microcirculation and lymphatics

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Microcirculation and Lymphatics Dr. I. Avinash MD, DTCD : 

Microcirculation and Lymphatics Dr. I. Avinash MD, DTCD

Reading : 

Reading Klabunde, Cardiovascular Physiology Concepts Chapter 8 (Exchange Function of the Microcirculation) pages 171-183.

Circulatory System : 

Circulatory System Cardiac output is determined by the metabolic needs of the body Functional Anatomy The large arteries serve as blood conduits Arterioles are the “Stopcocks” of the circulation Capillaries are the site of exchange of gases, water, and solutes The veins serve as storage vessels and also as blood conduits

Anatomy of Microcirculation : 

Anatomy of Microcirculation

Arterioles are the “Stopcocks” of the circulation : 

Arterioles are the “Stopcocks” of the circulation Arterioles Thick smooth muscle Give rise to metarterioles, then to capillaries Metarterioles Precapillary sphincters regulate flow into the capillaries Local conditions in the tissues regulate the opening and closing of the precapillary sphincters

Capillary Trivia : 

Capillary Trivia

Capillaries : 

Capillaries Thin-walled Smallest vessels in the circulation Have the greatest cross-sectional area because they are so numerous Have the greatest surface area for exchange

Capillaries : 

Capillaries True capillaries are devoid of smooth muscle and are incapable of active constriction Capillary distribution varies from tissue to tissue

Why can capillaries withstand high intravascular pressures? : 

Why can capillaries withstand high intravascular pressures? Laplace’s Law Wall Tension = Pr Wall Stress = Pr/w P = transmural pressure r = radius of the vessel w = wall thickness

LaPlace’s Law : 

LaPlace’s Law

Mechanisms of Exchange Across The Capillary Endothelium : 

Mechanisms of Exchange Across The Capillary Endothelium Diffusion Oxygen, Carbon Dioxide, Lipid-Soluble Substances Bulk flow (via intercellular clefts or “pores”) Water, Electrolytes, Small Molecules Vesicular transport Proteins Active transport Ions, Glucose, Amino Acids

Diffusion Through the Capillary Membrane : 

Diffusion Through the Capillary Membrane

Capillaries : 

Capillaries The permeability of the capillary endothelial membrane is not the same in all body tissues

Body Fluid Compartments : 

Body Fluid Compartments

Slide 19: 

Intracellular Fluid 28 L 281 mOsm/L Plasma 3 L 282 mOsm/L Interstitial Fluid 11 L 281 mOsm/L Cell Membrane Capillary Membrane Lymphatics Extracellular Fluid 14 L Fluid Intake Fluid Losses

Distribution of Various IV Fluids Throughout the Body Fluid Compartments : 

Distribution of Various IV Fluids Throughout the Body Fluid Compartments ICF = 28 L Interstitial Fluid = 11 L Plasma = 3 L RBC = 2 L ECF = 14 L TBW = 42 L 5 % Dextrose 0.9 % Saline or LR Colloid

Transcapillary Fluid Exchange : 

Transcapillary Fluid Exchange

Factors Determining Fluid Movement : 

Factors Determining Fluid Movement

Net Driving Forces : 

Capillary Pressure Plasma Colloid Osmotic Pressure Interstitial Fluid Colloid Osmotic Pressure Interstitial Fluid Pressure CAPILLARY INTERSTITIAL FLUID Net Driving Forces

Filtration - Reabsorption : 

Filtration - Reabsorption Hydrostatic Forces Pc = Capillary hydrostatic pressure Higher at the arteriolar end than at the venule end. Tends to force fluid outward through the capillary membrane. Pif = Interstitial fluid hydrostatic pressure Normally interstitial fluid hydrostatic pressure is negative. This is usually an outward force for fluid movement.

Filtration-Reabsorption : 

Filtration-Reabsorption Osmotic Forces p = plasma colloid osmotic pressure Plasma proteins are the major determinan. Albumin generates about 70% of the oncotic pressure. Tends to cause inward movement of fluid. if = interstitial fluid osmotic pressure Caused by the small amount of plasma proteins that leak into the interstitial space. Tends to cause outward movement of fluid.

Gibbs-Donnan Effect : 

Gibbs-Donnan Effect Albumin exerts a greater osmotic force than expected when only the number of albumin molecules in the plasma are considered. The reason is Albumin has a negative charge Attracts sodium ions

Osmosis : 

Osmosis Semi-Permeable Membrane

Osmosis : 

Osmosis Water moves from a region of high concentration to one that has a lower concentration of water

Filtration - Reabsorption : 

Filtration - Reabsorption Filtration The sum of the hydrostatic and osmotic forces favors the net movement of water from the capillary to the interstitial space Reabsorption The sum of the hydrostatic and osmotic forces favors the net movement of water from the interstitial space to the capillary

Starling Forces : 

Starling Forces Net Driving Force = Filtration: NDF>0 Reabsortion: NDF<0

Filtration-Reabsorption:Baseline Beginning of Capillary : 

Filtration-Reabsorption:Baseline Beginning of Capillary Pc = 30 mm Hg Pi = -3 mm Hg c = 26 mm Hg I = 6 mm Hg rc = 0.9 NDF = [30-(-3)]-0.9[(26-6)] = 15 mm Hg Pc Capillary Length 0 30 10

Filtration-Reabsorption:Baseline End of Capillary : 

Filtration-Reabsorption:Baseline End of Capillary Pc = 10 mm Hg Pi = -3 mm Hg c = 26 mm Hg I = 6 mm Hg rc = 0.9 NDF = [10-(-3)]-0.9[(26-6)] = -5 mm Hg Pc Capillary Length 0 30 10

Filtration-Reabsorption:Baseline : 

Filtration-Reabsorption:Baseline Filtration NDF 0 15 -15 Reabsorption

Clinical Correlates : 

Clinical Correlates Dehydration Decreased capillary hydrostatic pressure Increased oncotic pressure

Filtration-Reabsorption:Dehydration Beginning of Capillary : 

Filtration-Reabsorption:Dehydration Beginning of Capillary Pc = 25 mm Hg Pi = -3 mm Hg c = 31 mm Hg I = 6 mm Hg rc = 0.9 NDF = [25-(-3)]-0.9[(31-6)] = 5.5 mm Hg Pc Capillary Length 0 25 5

Filtration-Reabsorption:Dehydration End of Capillary : 

Filtration-Reabsorption:Dehydration End of Capillary Pc = 5 mm Hg Pi = -3 mm Hg c = 31 mm Hg I = 6 mm Hg rc = 0.9 NDF = [5-(-3)]-0.9[(31-6)] = -14.5 mm Hg Pc Capillary Length 0 25 5

Filtration-Reabsorption:Dehydration : 

Filtration-Reabsorption:Dehydration Reabsorption NDF 0 15 -15 Filtration

Lymphatics : 

Lymphatics

Lymphatics : 

Lymphatics Close-ended network of highly permeable lymph capillaries Lacking in tight junctions Fine filament anchors to connective tissue Muscular contraction: Distortion and opening of spaces

Lymphatics : 

Lymphatics Pumping by the lymphatic system is the basic cause of the negative pressure in the interstitial fluid space

Lymphatics : 

Lymphatics Plasma filtrate is returned to the circulation by: Tissue pressure Intermittent skeletal muscle activity Lymphatic vessel contraction System of one-way valves Returns the following to the circulation: Protein (Albumin) Bacteria Fat Excess fluid

Clinical Correlates : 

Clinical Correlates Edema Lymphatic obstruction Change in capillary permeability Reduction in plasma protein Increased capillary hydrostatic pressure

THE END : 

THE END

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