Urine Formation

The kidneys are one of the main organs of the urinary system. Millions of tiny structures, which form the basic unit of each kidney, are known as nephrons. This nephron consists of a cup-like structure, known as glomerulus, which then merges into a tube known as the proximal tubule.

The proximal tubule then gives rise to a U-shaped structure known as loop of Henle or Henle's loop, which finally merges into the distal convoluted tubule, which carries the urine into the collecting tubules.

The first step in formation of urine is filtration. Filtration is the process by which the blood that passes through the glomerulus is filtered out, so that only certain structures pass into the proximal convoluted tubule. The rate at which the blood is filtered is known as the glomerular filtration rate, which is normally 125 ml/minute or 180 liters/day.

The glomerulus lining only allows small molecules to filter through, like glucose, plasma, ions like sodium, potassium, urea etc, but not larger molecules like blood cells and proteins. Thus, in kidney diseases, glomerulus lining is affected,  due to which protein molecules also pass through, leading to blood and protein in urine.

As there is only crude separation of waste products and water in filtration, some essential materials also pass through. They need to be reabsorbed in the body. This is known as selective reabsorption as only some elements are reabsorbed. The process occurs in two stages, active reabsorption (energy needed) and passive reabsorption(energy not needed).

Due to the difference in concentration of the fluid inside and outside the tubules, 99% of the water returns into circulation and thus, is passively absorbed, which is important for urine formation and flow.

If the glucose level is normal, most of it is reabsorbed into the blood from the proximal tubules. It is then is actively transported into the peritubular capillaries. But, if the glucose level is high, it is passed into the urine (a symptom of diabetes). Sodium ions are the only ions that are partially reabsorbed from the renal tubules into the blood.

This is the last step where the urine is concentrated by increasing the level of waste elements. Here, substances move into the distal and collecting tubules from blood in the capillaries around them.

These substances are secreted via active transport and include hydrogen ions, potassium ions, ammonia, and certain drugs or metabolic waste. Thus, the kidney tubules play a crucial role in maintaining the body's acid-base and electrolyte balance.

Then distal convoluted tubules drain urine into collecting tubules. Several collecting tubules join together to drain their contents into the collecting duct, which finally, after formation of urine, flows into the ducts of Bellini. This then eventually reaches the renal pelvis, from where the urine flows into the ureter to reach the urinary bladder.

Thus, these are the various steps that take place right from the time when blood flows into the kidneys, till urine is passed into the ureters. Urinary system diseases occur when there are problems with the functioning of the kidneys, which reflects in the color, odor, and concentration of urine.