Cardiovascular IV: Blood Volume Microcirculation & Fluid Exchange PDF

Cardiovascular IV: Blood Volume Microcirculation & Fluid Exchange ANSC 3080 G. Bedecarrats Learning Objectives  Describe the role of the microcirculation in fluid and macromolecular exchange  Explain the general mechanisms of transcapillary...

Cardiovascular IV: Blood Volume Microcirculation & Fluid Exchange ANSC 3080 G. Bedecarrats Learning Objectives  Describe the role of the microcirculation in fluid and macromolecular exchange  Explain the general mechanisms of transcapillary exchange  Explain the forces regulating the net movement of fluid across the capillary wall  Describe the basic mechanisms of edema formation Capillaries Contains only about 5 % of the blood Site of water-nutrients-wastes-gases exchanges with interstitial fluid and tissues 7-8 m diameter, 0.5 mm in length Red blood cells ~7.5 m in diameter Single layer of endothelial cells Dense networks each cell of a tissue within 100 m of a capillary Receive blood from smallest arterioles Also from metarterioles = connection between arteriole and capillaries network 1  Density of capillaries vary depending on the metabolic activity of the tissue  Metarterioles possess rings of smooth muscle tissue that can open and close on demand  In skeletal muscle: all open during exercising  Small diameter = very slow flow  Resistance reduced by the extent of the parallel ramifications Transcapillary Exchange Diffusion: depends on capillary type and substance properties – follows gradient between blood and interstitial fluid Lipid soluble substances exchange freely across cell membrane (O2 and CO2) Most capillaries have pores or clefts that allow transfer of water and lipid-insoluble molecules (Na+, Cl-, glucose, amino acids). Size and number of pores depend on the tissue Some capillaries are fenestrated (intestine, liver, kidneys) – vesicles fuse to form large gaps across endothelial cell membrane allowing water and water soluble macromolecules to pass 2  Bulk Flow = Fluid Exchange  Mass movement of water and dissolved substances  Takes place across the capillary walls  Towards interstitial fluid = filtration  Towards the intravascular fluid (blood) = absorption  Maintains fluid balance between intravascular and interstitial fluid  Dependent on:  Pressure gradients (Hydrostatic and osmotic)  Permeability of vessel (porosity)  Size of diffusion surface  Blood flow Pressure Gradients Hydrostatic pressure Filtration pressure Pressure from heart arterioles capillaries Pressure in interstitial fluid is close to 0 Pushes fluid out of vessel Colloid osmotic pressure (oncotic) Due to big proteins that stay in vessel Counter balances hydrostatic pressure 3  Starling Forces = Opposing forces Net movement of fluid depends on balance between filtration (hydrostatic) and osmotic (oncotic) pressures Hydrostatic pressure difference pushing fluid out Osmotic pressure difference pushing fluid in. Remains constant along capillary length Along capillary: In the first portion: hydrostatic stronger (more fluid out) Toward the end: hydrostatic  = less fluid out; and if hydrostatic < osmotic = fluid moves in a, b: normal situation c: arteriole dilation d: lower osmotic pressure e: arteriole constrict Where Does the Fluid Go? Net effect is generally accumulation of fluid into interstitial tissue Picked up by the lymphatic system Drained to the large veins Called lymphatic drainage Fluid that remained in the blood collects into venules and goes back to the heart If lymphatic system cannot keep up  edema formation 4  Physiological Significance Diffusion = nutrient and gas exchange Bulk flow = stabilize blood volume using interstitial fluid as a buffer Blood loss =  in pressure =  in hydrostatic = fluid enters back Excess fluid intake =  in pressure =  in hydrostatic = fluid accumulates in interstitial Low protein in plasma =  in osmotic = fluid out High protein in plasma =  in osmotic = fluid in Edema Formation Abnormal accumulation of interstitial fluid Four mechanisms responsible:  hydrostatic pressure in blood vessels  arterial pressure (hypertension)  venous pressure (right sided heart failure)  interstitial protein concentration Inflammation of capillaries, become “leaky”  oncotic pressure Loss of proteins (GI disease), liver dz., poor diet Obstruction of lymphatic vessels 5  Edema: Clinical Example  8-year-old horse with “dependent edema”  Swelling of legs and other “dependent” areas  Right sided heart failure due to tricuspid valve insufficiency  Venous congestion  Increased filtration pressure Plaque of edema 6