Each kidney in longitudinal section shows two distinct regions an outer renal cortex and an inner renal medulla.
The medulla is subdivided into 15 or 16 conical masses, the renal pyramids. Each pyramid has narrow renal papilla toward the pelvis and a broad base toward the cortex. Renal papillae open into a wide funnel-like structure, pelvis. The pelvis in turn leads into the ureter.
Nephron:
Each kidney consists of more than one million nephrons or uriniferous tubules. The total length of all tubules combined will be more than 600 km. Nephron is the functional unit of a kidney. Each nephron consists of a malpighian capsule or renal tubule, a nephric tubule (secretory part of uriniferous tubule) and a collecting tubule.
Malpighian capsule or Malpighian body it has two parts a Bowman’s capsule and a glomerulus.
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Bowman’s Capsule:
It forms the dilated blind end of the nephron. It is a double-walled cup-shaped
structure. It is lined by a thin semipermeable squamous epithelium. The outer concavity of Bowman’s cup contains a knot-like mass of blood capillaries called glomerulus.
Glomerulus:
The afferent arteriole enters in Bowman’s capsule and divides into a bunch of about 50 capillaries. This bunch is called glomerulus. Their diameter gets reduced to increase the blood pressure. These capillaries reunite to form efferent arteriole.
Nephric or Uriniferous Tubule:
It is a long, coiled tubule and can be divided into three major regions the proximal convoluted tubule (PCT), the loop of Henle and the distal convoluted tubule (DCT).
Proximal Convoluted Tubule (PCT):
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It is the initial (proximal) convoluted region of the nephric tubule. It is a highly coiled structure and is about 12-24 mm in length. It is lined by the columnar epithelium and is provided with microvilli. The cells have numerous mitochondria for active transport.
Loop of Henle:
It is a U-shaped loop formed in the middle of the nephric tubule. It has a thin descending limb and a thick ascending limb. The descending limb is lined with flattened epithelial cells, while the ascending limb is lined with cuboidal epithelium.
Distal Convoluted Tubule (DCT):
It is a convoluted structure and is present in the cortex region. It is lined by cuboidal epithelium. It opens into the collecting tubule which is lined by columnar epithelial cells with microvilli on its free surface.
Collecting Ducts and the Ducts of Bellini:
Collecting ducts are larger ducts, each receiving collecting tubules from several nephrons. These pass into the renal medulla and join with each other forming still larger ducts of Bellini. These drain the urine collected from the nephrons into the pelvis which leads to the ureter.