Renal tubules are long, cylindrical patchy tubules that extend from the base of Bowman’s capsule to the ends of the collecting ducts. Each renal tubule is approximately 3 cm in length and has an average diameter of 60 micrometers. It consists of three parts. These are-
(i) Proximal convoluted tubule: The tubule associated with Bowman’s capsule is called proximal convoluted tubule. Its length is about 14 cm and diameter is 57-60μ. These peritubules are covered by epithelial cells with capillaries or brush borders. At one end of the cells are numerous microscopic finger-like projections or microvilli. It is located in the cortex. This duct carries the filtrate from the glomerulus to the loop of Henley. The proximal tubule reabsorbs water, sodium ions, potassium, amino acids, glucose, chloride, peptide creatine and bicarbonate ions from the glomerular filtrate.
(ii) Loop of henle: The U-shaped tube extending from the proximal convoluted tubule to the distal convoluted tubule is called loop of Henle. This is called nephron leakage. It is located in the medulla and cortex. Its length is 11 mm and diameter is 20 μm. It consists of two parts. Descending limb and Ascending limb. The descending arm extends from the end of the proximal tubule to the medulla and the ascending arm is located in the cortex region. It is called loop of Henle after German medical scientist Friedrich Henle.
(iii) Distal convoluted tubule: The part from the loop of Henley to the collecting duct is called distal convoluted tubule. Its length is 5 mm and diameter is 22.5 μ. It is surrounded by peritubule capillary or epithelial cells. It is located in the cortex. The next part of the distal patchy duct is called the collecting duct. Several collecting ducts join to form the Duct of Bellini.

The bundle of blood vessels located in Bowman’s capsule is called glomerulus. An afferent arteriole from the renal artery enters Bowman’s capsule and divides into a network of 50-60 capillaries to form the glomerulus. Later these capillary networks join to form the efferent arteriole and exit Bowman’s capsule. The diameter of the afferent artery is 50 mm and the diameter of the efferent artery is 25 mm. Due to the difference in the diameter of the two arteries, the blood pressure in the glomerulus is higher. This blood pressure helps in the formation of urine. Glomerulus acts as ultra-filter and causes ultrafiltration.

The cup-like part located in the Malpighian body is called Bowman’s capsule. Its wall diameter is 0.2 mm. It is covered by squamous epithelium cells. Visceral and parietal layers. The outer layer is called parietal and the inner layer is visceral. The parietal layer is composed of epithelial cells. The visceral layer is composed of podocyte cells with flagella. Podocyte cells have finger-like pedicels at their free ends. Due to the position of the pedicels numerous pores are formed in the visceral layer. These holes are called filtration pore. Glomerular filtrate enters Bowman’s capsule through these pores. The space between the parietal and visceral layers is called capsular space. It is named Bowman’s capsule after Sir William Bowman.

The funnel-shaped part in front of the nephron is called Malpighian body. Its diameter is 200 microns. It consists of two parts. These are-
(i) Bowman’s capsule: The cup-like part located in the Malpighian body is called Bowman’s capsule. Its wall diameter is 0.2 mm. It is covered by squamous epithelium cells. Visceral and parietal layers. The outer layer is called parietal and the inner layer is visceral. The parietal layer is composed of epithelial cells. The visceral layer is composed of podocyte cells with flagella. Podocyte cells have finger-like pedicels at their free ends. Due to the position of the pedicels numerous pores are formed in the visceral layer. These holes are called filtration pore. Glomerular filtrate enters Bowman’s capsule through these pores. The space between the parietal and visceral layers is called capsular space. It is named Bowman’s capsule after Sir William Bowman.
(ii) Glumerulas: The bundle of blood vessels located in Bowman’s capsule is called glomerulus. An afferent arteriole from the renal artery enters Bowman’s capsule and divides into a network of 50-60 capillaries to form the glomerulus. Later these capillary networks join to form the efferent arteriole and exit Bowman’s capsule. The diameter of the afferent artery is 50 mm and the diameter of the efferent artery is 25 mm. Due to the difference in the diameter of the two arteries, the blood pressure in the glomerulus is higher. This blood pressure helps in the formation of urine. Glomerulus acts as ultra-filter and causes ultrafiltration.

