Glomerular filtration occurs in the glomeruli of the kidney, which act as non‐selective filters. Filtration occurs as a result of glomerular blood pressure, which is determined by the difference in diameter between afferent and efferent arterioles (Tortora and Derrickson [199]). The effect is a simple mechanical filter that permits substances smaller than plasma proteins to pass from the glomeruli to the glomerular capsule (Marieb and Hoehn [110]).

Tubular reabsorption then occurs, where substances such as sodium ions and water are reabsorbed, primarily in the proximal convoluted tubules but also in the distal tubules and collecting ducts (Marieb and Hoehn [110]). Necessary substances are removed from the filtrate and returned to the peritubular capillaries, where they go back into the circulation (Waugh and Grant [207]). Others, such as creatinine and drug metabolites, are not reabsorbed because of their size, insolubility or a lack of carriers (Marieb and Keller [111]).

Tubular reabsorption is hormonally controlled as aldosterone increases the reabsorption of sodium, and antidiuretic hormone (ADH) enhances water reabsorption by the collecting ducts (Marieb and Hoehn [110]).

Tubular secretion is an active process that is important in eliminating drugs, certain wastes and excess ions and in maintaining the acid/base balance of blood (Marieb and Hoehn [110]).

Regulation of urine concentration and volume occurs in the loop of Henle, where the osmolarity (concentration) of the filtrate is controlled (Tortora and Derrickson [199]). As the filtrate flows through the tubules, the permeability of the walls controls how diluted or concentrated the resulting urine will be (Waugh and Grant [207]). In the absence of ADH, diluted urine is formed because the filtrate is not reabsorbed as it passes through the kidneys (Marieb and Keller [111]). As levels of ADH increase, the collecting ducts become more permeable to water, and water moves out of the filtrate back into the blood; consequently, more concentrated urine is produced, and in smaller amounts, as a response to the physiological need for increased intravascular volume and/or maintaining electrolyte balance (Marieb and Hoehn [110]).

The composition, smell and colour of urine can change dramatically as a result of disease processes (Eyer et al. [61]). An unusual colour, such as pink or brown, may result from eating certain foods (beetroot or rhubarb), it or may be due to the presence of bile products or blood (Marieb and Hoehn [110]). A turbid (cloudy) colour can mean an infection of the urinary tract, which is one of the most common sites of bacterial infection (Wilkinson et al. [215]). Risk factors for urinary tract infections (UTIs) include the presence of a urinary catheter, female sex, diabetes and advanced age (Sheerin [187]). See Figure 14.40 for other predisposing factors. A fruity smell, instead of the natural slightly aromatic smell, can indicate the presence of ketones, a result of disease processes such as diabetes mellitus (Marieb and Hoehn [110]).

Urinalysis allows the assessment of the composition of urine, giving an indication of renal function and the effectiveness of the mechanisms described above.