Structure of Nephron

Introduction

The name of the structural and functional unit of the human kidney, which is very minute in shape and size, is called the nephron. Each of the two kidneys comprises more than two million nephrons. The nephron structure begins from the renal cortex, tubules in the renal medulla and moves back to the cortex before reaching the collecting duct.

The nephron constitutes several parts – 

• The renal corpuscle 
• The proximal convoluted tubule 
• Loop of Henle
• The distal convoluted tubule

The renal cortex includes the renal corpuscle and the proximal and the distal convoluted tubules. The loop of Henle is present in the renal medulla as medullary rays. 

Structure of Nephron

• Renal corpuscle

Blood plasma is filtered in the renal corpuscle. The glomerulus and the glomerular capsule, known as Bowman’s capsule, make up the renal corpuscle. There are two poles to the renal corpuscle: a vascular pole and a tubular pole. The arterioles from the renal circulation enter and exit the glomerulus. Renal tubule is at the urinary pole from where the glomerular filtrate exits the Bowman’s capsule. 

• Glomerulus

The glomerulus is a network of filtering capillaries near the vascular pole of the renal corpuscle and Bowman’s capsule known as a tuft. An afferent arteriole of the renal circulation supplies blood to each glomerulus. The glomerular blood pressure acts as a driving force to filter water and solutes from the blood plasma into Bowman’s space, which is the interior of Bowman’s capsule.

Approximately one-fifth of the plasma gets filtered in the glomerulus. The remaining is sent to a different arteriole. The efferent arteriole has a smaller diameter than the afferent, which causes increase in the hydrostatic pressure of the glomerulus.

• Bowman’s capsule

The glomerulus is surrounded by the Bowman’s capsule, also known as the glomerular capsule. It is made up of three layers: The lamina rara externa near podocyte processes, followed by lamina rara interna near the endothelial cells and lamina densa, a darker central zone of the basal lamina. The filtrate is made up of fluids from the glomerulus that have been ultra-filtered through numerous layers. The filtrate is then sent to the renal tubule, where it is further processed into urine. The tubular fluid refers to all stages of this fluid.

• Renal tubule

The tubular fluid filtered via the glomerulus is present in the renal tubule, a long pipe-like structure. The filtrate continues to the collecting duct system after passing through the renal tubule. The renal tubule is made up of the following components:

• Tubule twisted at the proximal end (lies in the cortex and lined by simple cuboidal epithelium with brush borders which help to increase the area of absorption)
• Henle’s loop (U-shaped and lies in the medulla)- Henle loop’s descending limb and Henle loop’s ascending limb. The ascending limb of the Henle loop is made up of two segments: The ascending limb’s lower end is relatively thin and lined with simple squamous epithelium, whereas the distal section is thick and lined with simple cuboidal epithelium
• Tubule convoluted (proximal and Distal)
• Collecting tubule

• Proximal convoluted tubule

The proximal convoluted tubule is the initial part of the renal tubule. It starts at the glomerulus’s urinary pole. Most of the glomerular filtrate or Na+ ions (65 percent) are reabsorbed to provide a small amount of Na+ ions to downstream sites, which may then modify their rate of reabsorption of Na+ ions to attain a steady-state balance for this cation. The tubule’s convoluted section leads to a straight segment that descends into the medulla within a medullary ray and creates the Henle loop.

• Loop of Henle

Henle’s loop is shaped like a hairpin and descends into the medulla. The four segments, which make the pars recta are the thin descending limb, thin ascending limb, thick descending limb and thick ascending limb. The tubule ascends parallel to the descending limb after the loop of Henle turns in the narrow segment within the medulla. 

The distal convoluted tubule, which lies near the glomerulus, is formed by the termination of the Henle’s loop. The Henle loops are connected to the vasa recta, which are capillary loops. The reabsorption of water in the latter segments of the renal tubule is dependent on such high osmotic pressure.

• Distal convoluted tubule

The distal convoluted tubule is less convoluted than the proximal convoluted tubule and is shorter. This segment secretes and reabsorbs more ions. The juxtaglomerular apparatus is formed by the first segment of the distal convoluted tubule, which lies adjacent to the glomerulus.

Type of Nephrons

According to their position, nephrons are of two types. 

(i) Cortical nephrons:

These constitute about 85% of total. (75-85%)

Their loops of Henle are mostly confined to the cortex and a very small part of it runs in the medulla.

Only peritubular capillary network is present & vasa recta is absent.

(ii) Juxtamedullary nephrons:

About 15% of the total. (15-25%) .

The loop of Henle of these nephrons are long, dipping deep down into the medulla. 

Both PTC network and vasa recta are present.

In these nephrons the initial part of the DCT is located close to the mouth of Bowman’s capsule, in contact with both afferent and efferent arterioles. 

Conclusion

Although human excretion occurs through the lungs, skin, and liver, the kidneys are the primary organs of the excretory system. They are bean-shaped organs that weigh between 150 and 170 grams and measure 4 to 5 inches in length. These bean-shaped organs are made up of millions of nephrons.

Kidney is found in the retroperitoneal area of the abdominal cavity, directly below the rib cage, on both the left and right sides of the body. The right kidney is smaller and sits lower in the body than the left. Female kidneys have a lesser total area, size, and weight when compared to male kidneys.