What is the anatomy of a nephron?

Anatomy of the Nephron

The nephron is the basic structural and functional unit of the kidney, consisting of two main components: a glomerulus (filtration apparatus) and a tubular system that processes the filtered fluid through sequential segments spanning from the renal cortex to the medulla. 1, 2

Major Structural Components

Glomerulus

• The glomerulus serves as the initial filtration site where plasma is filtered to create the glomerular filtrate 1
• Contains specialized epithelial cells called podocytes that form the filtration barrier and communicate with other nephron cells through extracellular vesicles 3
• The glomerular filtration surface area is significantly smaller in term neonates compared to adults, affecting filtration capacity 4

Tubular System

The tubule collects glomerular filtrate and performs reabsorption and secretion to produce final urine 1. The tubular segments include:

Proximal Tubule

• Reabsorbs approximately 65% of filtered load including nearly all filtered amino acids, glucose, solutes, and low molecular weight proteins 5
• Reabsorbs approximately 80% of filtered bicarbonate, playing a critical role in acid-base balance 5
• Represents the most metabolically active segment of the nephron 5

Loop of Henle (Thick Ascending Limb and Thin Segments)

• The thick ascending limb (TAL) and early distal convoluted tubule (DCT) constitute the "free water generating segment" where urine is diluted 6
• Uromodulin expression increases 8-fold from medullary TAL to cortical TAL, correlating with increasing osmotic gradient generation 6
• This segment is impermeable to water, allowing for concentration and dilution of urine 6

Distal Convoluted Tubule and Collecting Duct

• Performs fine-tuning of electrolyte balance and final urine concentration 1
• Responds to hormonal regulation for final adjustments in sodium and water handling 5

Spatial Distribution and Zonation

• Nephrons span from the renal cortex to the medulla, with distinct functional zones 6
• The outer medulla (particularly the inner stripe) is highly enriched in immune cells and uromodulin expression, making it a key area for immune sensing and protection from ischemic injury 6
• The outer medulla is particularly sensitive to perfusion changes and ischemic injury 6

Functional Integration

• The nephron regulates body homeostasis through three sequential processes: filtration (at the glomerulus), reabsorption, and secretion (along the tubule) 1, 2, 7
• The complex interplay between tubular and vascular components results in excretion of unwanted fluid and solutes as urine 2, 7
• Nephron patterning during development is controlled by integrated WNT/BMP/FGF signaling, demonstrating plasticity in establishing proximal versus distal nephron identities 8

Clinical Relevance

• Approximately 1 million nephrons form in each human kidney during embryonic and fetal development, and this number does not increase after birth 8
• Nephron number may play a role in human health and disease susceptibility 5
• Understanding nephron anatomy is essential before addressing disorders affecting kidney function 1