A typical nephron consists of two main parts.
1. Malpighian body: The funnel-shaped part in front of the nephron is called Malpighian body. Its diameter is 200 microns. It consists of two parts. These are-
(i) Bowman’s capsule: The cup-like part located in the Malpighian body is called Bowman’s capsule. Its wall diameter is 0.2 mm. It is covered by squamous epithelium cells. Visceral and parietal layers. The outer layer is called parietal and the inner layer is visceral. The parietal layer is composed of epithelial cells. The visceral layer is composed of podocyte cells with flagella. Podocyte cells have finger-like pedicels at their free ends. Due to the position of the pedicels numerous pores are formed in the visceral layer. These holes are called filtration pore. Glomerular filtrate enters Bowman’s capsule through these pores. The space between the parietal and visceral layers is called capsular space. It is named Bowman’s capsule after Sir William Bowman.
(ii) Glumerulas: The bundle of blood vessels located in Bowman’s capsule is called glomerulus. An afferent arteriole from the renal artery enters Bowman’s capsule and divides into a network of 50-60 capillaries to form the glomerulus. Later these capillary networks join to form the efferent arteriole and exit Bowman’s capsule. The diameter of the afferent artery is 50 mm and the diameter of the efferent artery is 25 mm. Due to the difference in the diameter of the two arteries, the blood pressure in the glomerulus is higher. This blood pressure helps in the formation of urine. Glomerulus acts as ultra-filter and causes ultrafiltration.
2. Renal tubules
Renal tubules are long, cylindrical patchy tubules that extend from the base of Bowman’s capsule to the ends of the collecting ducts. Each renal tubule is approximately 3 cm in length and has an average diameter of 60 micrometers. It consists of three parts. These are-
(i) Proximal convoluted tubule: The tubule associated with Bowman’s capsule is called proximal convoluted tubule. Its length is about 14 cm and diameter is 57-60μ. These peritubules are covered by epithelial cells with capillaries or brush borders. At one end of the cells are numerous microscopic finger-like projections or microvilli. It is located in the cortex. This duct carries the filtrate from the glomerulus to the loop of Henley. The proximal tubule reabsorbs water, sodium ions, potassium, amino acids, glucose, chloride, peptide creatine and bicarbonate ions from the glomerular filtrate.
(ii) Loop of henle: The U-shaped tube extending from the proximal convoluted tubule to the distal convoluted tubule is called loop of Henle. This is called nephron leakage. It is located in the medulla and cortex. Its length is 11 mm and diameter is 20 μm. It consists of two parts. Descending limb and Ascending limb. The descending arm extends from the end of the proximal tubule to the medulla and the ascending arm is located in the cortex region. It is called loop of Henle after German medical scientist Friedrich Henle.
(iii) Distal convoluted tubule: The part from the loop of Henley to the collecting duct is called distal convoluted tubule. Its length is 5 mm and diameter is 22.5 μ. It is surrounded by peritubule capillary or epithelial cells. It is located in the cortex. The next part of the distal patchy duct is called the collecting duct. Several collecting ducts join to form the Duct of Bellini.

Nephron can be divided into three parts.
1. Superficial cortical nephron: The corpuscle of the nephron located one mm deep in the cortex is called Superficial cortical nephron. It contains 85% Nephron.
2. Midcortical nephron: The corpuscle of the nephron located in the middle of the cortex is called Midcortical nephron. It contains 5% Nephron.
3. Juxtamedullary nephron: The corpuscle of the nephron located deep in the cortex is called Juxtamedullary nephron. It contains 10% Nephron.

The structural and functional unit of kidney is called Nephron. Nephron is an individual unit that produces urine. Its length is 3-5 cm. Each kidney has 10-12 lakh Nephrons. The tubules of the nephron in each kidney are collectively 36 km or 22.5 miles long. In both kidneys 50% of nephrons are always active and 50% of nephrons are always at rest.

Renin is an enzyme-like hormone secreted from the juxtaglomerular cells of the kidney. It converts inactive angiotensinogen to active angiotensin. Angiotensin helps increase blood pressure.

1. Blood Purification: As a result of metabolism, nitrogenous waste products – urea, uric acid, creatinine etc. are created in the human body. These waste materials are excreted from the body through the kidneys. As a result blood is purified.
2. Acid-base balance: Kidneys regulate the levels of mineral salts and bicarbonate in the body. It regulates the acid-base balance in the blood.
3. Water balance: Kidneys regulate water balance in the body through osmoregulation.
4. Temperature regulation: It regulates temperature throughout the body.
5. Vitamin production: Kidneys produce vitamin D in the body.
6. Blood cell production: Erythropoietin hormone is produced in kidney. Erythropoietin makes red blood cells in the body.
7. Blood pressure regulation: Kidneys regulate blood pressure in the body with the help of various hormones.
8. Hormone production: Kidney produces hormones like erythropoietin, prostaglandin, angiotensin, calcitriol etc. in the animal body.
9. Removal of toxins: It removes toxins from the body.
10. Mineral salts: Kidney controls the amount of sodium, potassium, calcium, phosphate, chloride etc. in the human body.
11. Gluconeogenesis: In the process of gluconeogenesis, sugar is produced from proteins and lipids. Gluconeogenesis occurs during prolonged starvation. During this time the kidney can supply about 20% glucose.
12. Enzyme secretion: Renin enzyme is secreted from kidney. Renin acts like an enzyme hormone.
13. Regulation of Osmosis: Kidneys regulate the osmotic pressure between blood and cells-tissues in the body.
14. Bone and teeth formation: Kidney plays a major role in vitamin-D production. Vitamin-D regulates calcium levels in the blood. Calcium forms bones and teeth.
15. Homeostasis: Homeostasis is the internal orderly environment of the body. Kidneys maintain homeostasis in the body by removing waste products from the blood.

The central part of kidney wall is called medulla. It is light red in color. It consists of 8-18 pyramidal structures. It is called renal pyramid. The base of the pyramid faces the cortex and the apex faces the renal sinus. The top of the pyramid is called Papilla. Papilla with 10-25 pores. Apex of papilla opens into minor calyx. The minor calyxes open together into the major calyx. Also, the pillar-like part of the cortex between the two renal pyramids is called the Renal column. (iii) Pelvis: The large collecting space inside the kidney is called the pelvic floor. There are two types of branchial parts in the pelvis. There are 2-3 main branches. Major branches are called major calyxes. 8-14 minor calyxes arise from the major calyx.