The kidney's microscopic anatomy is a marvel of intricate structures working in harmony. From the renal cortex's filtering units to the medulla's concentrating tubules, each component plays a crucial role in maintaining our body's balance.

Understanding the kidney's microstructures is key to grasping how our bodies filter blood, regulate fluids, and remove waste. The renal corpuscles, tubules, and capillaries work together in a complex dance of filtration, reabsorption, and secretion to keep us healthy.

Microscopic Anatomy of the Kidney

Renal cortex vs medulla histology

• Renal cortex
  • Contains renal corpuscles (Bowman's capsules and glomeruli) and convoluted tubules (proximal and distal)
  • Appears lighter in color due to the numerous renal corpuscles scattered throughout the cortical tissue
  • Houses the initial and final segments of the nephron, the proximal convoluted tubule (PCT) and distal convoluted tubule (DCT)
• Renal medulla
  • Consists of renal pyramids composed of straight tubules (loops of Henle) and collecting ducts arranged in parallel
  • Appears darker and striated due to the parallel arrangement of the tubules and blood vessels (vasa recta)
  • Contains the intermediate segments of the nephron, the descending and ascending limbs of the loop of Henle, as well as the collecting ducts that converge to form the renal papilla

Structure of filtration membrane

• Filtration membrane components
  • Fenestrated endothelium of glomerular capillaries allows passage of small molecules (ions, glucose) and fluid while restricting larger particles (blood cells, proteins)
  • Basement membrane provides structural support and acts as a selective barrier based on molecular size and charge
  • Filtration slits between interdigitating podocyte foot processes regulate the passage of molecules, preventing the loss of essential proteins (albumin) while allowing filtration of waste products (urea, creatinine)
• Functions of the filtration membrane
  • Selectively filters blood to form an ultrafiltrate containing water, ions, glucose, amino acids, and small waste products
  • Retains larger molecules such as proteins (hemoglobin) and blood cells within the capillaries, maintaining the colloid osmotic pressure of blood
  • Maintains the balance of electrolytes and fluid in the body by regulating the composition of the filtrate entering the renal tubules
  • Determines the glomerular filtration rate (GFR), which is the volume of fluid filtered from the glomerular capillaries into Bowman's capsule per unit time

Renal Microstructures and Their Functions

Components of renal structures

• Renal corpuscle
  • Bowman's capsule: Double-walled epithelial structure surrounding the glomerulus, with the parietal layer forming the outer wall and the visceral layer (podocytes) in contact with the glomerular capillaries
  • Glomerulus: Tuft-like network of capillaries arising from the afferent arteriole and draining into the efferent arteriole, responsible for the filtration of blood
  • Podocytes: Specialized epithelial cells with long, interdigitating foot processes that form the filtration slits, contributing to the selective permeability of the filtration barrier
• Renal tubules
  • Proximal convoluted tubule (PCT) reabsorbs the majority of the filtrate components, including nutrients (glucose, amino acids), ions (sodium, chloride), and water
  • Loop of Henle consists of the descending limb (permeable to water) and ascending limb (impermeable to water, active transport of ions), creating a concentration gradient in the medulla
  • Distal convoluted tubule (DCT) fine-tunes the composition of the filtrate by selectively reabsorbing or secreting ions (sodium, potassium, calcium) and regulating acid-base balance
  • Collecting duct receives filtrate from multiple nephrons and conducts it to the renal pelvis, responsive to hormones (ADH, aldosterone) for final adjustments of water and electrolyte balance
• Renal capillaries
  • Peritubular capillaries surround the renal tubules, participating in the reabsorption of essential substances and secretion of waste products and excess ions
  • Vasa recta are parallel arrangements of capillaries in the medulla that maintain the osmotic gradient necessary for urine concentration

Roles of renal capillaries

• Peritubular capillaries
  • Reabsorb essential substances from the renal tubules, such as glucose, amino acids, and ions (sodium, potassium, calcium), returning them to the bloodstream
  • Secrete waste products and excess substances into the renal tubules, such as hydrogen ions, potassium, and organic acids (uric acid), for elimination in the urine
  • Maintain the concentration gradient necessary for efficient reabsorption and secretion processes by equilibrating with the interstitial fluid
• Vasa recta
  • Countercurrent exchange system that preserves the medullary osmotic gradient established by the loop of Henle
  • Descending vasa recta passively loses water and gains solutes (sodium, chloride, urea) as it enters the hyperosmotic medulla
  • Ascending vasa recta passively gains water and loses solutes as it returns to the cortex, maintaining the concentration gradient
  • Helps maintain the high osmolarity of the medullary interstitium, which is essential for the production of concentrated urine

Juxtaglomerular apparatus composition

• Location: Juxtaglomerular apparatus (JGA) is found at the point where the DCT comes in close contact with the afferent and efferent arterioles of its own nephron
• Cellular composition
  • Macula densa: Specialized cells of the DCT that sense changes in the tubular fluid composition (sodium chloride concentration) and transmit signals to the JG cells
  • Juxtaglomerular cells (JG cells): Modified smooth muscle cells in the walls of the afferent arteriole that secrete renin in response to decreased blood pressure, reduced sodium chloride delivery to the macula densa, or sympathetic stimulation (β1-adrenergic receptors)
  • Extraglomerular mesangial cells: Cells located between the macula densa and the JG cells that provide structural support and help regulate glomerular filtration rate through contraction and relaxation
• Function: Participates in tubuloglomerular feedback, a mechanism that adjusts renal blood flow and GFR based on the composition of the tubular fluid

Histology of renal tubule segments

• Proximal convoluted tubule (PCT)
  • Simple cuboidal epithelium with prominent microvilli forming a brush border, increasing the surface area for reabsorption
  • Acidophilic cytoplasm due to the presence of numerous mitochondria, reflecting the high energy demand for active transport processes
• Loop of Henle
  1. Descending thin limb: Simple squamous epithelium, highly permeable to water due to the presence of aquaporin channels, allowing passive water reabsorption
  2. Ascending thin limb: Simple squamous epithelium, impermeable to water but permeable to ions, contributing to the generation of the medullary concentration gradient
  3. Thick ascending limb: Simple cuboidal epithelium with tight junctions, actively transports ions (sodium, chloride, potassium) out of the tubule, diluting the filtrate and generating the medullary osmotic gradient
• Distal convoluted tubule (DCT)
  • Simple cuboidal epithelium with less prominent microvilli compared to PCT, reflecting its role in fine-tuning the composition of the filtrate
  • Lighter staining cytoplasm compared to PCT due to fewer mitochondria, as the DCT has lower energy requirements for its transport processes
• Collecting duct
  • Simple cuboidal epithelium in the cortex, transitioning to columnar epithelium in the medulla, allowing for greater water permeability in the presence of antidiuretic hormone (ADH)
  • Presence of two cell types: Principal cells reabsorb water and secrete potassium in response to ADH and aldosterone, while intercalated cells (α and β subtypes) regulate acid-base balance by secreting hydrogen ions or bicarbonate

Renal Physiology and Regulation

• Urine concentration: Process by which the kidneys produce concentrated urine through the countercurrent multiplication system in the loop of Henle and the action of ADH on the collecting ducts
• Osmoregulation: The kidney's role in maintaining fluid and electrolyte balance through selective reabsorption and secretion of water and solutes
• Renal clearance: Measure of how efficiently the kidneys remove substances from the blood, used to assess kidney function and determine the renal plasma threshold for various substances