Nephrology

 

Each kidney is covered by three layers.

The outermost layer is renal fascia, made of thin fibrous tissue, and it fixes the kidney to the abdominal wall.
The middle layer is the adipose capsule, which is fat tissue that protects the kidney from shock.
The innermost layer is the renal capsule, a smooth membrane that protects the kidney from infections and continues with the ureter.

Inside the kidney, there are two main regions.
The outer region is the renal cortex, which is red and granular.
It contains Malpighian bodies (filtering units), convoluted tubules (twisted tubes), and blood vessels.
The inner region is the renal medulla, which is pale red and looks striped.
It contains Loops of Henle and collecting ducts.
These structures form cone-shaped parts called renal pyramids.
The cortex extends between pyramids as renal columns (Columns of Bertini).
The narrow tip of each pyramid is called the renal papilla.

There are many renal pyramids in the kidney.
The tip of each pyramid, called renal papilla, opens into a minor calyx.
Many minor calyces join together to form a major calyx.
All major calyces unite to form the renal pelvis.
The renal pelvis (renal sinus) is a funnel-shaped space in the medulla region.
The renal pelvis continues as the ureter, which leaves the kidney through the hilus (entry and exit point of kidney).

Nephron is the structural and functional unit of kidney (basic working unit).

Each nephron has:
• A thin tube called renal tubule (about 4–6 cm long)
• A group of capillaries (tiny blood vessels) called glomerulus

The renal tubule wall is made of a single layer of epithelial cells.

The starting part is wide and cup-shaped, called Bowman’s capsule.
The end part is open.

The nephron is divided into:
• Bowman’s capsule
• Neck
• Proximal Convoluted Tubule (PCT)
• Loop of Henle (LoH)
• Distal Convoluted Tubule (DCT)
• Collecting Tubule (CT)

The glomerulus lies inside Bowman’s capsule.
Together, they form the renal corpuscle or Malpighian body.

Each Malpighian body is about 200 micrometres (μm) in diameter and contains Bowman’s capsule and glomerulus.

Glomerulus is a group (bunch) of tiny blood capillaries present inside Bowman’s capsule.

A small branch of the renal artery called the afferent arteriole enters Bowman’s capsule.
It divides into many small capillaries to form a network called the glomerulus.

The walls of these capillaries are fenestrated (having tiny pores).

All the capillaries then join together again to form the efferent arteriole, which leaves the capsule.

The afferent arteriole is wider than the efferent arteriole.
This difference creates high blood pressure (hydrostatic pressure) inside the glomerulus.

This high pressure is necessary for ultrafiltration (filtering of blood to form urine).

Bowman’s capsule is a cup-shaped structure with a double wall.

Both walls are made of simple squamous epithelium (flat thin cells).

The outer wall is called the parietal layer.
The inner wall is called the visceral layer.

The space between the two walls is called the capsular space or urinary space.

The visceral layer has special cells called podocytes.
Podocytes have foot-like projections (pedicels).
These pedicels are closely attached to the capillaries of the glomerulus.

There are small gaps between podocytes called filtration slits.
These slits help in filtering blood.

The parietal wall is thin and continues into the neck region.

The neck is lined by ciliated epithelium (cells with hair-like structures).
The cavity of the neck is called the urinary pole.
The neck then continues into the proximal convoluted tubule (PCT).

Proximal Convoluted Tubule (PCT) is a highly coiled (twisted) part of the nephron.

It is lined by cuboidal cells (cube-shaped cells).

These cells have a brush border (microvilli – tiny finger-like projections).
Microvilli increase the surface area for absorption.

The PCT is surrounded by peritubular capillaries (tiny blood vessels around the tubule).

It is the main place for selective reabsorption (taking back useful substances into blood).

Because it is highly coiled, the filtrate moves slowly.
This slow movement allows maximum reabsorption of useful substances like glucose, amino acids, water, and salts.