THE AUSTRALIAN NATIONAL UNIVERSITY

1. THE AUSTRALIAN NATIONAL UNIVERSITY Introduction to Vascular FiltrationChristian StrickerAssociate Professor for Systems PhysiologyANUMS/JCSMR - ANUChristian.Stricker@anu.edu.auhttp://stricker.jcsmr.anu.edu.au/Vasfilt.pptx

3. Aims The students should • appreciate capillary organisation and specialisation; • be cognisant of the concepts of vascular diffusion and permeation; • know factors determining capillary permeability; • realise how blood flow determines solute transfer; • be familiar with how Starling “forces” determine fluid exchange; and • recognise how fluid balance in tissue is maintained.

4. Contents • Microcirculation and solute exchange • Organisation and histology of capillaries • Diffusion and permeation of solute • Blood flow and solute transfer • Fluid circulation between plasma, interstice and lymph • Starling’s principle of fluid exchange • Capillary pressure (Pc) and its regulation • Colloid osmotic pressure in capillary (πp) • Interstitial colloid osmotic pressure (πi) • Interstitial fluid pressure (Pi) • Tissue fluid balance • Lymph

5. 1. Microcirculation and Solute Exchange

6. Organization of Capillaries Levick, 5th ed., 2010 • Capillaries account for majority of solute and fluid exchange: 0.5 – 1 mm long and 4 – 8 µm thick; are “porous” (see later). • Originate as a module of capillaries from terminal arterioles. • Reunite to form pericytic venules (~15 µm thick), which have smooth muscle and are highly water permeable. • Capillary density highly adapted to tissue function: 300-1000 / mm2 in muscle; 3’000 in brain and heart; highest in lung → diffusional distance↓.

7. Vasomotion • Capillary flow tends to fluctuate: wax and wave every ~ 15 s (vasomotion). • Can stop for a while in “closed” capillaries. • Capillary transit time governs time available for gas and fluid exchange. • Upstream and downstream regulation (see later in Block 2).

8. Three Types of Capillary • Continuous capillary: “standard” • Lined out by endothelial cells with basal membrane delineating. • Pericytes between basal membranes. • Transcapillary diffusion distance ~ 0.3 µm. • Features for solute exchange: • Intercellular cleft • Glycocalyx • Caveola-vesicle system • Fenestrated capillary: fluid filtration • In kidneys, intestines, synovia, choroid plexus. • Very permeable to water. • Diaphragm of 4 – 5 nm thick (cartwheel); form due to vascular endothelial growth factor (VEGF). • Discontinuous capillary: Blood cell turnover • Found in liver, spleen and bone marrow. • Sinusoidal capillaries. • Endothelial gaps over 100 nm wide; discontinuity in basal membrane. Levick, 5th ed., 2010

9. Vascular Permeability • Vessels have semiperm. membrane • only parts of solute can permeate (size). • Permeability [cm/s] = capillary “diffusion” * concentration difference • Depends on properties of both membrane and solute. • Lipid soluble molecules: O2, CO2, general anaesthetics. • Transcellular diffusion across endothelial membrane • Small, lipid-insoluble molecules: salts, glucose, AA, most drugs, etc. • Diffusion through aqueous path (intercell. cleft and fenestrations; slow permeation due to limited space) • Large, lipid-insoluble molecules: protein • Diffuse slowly via large pore system (endothelial gaps, vesicular transport and transendothelial channels) • Mostly, specific transporters contribute little to transcapillary exchange. • Exchange via intercellular clefts ≫ transport capacities.

10. Fibre Matrix on Endothelial Surface • Glycocalyx covers fenestrae, endothelium, intercellular junctions: sieves out plasma protein. • Proteoglycans and sialoglycoproteins bind to + charged arginines on albumin creating a 3D sieve reflecting cells and protein. • Reflection governed primarily by glycocalyx mesh size, secondarily by negative charge on proteoglycans. • Large pore system represented via multivesicular transcellular channel (MVC) and vesicles (V). • Caveolins, proteins that interact with cholesterol and polymerize to build caveolae forming invaginations for macromolecular exchange across endothelium. • Cap. permeability given by number of open junctions and fenestrae. Levick, 5th ed., 2010 Guyton & Hall, 12th ed., 2011

11. Solute Transfer and Blood Flow • Effect of increased blood flow depends on whether solute exchange is • flow limited: if diffusion capacity > solute delivery rate, blood (Ca) equilibrates with pericapillary fluid (Ci) before capillary end. • Transfer rate ~ blood flow (O2 uptake in lung; see later). • diffusion limited (permeation ↑): if diffusion capacity < solute delivery rate, no equilibration before capillary end (Cv). • Transfer rate ~ constant (glucose uptake in exercising muscle). Levick, 5th ed., 2010

12. 2. Fluid Circulation between Plasma, Interstitium and Lymph Starling’s principle of fluid exchange Ultrafiltration across semipermeable membrane

13. Starling Principle of Fluid Exchange • Pressures determine solute flow (simple formulation). • Hydrolic push = Pc– Pi • Osmotic suction = πp – πi • Cap. filtration rate ∞ (hydrolic push – osmotic suction) • If hydrolic push > osmotic suction: filtration into interstitium: normal. • If hydrolic push < osmotic suction: fluid absorption from interstitium.

