pulls fluid in (26mmHg)

in Interstitial: Pulls fluid out (1mmHg)

Question 11

Increasing Pc (capillary pressure) results in more filtration. What affects Pc?

Answer 11

Increasing arterial or venous pressure increases Pc

Increasing upstream R decreases Pc
Increasing downstream resistance increases Pc

Question 12

Heart failure

Answer 12

Reduced CO
Poor peripheral perfusion  resulting in neural and hormonal adaptations
Fluid retention  
increased end diastolic P
Increased Central Venous P
Increased Capillary pressure
Increased net filtration= oedema

Question 13

Large molecule movement

Answer 13

via vesicles or fenestrations

Question 14

Lymphatic vessel Function

Answer 14

1) Return blood component to circulation
2) absorption from gut
3)Removal of RBCs from tissue
Immunological removal/isolation of bacteria

NF out: 24L
NF in: 20.4L

Net 3.6L out into lymphatics

Question 15

Lymphatic Drainage: Terminal Lymphatics

Answer 15

Composed of endothelium with intercellular gaps surrounded by highly permeable BM, Blind end sacs.
Similar size to venules
Spontaneous and stretch activsted vasomotion “pumps” lymph. ALso sympathetic nerves modulate vaso motion.
Larger Lymphatics have SM cells

Question 16

How much lymph returned to systemic circulation via thoracic duct

Answer 16

2-4 L/day. Returned to heart

Question 17

Elephantiatis edema

Answer 17

Consequence of lymphatic vessel block