The kidneys are the organs in the body that filter out toxic and other waste materials from the bloodstream and maintain the body's fluid balance (Fig.1-10). Blood passes from the aorta into the kidney where it is filtered and returned back into the blood stream. The toxic products are then passed with fluid into the ureters which lead down to the bladder. Urine leaves the bladder via the urethra to the exterior.

Terminology
Calculi - These may be seen as powder like deposits on the vulva of sow or as small stones in the urine. They are due to the crystallisation of mineral deposits and are not usually of any clinical significance. They are particularly striking in the kidneys of piglets which have died of TGE or greasy pig disease and are exaggerated in mercury poisoning.
Cystitis - Inflammation usually due to infection of the lining of the bladder. The normal thickness of the bladder is approximately 10mm but in severe cases it may be up to 50mm. Haemorrhage often occurs and in such cases mortality in sows can be high.
Haematuria - Blood in urine. Always consider this as serious, a sign of severe cystitis/pyelonephritis.
Haemoglobinuria - Free haemoglobulin in the urine.
Nephritis - Inflammation of the kidney. It can be associated with several different bacteria that are transmitted either via the bloodstream (septicaemia) or reflux from the bladder. Bacteria or their toxins can damage the delicate filtering mechanism of the kidney. These include Staphylococcus hyicus (greasy pig disease), salmonella, streptococci and erysipelas. Actinobaculum suis (formally known as Eubacterium suis or Corynebacterium suis) is the commonest specific cause of ascending (i.e. reflux) nephritis and cystitis. Haemorrhage into the kidneys is common in the swine fevers. Fungal mycotoxins may also damage the kidneys but do not cause inflammation.
pH - Urine is normally slightly acid, pH5 to 6.6. (Neutral is pH7). After weaning however, urine becomes alkaline (>pH7) for up to 3 weeks. Sows with pH more than 7.5 probably have pyelonephritis and mortality in such animals can be high particularly if it reaches a level of 8.
Proteinuria - Protein in the urine. Normal levels are 6-20mg/100 ml. Levels are elevated in kidney disease.
Pyelonephritis - The ureters arise from the cup-shaped pylorus or collecting area in the kidneys. Infection of this area together with the kidney is called pyelonephritis. It is a common disease in the sow. Bacteria associated with this include E. coli, streptococci and Actinobaculum suis the latter being the most common and important.
Pyuria - Pus in the urine.
Urethritis - Inflammation of the urethra.