Urine Formation: Filtration, Reabsorption, and Secretion

Urine is the means through which the chemicals are filtered out of the body. Urine is a liquid excreted through the urethra that is produced by the kidneys, collected in the bladder, and discharged by the urethra. Urine is used to remove surplus minerals and vitamins from the body, as well as blood corpuscles.

Mechanism of Urine Formation

Filtration, Reabsorption, and Secretion are the three stages that lead to the production of urine.


The afferent arteriole delivers blood to the glomerulus. The glomerulus blood contains both filterable and non-filterable blood components. Non-filterable blood components bypass the filtration process by departing through the efferent arteriole, while filterable blood components flow toward the interior of the glomerulus. Filterable blood components now take the form of glomerular filtrate, which resembles plasma. Water, nitrogenous waste, nutrients, and salts are some of the filterable blood components (ions). Formed elements such as blood cells and platelets, as well as plasma proteins, are nonfilterable blood components. Because much of the glomerular filtrate is reabsorbed into the blood as it passes through the tubules of the nephron, it is not the same consistency as urine.



Molecules and ions are reabsorbed back into the bloodstream through the peritubular capillary network. In comparison to the glomerular filtrate, sodium chloride reabsorbed into the system raises the osmolarity of blood. This reabsorption process permits water (H2O) to flow back into the circulatory system from the glomerular filtrate. Glucose and a variety of amino acids are also reabsorbed into the bloodstream. These nutrients contain carrier molecules that bind to the glomerular molecule and release it back into circulation. Excess glucose or amino acids are released into the urine if all of the carrier molecules have been used up. The inability of the body to reabsorb glucose is a complication of diabetes. When there is too much glucose in the glomerular filtrate, the osmolarity of the filtrate rises, causing water to be discharged into the urine instead of reabsorbed by the circulatory system. Because water is not reabsorbed, frequent urination and unexplained thirst are warning indications of diabetes. Reabsorbed Filtrate and Non-reabsorbed Filtrate have now been separated from the glomerular filtrate. As it goes through the collecting duct to be processed into the urine, the non-reabsorbed filtrate is now known as tubular fluid.


The peritubular capillary network transports some compounds from the bloodstream to the distal convoluted tubule or collecting duct. Hydrogen ions, creatinine, and medicines are examples of these substances. Urine is a collection of substances that have not been reabsorbed by the kidneys or tubules.

Keeping the Water-Salt Balance

The kidneys’ job is to keep the blood’s water-salt balance in check. They also keep blood volume and blood pressure in check.

Water reabsorption

The anti-diuretic hormone (ADH), secreted by the posterior lobe of the pituitary gland, exerts direct control over water excretion in the kidneys. ADH stimulates the insertion of water channels into the membranes of the cells that line the collecting ducts, allowing water to be reabsorbed. Without ADH, only a small amount of water is reabsorbed in the collecting ducts, resulting in dilute urine. The first occurs when blood plasma becomes overly concentrated. Special receptors in the hypothalamus release ADH when this happens. Stretch receptors in the aorta and carotid arteries stimulate ADH secretion to increase blood volume when blood pressure drops.

Reabsorption of Salt

 The kidneys also manage the excretion and reabsorption of different ions, which regulates the salt balance in the blood. As previously stated, ADH aids in the dilation of body fluids by promoting water reabsorption in the kidneys. In the distal nephron, the kidneys have a controlled system for reabsorbing sodium. Aldosterone, a steroid hormone produced by the adrenal cortex, regulates this mechanism. Aldosterone causes potassium ions to be excreted and sodium ions to be reabsorbed. The kidneys initiate the release of Aldosterone. The macula densa, mesangial cells, and juxtaglomerular cells make up the juxtaglomerular apparatus, which is a kidney structure. Following the reabsorption of sodium ions, water is reabsorption. Blood pressure and blood volume both rise as a result of this.

When cardiac cells are stretched due to increased blood volume, atrial natriuretic hormone (ANH) is released by the atria of the heart. The juxtaglomerular apparatus secretes renin, and the adrenal cortex secretes aldosterone, which is inhibited by ANH. This encourages sodium excretion. Water is expelled along with sodium. Blood pressure and volume both drop as a result of this.

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