13.13 Physical Characteristics of Urine

The renal system’s ability to filter the blood resides in about 2 to 3 million tufts of specialised capillaries—the glomeruli—distributed more or less equally between the two kidneys. Because the glomeruli filter the blood based mostly on particle size, large elements like blood cells, platelets, antibodies, and albumen are excluded. The glomerulus is the first part of the nephron, which then continues as a highly specialised tubular structure responsible for creating the final urine composition. All other solutes, such as ions, amino acids, vitamins, and wastes, are filtered to create a filtrate composition very similar to plasma.

Characteristics of the urine vary, depending on influences such as water intake, exercise, environmental temperature, nutrient intake, and other factors (Table 13.7).

Table 13.7 Normal urine characteristics

Urinalysis (urine analysis) often provides clues to renal disease. Normally, only traces of protein are found in urine, and when higher amounts are found, damage to the glomeruli is the likely basis. The colour of urine is determined mostly by the breakdown products of red blood cell destruction (Figure 13.25). The “haem” of haemoglobin is converted by the liver into water-soluble forms that can be excreted into the bile and indirectly into the urine. This yellow pigment is urochrome. Urine colour may also be affected by certain foods or medications. A kidney stone or a cancer of the urinary system may produce sufficient bleeding to manifest as pink or even bright red urine. Diseases of the liver or obstructions of bile drainage from the liver impart a dark “tea” or “cola” hue to the urine. Dehydration produces darker, concentrated urine that may also possess the slight odour of ammonia. Most of the ammonia produced from protein breakdown is converted into urea by the liver, so ammonia is rarely detected in fresh urine. The strong ammonia odour you may detect older urine is due to the breakdown of urea into ammonia by bacteria in the environment.

The kidneys must produce a minimum urine volume to rid the body of wastes. Output below this level may be caused by severe dehydration or renal disease and is termed oliguria. The virtual absence of urine production is termed anuria. Causes of anuria can include kidney failure, obstruction (such as kidney stone or tumour) or enlarged prostate. Excessive urine production (polyuria) may be due to diabetes mellitus or diabetes insipidus, excess caffeine or alcohol, kidney disease, certain drugs, such as diuretics, sickle cell anaemia, or excessive water intake.

The pH (hydrogen ion concentration) of the urine can vary more than 1000-fold, from a normal low of 4.5 to a maximum of 8.0. Diet can influence pH; meats lower the pH, whereas citrus fruits, vegetables and dairy products raise the pH. Chronically high or low pH can lead to disorders, such as the development of kidney stones or osteomalacia.

Specific gravity is a measure of the quantity of solutes per unit volume of a solution and is traditionally easier to measure than osmolarity. Urine will always have a specific gravity greater than pure water (water = 1.0) due to the presence of solutes. Laboratories can now measure urine osmolarity directly, which is a more accurate indicator of urinary solutes than specific gravity. Urine osmolarity ranges from a low of 50–100 mOsmol/L to as high as 1200 mOsmol/L H₂O.

Cells are not normally found in the urine. The presence of leukocytes may indicate an infection.

Protein does not normally leave the glomerular capillaries, so only trace amounts of protein should be found in the urine, approximately 10 mg/100 mL in a random sample. If excessive protein is detected in the urine, it usually means that the glomerulus is damaged and is allowing protein to “leak” into the filtrate.

Ketones are by-products of fat metabolism. Finding ketones in the urine suggests that the body is using fat as an energy source in preference to glucose. In diabetes mellitus when there is not enough insulin (Type I diabetes mellitus) or because of insulin resistance (Type 2 diabetes mellitus), there is plenty of glucose, but without the action of insulin, the cells cannot take it up, so it remains in the bloodstream. Instead, the cells are forced to use fat as their energy source, and fat consumed at such a level produces excessive ketones as by-products. These excess ketones will appear in the urine. Ketones may also appear if there is a severe deficiency of proteins or carbohydrates in the diet.

Nitrates (NO₃^–) occur normally in the urine. Gram-negative bacteria metabolise nitrate into nitrite (NO₂^–), and its presence in the urine is indirect evidence of infection.

There should be no blood found in the urine. It may sometimes appear in urine samples as a result of menstrual contamination, but this is not an abnormal condition.

Case study

Maynard, a 6-year-old male Irish Wolfhound, presents with polyuria (increased urination), polydipsia (increased thirst), and urinary incontinence. His owner reports frequent urination indoors and a noticeable change in urine odour. Urinalysis reveals bacteriuria (bacteria in urine), pyuria (leukocytes in urine), and a decreased urine specific gravity, consistent with a urinary tract infection (UTI). Large breeds like Irish Wolfhounds may be predisposed to UTIs due to incomplete bladder emptying or underlying systemic conditions. Maynard is treated with targeted antibiotics following culture and sensitivity testing.

Irish Wolfhound by Fitzkarl via Wikimedia Commons CC BY SA 4.0

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