Biochemistry – ICF/ECF

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Intracellular fluids
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All fluids within body cells ~40% of total body weight
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Extracellular fluids
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All fluids outside cells ~20% of total body weight Sub compartments – interstitial fluid and plasma, lymph, cerebro-spinal fluid and synovial fluid
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Water content regulation
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Content regulated so total volume of water in body remains constant Kidneys are primary regulatory of water excretion
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Regulation processes
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Osmosis Osmolality Baroreceptors Learned behaviour
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Sources of water
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Ingestion Cellular metabolism
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Routes of water loss
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Urine Evaporation (perspiration and respiratory passages) Faeces
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Osmolarity
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Osmoles of solute per L of solution Volume of solution changes with the amount of solute added as well as with changes in temperature and pressure Difficult to determine
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Osmolality
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Moles (or osmoles) of solute per kg of solvent Expressed as mol/kg, molal or m Amount of solvent will remain constant regardless of changes in temperature and pressure, thus osmolality is easier to evaluate and is more commonly used and often preferred in practical osmometry Adding or removing water from a solution changes this
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Increased osmolality
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Triggers thirst and ADH (vasopressin) secretion
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Decreased osmolality
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Inhibits thirst and ADH secretion
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Nonelectrolytes
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Typically polar covalent substances as they dissolves in water as molecules instead of ions E.g. sugar (C12H22O11), urea (CH4N2O) and creatine (C4H9N3O2)
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Electrolytes
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When dissolved in a body fluid, they dissociate into cations (+vely charged ions) and anions (-vely charged ions) Most are inorganic compounds but a few are organic (e.g. citric acid, oxaloacetic acid, lactic acid and several AA’s) In solution these molecules lose and ion and the rest of the molecule carries the opposite charge, so they can carry an electrical current
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Regulation of intra/extracellular fluid
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1) Large organic molecules 2) Ion transport (e.g. Na+, K+ and Ca2+) 3) Electric charge difference 4) Water moves by osmosis
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Regulation of electrolytes in extracellular fluid
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Molecules or ions with an electrical charge (water ingestion adds electrolytes to the body, kidneys, liver, skin, lungs remove from the body) Concentration changes only when growing, gaining or losing weight
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Regulation of Na+ ions in extracellular fluid
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Dominant ECF cations Responsible for 90-95% of osmotic pressure Kidneys major route of excretion Small quantities lost in sweat
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Hypernatremia
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When Na concentration in the blood is abnormally high (to a value exceeding 145 mmol/L)
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Hyponatremia
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When the level of Na in the blood is abnormally low

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