Microcirculation and Exchange of Materials

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1 Microcirculation and Exchange of Materials Jonas Addae Medical Sciences, UWI Summary: Microvascular Transport Glucose, O2 & CO2 Diffusion Movement of Water Ultrafiltration Large molecules Exocytosis

Lecture Outline 1. Microvascular architecture and blood flow in organs & tissues 2. Diffusion of gases & solutes 3. Ultrafiltration forces 4. Lymphatics function 5.

2 Lecture Outline 1. Microvascular architecture and blood flow in organs & tissues 2. Diffusion of gases & solutes 3. Ultrafiltration forces 4. Lymphatics function 5. Oedema MICROVASCULAR ARCHITECTURE Proximal ARTERIOLES (50-20 µm) Control total blood flow to exchange vessels Terminal & Metaarterioles ( 20-8 µm) Precapillary sphincter Terminal arterioles and precapillary sphincters control distribution of blood to exchange vessels

3 MICROVASCULAR ARCHITECTURE CAPILLARIES ( 8-4 µm ) = Exchange vessels Capillary density in an organ Depends on metabolic and functional requirements of the organ SMALLEST VENULES ( µm ) = Exchange vessels MICROVASCULAR ARCHITECTURE MUSCULAR VENULES Major site of postcapillary resistance ARTERIO-VENOUS ANASTOMOSIS Present in skin and subcutaneous tissue of ears, fingers, and toes Promote heat loss but not exhange of nutrients LYMPHATICS

4 MICROVASCULAR HAEMODYNAMICS Flow is generally laminar Local turbulence at some branches Plasma skimming Influences distribution of RBCs, WBCs, & Plasma. A pre-capillary sphincter regulates the flow of blood through the capillary

5 MICROVASCULAR TRANSPORT MECHANISMS Diffusion Glucose, O2 & CO2 move from regions of higher concentration to lower ones Ultrafiltration Movement of fluid with solutes thro capillary wall Determined by difference between hydrostatic and osmotic pressures across capillary wall. DETERMINANTS OF DIFFUSION What factors will promote diffusion of O2 from blood to extracellular space? FICK S LAW OF DIFFUSION J = - D A ( C / X) J = rate of diffusion of a solute from capillary D = diffusion coefficient for the solute A = area through which diffusion takes place Opening more precapillary sphincters increases value of A C / X = Difference in concentration over a given distance from the capillary. When c decreases with distance, C / X is negative.

6 DETERMINANTS OF DIFFUSION It is difficult to measure D & X independently => when membrane permeability to a solute P = D / X J = P A ( Cp - Ci ) Relation is particularly true for solutes with relatively low permeability e.g. Glucose, sucrose where transport is DIFFUSION LIMITED DIFFUSION AND BLOOD FLOW If permeability of a given solute ( P ) and surface area (A) are high enough ( e.g. O2, CO2, lipid soluble substances ) transport of the solute is not limited by its diffusibility but on the blood flow i.e. Solute transport is FLOW LIMITED J ~ F ( Cp - Ci )

MICROVASCULAR TRANSPORT MECHANISMS Diffusion Glucose, O2 & CO2 move from regions of higher concentration to lower ones Ultrafiltration Movement of fluid Determined by Starling s

vs ABSORPTION FORCES FILTRATION FORCES P c + ΙΙ i ABSORPTION FORCES P i + ΙΙ c e.g.

7 MICROVASCULAR TRANSPORT MECHANISMS Diffusion Glucose, O2 & CO2 move from regions of higher concentration to lower ones Ultrafiltration Movement of fluid Determined by Starling s Forces hydrostatic and osmotic pressures across capillary wall P = hydrostatic pressure ΙΙ = osmotic pressure c = capillary i = interstitial STARLING S FORCES FILTRATION FORCES vs ABSORPTION FORCES FILTRATION FORCES P c + ΙΙ i ABSORPTION FORCES P i + ΙΙ c e.g. when (in mmhg) Pc is (35-15), ΙΙ i (3), Pi ( -2), ΙΙc (25 ) there is: filtration at arteriolar end (-2) -25 = +15 mmhg absorption at venular end of capillary (-2) -25 = -5 mmhg Forces moving fluid out of capillary are +ve Forces moving fluid into capillary are -ve

8 DETERMINANTS OF MICROVASCULAR PRESSURES CAPILLARY HYDROSTATIC PRESSURE ( Pc ) State of contraction of arteriole vs venule Rv / Ra Pc Rv ( venoconstriction ) Ra (arteriodilatation ) ULTRAFILTRATION Negligible importance for supplying O 2 & nutrients to tissues and removing waste products Important for maintaining plasma and intersitial fluid volumes N.B. Ultrafiltration & diffusion take place simultaneously

9 LYMPHATIC DRAINAGE & FLOW Fluid, solutes & proteins in interstitial space enter terminal lymphatics About 4 litres of fluid is returned to the circulation thro the lymphatics Systemic vs Pulmonary Circulation Note: Values in Pulmonary Circulation are much lower than those in the Systemic Circulation

10 Starling s Forces at Arteriole end of Capillary: Systemic vs Pulmonary Circulation Syst. Pulm Filtration Forces Pc ΙΙi Absorption Forces ΙΙc Pi -1-1 Lymphatic Forces P L -1-2 ΙΙ L The low capillary hydrostatic pressure in the pulmonary circulation is aided by a high tissue osmotic pressure. A high lymphatic osmotic pressure keeps the lungs dry EFFECTS OF STANDING VENOUS PRESSURE CAPILLARY HYDROSTATIC PRESSURE OEDEMA

11 SAFETY FACTORS AGAINST OEDEMA Ultrafiltration changes that tend to decrease filtration i.e. there is: Πc Πi Pi Pc (reflex arteriolar constriction and venular dilatation) CAUSES OF OEDEMA Abnormal increase in interstitial fluid Venous congestion increase in capillary pressure Hypoproteinaemia decrease in colloid osmotic pressure Arteriolar dilatation ( or venular constriction) increase in capillary pressure Increase in endothelial permeability to proteins (e.g. from burns) IIc AND IIi Lymphatic obstruction

12 Summary: Microvascular Transport Glucose, O2 & CO2 Diffusion Movement of Water Ultrafiltration Large molecules Exocytosis

Unit II Problem 4 Physiology: Diffusion of Gases and Pulmonary Circulation

Unit II Problem 4 Physiology: Diffusion of Gases and Pulmonary Circulation - Physical principles of gases: Pressure of a gas is caused by the movement of its molecules against a surface (more concentration