Quick Jump to Sections

Location Gross Anatomy Surfaces Coverings Internal Nephron Blood Supply Nerve Supply Ureter Functions Clinical NEET Points
Section A

Location & Position

Exact Location

  • Position: Retroperitoneal — behind the peritoneum, on posterior abdominal wall
  • Region: Lumbar region, on either side of vertebral column
  • Vertebral level: T12 to L3 (right kidney slightly lower due to liver above)
  • Right kidney: T12–L3 vertebral level
  • Left kidney: T11–L2 vertebral level (slightly higher than right)
  • Hilum level: Transpyloric plane (L1) — both kidneys
  • Movement: Move 2–3 cm during respiration (descend on inspiration)

Relations

Right Kidney Relations

  • Anteriorly: Right suprarenal gland (upper pole), liver (upper 2/3), hepatic flexure of colon, 2nd part of duodenum
  • Posteriorly: Diaphragm, 12th rib, psoas major, quadratus lumborum, transversus abdominis
  • Medially: IVC, right suprarenal gland

Left Kidney Relations

  • Anteriorly: Left suprarenal gland (upper pole), stomach, spleen, body of pancreas, left colic flexure, jejunum
  • Posteriorly: Diaphragm, 11th and 12th ribs, psoas major, quadratus lumborum
  • Medially: Aorta, left suprarenal gland

Posterior Relations (Both)

  • Diaphragm: Separates from pleural cavity
  • Psoas major: Medial muscle
  • Quadratus lumborum: Lateral muscle
  • Transversus abdominis: Most lateral
  • Subcostal nerve (T12): Passes diagonally
  • Iliohypogastric & ilioinguinal nerves (L1): Below 12th rib

Why Right is Lower?

  • Liver occupies right hypochondrium
  • Pushes right kidney inferiorly
  • Right kidney: T12–L3
  • Left kidney: T11–L2
  • Right kidney ~1.5 cm lower than left
Section B

Gross Anatomy

Physical Characteristics

Weight
120–170 g
Each kidney
Length
10–12 cm
Vertically
Width
5–7 cm
Horizontally
Thickness
2.5–3 cm
Anteroposterior
Color
Reddish-brown
Due to vascularity
Shape
Bean-shaped
Medial concavity

Surfaces, Poles & Borders

Anterior Surface

  • Convex, covered by peritoneum (except for small area)
  • Shows impressions of overlying organs
  • Right: liver, duodenum, colon
  • Left: stomach, spleen, pancreas, colon, jejunum

Posterior Surface

  • Flat, devoid of peritoneum
  • Rests on posterior abdominal wall muscles
  • Related to diaphragm, ribs 11 & 12, and lumbar muscles

Upper (Superior) Pole

  • Broad and rounded
  • Capped by suprarenal gland
  • Related to diaphragm posteriorly

Lower (Inferior) Pole

  • Narrower and more pointed
  • 2.5 cm above iliac crest
  • Ureter begins here (renal pelvis)

Medial Border

  • Concave — like inner curve of bean
  • Middle part = Hilum (renal hilum)
  • Hilum transmits renal vessels, ureter, nerves

Lateral Border

  • Convex, smooth, rounded
  • No major structures here
  • Outer surface of the bean shape

Renal Hilum — Gateway of Kidney

Hilum = vertical slit on medial border where structures enter/exit the kidney

  • Location: Middle of medial border, at L1 level (transpyloric plane)
  • Leads to: Renal sinus (a fat-filled space inside kidney)
  • Contents (Anterior to Posterior — "VAD"):
    • Vein — Renal vein (most anterior)
    • Artery — Renal artery (middle)
    • Duct — Ureter / Renal pelvis (most posterior)
    • Also: lymphatics and autonomic nerves

Memory: "VAD" at hilum — Vein (front), Artery (middle), Duct/ureter (behind)

Section C

Coverings of Kidney

Three Coverings (From Inside to Outside)

Kidney has 3 coverings arranged concentrically — like layers of an onion:

1

Fibrous Capsule (Renal Capsule)

Innermost layer. Thin, tough fibrous coat. Strips easily from healthy kidney (difficult in disease/scarring). Continuous with outer coat of ureter at hilum.

