Tubular Portion Of Nephron Notes

Nephron Introduction

Nephron is defined as the structural and functional unit of the kidney. Each kidney consists of 1-1.3 million nephrons.

The number of nephrons starts decreasing after about 45-50 years of age at the rate of 0.8-1% every year.

Each nephron is formed by two parts:

1. A blind end called renal corpuscle or Malpighian corpuscle
2. A tubular portion called the renal tubule.

Read And Learn More: Medical Physiology Notes

Renal Corpuscle

• The renal corpuscle is otherwise known as the Malpighian corpuscle. It is a spheroidal and slightly flattened structure with a diameter of about 200 μ.
• The 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

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

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

1. Cortical nephrons or superficial nephrons
2. Juxtamedullary nephrons.

1. Cortical Nephrons

• Cortical nephrons are the nephrons, which have their corpuscles in the outer cortex of the kidney near the periphery.
• In human kidneys, 85% of nephrons are cortical nephrons.

2. Juxtamedullary Nephrons

Juxtamedullary nephrons are the nephrons that have their corpuscles in the inner cortex near the medulla or corticomedullary junction.

The features of the two types of nephrons

Structure Of Renal Corpuscle

The renal corpuscle is formed by two portions:

1. Glomerulus
2. Bowman’s capsule, which encloses the glomerulus.

Glomerulus

• Glomerulus is a tuft of capillaries enclosed by Bowman’s capsule.
• It consists of a network of glomerular capillaries which are connected to an afferent arteriole on one end and to an efferent arteriole on the other end.
• Thus, the vascular system in the glomerulus is purely arterial.
• The capillaries of the glomerulus arise from the afferent arteriole.
• After entering the Bowman’s capsule, the afferent Afferent arteriole divides into 4 or 5 large capillaries. Each large capillary subdivides into many small capillaries.

• The 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’s capsule.
• The diameter of the efferent arteriole is less than that of the afferent arteriole.
• This difference in diameter may be correlated with functional values.
• The capillaries are made up of a single layer of endothelial cells which are attached to a basement membrane.
• The endothelium has many pores called fenestra or filtration pores. The diameter of each pore is 0.1 μ.
• The presence of the Fenestra is evidence of the filtration function of the glomerulus.

Bowman’s Capsule

Bowman’s capsule is a capsular structure that encloses the glomerulus.

Bowman’s capsule is formed by two layers:

1. The inner visceral layer
2. The outer parietal layer.

• The visceral layer covers the glomerular capillaries. It is continued as the parietal layer at the visceral pole.
• The parietal layer is continued with the wall of the tubular portion of the nephron.
• The cleft-like space between the visceral and parietal layers is continued as the lumen of the tubular portion.
• The functional anatomy of Bowman’s capsule can be compared to a funnel with filter paper.
• Diameter of It Bowman’s capsule is 200 μ.

Bowman’s Capsule Histology

• Both the layers of Bowman’s capsule are composed of a single layer of flattened epithelial cells resting on a basement membrane.
• The 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 the visceral layer of Bowman’s capsule.
• The epithelial cells of the visceral layer fuse with the basement membrane but the fusion is not complete.
• Each cell is connected with the basement membrane by cytoplasmic extensions of epithelial cells called pedicles or feet.
• These pedicles are arranged in an interdigitating manner leaving small cleft-like spaces in between. The cleft-like space is called a slit pore.
• The epithelial cells with pedicles are called podocytes.

Tubular Portion Of Nephron

The tubular portion of the nephron is the continuation of Bowman’s capsule.

Tubular Portion Of The Nephron is made up of three parts:

1. The proximal convoluted tubule
2. Loop of Henle
3. The distal convoluted tubule

Proximal Convoluted Tubule

• Proximal Convoluted Tubule is the coiled portion arising from Bowman’s capsule?
• Proximal Convoluted Tubule is situated in the cortex.
• Proximal Convoluted Tubule is continued as descending limb of the loop of Henle. The length of the proximal convoluted tubule is 14 mm and the diameter is 55 μ.

Proximal Convoluted Tubule Histology

• Proximal convoluted tubule is formed by single layer of cuboidal epithelial cells.
• The feature of these cells is the presence of hair-like 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:

1. Descending limb
2. Hairpin bend
3. Ascending limb

Descending Limb

Descending limb or segment of the loop of Henle is made up of a thick descending segment and a thin descending segment.

• The thick descending segment is the direct continuation of the proximal convoluted tubule.
• Descending Limb descends down into the medulla. It has a length of 6 mm and a diameter of 55 μ.
• The thick descending segment of Henle’s loop is continued as a thin descending segment.

Hairpin Bend

The thin descending segment is continued as a hairpin bend of the loop. The hairpin bend is continued as the ascending segment of the loop of Henle.

Ascending Limb

• Ascending limb or segment of Henle’s loop has two parts, a thin ascending segment, and a thick ascending segment.
• A thin ascending segment is the continuation of the hairpin bend.
• The total length of a thin descending segment, hairpin bend, and thin ascending segment of Henle’s loop is 10-15 mm, and the diameter is 15 μ.
• The thin ascending segment is continued as a thick ascending segment. It is about 9 mm long with a diameter of 30 μ.
• A thick ascending segment ascends to the cortex and continues as a distal convoluted tubule.

Length and Extent of Loop of Henle

1. In cortical nephrons, it is short and the hairpin bend penetrates only up to the outer medulla
2. In juxtamedullary nephrons, this is long and the hairpin bend extends deep into the inner medulla. In some nephrons, it even runs up to the papilla.

Hairpin Bend Histology

• The thick descending segment is formed by brush-bordered cuboidal epithelial cells.
• The thin descending segment, hairpin bend, and thin ascending segment are lined by flattened epithelial cells without brush border.
• The thick ascending segment is lined by cuboidal epithelial cells without a brush border.
• The terminal portion of the thick ascending segment which runs between the afferent and efferent arterioles of the same nephrons forms the macula densa.
• Macula densa is part of the juxtaglomerular apparatus.

Distal Convoluted Tubule

• Distal Convoluted Tubule is the continuation of a thick ascending segment and occupies the cortex of the kidney.
• Distal Convoluted Tubule is continued as a collecting duct. The length of the distal convoluted tubule is 14.5-15 mm. It has a diameter of 22-50 μ.

Distal Convoluted Tubule Histology

• The length and the extent of the loop of Henle vary in the Distal convoluted tubule lined by a single layer of different nephrons. cuboidal epithelial cells without brush border.
• The epithelial cells in the distal convoluted tubule are called in-intercalated cells (cells).

Collecting Duct

• The distal convoluted tubule continues as the initial or arched collecting duct, which is in the cortex.
• The lower part of the collecting duct lies in the medulla. Seven to ten initial collecting ducts unite to form the straight collecting duct, which passes through the medulla.
• At the inner zone of the medulla, the straight collecting ducts from each medullary pyramid unite to form papillary ducts or ducts of Bellini, which open into the papilla.
• Papilla is a V-shaped area that collects urine from each medullary pyramid. Each papilla drains into a minor calyx. Three or four minor calyces unite to form one major calyx.
• Each kidney has got about 8 minor calyces and 2-3 major calyces. The major calyces open into the pelvis of the ureter.
• The pelvis is the expanded portion of the ureter present in the renal sinus. The length of the collecting duct is 20-22 mm. The diameter of the collecting duct varies between 40 and 200

Collecting Duct Histology

The collecting duct is formed by cuboidal or columnar epithelial cells.

The epithelial cells of collecting duct are of two types:

1. The principal or P cells
2. Intercalated or I cells.

These two types of cells have some functional significance.