JUST for Little Hawk:

Urine is the fluid excreted by the kidneys. It consists of water, carrying in solution the body's waste products such as urea, uric acid, creatinine, organic acids, and also other solutes such as Na+, K+, Ca2+, Mg2+, Cl-, the body fluid concentrations of which are regulated by the kidneys.

After being produced by the kidneys, urine passes along the ureters to be stored in the bladder, until it is allowed to flow out of the body through the urethra, in the process of micturition (urination). The smooth muscle of the bladder forms an internal sphincter at its junction with the urethra, and further along the urethra is the voluntary-control external sphincter. The bladder begins to contract (micturition reflex), and produces the desire to urinate, when its volume exceeds about 200 ml. However, if we do not relax the external sphincter, the contractions subside, but return with increasing force and frequency as the bladder continues to fill. When the bladder volume is about 500 ml the micturition reflex may force open the internal sphincter and lead to a reflex relaxation of the external sphincter, so that urination occurs involuntarily.

Voluntary urination involves relaxation of the external sphincter and tensing of the abdominal muscles to increase abdominal pressure and compress the bladder, to initiate bladder contraction and relaxation of the internal sphincter.

Most people excrete about 1.5 litres of urine per day, but the volume can range (in healthy adults) from 400 ml up to about 25 litres, depending on fluid intake. In renal failure, there may be no urine production, and in the rare condition of untreated diabetes insipidus, the urine volume is consistently 25 litres/day. Urine is termed 'dilute' if its solute concentration (osmolality) is lower than that of the blood plasma, and 'concentrated' if its solute concentration is greater than that of the plasma.

Humans who are maximally conserving water — when their kidneys are reabsorbing as much as possible — can produce urine with a solute concentration (osmolality) about five times that of blood plasma. Many other animals can conserve water much more effectively. For example, cats, dogs, and rats can produce urine of ten times the plasma osmolality, and gerbils twenty times!

When voided, urine is normally sterile and clear, although it has a yellow colour due to the presence of pigments. However, small amounts of particulate matter such as epithelial cells and lipids may be present; these are 'casts'. Protein is not normally filtered from the blood plasma by the kidneys, so protein in the urine — proteinuria — is generally indicative of damage to the glomeruli, at the blind inner ends of the kidney tubules, where filtration occurs. The urine may also appear to contain blood (haematuria). This may be due to haemolysis in the bloodstream (breakdown of red cells) so that some haemoglobin is released from them and excreted, or it may be due to the presence of whole red cells, as a result of bleeding in the kidneys or urinary tract.