Urine is a liquid by-product formed from excess water and metabolic waste molecules during filtration by kidneys.

Urine formation occurs during three processes:

Filtration
Reabsorption
Secretion

Filtration

In the first step of urine production, water and most solutes in blood plasma enters the afferent arteriole and flows into the glomerulus where filterable blood components, such as water and then into the renal tubule.These filterable components accumulate in the glomerulus to form the glomerular filtrate.

Normally, about 20% of the total blood pumped by the heart each minute will enter the kidneys to undergo filtration; this is called the filtration fraction. The remaining 80% of the blood flows through the rest of the body to facilitate tissue perfusion and gas exchange.

Reabsorption

During Reabsorption, the molecules and ions are reabsorbed into the circulatory system. The fluid passes through the components of the nephron as water and ions are removed as the fluid osmolarity (ion concentration) changes. In the collecting duct, secretion will occur before the fluid leaves the ureter in the form of urine.

Secretion

During secretion some substances such as hydrogen ions, creatinine, and drugs are removed from the blood through the peritubular capillary network into the collecting duct. The end product of all these processes is urine, which is essentially a collection of substances that has not been reabsorbed during glomerular filtration or tubular reabsorbtion.

Urine is mainly composed of water that has not been reabsorbed, which is the way in which the body lowers blood volume, by increasing the amount of water that becomes urine instead of becoming reabsorbed. The other main component of urine is urea, a highly soluble molecule composed of ammonia and carbon dioxide, and provides a way for nitrogen (found in ammonia) to be removed from the body. Urine also contains many salts and other waste components. Red blood cells and sugar are not normally found in urine but may indicate glomerulus injury and diabetes mellitus respectively.