…differences, the total number of all cations and anions in body fluids are equal to maintain electroneutrality. Osmolality Osmolality refers to the number of osmotically active particles (osmoles) per kilogram of solvent. In clinical veterinary medicine, osmolality is expressed as milli-osmoles per kg. Osmolarity is simply the concentration per liter of solvent. In biologic fluids, there is a negligible difference between osmolality and osmolarit…
Urine specific gravity (USG) and osmolality are measures of the solute concentration in urine and are used to assess tubular function, i.e. the ability of the renal tubules to dilute (loop of Henle) or concentrate (distal tubules) the glomerular filtrate. Renal concentrating ability Knowledge of urinary solute concentration is essential for proper interpretation of urea and creatinine, which are indicators of glomerular filtration rate. A wide…
Concentrating ability Read More »
Physiology Sodium (Na+) is the major extracellular cation and is a primary determinant of plasma osmolality and extracellular fluid (ECF) volume. Sodium concentration is inextricably linked with ECF volume, therefore interpretation of sodium levels should always include consideration of the hydration status of the patient (and, therefore, changes in “free” water). The body attempts to maintain a constant ECF volume, as major changes…
…diagrams under acid-base below). Aldosterone is stimulated by angiotensin II (in response to hypovolemia, hypo-osmolality, low NaCl [latter sensed by the macula densa]), hyperkalemia and ACTH. It is inhibited by dopamine and atrial natriutic peptide, which causes sodium loss. Elimination of nitrogenous waste Most nitrogenous waste is formed by protein catabolism, which is excreted into the urine via glomerular filtration (urea and creatinine…
…acidosis caused by metabolites of ethylene glycol. They also have a high osmolal gap (measured minus calculated osmolality > 30) because ethylene glycol and its metabolites are unmeasured osmols but are not included in the equation to calculate osmolality (based on sodium, potassium, urea nitrogen and glucose). Bicarbonate loss metabolic acidosis: This is also called a hyperchloremic metabolic acidosis because it is associated with a hi…
Physiology Chloride is the major extracellular anion, found together with sodium. Chloride is important for osmolality and acid-base balance. Because chloride is found in a 1:1 ratio with sodium (although is slightly lower than sodium in plasma), changes in chloride should always be interpreted with sodium. Changes in free water and the gain or loss of chloride and sodium are the major mechanisms for a change in chloride concentration. A loss o…
…ples. The increases in MCV will falsely increase the HCT, which is a calculated value (see above formula). Hyperosmolality: With the ADVIA hematology analyzer (and potentially other hematologic analyzers), macrocytosis can be observed in animals with severe hyperosmolality, e.g. hypernatremia. This is attributed to dehydration of red blood cells which occurs in vivo due to the hyperosmotic environment. Once these dehydrated cells are pl…
…ability: How we assess the ability of the kidneys to concentrate urine by measuring USG or osmolality Chemical constituents: pH, protein (Dipstick, SSA, Bence-Jones), protein-to-creatinine ratio, glucose, ketones, bilirubin, heme (“blood”) and uric acid. Cellular constituents: Those that are normally seen in urine including leukocytes, erythrocytes, various types of epithelial cells and s…
…scatter light if in suspension). Concentrating ability can also be assessed more accurately by measuring urine osmolality, but this is not usually done as part of the routine urinalysis. Dipstick analysis: Dipsticks consist of various pads containing chemical ingredients which provide a color change when a particular analyte is present in urine. This color change is converted to a semi-quantitative result for the analyte in question. In animals,…
…is RBC dehydration if there is excess EDTA for the amount of blood in the sample (EDTA dehydrates RBC) or hypo-osmolality – in the latter cases, the high MCHC can be accompanied by a lower MCV (MCV will be lower than it “should be” even if not below the reference interval). CHCM: Excess EDTA or hyponatremia. Decreased values (hypochromic) Artifact: MCHC/CHCM: RBC swelling with storage and causes of hyperosmolality su…
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