14. Regulation of PC • Measured using micropipettes • Capillary blood pressure (PC): • Most variable Starling parameter • Vascular resistance (see last lecture) • Arterial pressure • Venous pressure • Gravity (hydrostatic pressure) • Distance along capillary axis • Blood pressure ↓ along capillary • at inflow: ~35 torr • middle: ~25 torr • at outflow: ~12 torr • In glomerular capillary ~60 torr. Levick, 5th ed., 2010

15. Interstitial Fluid Pressure (Pi) • 3D network of negatively charged biopolymer fibres, a solid phase and a space-filling solution of electrolytes and escaped plasma proteins. • Quite difficult to measure. • Determined by fluid volume and compliance of tissue. • Slightly negative (subatmospheric) in many tissues: ~ -3 torr (loose subcutaneous tissue, eye lid). • Holds certain tissues together. • Slightly positive (~ 6 torr) in tightly encased tissues (kidney, brain, sclera, around muscle), but still more negative than capsule pressure. • In most tissues, Pi is directly exposed to gravity and, therefore, scales with hydrostatic level (like Pc). Guyton & Hall, 12th ed., 2011 Boron & Boupaep, 2th ed., 2009

16. Plasma Colloid Osmotic Pressure (πp) • Colloid osmotic pressure (COP) caused by impermeable protein in plasma. • Is about ~ 28 torr; 80% is caused by albumin. • Albumin contributes dyspropor-tionately (19% protein and 9% Gibb-Donnan, i.e. net negative charge of protein attracts Na+). • Other proteins contribute little (20%). • Variable as solute is filtered along capillary. Levick, 5th ed., 2010

17. Interstitial COP (πi) • Impossible to measure; is inferred value. • Is typically about ⅓ of plasma COP due to escaped plasma protein via pores and transcytosis. • Significant protein content in interstice. • Average value ~8 torr. • Not a fixed quantity; i.e. drops with capillary filtration rate (“dilution”). Levick, 5th ed., 2010

18. Fluid Balance Along Capillary • Arterial end: net outward force (~13 torr) as Pc is high. • Mid-capillary: net outward force (0.3 torr). • Venous end: net inward force (~7 torr) for absorption as Pcsmall. • In most capillaries, amount of filtration ~ volume returned by absorption. • ~90% of fluid is reabsorbed, remainder in lymphatics (~ 2 mL/min). Modified from Boron & Boupaep, 2th ed., 2009

19. Lymph Guyton & Hall, 12th ed., 2011 • Formation as a filtrate (~2 - 3 L/d); almost same as interstitial fluid; rich in protein. • Composition variable in different areas: high fat content in GI tract. • Specialisation of lymph vessels: anchoring filaments can keep pores open; valves direct flow. • Lymph flow increases if Pc ↑, πp ↓, πi ↑, capillary permeability ↑. • Lymph flow limited by Pi : > Patm vessel diam.↓ (compression) → R ↑.

20. Overview of Microcirculation • 3 convective loops to fluid circulation: • 1st loop: circulation proper • 7200 L/d as CO and VR • 2nd loop: interstitial exchange • Filtered in capillaries: 20 L/d • Reabsorbed: 16 – 18 L/d (very little protein) • 3rd loop: lymph flow • 2 – 4 L/d • Achieves fluid homeostasis Modified from Boron & Boupaep, 2th ed., 2009

21. Take-Home Messages • Vasomotion determines capillary flow. • 3 type of different capillaries. • Vascular permeability << diffusion (100x). • Vascular permeability different for various solute properties (lipid soluble, -insoluble, large and small). • Solute transfer across capillary can be flow- or diffusion-limited. • In most capillaries, amount of filtration is about volume returned by absorption. • Pi is slightly negative in many tissues. • Lymph is produced as a consequence of filtration.

22. Barbara Jones, a 39 year-old has radiation therapy for breast cancer of the right axillar region. She is concerned about peripheral oedema as a consequence of radiation. Which of the following changes favours filtration at the arteriolar end of the capillary bed? • Decrease in hydrostatic pressure of capillaries. • Increase in hydrostatic pressure of capillaries. • Decrease in oncotic pressure of interstitium. • Increase in oncotic pressure of capillaries. • Increase in capillary flow.

23. That’s it folks…

24. Barbara Jones, a 39 year-old has radiation therapy for breast cancer of the right axillar region. She is concerned about peripheral oedema as a consequence of radiation. Which of the following changes favours filtration at the arteriolar end of the capillary bed? • Decrease in hydrostatic pressure of capillaries. • Increase in hydrostatic pressure of capillaries. • Decrease in oncotic pressure of interstitium. • Increase in oncotic pressure of capillaries. • Increase in capillary flow.