Urine Formation: In humans, Urine is generated in the nephrons and the elimination process is divided into the following steps:

• Glomerular Filtration

• Tubular Reabsorption

• Secretion

Urine Formation

Glomerular Filtration

The first step of urine formation occurs in the glomerulus of the kidneys. The blood enters the glomerular capillaries by afferent arteriole and leaves by an efferent arteriole. In this process excess water, ions, glucose, and waste products are eliminated from the blood into the urine collection which is then eliminated from the body.

The glomerular filtrate consists of water, glucose, ions along urea. Therefore, these necessary substances need to be reabsorbed into the body. It happens by tubular reabsorption.

The rate at which the glomerulus filters blood and produces a filtrate is called the glomerular filtration rate.

Tubular Reabsorption 

It is the second step of urine formation and is a very important step, as it makes sure that essential substances and ions are excreted from our bodies. In this process, ions and substances such as sodium ions, glucose, amino acids, water, etc are absorbed. It also ensures less water loss. There are two types of transport taking place during tubular reabsorption. 

1. Active transport- Glucose and Ions are absorbed by active transport.

2. Passive transport- Water is absorbed by passive transport.

Various tubules that carry out the process of reabsorption are as follows. 

1. Proximal Convoluted Tubule (PCT) – The PCT reabsorbs ion, water, and nutrients from the filtrate. By secreting potassium, hydrogen, and ammonia to filtrate and reabsorbing bicarbonate ions from the filtrate. It also eliminates toxins from the filtrate aid in regulating the ionic balance and pH of bodily fluids. thus maintaining the pH of the filtrate.

2. Ascending loop of Henle – Allows the reabsorption of sodium and chloride ions from the filtrate.

3. Descending Loop of Henle is impervious to water but permeable to electrolytes. The passage of electrolytes from the filtrate to the medullary fluid dilutes the filtrate. It allows the reabsorption of water. 

4. Distal Tubule – Reabsorbs selective ions from the filtrate including sodium chloride ions. The Distal Convoluted Tubule (DCT) is responsible for the reabsorption of water and sodium ions. It also aids in the maintenance of pH and ionic equilibrium by the secretion and reabsorption of ions such as PCT.

5. Collecting duct – Connect the nephrons to the minor calyx or renal pelvis and reabsorbs a substantial amount of solutes and water from the filtrate.

Secretion

Along with the formation of urine, the waste ions like hydrogen and potassium, and ammonia are secreted out which are then mixed with the urine and expelled from the body. This is called secretion.

After a series of these processes, the bladder gets filled with urine. Upon signaling from the brain, the bladder relaxes and releases urine from the body by the process of micturition or urination.

Work of different tubules involved in the process are – 

• Glomerulus – Helps in the filtration of blood.
• Proximal Convoluted Tubules (PCT) – Remove unwanted material and help in maintaining the ionic balance pH of the body fluids by eliminating potassium, ammonia, and hydrogen.
• Descending loop of Henle – This portion of Henle is greatly permeable to water and less permeable to ions, so it easily absorbs all water.
• Ascending loop of Henle – These are less permeable to water and easily reabsorb solutes from the luminal fluid.
• Distal Convoluted Tubules (DCT) & Collecting Duct – Distal Convoluted Tubules and Collecting duct are two final divisions of the kidneys nephron. They absorb ions and water and can further subdivide them with their own function.

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By Team Learning Mantras