2

Perinephric Fat (Adipose Capsule)

Middle layer. Perirenal fat — fatty cushion around kidney. Protects kidney from trauma. Thickest posteriorly. Absent at hilum. Reduced in starvation → "floating kidney" (nephroptosis).

3

Renal Fascia (Gerota's Fascia)

Outermost layer. Condensed connective tissue. Two layers: anterior (Toldt's fascia) and posterior (Zuckerkandl's fascia). Closed above and laterally; open below (important clinically). Contains suprarenal gland within the same fascial envelope.

Clinical Importance of Coverings

Nephroptosis (Floating Kidney)

Loss of perinephric fat → kidney descends. Common in thin women. Causes intermittent ureteric obstruction → loin pain.

Perinephric Abscess

Pus collects in perinephric fat space. Often secondary to renal carbuncle. Confined by Gerota's fascia. Can track downwards (fascia open inferiorly).

Retroperitoneal Hematoma

Blood collects in perirenal fat after trauma. Contained by fascia. Can cause flank ecchymosis (Grey Turner's sign).

Renal Transplant

Transplanted kidney placed in iliac fossa (extraperitoneal). Gerota's fascia knowledge essential for surgery.

Section D

Internal Structure

Cortex vs Medulla

Renal Cortex (Outer)

  • Reddish-brown, granular appearance
  • ~1 cm thick outer rim
  • Contains: Glomeruli, Bowman's capsule, PCT, DCT
  • Cortical columns (of Bertin) dip between pyramids
  • Highly vascular
  • Site of filtration and reabsorption

Renal Medulla (Inner)

  • Pale, striated appearance
  • Contains 8–18 renal pyramids
  • Each pyramid: base faces cortex, apex = renal papilla
  • Contains: Loop of Henle, collecting ducts
  • Site of concentration of urine
  • Medullary rays project into cortex

Collecting System

  • Renal papilla: Apex of each pyramid; 8–18 per kidney; site where collecting ducts open
  • Minor calyx: Cup-shaped; surrounds 1–3 papillae; 7–14 per kidney
  • Major calyx: Formed by union of 2–3 minor calyces; 2–3 per kidney (upper, middle, lower)
  • Renal pelvis: Funnel-shaped; formed by major calyces joining; capacity ~8 mL; narrows to ureter at PUJ (pelvi-ureteric junction)
  • PUJ (Pelvi-Ureteric Junction): At L1–L2 level; common site of obstruction; important surgically

Renal Columns (Columns of Bertin)

  • Extensions of cortical tissue that dip between medullary pyramids
  • Contain interlobar vessels (arcuate arteries/veins)
  • Important landmark in ultrasound and CT
  • Prominent column of Bertin: Can mimic a renal mass on imaging — important normal variant to recognize
Section E

The Nephron — Functional Unit

Overview

Each kidney has ~1 million nephrons. Nephron = structural and functional unit of kidney. Once lost, nephrons cannot regenerate (hence chronic kidney disease is progressive).

Cortical Nephrons (85%)

  • Glomerulus in outer cortex
  • Short loop of Henle (doesn't reach medulla)
  • Primary role: Filtration and reabsorption
  • Efferent arteriole → peritubular capillaries

Juxtamedullary Nephrons (15%)

  • Glomerulus near cortico-medullary junction
  • Long loop of Henle (deep into medulla)
  • Primary role: Urine concentration
  • Efferent arteriole → vasa recta

1. Renal Corpuscle

Filtration Unit

Glomerulus

  • Tuft of capillaries from afferent arteriole
  • High pressure capillaries (~60 mmHg)
  • Covered by visceral layer of Bowman's capsule (podocytes)
  • Filtration membrane: Endothelium + Basement membrane + Podocytes

Bowman's Capsule

  • Double-walled cup surrounding glomerulus
  • Visceral layer: Podocytes (with foot processes/pedicels)
  • Parietal layer: Simple squamous epithelium
  • Bowman's space: Filtrate collects here → enters PCT

Glomerulonephritis: Inflammation of glomeruli → protein in urine (proteinuria), haematuria. Causes: IgA nephropathy (most common), post-streptococcal, lupus nephritis.

