Saturday, April 17, 2010

Microscopic Structure of a Nephron

The nephron is the structural and functional unit of the kidney. Each nephron functions as an independent unit and produces a miniscule quantity of urine. The nephron can be differentiated into the following regions:

Malpighian Corpuscle

This consists of two parts:

detailed structure of nephron with its parts
Structure of a Nephron

Bowman's Capsule

This is a cup - shaped structure which is double walled in the hollow of which is a net work of capillaries called the glomerulus.


This is a knotted mass of blood capillaries formed by the afferent arteriole (incoming) and the efferent arteriole (outgoing).

Renal Tubule

This is the remaining part of the nephron, continuous with the Bowman's capsule. It is lined with ciliated epithelium and differentiated into the following regions:

Proximal Convoluted Tubule (PCT)

This is the region behind the Bowman's capsule and consists of a coiled tube that descends to form the Henle's loop. The proximal convoluted tubule is present in the cortex region.

Henle's loop

This is continuous with the proximal convoluted tubule and is U-shaped having a narrow descending limb and a thick ascending limb. This part of the nephron descends from the cortex into the medulla region.

classification of regions in nephron

Distal Convoluted Tubule (DCT)

This is another coiled and twisted tubule that continues from the ascending limb of loop of Henle found in the renal cortex.

Collecting Tubule

The distal convoluted tubule continues to form the collecting tubule.

Collecting Ducts

Several collecting tubules fuse to form large collecting ducts which pass downwards from the cortex to the medulla region.

Ducts of Bellini

Several collecting ducts join to form the larger ducts of Bellini in the medulla region. Running through the pyramids they finally open in the renal pelvis draining all the urine collected from the various nephrons. The renal pelvis is an expanded region which opens into the ureter.

Vasa Rectae

The efferent arteriole that exits from the glomerulus does not merge into a vein but breaks up into a system of capillaries called vasa rectae or peritubular capillaries which envelop the proximal convoluted tubule, the Henle's loop and distal convoluted tubule. These capillaries rejoin and drain into the venule. Venules join together to form the renal vein which drain blood from the kidneys.

illustration of stages of blood flow in kidney

Blood Flow in Kidney

Composition of Urine

The volume and composition of urine varies widely from day to day even in a healthy person depending on the following factors:
  • Urine output increases with increase in fluid intake. Composition of urine changes depending on the food consumed.
  • Excess physical activity, profuse sweating and warm temperature decreases the urine output.

On an average the urine output in a healthy person is 1 to1.8 litres per day.

Characteristics of Urine

Colour Yellow due to the presence of a pigment called urochrome formed by the breaking down of the haemoglobin of the red blood corpuscles
Specific gravity 1.003 - 1.040
Tonicity Hypertonic
pH Acidic
Odour Ammoniacal particularly is stale. This is because bacteria convert urea to ammonia
The nitrogenous wastes found in urine are urea, ammonia, uric acid, creatinine and hippuric acid. Salts like sodium chloride, traces of mineral ions like sulphates, chlorides, phosphates, sodium, calcium, magnesium, iodine, vitamin C, oxalic acid, alcohol, hormones and enzymes are also present.

Glucose and proteins are normally absent. The levels go up in the case of diseases like diabetes mellitus or in kidney failure.

Chemical Composition of Urine

Organic Constituents Inorganic Constituents
Nitrogen 23 - 35 g Chloride 6 – 9g
Urea 25 - 30 g Chloride (NaCI) 10 – 15g
Creatine 60 - 150 mg Phosphate 0.8 - 1.3 g
Creatinine 1.2 - 1.7g Sulphate 0.8 - 1.3 gm
Ammonia 0.3 – 1.0 g Potassium 2.5 - 3.0 g
Uric acid - 0.5 - 0.8 g Sodium 4.5 g
Hippuric acid 0.1- 1.0 g Calcium 0.1- 0.3g
Oxalic acid 10 - 30 mg Magnesium 0.1 - 0.2 g
Amino acid 150 - 200 mg Iodine 50 – 250μg
Allantion - traces Arsenic 50μg
Vitamins, hormones, enzymes - traces Lead 50μg


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