17.1 Physical Characteristics of Urine
Learning Objectives
By the end of this section, you will be able to:
- Compare and contrast blood plasma, glomerular filtrate, and urine characteristics
- Describe the characteristics of a normal urine sample, including normal range of pH, osmolarity, and volume
The urinary 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. The glomeruli create about 200 litres of this filtrate every day, yet you excrete less than two litres of waste per day of urine.
Characteristics of the urine vary, depending on influences such as water intake, exercise, environmental temperature, nutrient intake, and other factors (Table 17.1.1.). Some of the characteristics such as colour and odour are rough descriptors of your state of hydration, for example, if you exercise or work outside, and sweat a great deal, your urine will turn darker and produce a slight odour, even if you drink plenty of water. Athletes are often advised to consume water until their urine is clear. This is (mostly) good advice; however, it takes time for the kidneys to process body fluids and store it in the bladder. Another way of looking at this is that the quality of the urine produced is an average over the time it takes to make that urine. Producing clear urine may take only a few minutes if you are drinking a lot of water or several hours if you are working outside and not drinking much.
Table 17.1.1. Normal urine characteristics
| Characteristic | Normal values |
| Colour | Pale yellow to deep amber |
| Odour | Odourless |
| Volume | 750-2000mL/24h |
| pH | 4.5-8.0 |
| Specific gravity | 1.003-1.0032 |
| Osmolarity | 40-1350 mOsmol/kg |
| Urobilinogen | 0.2-1.0 mg/100mL |
| White blood cells | 0-2 HPF (per high-power field of microscope) |
| Leukocyte esterase | None |
| Protein | None or trace |
| Bilirubin | <0.3 mg/100mL |
| Ketones | None |
| Nitrites | None |
| Blood | None |
| Glucose | None |
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. Unusually large quantities of urine may point to diseases like diabetes mellitus or hypothalamic tumours that cause diabetes insipidus. The colour of urine is determined mostly by the breakdown products of red blood cell destruction (Figure 17.1.1). The “haem” of haemoglobin is converted by th