2. Proximal Convoluted Tubule (PCT)

Major Reabsorber

Structure

  • Longest part of nephron (~15 mm)
  • Located in renal cortex
  • Cuboidal cells with abundant microvilli (brush border)
  • Rich in mitochondria (active transport)

Functions

  • Reabsorbs 60–65% of filtered Na⁺, water
  • 100% reabsorption of glucose, amino acids
  • Reabsorbs HCO₃⁻ (bicarbonate)
  • Secretes H⁺, drugs, organic acids

Fanconi syndrome: Defect in PCT reabsorption → glucose, amino acids, phosphate lost in urine despite normal blood levels.

3. Loop of Henle

Concentrating Unit

Parts

  • Descending thin limb: Permeable to water; dips into medulla
  • Ascending thin limb: Permeable to Na⁺/Cl⁻; impermeable to water
  • Ascending thick limb: Active Na⁺-K⁺-2Cl⁻ transport; impermeable to water

Countercurrent Mechanism

  • Creates medullary osmotic gradient (300–1200 mOsm)
  • Enables concentration of urine up to 4x plasma
  • Vasa recta maintain this gradient

Loop diuretics (Furosemide) act here — block Na⁺-K⁺-2Cl⁻ cotransporter in thick ascending limb → powerful diuresis.

4. Distal Convoluted Tubule (DCT)

Fine Tuning

Structure

  • Shorter than PCT; located in cortex
  • Begins at macula densa (juxtaglomerular apparatus)
  • No brush border (unlike PCT)

Functions

  • Aldosterone acts here → Na⁺ reabsorption
  • PTH acts here → Ca²⁺ reabsorption
  • K⁺ secretion (regulated by aldosterone)
  • H⁺ secretion (acid-base regulation)

Thiazide diuretics act on DCT — block Na⁺-Cl⁻ cotransporter. Used in hypertension.

5. Collecting Duct

ADH Target

Structure

  • Not technically part of nephron (collecting system)
  • Receives urine from several DCTs
  • Passes through medulla → opens at papilla
  • Two cell types: Principal cells, Intercalated cells

Functions

  • ADH (Vasopressin): Acts on principal cells → aquaporin-2 insertion → water reabsorption → concentrated urine
  • No ADH: Impermeable to water → dilute urine
  • Final regulation of urine concentration

Diabetes Insipidus: ADH deficiency (central) or resistance (nephrogenic) → collecting duct impermeable → huge volume of dilute urine (polyuria).

6. Juxtaglomerular Apparatus (JGA)

BP Sensor

Components

  • Macula densa: Specialized DCT cells at afferent arteriole contact; senses NaCl in filtrate
  • JG cells (granular cells): Modified smooth muscle of afferent arteriole; secrete Renin
  • Extraglomerular mesangial cells (Lacis cells): Between macula densa and glomerulus; signaling role

RAAS Activation

  • Low BP/Na⁺ → JG cells release Renin
  • Renin → Angiotensinogen → Angiotensin I → (ACE) → Angiotensin II
  • Ang II → Vasoconstriction + Aldosterone release → ↑BP

ACE inhibitors (Ramipril, Enalapril) block RAAS → used in hypertension, heart failure, diabetic nephropathy.

Section F

Blood Supply

Arterial Supply

  • Renal artery: Branch of abdominal aorta at L1–L2 level; wide caliber for high flow
  • Right renal artery: Longer (crosses behind IVC); passes posterior to right renal vein, head of pancreas and 2nd part of duodenum
  • Left renal artery: Shorter; passes posterior to left renal vein, body of pancreas and splenic vein
  • Segmental arteries (5): Renal artery divides into 5 end arteries — superior, antero-superior, antero-inferior, inferior, posterior
  • End arteries: No anastomoses — blockage = infarction of that segment
  • Interlobar arteries: Run in columns of Bertin between pyramids
  • Arcuate arteries: Run along cortico-medullary junction
  • Interlobular arteries: Ascend into cortex; give off afferent arterioles to glomeruli

Venous Drainage

  • Renal veins: Drain into IVC; lie anterior to corresponding renal arteries
  • Left renal vein: Longer (3x longer than right); crosses anterior to aorta, posterior to superior mesenteric artery (SMA); receives left gonadal, left suprarenal, and left inferior phrenic veins
  • Right renal vein: Short; drains directly into IVC; receives no tributaries
  • Nutcracker syndrome: Left renal vein compressed between aorta and SMA → left flank pain, haematuria

Important: Left renal vein is more at risk during abdominal aortic aneurysm repair — it can be inadvertently ligated.

