Nephron- Ultra-Structure and Functions
Introduction to the Nephron
The nephron is the functional unit of the kidney, responsible for blood filtration, reabsorption of essential substances, and excretion of waste products. Each human kidney contains approximately 1 million nephrons, working in concert to maintain fluid balance, electrolyte homeostasis, and blood pressure regulation. Understanding the ultra-structure and functions of the nephron is crucial for comprehending kidney physiology and pathology.
Anatomy of the Nephron
The nephron consists of several distinct segments, each with specialized structures and functions:
- Renal Corpuscle (Bowman's capsule and glomerulus)
- Proximal Convoluted Tubule (PCT)
- Loop of Henle (descending and ascending limbs)
- Distal Convoluted Tubule (DCT)
- Collecting Duct
These segments work together to filter blood, reabsorb essential substances, and concentrate urine.
Renal Corpuscle
The renal corpuscle consists of the Bowman's capsule and the glomerulus:
Bowman's Capsule:
- A cup-shaped structure that encapsulates the glomerulus
- Composed of a single layer of epithelial cells
- Contains podocytes with foot processes that interdigitate to form filtration slits
Glomerulus:
- A network of capillaries derived from the afferent arteriole
- Lined with fenestrated endothelium to allow filtration
- Surrounded by mesangial cells that provide structural support and regulate blood flow
The renal corpuscle is the site of ultrafiltration, where blood pressure forces plasma through the filtration barrier (endothelium, basement membrane, and podocyte filtration slits) to form the glomerular filtrate.
Proximal Convoluted Tubule (PCT)
The PCT is the first segment of the tubular system and is responsible for reabsorbing approximately 65% of the filtered load.
Ultrastructure:
- Lined with cuboidal epithelial cells with a brush border (microvilli) on the apical surface
- Numerous mitochondria to support active transport processes
- Extensive basolateral membrane infoldings to increase surface area for transport
Functions:
- Reabsorption of glucose, amino acids, and small peptides
- Reabsorption of sodium, chloride, potassium, and bicarbonate
- Reabsorption of water (approximately 65% of filtered water)
- Secretion of organic acids and bases
- Production of ammonia for acid-base balance
Loop of Henle
The Loop of Henle consists of the descending limb, thin ascending limb, and thick ascending limb. It plays a crucial role in concentrating urine and maintaining the medullary osmotic gradient.
Descending Limb:
- Thin epithelium with few mitochondria
- Highly permeable to water, less permeable to solutes
- Contributes to the concentration of urine
Thin Ascending Limb:
- Similar structure to the descending limb
- Impermeable to water, permeable to certain ions
Thick Ascending Limb:
- Cuboidal epithelium with numerous mitochondria
- Contains the Na-K-2Cl cotransporter for active salt reabsorption
- Impermeable to water
- Responsible for diluting the tubular fluid and contributing to the countercurrent multiplication system
Distal Convoluted Tubule (DCT)
The DCT is responsible for fine-tuning electrolyte balance and is a key site for hormone-mediated regulation of renal function.
Ultrastructure:
- Cuboidal epithelium with extensive basolateral membrane infoldings
- Numerous mitochondria to support active transport
- Lacks a brush border
Functions:
- Reabsorption of sodium and chloride (through the thiazide-sensitive Na-Cl cotransporter)
- Secretion of potassium and hydrogen ions
- Reabsorption of calcium (regulated by parathyroid hormone)
- Reabsorption of magnesium
- Site of action for aldosterone and antidiuretic hormone (ADH)
Collecting Duct
The collecting duct is the final segment of the nephron and is crucial for water balance and pH regulation.
Ultrastructure:
- Composed of principal cells and intercalated cells
- Principal cells: responsible for water and sodium reabsorption
- Intercalated cells: involved in acid-base balance
Functions:
- Water reabsorption (regulated by ADH)
- Fine-tuning of sodium and potassium balance
- Acid-base regulation through hydrogen ion secretion and bicarbonate reabsorption
- Urea reabsorption (contributes to the medullary osmotic gradient)
Functions of the Nephron
The nephron performs several critical functions in maintaining homeostasis:
- Filtration: The glomerulus filters blood to produce an ultrafiltrate free of cells and large proteins.
- Reabsorption: Essential substances such as glucose, amino acids, water, and electrolytes are selectively reabsorbed along the tubular system.
- Secretion: Waste products, excess ions, and certain drugs are actively secreted into the tubular lumen.
- Concentration and dilution of urine: The loop of Henle and collecting duct system allow for the production of concentrated or dilute urine based on the body's needs.
- Acid-base balance: The nephron regulates blood pH by excreting or retaining hydrogen ions and bicarbonate.
- Blood pressure regulation: Through sodium and water handling, as well as the renin-angiotensin-aldosterone system.
- Hormone production: The nephron produces erythropoietin (stimulates red blood cell production) and calcitriol (active form of vitamin D).
