Structure of Nephron

The human excretory system relies on a complex network of filtration units to maintain homeostasis, balance electrolytes, and remove toxins from the blood. At the heart of this system lies the nephron. Understanding the structure of the nephron is fundamental for students preparing for competitive exams like NEET, as well as those studying anatomy and physiology.

This guide provides a detailed breakdown of the nephron's anatomy, histology, and functional components.

1.0What is a Nephron?

The nephron is defined as the structural and functional unit of the kidney. Each human kidney contains approximately 1 million nephrons. These microscopic structures are responsible for two vital processes:

1. Filtration: Removing waste products from the blood.
2. Reabsorption/Secretion: Retaining essential substances like glucose, amino acids, and water while secreting excess ions.

Physically, the nephron is a long, fine tubule closed at one end and open at the other. It spans two main regions of the kidney: the outer renal cortex and the inner renal medulla.

2.0Detailed Structure of the Nephron

The nephron is broadly divided into two major portions based on its anatomy and function:

1. Renal Corpuscle (Malpighian Body)
2. Renal Tubule

Let us explore these sections in detail.

1. Renal Corpuscle (Malpighian Body)

The Renal Corpuscle is the initial filtering component of the nephron, located in the renal cortex. It consists of two structures: the Glomerulus and Bowman’s Capsule.

A. Glomerulus

The glomerulus is a tuft of capillaries formed by the afferent arteriole (a fine branch of the renal artery). Blood enters the glomerulus under high pressure through the afferent arteriole and leaves through the efferent arteriole.

• Key Feature: The efferent arteriole has a narrower diameter than the afferent arteriole. This diameter difference creates high hydrostatic pressure within the glomerular capillaries, which is essential for ultrafiltration.
• Endothelium: The capillary walls here possess small pores called fenestrae, making them highly permeable.

B. Bowman’s Capsule

This is a double-walled, cup-like structure that encloses the glomerulus. It collects the filtrate squeezed out of the glomerular capillaries.

• Layers:
• • Parietal Layer: The outer wall composed of simple squamous epithelium.
  • Visceral Layer: The inner wall that adheres to the glomerulus. It contains specialized cells called Podocytes.
• Podocytes: These cells have foot-like projections (pedicels) that wrap around the capillaries. The spaces between these projections are called filtration slits or slit pores, through which blood is filtered.

2. Renal Tubule

The renal tubule is a continuous tube connected to Bowman’s capsule. It modifies the filtrate as it passes through, eventually converting it into urine. It is divided into three main segments:

A. Proximal Convoluted Tubule (PCT)

This is the highly coiled first section of the renal tubule, located entirely within the renal cortex.

• Histology: The PCT is lined by simple cuboidal brush border epithelium.
• Significance of Brush Border: The microvilli (brush border) significantly increase the surface area for reabsorption.
• Function: Nearly 70-80% of electrolytes and water, and 100% of essential nutrients (like glucose and amino acids), are reabsorbed here.

B. Loop of Henle (Henle’s Loop)

The PCT leads into a hairpin-shaped loop called the Loop of Henle. This part of the nephron dips into the renal medulla and plays a crucial role in concentrating urine. It has two distinct limbs:

Descending Limb:

• It carries filtrate down into the medulla.
• Permeability: It is permeable to water but almost impermeable to electrolytes. As filtrate moves down, water moves out, making the filtrate hypertonic (concentrated).

Ascending Limb:

• • It carries filtrate back up toward the cortex.
  • Permeability: It is impermeable to water but allows the transport of electrolytes (Na+, Cl−) actively or passively. As filtrate moves up, it becomes hypotonic (diluted).

C. Distal Convoluted Tubule (DCT)

The ascending limb re-enters the cortex to form another highly coiled region called the DCT.

• Histology: Lined by simple cuboidal epithelium (without a distinct brush border).
• Function: It is the site for "conditional reabsorption." Under the influence of hormones like Aldosterone and ADH (Antidiuretic Hormone), it regulates sodium and water balance. It also helps in pH maintenance by secreting H+and K+ ions.

Collecting Duct

Although not technically part of a single nephron (as multiple nephrons open into one), the collecting duct is the final pathway.

• The DCTs of many nephrons open into a straight tube called the Collecting Duct.
• These ducts extend from the cortex through the medulla and converge to open into the renal pelvis.
• Role: Large amounts of water can be reabsorbed here to produce concentrated urine.

3.0Histology of the Nephron Segments

For students tackling high-level biology questions, knowing the epithelial lining of each segment is crucial.

4.0Types of Nephrons

Not all nephrons in the kidney are identical. They are classified into two types based on the location of their renal corpuscle and the length of their Loop of Henle.

1. Cortical Nephrons

• Abundance: Constitute about 85% of total nephrons.
• Location: The Malpighian corpuscle is located close to the outer surface of the cortex.
• Loop of Henle: Very short; it extends only a little way into the medulla.
• Vasa Recta: Usually absent or highly reduced.
• Primary Function: Basic filtration and urine formation.

2. Juxtamedullary Nephrons

• Abundance: Constitute about 15% of total nephrons.
• Location: The Malpighian corpuscle is located at the junction of the cortex and medulla.
• Loop of Henle: Very long; it runs deep into the medulla.
• Vasa Recta: Well-developed.
• Primary Function: Vital for the counter-current mechanism which concentrates urine and conserves water during dehydration.

5.0Blood Supply: The Peritubular Capillaries and Vasa Recta

The structure of the nephron is intimately tied to its blood supply, which facilitates the exchange of materials.

1. Afferent Arteriole: Supplies blood to the glomerulus.
2. Efferent Arteriole: Carries filtered blood away from the glomerulus.
3. Peritubular Capillaries: The efferent arteriole emerging from the glomerulus forms a fine capillary network around the renal tubule (PCT and DCT) called peritubular capillaries.
4. Vasa Recta: In Juxtamedullary nephrons, a vessel from the peritubular network runs parallel to the long Loop of Henle, forming a "U" shape called the Vasa Recta. This is essential for maintaining the medullary concentration gradient.