Intrarenal Circulation Summary

Flow sequence: Renal artery → Segmental arteries → Interlobar arteries → Arcuate arteries → Interlobular arteries → Afferent arteriole → Glomerulus → Efferent arteriole → Peritubular capillaries / Vasa recta → Interlobular veins → Arcuate veins → Interlobar veins → Renal vein → IVC

Two capillary beds in series is unique to kidneys — allows independent regulation of filtration pressure (glomerular) and reabsorption (peritubular).

Section G

Nerve Supply & Lymphatics

Nerve Supply

  • Sympathetic: T10–L1 via renal plexus (from celiac and aorticorenal ganglia); vasomotor (controls blood flow); inhibits renin secretion at high activity
  • Parasympathetic: Vagus nerve (minor role); not well established
  • Afferent (pain): T10–L1 dermatomes; referred pain to loin, groin, scrotum/labia (ureteric colic)
  • Renal pain: Loin (flank) pain; dull ache (capsule distension) or colicky pain (ureteric spasm)

Lymphatic Drainage

  • Drain to: Lateral aortic (para-aortic/lumbar) lymph nodes at L1–L2
  • Follow renal vessels
  • Important in spread of renal cell carcinoma
Section H

Ureter

Overview

  • Length: ~25 cm (10 cm abdominal + 15 cm pelvic)
  • Diameter: ~3 mm; muscular tube with peristaltic movement
  • Course: From renal pelvis (L1) → pelvi-ureteric junction → bladder
  • Retroperitoneal throughout its course

Three Sites of Narrowing (Stone Gets Stuck Here!)

1. Pelvi-Ureteric Junction (PUJ)

Where renal pelvis meets ureter. At L1–L2 level. Most common site of congenital obstruction.

2. Pelvic Brim

Where ureter crosses bifurcation of common iliac artery. At level of sacroiliac joint.

3. Vesico-Ureteric Junction (VUJ)

Where ureter enters bladder wall obliquely. Narrowest point. Most common site for ureteric stones to lodge.

Ureteric Colic: Severe, colicky, loin-to-groin pain radiating to scrotum/labia. Due to smooth muscle spasm from a stone. Treated with NSAIDs (first-line) + alpha-blockers (tamsulosin) to facilitate passage.

Important Crossings (Exam Favourite!)

  • Gonadal vessels: Cross anterior to ureter (both sides); important during gonadectomy
  • Bifurcation of common iliac artery: Ureter crosses here at pelvic brim
  • Vas deferens (male): Crosses anterior to ureter just before bladder — "water under the bridge"
  • Uterine artery (female): Crosses anterior to ureter at level of cervix — "water under the bridge"; risk during hysterectomy (ureter may be ligated accidentally!)

Surgical Risk: Uterine artery crosses ureter 2 cm lateral to cervix during hysterectomy. If ureter is accidentally ligated = post-op hydronephrosis/urinoma. Must be identified before ligation.

Section I

Functions of Kidney

Major Functions

1. Filtration & Excretion

Filters 180 L/day plasma; GFR ~125 mL/min; removes urea, creatinine, drugs, toxins.

2. Fluid Balance

Regulates body water volume; adjusts urine output from 0.5–20 L/day based on ADH levels.

3. Electrolyte Regulation

Na⁺, K⁺, Ca²⁺, Mg²⁺, PO₄³⁻ balance via aldosterone, PTH, and direct tubular mechanisms.

4. Acid-Base Balance

Excretes H⁺, reabsorbs HCO₃⁻; maintains blood pH 7.35–7.45. Kidney is slower but more powerful than lungs.

5. BP Regulation (RAAS)

Renin → Ang II → vasoconstriction + aldosterone. Long-term BP control via volume regulation.

6. Erythropoietin (EPO)

Peritubular cells produce EPO in response to hypoxia → stimulates RBC production in bone marrow. CKD → EPO deficiency → anaemia.

7. Vitamin D Activation

Kidney converts 25-hydroxyvitamin D → 1,25-dihydroxyvitamin D (calcitriol) via 1-α-hydroxylase. CKD → renal osteodystrophy.

