Body Fluids

Prof. K. Sivapalan

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Body Composition

• Carbon, oxygen, hydrogen, nitrogen, …….

• Muscles, bones, liver, spleen, brain….

• Water 60 %.

• Protein 18-20 %

• Fat: M-15 %, F- 25 %

• Carbohydrate 2 %

• Minerals.

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Body Fat

• Structural fat [6-7%] and storage fat

• Lean body mass

• Body mass index-

– Weight in Kg/height in M2

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Fluid Compartments

• Intra cellular fluid

• Extra cellular fluid-Inter cellular fluid

– Tissue fluid

– Plasma

– Transcellular fluids

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Major Components

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Definitions - Solutions

• A mole is the gram-molecular weight of a substance- NaCl- 23+35.5=58.5g

• One electrical equivalent (eq) is 1 mol of an ionized substance divided by its valence.

• Gram equivalent is the weight of a substance that is chemically equivalent to 8.000 g of oxygen

• The normality (N) of a solution is the number of gram equivalents in 1 liter

• Molar solution contains one gram mole of a substance in one liter.

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Diffusion

• Diffusion is the process by which a gas or a substance in a solution expands, because of the motion of its particles, to fill all the available volume.

• The rate depends on ,

– Concentration [chemical] gradient

– Electrical gradient for charged particles

– Cross-sectional area

– Distance

– Permeability of the boundaries

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Osmosis

• Diffusion of solvent molecules into a region in which there is a higher concentration of a solute to which the membrane is impermeable—is called osmosis.

• P=nRT/V [n is the number of particles, R is the gas constant, T is the absolute temperature, and V is the volume]

• One osmole (Osm) equals the gram-molecular weight of a substance divided by the number of freely moving particles that each molecule liberates in solution

• Osmolar solution- 1 osmole in 1 liter [op-22.4 atmosphears]

• Glucose- mole=Osmole, NaCl- 1 mole = 2 osmoles, Na2so4- 1 mole = 3 osmoles.

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Osmolality - Tonisity• The osmolarity is the number of osmoles per liter of

solution (eg, plasma)

• The osmolality is the number of osmoles per kilogram of solvent.

• Measured by the degree to which the freezing point is depressed [1.86 ºC]

• Osmolality of plasma is 290 mOsm/L

• The term tonicity is used to describe the osmolality of a solution relative to plasma- hypotonic, isotonic, hypertonic.

• Osmotic Pressure is the pressure exerted by the solution when separated from water by semi-permiable membrane or the pressure required to prevent net movement of water into the solution.

• Osmotic pressure of 1 osmolar solution is 22.4 atmospheres = 17024 mmHg, plasma= 17024x0.290=4936 mmHg.

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Importance of Osmolality

• Cells swell when exposed to extracellular hypotonicity and shrink when exposed to extracellular hypertonicity because cell membrane is freely permiable to water. [Fragility test for red cells]

• If the concentration increases, reactions are altered and if the volume increases beyond the limit- lyses or contents diffuse out.

• All but about 20 of the 290 mOsm in each liter of normal plasma are contributed by Na+ and its accompanying anions, principally Cl– and HCO3–.

• Osmolality of plasma (mOsm/L) = 2[Na+] (mEq/L) + 0.055[Glucose] (mg/dL) + 0.36[Blood Urea Nitrogen] (mg/dL)

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Exchange Between ICF and ECF

• The properties of the cell membrane determine the exchange.

• Water is freely permeable

• The permeability to solutes is determined by fat solubility and mollicular size.

• Many substances are transported across the membrane by facilitated diffusion, active transport, secondary active transport, pinocytosis, phagocytosis etc.

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TRANSPORT ACROSS MEMBRANE

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PINOCYTOSIS

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• The exchange occurs through capillaries which lie between arteriol and venule.

• Precapillary sphinctors determine the flow into specific capillaries which is determined by local metabolites.

• The capillaries have clefts between endothelial cells [slit-pores] with diameter of 6-7 nm- smaller than albumin.

• Fluid and solutes pass freely through these pores.

• Plasmalemmal vesicles also transport small amounts.

