Loss of any substance from the body occurs by excretion in urine, feces or by insensible routes (respiration is the only significant site of insensible loss in the dog and cat) and by consumption in metabolic reactions. The body can "gain" substances by ingestion, parenteral administration (e.g. IV or IM injection) or by production in a metabolic reaction.

The kidneys are responsible for excretion of metabolic wastes including:

• uric acid which is derived from nucleic acids
• creatinine from muscle creatine
• bilirubin derived from hemoglobin
• urea nitrogen derived from dietary and endogenous proteins

The kidneys excrete many foreign chemicals including pharmacologics and toxins.

The kidneys play an important role in the regulation of arterial blood pressure. The following factors influence blood pressure:

• sodium and water balance
• the renin-angiotensin-aldosterone system
• vasoactive prostaglandins which are produced in the kidney

Decreased oxygen delivery to the kidney stimulates the release of renal erythropoietic factor which acts on a liver globulin to produce erythropoietin. Erythropoietin stimulates bone marrow stem cells to differentiate along RBC lines.

The kidneys produce the active form of vitamin D; 1,25 dihydroxycholecalciferol.

The kidneys are capable of gluconeogenesis (glucose production) during a prolonged fast, although this is a minor function.

The following hormones are degraded or excreted by the kidneys:

• parathyroid hormone
• growth hormone
• secretin
• cholecystokinin
• glucagon
• gastrin
• insulin
• antidiuretic hormone

Any of these hormones may increase in circulation in the patient with renal dysfunction if the kidney is unable to excrete or degrade the hormone.