8. Gluconeogenesis

Renal cortex can produce glucose (especially during fasting/prolonged exercise). About 20% of total gluconeogenesis.

Section J

Clinical Anatomy

Renal Cell Carcinoma (RCC)

Most common renal malignancy (85%). Clear cell type most frequent. Classic triad: haematuria, loin pain, flank mass. RCC invades renal vein → IVC → right atrium ("tumour thrombus").

Hydronephrosis

Dilation of renal pelvis and calyces due to obstruction. Causes: PUJ obstruction, stones, tumour, stricture. Long-standing → cortical thinning → renal failure.

Polycystic Kidney Disease (ADPKD)

Autosomal dominant; PKD1 (chromosome 16) most common. Bilateral cysts. Associated with: berry aneurysms (subarachnoid haemorrhage), mitral valve prolapse, liver cysts. Leads to CKD.

Horseshoe Kidney

Fusion of lower poles during fetal development. Most common renal fusion anomaly. Ascent blocked by inferior mesenteric artery at L3. Increased risk: PUJ obstruction, stones, Wilms tumour.

Renal Biopsy

Lower pole of kidney preferred (avoids hilum). Posterior approach in prone position. Real-time ultrasound guidance. Complication: Haematuria (most common), AVF, perirenal haematoma.

Nephrotic vs Nephritic Syndrome

Nephrotic: Massive proteinuria (>3.5g/day), oedema, hypoalbuminaemia, hyperlipidaemia. Nephritic: Haematuria, oliguria, hypertension, mild proteinuria, RBC casts.

Acute Kidney Injury (AKI)

Pre-renal (most common — dehydration, shock), intrinsic (ATN, glomerulonephritis), post-renal (obstruction). Managed by treating cause + fluids. Dialysis if severe.

Renal Transplant Anatomy

Placed in right iliac fossa (extraperitoneal). Renal artery anastomosed to external iliac artery; renal vein to external iliac vein; ureter to bladder. Right iliac fossa preferred (shorter right external iliac vein).

Section K

NEET / Exam High-Yield Points

Must-Know Facts

Position & Levels

  • Kidney = retroperitoneal
  • Right lower than left (liver above)
  • Hilum at L1 = transpyloric plane
  • Left kidney: T11–L2; Right: T12–L3

Hilum Contents — "VAD"

  • Vein — most anterior
  • Artery — middle
  • Duct (ureter) — most posterior
  • Plus lymphatics and nerves

Three Narrowings of Ureter

  • PUJ (pelvi-ureteric junction)
  • Pelvic brim (common iliac crossing)
  • VUJ (vesico-ureteric junction) — narrowest

Nephron Diuretic Sites

  • Loop diuretics → Thick ascending limb
  • Thiazides → DCT
  • Aldosterone antagonists → Collecting duct
  • Osmotic diuretics → PCT

Coverings Mnemonic

  • "FCG" inside-out: Fibrous capsule, perinephric Fat (adipose), Gerota's fascia
  • Gerota's fascia open inferiorly (pus tracks down!)

Special Kidney Facts

  • Kidney gets 25% of cardiac output
  • Filters 180 L/day; excretes ~1.5 L urine
  • ~1 million nephrons per kidney
  • Renal artery = end arteries → infarction if blocked

Common NEET Questions

QuestionAnswer
Which kidney is lower and why?Right kidney — pushed down by liver
Contents of renal hilum (order)?Vein (anterior), Artery, Ureter (posterior) — VAD
Functional unit of kidneyNephron (~1 million per kidney)
Site where ADH acts?Collecting duct (principal cells)
Site where aldosterone acts?DCT and collecting duct
Narrowest part of ureter?Vesico-ureteric junction (VUJ)
Uterine artery and ureter relation?Uterine artery crosses anterior to ureter ("water under bridge")
Which nephrons concentrate urine?Juxtamedullary nephrons (long loop of Henle)
Renal biopsy site?Lower pole of right kidney (posterior approach)
Horseshoe kidney: fusion blocked by?Inferior mesenteric artery (at L3)
CKD anaemia cause?Decreased EPO production by peritubular cells
Gerota's fascia: where is it open?Inferiorly — pus/infection can track down