Various nitrogenous waste products are produced as a result of metabolism and this waste is excreted from the human body mainly in the form of urine. Kidneys play a major role in the process of excretion and the purification of blood. Normal human urine comprises 95 percent water and 5 percent solid waste products. It is produced in the nephron, which is the functional unit of the kidney. Urine formation in our body is mainly carried out in three phases:

  1. Glomerular filtration
  2. Reabsorption
  3. Secretion

Mechanism of Urine Formation

The mechanism of urine formation involves the following steps:

Glomerular Filtration

Each kidney contains over 1 million nephrons. Nephron has glomerulus, tubules, and collecting duct. The glomerulus is the site of blood filtration. The glomerulus is a tuft of capillaries that is surrounded by a cup-like structure, called Bowman’s capsule. As blood flows through the glomerulus with pressure, it pushes water and solutes from the capillaries into the filtration membrane. 

The filtration membrane allows water and small solutes to pass but blocks blood cells and large proteins due to which these substances remain in the bloodstream. The filtrate which is the fluid that has passed through the membrane flows from the Bowman’s capsule further into the nephron.

This process is known as ultrafiltration.


The glomerulus filters water and small solutes out of the bloodstream. The resulting filtrate contains waste but also other substances like electrolytes, glucose, amino acids, and smaller proteins (these substances are useful for our body). When the filtrate exits the glomerulus, it flows into the renal tubule. As it moves, the needed substances and some water are reabsorbed. This reabsorption of vital nutrients from the filtrate is the second step in urine formation. Around 99 percent of the filtrate obtained is reabsorbed by the renal tubules. This is known as reabsorption. 


The next step in urine formation is tubular secretion. In this step, tubular cells secrete substances like hydrogen ions, potassium ions, etc into the filtrate. The secreted ions combine with the filtrate and form urine. Through this process, the acid-base and the balance of other body fluids are maintained. The urine passes out of the nephron tubule into a collecting duct. The urine flows out of the nephron tubule into a collecting duct. It passes out of the kidney through the renal pelvis, into the ureter, and down to the bladder.


The urine produced is 95% water and 5% nitrogenous wastes (such as urea, ammonia, and creatinine) excreted in the urine. Apart from these, the potassium, sodium, and calcium ions are also excreted.