nephron, types of nephrons, Structure of nephrons, Short notes of nephron,

  Nephron  

A nephron is a unit of structure and function in a kidney. Number and Size. A kidney contains about a million nephrons, each approximately 3 cm. long There is, thus, an enormous surface area for the exchange of materials in a kidney.

Types of Nephrons: -

Nephrons are of two types: - Cortical and Juxtamedullary, with regard to their location in the kidney. The cortical nephrons form about 80% of the total nephrons. They lie in the renal cortex and have very short loops of Henle that extend only a little into the medulla. The juxtamedullary nephrons have their Bowman's capsules close to (juxta) the junction of the cortex and the medulla, and have very long loops of Henle, extending deep into the medulla The cortical nephrons control the plasma volume when water supply is normal. The juxtamedullary nephrons regulate the plasma volume when water is in short supply.

Structure of Nephron: -

A nephron 1s a long tubule differentiated into four regions having different anatomical features and physiological role: Bowman's capsule. proximal convoluted tubule (PCT), loop of Henle, and distal convoluted tubule (DCT). The latter opens into one of the collecting ducts.

(i) Bowman's Capsule.: - The Bowman's capsule is a large, double-walled cup. It lies in the renal cortex and forms the beginning of the nephron. It contains a glomerulus in it. Inner wall of the Bowmans capsule, called the visceral wall is in close contact with the glomerular capillaries its outer wall is continuous with the rest of the nephron. The space between the two walls of the Bowman's capsule is continuous with the lumen (Cavity) of the next part of the nephron The Bowman's capsule and the glomerulus (a group of about 50 capillaries) together form a globular body called renal corpuscle, or Malpighian body. The endothelial cells forming the wall of the glomerular capillaries have pores between them called fenestrae. The cells rest on a basement membrane. The podocytes forming the inner wall of the Bowman's capsule have gaps, the stilt-pores, between them. Thus, the basement membrane of the capillary wall is the only continuous layer between the blood in the glomerular capillaries and the lumen of the Bowman's capsule. This facilitates the process of ultrafiltration. The outer wall of the Bowman's capsule consists of unspecialized squamous epithelium.

ii) Proximal Convoluted Tubule (PCT).: - I starts from the neck of the Bowman's capsule and i greatly twisted. It also lies in the renal cortex. Its wall consists of a single layer of columnar cells bearing microvilli on the free surface and resting on a basement membrane (hg 20.14)

iii)Loop of Henle.: - It is a U-shaped segment of the nephron located in the renal medulla. It consists of two straight parallel limbs: a descending limb which is aa continuation of PCT into the renal medulla and an ascending limb which reenters the renal cortex and joins the DCT Each limb has a thick region toward the cortex and a thin region on the other side. The thick regions are walled by columnar cells, and the thin regions by flat cell.

iv)Distal Convoluted Tubule (DCT).: -Like the PCT. it is greatly twisted and lies in the renal cortex into A short a terminal relatively straight part of the DCT is called collecting tubule. The 1a DT hag collecting duct. The cells forming the wall of the DCT are similar to those in the PCT wall. The D JUxtaglomerular apparatus (JGA). It is a special sensitive region formed by cellular modifications in distal cOnvoluted tubule and the afferent arteriole at the location of their contact. This mechanisim playo role in the regulation of glomerular filtration rate. Collecting Ducts. These are larger tubes, each receiving the collecting tubules of several Deprons. They pass into the renal medulla and join each other, forming still larger ducts of Bellini. Tnese ru through the medullary pyramids and open into calyces, which lead into the pelvis. The collecting ducts are lined by cuboidal and columnar cells in different regions. At intervals, the cuboidal cells are ciliated.