Kidney Anatomy and Nephron Function

• There are approximately 1.3 million nephrons in a kidney

Gross Anatomy

• Renal corpuscle: Located in the cortical region and serves as site of plasma filtration.
• Glomerular filtrate: Fluid within the renal corpuscle
• Vascular pole: Location where blood vessels (afferent/efferent arterioles) enter and exit.
• Glomerulus: Ball-like capillary bed where plasma is taken from the blood.
• Glomerular (Bowman's) capsule: Site where glomerular fluid is collected before entering the tubules.
• Tubular pole: Continuous with the renal tubule.
• Renal tubule: Components are added and removed here
• Tubular fluid: Fluid within the renal tubule, which eventually becomes urine at the papillary ducts.
• Proximal convoluted tubule (PCT): Exits the renal corpuscle, remains in the renal cortex; contains many peritubular capillaries to alter solute/ion concentration and has a brush border for filtrate reabsorption.
• Nephron loop (loop of Henle): descends into the medulla (descending limb) and ascends back into the cortex (ascending limb).
• Distal convoluted tubule (DCT): Located in the cortex, surrounded by peritubular capillaries.
• Efflux: DCT > collecting ducts > papillary ducts

Cortical Nephrons

• Make up 80-85% of nephrons.
• Have their corpuscle on the periphery of the cortex, and a short nephron loop
• Ascending and descending limbs have similar structure.
• Peritubular capillaries: Disorganized blood supply wrapping around the tubules, both in the cortex region and a little bit in the nephron loop

Juxtamedullary Nephrons

• Comprise 15-20% of nephrons.
• Have their corpuscle near the medulla, and a long nephron loop
• Ascending limb: A thick region and a thin region (different permeabilities)
• Descending limb: Similar to the thin ascending limb
• Peritubular capillaries: Supply blood to the cortical region
• Vasa recta: Supply blood to the nephron loop; highly organized
• Function: Works with ascending limb to establish a concentration gradient to control the movement of ions/fluids; controls dilute vs. concentrated urine.

Renal Capsule

• Glomerulus: Walls are made of endothelial cells forming fenestrated capillaries.
• Efferent arteriole: Contains thicker blood (more cellular components) since plasma is pulled out into glomerulus
• Peritubular capillaries: Thin blood back out by returning fluids
• Parietal layer of (Bowman's) capsule: Simple squamous epithelium that acts as a pocket to catch filtrate as it leaves the glomerulus.
• Visceral layer of capsule: Tightly adheres to the endothelium of the glomerulus and is made of podocytes.
• Podocyte: Modified simple squamous epithelium with finger-like projections (pedicels) to create an extra barrier of filtration.
• Capsular space: Region between the parietal and visceral layers where filtrate collects.

Juxtaglomerular Apparatus

• Juxtaglomerular apparatus: Region where ascending limb (just before DCT) comes in direct contact with the afferent arteriole
• Function: Regulates filtrate production through a fast feedback loop.
• Macula densa: Ascending limb side composed of densely packed cells with sensory receptors.
• Function: Detects water proportions, and ion (sodium, chloride) concentrations in the ascending limb; too high, it means we're making too much filtrate too fast
• Juxtaglomerular cells: Afferent arteriole side composed of modified smooth muscle cells
• Function: Contracts when signaled by macula densa, reducing blood flow into the glomerulus and the amount of filtrate being produced.
• Extraglomerular mesangial cells: Located between juxtaglomerular cells, afferent/efferent arterioles, and glomerulus; consist of modified smooth muscle cells.
• Function: Contracts to reduce SA and blood flow for filtrate production regulation.

Filtration Membrane

• Endothelium: Of fenestrated capillary walls (innermost layer), which prevents cellular components from leaving, but allows most plasma through.
• Basal lamina: Of glomerulus which prevents larger proteins from leaving.
• Podocytes: Visceral layer of glomerulus consisting of modified simple squamous epithelium with pedicels that create filtration slits.
• Filtration slit membrane: Found between pedicels and prevents medium-small proteins (e.g. albumin) from leaving.

Renal Tubule & Ducts

• Function: Allows reabsorption and secretion into filtrate.
• PCT: Consists of simple cuboidal cells with microvilli brush border (for reabsorption).
• Descending and thin ascending limb: Consists of simple squamous epithelium (allows movement of ions and fluids).
• Thick ascending limb: Consists of simple cuboidal and low columnar epithelium (limits permeability).
• DCT (most): Simple cuboidal
• DCT (end) and collecting duct: Simple cuboidal cells interspersed with specialized intercalated and principal cells to regulate ion concentrations.
• Intercalated: Monitor H+ ions to regulate pH.
• Principal: Under hormonal control regulates Na+ and K+ levels in blood and renal tubule.