Nephron; structure and function

 Nephron is defined as the structural and functional unit of kidney. Each kidney consists of 1 to 1.3 millions of nephrons. The number of nephrons starts decreasing after about 45 to 50 years of age at the rate of 0.8% to 1% every year. Each nephron is formed by two parts :

1. A blind end called renal corpuscle or Malpighian corpuscle

2. A tubular portion called renal tubule.

RENAL CORPUSCLE

Renal corpuscle or Malpighian corpuscle is a spheroidal and slightly flattened structure with a diameter of about 200 μ. Function of the renal corpuscle is the filtration of blood which forms the first phase of urine formation.

SITUATION OF RENAL CORPUSCLE AND TYPES OF NEPHRON

Renal corpuscle is situated in the cortex of the kidney either near the periphery or near the medulla.

Classification of Nephrons

Based on the situation of renal corpuscle, the nephrons are classified into two types:

1. Cortical nephrons or superficial nephrons: Nephrons having the corpuscles in outer cortex of the kidney

near the periphery. In human kidneys, 85% nephrons are cortical nephrons.

2. Juxtamedullary nephrons: Nephrons having the corpuscles in inner cortex near medulla or corticomedullary junction.

STRUCTURE OF RENAL CORPUSCLE

Renal corpuscle is formed by two portions:

1. Glomerulus

2. Bowman capsule.

Glomerulus

Glomerulus is a tuft of capillaries enclosed by Bowman capsule. It consists of glomerular capillaries interposed between afferent arteriole on one end and efferent arteriole on the other end. Thus, the vascular system in the glomerulus is purely arterial.

Glomerular capillaries arise from the afferent arteriole. After entering the Bowman capsule, the afferent arteriole divides into 4 or 5 large capillaries. Each large capillary subdivides into many small capillaries. These small capillaries are arranged in irregular loops and form anastomosis. All the smaller capillaries finally reunite to form the efferent arteriole, which leaves the Bowman capsule.

Diameter of the efferent arteriole is less than that of afferent arteriole. This difference in diameter has got functional significance.

Functional histology

Glomerular capillaries are made up of single layer of cuboidal epithelial cells. Characteristic feature of these cells is the presence of hairlike projections directed towards the lumen of the tubule. Because of thepresence of these projections, the epithelial cells are called brush-bordered cells.

LOOP OF HENLE

Loop of Henle consists of:

i. Descending limb

ii. Hairpin bend

iii. Ascending limb.

endothelial cells, which are attached to a basement membrane. Endothelium has many pores called fenestrae or filtration pores. Diameter of each pore is 0.1 μ. Presence of the fenestra is the evidence of the filtration function of the glomerulus.

Bowman Capsule

Bowman capsule is a capsular structure, which encloses the glomerulus.

It is formed by two layers:

i. Inner visceral layer

ii. Outer parietal layer.

Visceral layer covers the glomerular capillaries. It is continued as the parietal layer at the visceral pole. Parietal layer is continued with the wall of the tubular portion of nephron. The cleftlike space between the visceral and parietal layers is continued as the lumen of the tubular portion. Functional anatomy of Bowman capsule resembles a funnel with filter paper. Diameter of Bowman capsule is 200 μ.

Functional histology

Both the layers of Bowman capsule are composed of a single layer of flattened epithelial cells resting on a basement membrane. Basement membrane of the visceral layer fuses with the basement membrane of glomerular capillaries on which the capillary endothelial cells are arranged. Thus, the basement membranes, which are fused together, form the separation between the glomerular capillary endothelium and the epithelium of visceral layer of Bowman capsule. Epithelial cells of the visceral layer fuse with the basement membrane but the fusion is not complete. Each cell is connected with basement membrane by cytoplasmic extensions of epithelial cells called pedicles or feet. These pedicles are arranged in an interdigitating manner leaving small cleftlike spaces in between. The cleftlike space is called slit pore. Epithelial cells with pedicles are called podocytes .

TUBULAR PORTION OF NEPHRON

Tubular portion of nephron is the continuation of Bowman capsule.

It is made up of three parts:

1. Proximal convoluted tubule

2. Loop of Henle

3. Distal convoluted tubule.

PROXIMAL CONVOLUTED TUBULE

Proximal convoluted tubule is the coiled portion arising from Bowman capsule. It is situated in the cortex. It is continued as descending limb of loop of Henle. Length of proximal convoluted tubule is 14 mm and the diameter is 55 μ. Proximal convoluted tubule is continued as loop of Henle.

Functional histology

Proximal convoluted tubule is formed by single layer of cuboidal epithelial cells. Characteristic feature of these cells is the presence of hairlike projections directed towards the lumen of the tubule. Because of the presence of these projections, the epithelial cells are called brush-bordered cells.

LOOP OF HENLE

Loop of Henle consists of:

i. Descending limb

ii. Hairpin bend

iii. Ascending limb.