Microvascular Exchange Flashcards
1
Q
What is a microcirculatory unit?
A
- Decreasing size: arteries–> terminal arterioles–> capillaries
- Small capillaries; low press, RBC width
- Arterioles, capillaries bed, post-capillary venules
2
Q
Describe the surface area of capillaries?
A
- Thin walls and large SA
- Many present
3
Q
Describe the structure of capillaries
A
- All have single endothelial layer with some pericytes (contractile abilities, no smooth muscle)
- Relationship between endothelial cells: continuous, fenestrated, sinusoidal/ discontinuous
4
Q
Describe the sinusoidal capillary structure
A
- Huge gaps for red cells
- Liver, GI tract
- Large lipophobic molecules can move out
- Very specialised type
5
Q
Describe the fenestrated capillary structure
A
- Less leaky (renal)
- Quite large openings
- Small lipophobic molecules and small proteins
- Quite specialised
6
Q
Describe the continuous capillary structure
A
- Diffusion of lipophilic, slow diffusion for lipophobic (Pino/phagocytosis)
- Ubiquitous
- Fused invaginations
- Standard for most capillaries
- Tight junctions prevent paracellular transport- proteins anchor and orientate cells
- A lot of unwanted movement- especially in BBB
7
Q
Describe transport at the endothelium
A
- Discontinuous- between cells (no membrane crossing, transporters not needed, similar in fenestrated)
- Continuous- transporters required (cell packed, more substances need to cross membrane, active transport, facilitated diffusion etc.)
8
Q
What is Fick’s Law of diffusion?
A
- Rate of diffusion dependent on :
- Proportoional to conc gradient (∆C)
- Inversely proportional to distance (∆x)
- Proportional to SA (A)
- Proportional to solute diffusivity (D)
9
Q
What is the equation of rate of diffusion (Fick’s Law)?
A
- Js = -D A ∆C/∆x
10
Q
Describe capillary diffusion
A
- Js = -P (permeability)A∆C
- Lipid soluble- diffuse
- Water soluble, non-lipid soluble- intracellular pores (gaps or channels)
- Large solutes- large pore
- Gases diffuse easily
- Exchangeable proteins- pinocytosis and exocytosis
- Plasma proteins stuck- exert oncotic pressure
11
Q
What are other forms of solute exchange?
A
- Some water-soluble solutes ‘swept’ along with fluid exchange- convective transport
- Other mechaniss: pinocytosis, GLUTs, true active transport generally limited to brain
- Water moves freely- small and sufficient pores
12
Q
What are the limitations of diffusion?
A
- Relate to Fick’s law
- Solute properties and conc grads unlikely to change, temp unlikely to change- stable
- Increased capillary recruitment increases SA and decreases diffusion distance
- Muscle fibres- single capillaries- 2 fibres, with exercise demand for oxygen rises, increased SA with capillary recruitment
13
Q
How are capillaries recruited?
A
- Terminal arteriole initially constricted, smooth muscle contracted, module not perfused
- During exercise, dilated, VSM relaxed, module becomes perfused
14
Q
Describe fluid compartments
A
- Water 60% total body mass
- ~42L- 60% intracellular, 40% extracellular
- Fluid movement from vascular to interstitium and into cell
- 2/3 intracellular
- 1/9 total body fluid- vascular space
- 2/9 in extracellular interstitial space
15
Q
How does fluid move into capillaries?
A
- Starling’s capillaries
- Flow out capillary depends on oncotic pressure in cap + interstium + hydrostatic press in capillary and tissue
- Net filtration pressure=
(Pc-Pi)-(πc-πi) - Oncotic press holds fluid in
- Plasma protein cannot leave vascular compartment
- Protein in interstitial space- opposes hydrostatic press holds fluid in that space
- Fluid does not re-enter- slow loss of fluid across capillary
