Except for urinary bladder which is endodermal in origin, the whole excretory system is Except urinary bladder which is endodermal in origin, the whole excretory system is Except mesodermal.
In human the kidney is retroperitoneal i.e., the kidney is located outside the coelomic cavity and is covered by peritoneum (coelomic epithelium) from the ventral side only.
The size of each kidney is ~10 cm and it weighs is ~150 g.
The two kidneys are asymmetrical, the Rt. being posterior to the Lt.
Each kidney is bean-shaped with a groove (hilus) in the middle. The hilus is absent in frog’s kidney.
The white ﬁbrous connective tissue-covering around kidney is called renal or ﬁbrous capsule.
Each kidney (Metanephros type) is differentiated into 2 −regions:
- Outer −cortex
At certain points, the cortex extends into medulla forming ‘Columns of Bertini’.
Medulla contains 4−14 pyramids, each extending into pelvis.
The tip of the pyramid is called papilla, and the major collecting duct (Duct of Bellini) opens at this point.
The spaces, larger and smaller, into which pyramids open are called major and minor calyx.
Nephron (uriniferous tubule)
As mentioned above, the Nephron is the structural and functional unit of the kidney.
The nephrons in human are of 2−types:
- About 85% of the nephrons are of such type.
- The glomeruli of these nephrons are present in outer cortex.
- The Henle’s loop in these nephrons is shorter.
- Vasa recta are lacking.
- About 15% of the nephrons are of such type.
- The glomeruli of these nephrons are present in the inner cortex.
- The Henle’s loop in these nephrons is longer.
- Vasa recta are well developed.
Each nephron consists of the following prominent parts.
- It is a double-walled and blind (closed) structure. The inner side of this capsule has modiﬁ ed squamous epithelial layer (podocyte layer).
- Inside the capsule, there is a bunch of capillaries (all arterial in nature) called Glomerulus. These capillaries (a number ranging from 20 to 40) are more permeable than other capillaries of the body, due to the presence of ﬁne endothelial pores (fenestrae). These capillaries arise from Afferent arteriole (more in diameter) and join to form Efferent arteriole (lesser in diameter).
- The glomerulus along with Bowman’s capsule is called Malpighian body or Renal Corpuscle
- The endothelial lining of glomerular capillaries with basement membrane and podocyte layer of Bowman’s capsule, form Filtration membrane.
Proximal convoluted tubule (PCT)
- It is lined with columnar epithelium, though the cells, due to presence of microvilli (not cilia), appear to be cuboidal.
- This specialised epithelium of PCT is called Brush border epithelium. It increases surface area for absorption.
- It is Hair-pin like and has 2−limbs − Descending limb and Ascending limb.
- The Descending limb of Henle is lined with ﬂat cells (simple squamous epithelium). This epithelium is permeable to water but impermeable to sodium chloride (salts).
- Ascending limb of Henle is lined with cubical or cuboidal cells (particularly the thick portion). This lining is permeable to sodium but impermeable to water.
- The vasa recta, present around Henle’s loop of Juxta medullary nephrons, is absent in the Cortical nephrons.
Distal convoluted tubule (DCT)
- It is lined with cuboidal epithelium. The cells of this epithelium do not have well-developed microvilli.
- permeability of this epithelium can be altered by the action of Aldosterone and Vasopressin (ADH) hormones.
- The collecting tubules of different nephrons join to form the collecting duct. The water absorption occurs both from, tubule and ducts