Anatomy of the Kidney

1. Position and General Features

The Hilum

• Renal artery (enters)
• Renal vein (exits)
• Ureter (exits)
• Lymphatics and nerve plexus

2. Gross Internal Structure

Cortex

• Outer reddish-brown, granular zone (~1 cm thick)
• Contains: renal corpuscles (glomeruli + Bowman's capsule), proximal convoluted tubules, distal convoluted tubules, cortical collecting ducts
• Receives ~90-95% of total renal blood flow
• Columns of Bertin: extensions of cortex dipping between medullary pyramids
• Medullary rays (of Ferrein): striations projecting from medulla into cortex, containing straight tubules and collecting ducts

Medulla

• Inner, lighter-colored, striated zone
• Contains: loops of Henle (thin and thick limbs), medullary collecting ducts, vasa recta
• Lacks glomeruli
• Divided into outer medulla (outer stripe + inner stripe) and inner medulla

Renal Pyramids

• 8-18 conical structures in the medulla
• Base faces corticomedullary junction; apex (papilla) points toward renal sinus
• The papilla is perforated by collecting duct openings at the area cribrosa
• A single pyramid + its overlying cortical cap = renal lobe

Collecting System

• Minor calyces - cup-shaped structures receiving urine from papillae
• Major calyces (2-3) - formed by convergence of minor calyces
• Renal pelvis - expanded upper portion of the ureter, lined by urothelium (transitional epithelium)
• Ureter - 28-34 cm long, exits at the ureteropelvic junction

3. The Nephron - Functional Unit

Types of Nephrons

Components of the Nephron

• Spherical structure, ~200 µm diameter
• Located in the cortex
• Glomerulus: tuft of fenestrated capillaries fed by afferent arteriole, drained by efferent arteriole
• Bowman's capsule: double-walled epithelial cup surrounding the glomerulus
  • Parietal layer: simple squamous epithelium
  • Visceral layer: specialized podocytes with foot processes (pedicels) that interdigitate around capillaries
  • Urinary (Bowman's) space between layers collects filtrate

1. Fenestrated capillary endothelium (covered by glycocalyx - negatively charged)
2. Glomerular basement membrane (GBM) - type IV collagen, laminin, heparan sulfate
3. Podocyte foot processes with filtration slits bridged by the slit diaphragm (nephrin, podocin)

• Longest segment; begins at urinary pole of Bowman's capsule
• Proximal Convoluted Tubule (PCT): coils in the cortex; tall cells with abundant microvilli (brush border) maximizing reabsorptive surface
• Proximal Straight Tubule (PST): descends into medullary ray/outer stripe
• Reabsorbs ~2/3 of glomerular filtrate (Na+, glucose, amino acids, HCO₃⁻, water)

• Creates the medullary osmotic gradient essential for urine concentration
• Thin descending limb: permeable to water, impermeable to solutes; descends into medulla
• Thin ascending limb (juxtamedullary nephrons): impermeable to water, permeable to NaCl
• Thick ascending limb (TAL): actively transports NaCl (NKCC2 cotransporter); impermeable to water - the diluting segment

• Distal Convoluted Tubule (DCT): located in cortex; contains cells with basolateral Na/K-ATPase and apical Na-Cl cotransporter (NCC)
• Fluid is maximally dilute here (~100 mOsm/kg)
• The DCT contacts its own glomerulus at the juxtaglomerular apparatus (JGA)

• Connecting tubule → cortical collecting duct → outer medullary collecting duct → inner medullary collecting duct (duct of Bellini)
• Principal cells: regulated by aldosterone (Na+ reabsorption) and ADH/vasopressin (water reabsorption via aquaporin-2)
• Intercalated cells: regulate acid-base balance (H+ and K+ secretion/reabsorption)
• Urine drains from collecting duct openings at area cribrosa into minor calyces

4. Juxtaglomerular Apparatus (JGA)

5. Blood Supply

Arterial Supply (Hierarchical)

• Superficial nephrons: peritubular capillaries surround cortical tubules
• Juxtamedullary nephrons: vasa recta - long hairpin capillaries that descend into medulla parallel to loops of Henle, form the countercurrent exchange system for urine concentration

Venous Drainage

• The venous system has extensive collaterals (unlike arteries)

Clinical Note

6. Lymphatics and Innervation

• Lymphatics: drain into paraaortic (lumbar) lymph nodes; run with the blood vessels in the renal sinus
• Innervation: sympathetic fibers from T8-L1 via the renal plexus (derived from celiac and aorticorenal ganglia); control vascular tone, renin secretion, and tubular transport. The kidney has no parasympathetic innervation.

