Why kidney is considered as an excretory organ?

Kidneys is considered as the excretory organ because of its important role in the process of urine formation by filtering unwanted substances (or nitrogenous wastes) from the blood and producing urine to excrete these wastes.

Mechanism of urine formation

The mechanism of urine formation involves three main steps: glomerular filtration, reabsorption, and secretion and an additional step called water concentration. These processes ensure that only the harmful wastes materials (mostly the nitrogenous wastes) and excess water are removed from the body not the important substances (such as: electrolytes and essential nutrients, like proteins, amino acids, glucose).

1. Glomerular filtration (or ultrafiltration): 

Each kidney contains a large number of tiny structures called nephrons. Each nephron has a glomerulus, which is the site of blood filtration. The glomerulus is a complex network of capillaries which is surrounded by a cuplike structure called Bowman’s capsule (or glomerular capsule). As the blood flows through the glomerulus, blood pressure pushes water and solutes from the capillaries into the bowman’s capsule through filtration membrane. The product is called the nephric filtrate (or glomerular filtrate). This step starts the urine formation process.

2. Reabsorption (or tubular reabsorption or selective reabsorption): 

Nephric filtrates (also called primary urine) then passes mainly into proximal convoluted tubule which is surrounded by peritubular capillaries where the useful components of nephric filtrate, e.g., glucose, amino acids, vitamin C, calcium, potassium, sodium chloride, bicarbonate and water (75%) gets reabsorbed and are returned back to the blood.

3. Secretion (or tubular secretion):

The process of secretion occurs mainly in the distal convoluted tubule which is also surrounded by peritubular capillaries. Tubular secretion is the process of transfer of materials from peritubular capillaries to the renal tubular lumen. This process is a complete opposite process of reabsorption. This process removes additional wastes from the blood and adds them to the filtrate. This process of secretion is mainly caused by active transport and passive diffusion. The substances that are secreted into the tubular fluid from blood for removal from the body are:

• Potassium ions (K+)
• Hydrogen ions (H+)
• Ammonium ions (NH4+)
• Creatinine
• Urea
• Uric acid. 

4. Water concentration:

75% of water content of nephric filtrate is reabsorbed in the region of proximal convoluted tubule. Some 10% of water passes out of the filtrate through osmosis in the region of Henle’s loop as Henle’s loops are immersed in hyper-osmotic interstitial fluid. Moreover, concentration occurs in the area of collecting duct in the presence of a hormone called vasopressin or Antidiuretic hormone (ADH). Absence of ADH makes urine dilute. Thus, osmotic concentration of body fluid is maintained by hormone action and so deficiency of Antidiuretic hormone will cause excessive and repeated dilute urination. The process of expelling urine out of the body from the urinary bladder is called micturition.

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