Nephron: Introduction and Structure

• The structural and functional unit of kidney is called nephron.
• There are millions of nephrons (1-1.3 million) in each kidney.
• The count of nephron decreases after about 45 to 50 years of age at the rate of 0.8% to 1% every year.
• Nephron is generally formed by two parts:

a) Renal corpuscle or malphigian corpuscles known as blind end

• It has diameter of about 200µ and is slightly flattened and spheroidal structure.
• It helps in the filtration of the blood which forms the first phase of urine formation.
• This portion of nephron is generally present in the cortex of kidney either near the periphery or near the medulla.
• The nephrons are classified on the basis of the situation of the renal corpuscles into two types.

i. Cortical nephrons also called superficial nephrons

• Renal corpuscles are situated in the outer cortex of the kidney near the periphery in these neurons.
• 85% of the neurons in human beings are of these type i.e. cortical nephrons.

ii. Juxta-medullary nephrons

• Renal corpuscles in these neurons lie in inner cortex near medulla or cortico-medullary junction.

Structure of Renal Corpuscle

• It is made up of two portions. They are:

i) Glomerulus

• It is the structure covered by bowman capsule and is a tuft of capillaries.
• The glomerular capillaries are interposed between afferent arteriole on one end and efferent arteriole in another end.
• Thus, in the glomerulus vascular system is found to be purely arterial.
• Glomerular capillaries (which are 4 or 5 large capillaries) are formed form the afferent arteriole after entering the Bowman capsule.
• These large capillaries further subdivide into smaller capillaries which are arranged in irregular loops and form anastomosis.
• From these smaller capillaries efferent arteriole is formed by reunion which leaves the Bowman capsule.
• Afferent arteriole is more in diameter than efferent arteriole due to which the difference in the diameter of both arteries hold significance.

Functional histology

Glomerular capillaries are formed from a single layer of cells called endothelial cells which are attached to a basement membrane. Many pores are present in these endothelial cells called fenestrae or filtration pores. Due to the presence of these tiny pores of diameter 0.1µ, filtration function of glomerulus is possible.

Image source: iStock

ii) Bowman capsule

• Glomerulus gets enclosed by a capsular structure called Bowman capsule which is formed by two layers.
• One is known as inner visceral layer and the other as outer parietal layer.
• Glomerulus capillaries are covered by visceral layer which is continued as the parietal layer at the visceral pole.
• The parietal layer then continues with the wall of the tubular portion of the nephron.
• There is a cleft like space formed between the visceral and parietal layers which continue as the lumen of tubular portion.
• The functional anatomy of Bowman capsule is similar to a funnel with filter paper where the diameter of this capsule is 200µ.

Functional Histology

A single layer of flattened epithelial cells composes both the layers of Bowman capsule that rest on a basement membrane. Basement membrane of visceral layer and basement membrane of glomerular capillaries fuses with each other on which the capillary endothelial cells are arranged. Thus, this fusion forms a separation between the glomerular capillary endothelium and the epithelium of visceral layer of Bowman capsule. Though there is fusion between the epithelial cells of visceral layer and the basement membrane, the fusion remains incomplete. Each cell is connected with basement membrane by cytoplasmic extensions of epithelial cells called pedicles or feet which are arranged in an inter-digitating manner leaving small cleft-like spaces in between. The cleft like space is called slit pore and epithelial with pedicles are called podocytes.

b) Renal tubule i.e., a tubular portion

• It is the portion of the nephron that is a continuation of Bowman capsule made up of three parts.

i) Proximal convoluted tubule

• It is the coiled portion that arises from the Bowman capsule which is situated in the cortex.
• It continues as descending limb of loop of Henle and the size of the tubule is 14 mm long and 55µ in diameter.

Functional Histology

This convoluted tubule is formed by single layer of cuboidal epithelial cells characterized by presence of hair-like projections directed towards the lumen of the tubule. These epithelial cells with hairy projections are also called brush bordered cells.

ii) Loop of Henle

• This is the structure of nephron containing three parts. They are: descending limb, hairpin bend and ascending limb.

a) Descending limb

• This limb of loop of Henle is made up of two segments called thick descending segment and thin descending segment.
• Thick descending segment of this limb is the direct continuation of the proximal convoluted tubule which later descends down into the medulla.
• Its length is 6mm and diameter is 55µ and is formed by brush-bordered cuboidal epithelial cells.
• There is continuation of thick descending segment as thin descending segment which is formed by flattened epithelial cells without brush border.
• This thin descending segment continues as hairpin bend of the loop.

b) Hairpin bend

• It is formed by flattened epithelial cells which does not have brush border and continues as the ascending limb of loop of Henle.

c) Ascending limb

• This is the limb or segment of Henle loop having two parts. They are: thin ascending segment and thick ascending segment.

Thin ascending segment

• There is continuation of Hairpin bend as ascending segment that is lined by flattened epithelial cells without brush borders.
• The length of thin descending segment, hairpin bend and thin ascending segment of loop of Henle is 10mm to 15 mm and the diameter is 15µ.
• This segment continues as thick ascending segment.

Image source: GKScientist

Thick ascending segment

• This segment is about 9mm long with a diameter of 30µ and is lined by cuboidal epithelial cells without brush border.
• The terminal portion of thick ascending segment which runs between the afferent and efferent arterioles of the same nephrons forms the macula densa which is the part of juxtaglomerular apparatus.
• This segment ascends to the cortex and continues as distal convoluted tubule.

Length and Extent of Loop of Henle

• In different nephrons, the length and the extent of the loop of Henle vary.
• It is short and the hairpin bend penetrates only up to outer medulla in cortical nephrons.
• It is long and the hair pin bend extends deep into the inner medulla in juxtaglomerular nephrons. In some nephrons it even runs up to the papilla.

iii) Distal Convoluted tubule

• Thick ascending segment continues to form distal convoluted tubule which occupies the cortex of the kidney.
• It then continues as collecting duct.
• This tubule has length of 14.5-15mm and diameter of 22 to 50µ.
• Single layer of cuboidal epithelial cells without brush border lines the distal convoluted tubule.
• These epithelial cells are called intercalated cells (I cells) in distal convoluted tubule.

Collecting duct

• There is continuation of distal convoluted tubule as the initial or arched collecting duct which is in cortex.
• In medulla, the lower part of the collecting duct is situated.
• There is formation of the straight collecting duct from seven to ten initial collecting ducts which passes through medulla.
• Length of the collecting duct is 20-22mm and its diameter varies between 40 and 200µ.
• Collecting duct is formed by cuboidal or columnar epithelial cells.
• The epithelial cells forming collecting duct are principal or P cells and Intercalated or I cells which have some functional significance.

Passage of Urine

• Papillary ducts or ducts of Bellini are formed from the union of straight collecting ducts that arise from each medullary pyramid at the inner zone of medulla.
• These ducts finally open into “V” shaped area called papillae where urine gets collected from each medullary pyramid.
• The urine is drained into minor calyx from papillae which further unites to form major calyx.
• There are 8 minor calyces and 2 to 3 major calyces in each kidney where three to four minor calyces unite to form one major calyx.
• The urine passes from minor calyces to the expanded portion of ureter present in the renal sinus called pelvis of the ureter through major calyces.
• Urine then passes through remaining portion of ureter from the renal pelvis and is collected in the urinary bladder.

References:

i) https://en.wikipedia.org/wiki/Nephron

ii) Organismal Biology

iii) https://emedicine.medscape.com/article/1948775-overview

Nephron: Introduction and Structure