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By Root 1957 0

kidney, which act as nonselective filters. Filtration occurs as a result of increased glomerular blood pressure caused by the difference in diameter between afferent and efferent arterioles. 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 2010).

Tubular reabsorption

Tubular reabsorption then occurs, removing necessary substances from the filtrate and returning them to the peritubular capillaries. Tubular reabsorption is an active process that requires protein carriers and energy. Substances reabsorbed include nutrients and most ions. It is also a passive process, however, driven by electrochemical gradients. Substances reabsorbed in this way include sodium ions and water. Creatinine and the metabolites of drugs are not reabsorbed because of their size, insolubility or a lack of carriers. Most of the nutrients, 65% of the water and sodium ions, and the majority of actively transported ions are reabsorbed in the proximal convoluted tubules (Marieb and Hoehn 2010).

Reabsorption of additional sodium ions and water occurs in the distal tubules and collecting ducts and is hormonally controlled. Aldosterone increases the reabsorption of sodium, and antidiuretic hormone (ADH) enhances water reabsorption by the collecting ducts (Marieb and Hoehn 2010).

Tubular secretion

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 2010).

Regulation of urine concentration and volume occurs in the loop of Henle, where the osmolarity of the filtrate is controlled. As the filtrate flows through the tubules, the permeability of the walls controls how dilute or concentrated the resulting urine will be. In the absence of ADH, dilute urine is formed because the filtrate is not reabsorbed as it passes through the kidneys. As levels of ADH increase, the collecting tubules 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 (Marieb and Hoehn 2010).

Urine is a clear, strawcoloured fluid. The normal composition of urine includes water, urea, creatinine, sodium, potassium, organic acids, protein, small traces of glucose and cellular components. The colour of urine is due to a pigment called urochrome which is derived from the body’s destruction of haemoglobin. The more concentrated urine, the deeper yellow it becomes. An abnormal colour such as pink or brown may result from eating certain foods (beetroot or rhubarb), or may be due to the presence in the urine of bile products or blood (Marieb and Hoehn 2010). Often fresh urine appears turbid (cloudy), indicating that there may be an infection of the urinary tract. The urinary tract is the most common site of bacterial infection. Risk factors for urinary tract infections (UTI) include presence of a urinary catheter, female gender, diabetes and advanced age (Marini and Wheeler 2010). See Figure 12.23 for other UTI predisposing factors.

Figure 12.23 Predisposition to UTIs.

Bacteriuria is defined as the presence of bacteria in the urine (Rigby and Gray 2005). Urine specimens are rarely sterile, as a result of contamination with periurethral flora during collection. Infection is distinguished by counting the number of bacteria. Significant bacteriuria is defined as the presence of more than 105 organisms per mL of urine in the presence of clinical symptoms (Marini and Wheeler 2010). See Figure 12.24 for illustration of significant bacteriuria.

Figure 12.24 Significant bacteriuria. Specimens of urine are rarely sterile. A cutoff point is identified to distinguish true infection (significant bacteriuria) from effects of contamination from surrounding tissues.

Fresh urine is slightly aromatic. This can change as a result of disease processes such as diabetes mellitus, when acetone is present in the urine, giving it a fruity smell (Marieb and Hoehn