1. CS 2015 Introduction to Vascular Filtration Christian Stricker Associate Professor for Systems Physiology ANUMS/JCSMR - ANU Christian.Stricker@anu.edu.au http://stricker.jcsmr.anu.edu.au/Vasfilt.pptx Christian.Stricker@anu.edu.au http://stricker.jcsmr.anu.edu.au/Vasfilt.pptx THE AUSTRALIAN NATIONAL UNIVERSITY

2. CS 2015

3. Aims At the end of this lecture students should be able to appraise the capillary organisation and specialisation; describe the concepts of vascular diffusion and permeation; recognise factors determining capillary permeability; explain how blood flow determines solute transfer; show how Starling “forces” determine fluid exchange; and demonstrate how fluid balance in tissue is maintained.

4. CS 2015 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 (P c ) and its regulation Colloid osmotic pressure in capillary (π p ) Interstitial colloid osmotic pressure (π i ) Interstitial fluid pressure (P i ) –Tissue fluid balance –Lymph

5. CS 2015 1. Microcirculation and Solute Exchange

6. CS 2015 Organization of Capillaries 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 / mm 2 in muscle; 3’000 in brain and heart; highest in lung → diffusional distance↓. Levick, 5 th ed., 2010

7. CS 2015 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. CS 2015 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, 5 th ed., 2010

9. CS 2015 Vascular Permeability Vessels with 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: O 2, CO 2, 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: proteins 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. CS 2015 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, 5 th ed., 2010 Guyton & Hall, 12 th ed., 2011

11. CS 2015 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 (C a ) equilibrates with pericapillary fluid (C i ) before capillary end. Transfer rate ~ blood flow (O 2 uptake in lung; see later). –diffusion limited (permeation ↑): if diffusion capacity < solute delivery rate, no equilibration before capillary end (C v ). Transfer rate ~ constant (glucose uptake in exercising muscle). Levick, 5 th ed., 2010

12. CS 2015 2. Fluid Circulation between Plasma, Interstitium and Lymph Starling’s principle of fluid exchange Ultrafiltration across semipermeable membrane

13. CS 2015 Starling Principle of Fluid Exchange Pressures determine solute flow (simple formulation). Hydrolic push = P c – P i Osmotic suction = π p – π i Cap. filtration rate ∞ (hydrolic push – osmotic suction) –If hydrolic push > osmotic suction: filtration into interstitium: normal. –If hydrolic push < osmotic suction: fluid absorption from interstice.

14. CS 2015 Regulation of P C Measured using micropipettes Capillary blood pressure (P C ): –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, 5 th ed., 2010

15. CS 2015 Interstitial Fluid Pressure (P i ) 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, P i is directly exposed to gravity and, therefore, scales with hydrostatic level (like P c ). Guyton & Hall, 12 th ed., 2011 Boron & Boupaep, 2 th ed., 2009

16. CS 2015 Plasma Colloid Osmotic Pressure (π p ) 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, 5 th ed., 2010

17. CS 2015 Interstitial COP (π i ) 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, 5 th ed., 2010

18. CS 2015 Fluid Balance Along Capillary Arterial end: net outward force (~13 torr) as P c is high. Mid-capillary: net outward force (0.3 torr). Venous end: net inward force (~7 torr) for absorption as P c small. 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, 2 th ed., 2009

19. CS 2015 Lymph Formation as filtrate (~2 - 3 L/d); almost like interstitial fluid; protein rich. 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 P c ↑, π p ↓, π i ↑, capillary permeability ↑. Lymph flow limited by P i : > P atm vessel diam.↓ (compression) → R ↑. Guyton & Hall, 12 th ed., 2011

20. CS 2015 Overview of Microcirculation 3 convective loops to fluid circulation: –1 st loop: circulation proper 7200 L/d as CO and VR –2 nd loop: interstitial exchange Filtered in capillaries: 20 L/d Reabsorbed: 16 – 18 L/d (very little protein) –3 rd loop: lymph flow 2 – 4 L/d Achieves fluid homeostasis Modified from Boron & Boupaep, 2 th ed., 2009

21. CS 2015 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. P i is slightly negative in many tissues. Lymph is produced as a consequence of filtration.

22. CS 2015 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? A.Decrease in hydrostatic pressure of capillaries. B.Increase in hydrostatic pressure of capillaries. C.Decrease in oncotic pressure of interstitium. D.Increase in oncotic pressure of capillaries. E.Increase in capillary flow.

23. CS 2015 That’s it folks…

24. CS 2015 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? A.Decrease in hydrostatic pressure of capillaries. B.Increase in hydrostatic pressure of capillaries. C.Decrease in oncotic pressure of interstitium. D.Increase in oncotic pressure of capillaries. E.Increase in capillary flow.