7. Summary Table

Coverings of the Kidney

Overview - Layers from Inside Out

Schematic Cross-Section

1. Fibrous Capsule (Renal Capsule) - Innermost

• A tough, thin fibrous layer directly investing the renal parenchyma
• Composed of collagen and smooth muscle fibers
• Continuous at the hilum with the connective tissue of the renal sinus, where it surrounds the blood vessels and pelvis
• In health, it is smooth and easily stripped from the kidney surface
• In disease (e.g., chronic pyelonephritis, renal carcinoma), the capsule becomes adherent and cannot be cleanly removed
• Connected to the overlying renal fascia by fine connective tissue fiber bundles
• Clinical note: The non-distensible nature of the capsule means that acute renal swelling (e.g., pyelonephritis, obstruction) causes significant pain - the capsule has a rich pain nerve supply (sympathetic afferents T10-L1)

2. Perirenal (Perinephric) Fat - Perirenal Fat Capsule

• A layer of adipose tissue sitting between the fibrous capsule and the renal fascia
• Completely surrounds the kidney and fills the perinephric space
• Extends into the renal sinus through the hilum, surrounding the renal pelvis, calyces, and intrarenal vessels
• Fine, lighter yellow in texture - distinct from the coarser, yellow-orange pararenal fat (a useful surgical distinction during retroperitoneal dissection)
• Acts as a mechanical cushion protecting the kidney from trauma
• Also provides thermal insulation
• The perinephric space (bounded by renal fascia) also contains: the adrenal gland, ureter, renal vascular pedicle, and gonadal vessels

3. Renal Fascia (Gerota's Fascia) - The Key Investing Layer

Two Laminae

Attachments and Extents

• Superiorly: Anterior and posterior layers fuse above the suprarenal gland and blend with the fascia of the diaphragm - the compartment is closed superiorly
• Laterally: Anterior and posterior layers fuse to form the lateroconal fascia, which continues to the transversalis fascia
• Medially:
  • Anterior layer passes over the hilar vessels and fuses with connective tissue around the aorta and IVC; may cross the midline to the opposite side
  • Posterior layer passes medially between kidney and quadratus lumborum to fuse with psoas fascia
• Inferiorly: The compartment is open inferiorly - the two layers do not fuse; they taper into a cone-like shape and continue around the ureter as periureteric fascia descending into the pelvis

Contents of the Perirenal Space (inside Gerota's fascia)

• Kidney + fibrous capsule
• Perinephric fat
• Suprarenal (adrenal) gland (usually separated by a thin septum)
• Proximal ureter and gonadal vessels
• Renal vascular pedicle

Clinical Significance of Gerota's Fascia

1. Perinephric collections - the fascia limits spread of perinephric hematoma, urinoma, or abscess; they tend to spread inferiorly (the only open end)
2. Renal cell carcinoma (RCC) staging - invasion through Gerota's fascia = pT3b/T4 disease (upstages the tumor)
3. Radical nephrectomy - performed outside Gerota's fascia, removing kidney + fat + adrenal en bloc
4. Simple nephrectomy - performed inside Gerota's fascia
5. Renal transplantation - Gerota's fascia is opened to expose the kidney

4. Pararenal (Paranephric) Fat - Outermost

• Located outside the renal fascia
• Positioned between:
  • Posteriorly: renal fascia and quadratus lumborum fascia (anterior layer of thoracolumbar fascia)
  • Anteriorly: renal fascia and parietal peritoneum
• Most prominent posterolateral to the kidney
• Coarser, yellow-orange in colour
• The renal fascia sends collagen fiber bundles through the paranephric fat, anchoring the kidney in position
• Variable in thickness; can be substantial in obese individuals

Summary Diagram Relationships

Retroperitoneal Space Compartments

Structures That Must Be Incised to Reach the Kidney

1. Skin and subcutaneous fat
2. Muscles (flank approach: external oblique, internal oblique, transversus abdominis)
3. Transversalis fascia
4. Pararenal (paranephric) fat - posterior pararenal space
5. Renal fascia (Gerota's fascia) - mandatory for all renal surgery
6. Perirenal (perinephric) fat
7. Fibrous capsule - only if operating directly on the parenchyma