Exchange Between Blood and Tissues

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Diffusion in Capillaries

• Lipid soluble substances including oxygen and carbon dioxide diffuse across the endothelium according to concentration gradient.

• Water and water soluble substances such as sodium, chloride, bicarbonate and glucose pass through the pores.

• Larger molecules, albumin and other plasma proteins cannot pass through the pores

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Colloid Osmotic Pressure

• The capillary pore permit all solutes in plasma except the cells and proteins.

• It is similar to fluid separated by a semi-permiable membrane, the osmotic particle being the protein.

• The osmotic pressure exerted by plasma proteins amount to about 25 mmHg.

• Even though it is negligible contribution to the osmotic pressure of the plasma, it is the only difference across the capillary membrane.

• It is named colloid osmotic pressure or oncotic pressure.

• As the largest number of protein molecules are albumin, it is responsible for the colloid osmotic pressure

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Starling Forces forCapillary Exchange

• Pressure of the blood in the arteriolar end of the capillaries is 30 mm Hg.

• It reduces progressively towards the venous end and becomes 10 mmHg.

• Colloid osmotic pressure is 25 mmHg at the arteriolar end and becomes 28 mm Hg as concentration increases.

• Tissue pressure is slightly negative [-3 mm Hg] probably due to lymphatic suction.

• As some protein molecules escape into the tissue fluid, it also has mild colloid pressure [8 mm Hg].

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Starling Equilibrium forCapillary Exchange

• At the arteriolar end:

– Hydrostatic [blood] pressure= 30 mmHg

– Plasma Colloid osmotic pressure= -25 mmHg

– Tissue Colloid osmotic pressure= 8 mmHg

– Tissue pressure= -3 mmHg

• At the Venular end

– Hydrostatic [blood] pressure= 10 mmHg

– Plasma Colloid osmotic pressure= -28 mmHg

– Tissue Colloid osmotic pressure= 8 mmHg

– Tissue pressure= -3 mmHg

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Role of Lymphatics

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• Allmost all tissues have lymphatics channels which coalase and open into superior venacava through thoracic duct [2-3 liters / day].

• Exceptions include the superficial portions of the skin, the central nervous system, the endomysium of muscles, and the bones.

• Lymphatic channels originate in the tissues as blind ended tube.

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Formation of Lymph

• Lymphatic capillaries are made of endothelial cells which are attached to connective tissue through anchoring filaments.

• The edge of one endothelial cell overlaps the adjacent cell so that the edge forms a minute valve that opens to the interior of the lymphatic capillary.

• The lymphatics contract periodically and the valves prevent back flow and push the contents forwards.

• This permits sucking of excess tissue fluid along with the protein and other large particles.

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Summary of Microcirculation

• Filtration at the arteriolar end

• Re-absorption at venular end

• Balance fluid and proteins drained by lymphatics.

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Estimation of Fluid Volumes

• The volume of a fluid compartment in the body can be measured by placing an indicator substance in the compartment,

• allowing it to disperse evenly throughout the compartment’s fluid,

• and then analyzing the extent to which the substance becomes diluted.

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Conditions for Indicator Dilution Principle

• This method can be used to measure the volume of virtually any compartment in the body as long as :

• (1) the indicator disperses evenly throughout the compartment,

• (2) the indicator disperses only in the compartment that is being measured, and

• (3) the indicator is not metabolized or excreted.

• Several substances can be used to measure the volume of each of the different body fluids.

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Measurement of Total Body Water

• Radioactive water (tritium, 3H2O) or heavy water (deuterium, 2H2O) can be used to measure total body water.

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Measurement of Extracellular Fluid Volume.

• substances that disperse in the plasma and interstitial fluid but do not readily permeate the cell membrane are used.

• They include radioactive sodium, radioactive chloride, radioactive iothalamate, thiosulfate ion, and inulin

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Measurement of Plasma Volume

• Substance used should not penetrate capillary membranes but remains in the vascular system after injection.

• One of the most commonly used substances for measuring plasma volume is serum albumin labeled with radioactive iodine (125I-albumin).

• Also, dyes that avidly bind to the plasma proteins, such as Evans blue dye (also called T-1824), can be used.

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Measurement of Body Fluid Volumes

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Fluid Balance- ml/day

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