Kidneys and adrenal glands

Position and fascia

• Kidneys lie from about T12 to L3; the hilum is at L1 (transpyloric plane); the right kidney sits lower because of the liver. Adult length 10–12 cm.
• They lie on the sloping psoas, so the lower poles are more lateral than the upper poles and the hilum faces anteromedially. They move down 2–3 cm on deep inspiration.
• Renal (Gerota) fascia encloses kidney, adrenal and perirenal fat. It is closed above and laterally but open inferiorly, so perinephric pus or blood tracks down towards the iliac fossa and pelvis.
• Renal angle: below the 12th rib, lateral to erector spinae.

Relations

• The abdominal aorta lies to the left of the IVC on the vertebral bodies and divides at L4. An aortic aneurysm is commonest below the renal arteries (infrarenal); rupture gives a pulsatile, expansile mass above the umbilicus with back pain and shock, and the blood collects in the left retroperitoneum.

Hilum, vessels and internal structure

• Order at the hilum from front to back: vein, artery, pelvis.
• Right renal artery passes behind the IVC; left renal vein passes in front of the aorta, below the superior mesenteric artery, and receives the left gonadal and left adrenal veins.
• The renal artery divides into segmental arteries, which are end arteries, so blocking one gives a pale wedge infarct. Branches run interlobar (in the renal columns), then arcuate (arching over the bases of the pyramids at the corticomedullary junction), then interlobular (radially into the cortex) to the afferent arterioles.
• A relatively avascular plane lies just behind the lateral convex border between anterior and posterior segmental territories; percutaneous nephrostomy enters a posterior lower-pole calyx for this reason.
• Accessory renal arteries persist from the series of arteries used during ascent; one crossing the pelviureteric junction can cause PUJ obstruction (loin pain after a fluid load, dilated pelvis, normal ureter).

• The parts of the nephron most sensitive to ischaemia are the straight (S3) part of the proximal tubule and the thick ascending limb in the outer medulla: high oxygen demand in a region of low oxygen tension. This is acute tubular necrosis.

Normal imaging appearances and their physiology

• Ultrasound (adult): cortex less echogenic than liver, pyramids darker, sinus fat bright. Neonate: cortex as bright as liver with prominent dark pyramids is normal.
• Hypertrophied column of Bertin: cortex-like tissue projecting into the sinus with the same echogenicity as cortex (a pseudotumour). Fetal lobulation: smooth notches between pyramids; scars lie over calyces.
• CT phases: corticomedullary at 30–40 s (cortex receives most renal blood flow), nephrographic at about 100 s (uniform), excretory after several minutes.
• Obstructed kidney: the nephrogram appears late and grows progressively denser for hours. Tubular flow is slow, so more water is reabsorbed and the filtered contrast becomes concentrated in the tubules. Rupture of a calyceal fornix gives a separate sign, contrast leaking into the perirenal space.
• Rim sign: infarct with a preserved subcapsular rim supplied by capsular collaterals. Reverse rim: cortical necrosis (abruption with DIC): cortex dark, medulla and thin subcapsular rim enhancing.
• Page kidney: a subcapsular haematoma compresses the kidney, which becomes ischaemic and releases renin, causing hypertension. A perirenal haematoma outside the tough capsule rarely compresses enough to do this.

• Captopril renography: in a stenotic kidney, GFR is held up by angiotensin II constricting the efferent arteriole. An ACE inhibitor removes efferent tone, glomerular pressure falls and uptake and excretion on that side become delayed. The same mechanism causes acute kidney injury when ACE inhibitors are given in bilateral stenosis.

Development and variants

• Ureteric bud (from the mesonephric duct) forms ureter, pelvis, calyces and collecting ducts; metanephric mesoderm forms the nephrons. Nephrogenesis ends at 34–36 weeks. Ascent is apparent, from growth of the lumbar and sacral regions.
• Duplex kidney and ureter: caused by early splitting of the ureteric bud (bifid ureter) or two separate buds (complete duplication). In complete duplication the upper-moiety ureter opens lower and more medially (ectopic, often obstructed, with a ureterocele; in girls it may open below the sphincter or into the vagina, causing constant dribbling with otherwise normal voiding). The lower-moiety ureter refluxes (Weigert–Meyer rule).
• Horseshoe kidney: lower poles fused; ascent stopped by the inferior mesenteric artery. Pelvic kidney: supplied by the common iliac artery. Malrotation: hilum faces forwards.
• Retrocaval ureter: reversed-J swing of the upper right ureter at L3. Simple ureterocele: cobra-head with a lucent halo. Renal agenesis often accompanies a Müllerian anomaly on the same side.

Ureter

• 25–30 cm long; runs on psoas over the tips of the transverse processes, crossed in front by the gonadal vessels, and enters the pelvis in front of the common iliac bifurcation; in the pelvis it passes under the uterine artery about 1.5–2 cm lateral to the cervix.
• Narrowings: PUJ, pelvic brim, vesicoureteric junction (narrowest, commonest site for a stone to lodge). Obstruction raises pressure, dilates the system above and, if prolonged, causes pressure atrophy of the papillae and cortex (hydronephrosis).
• Colic is referred T11–L2, loin to groin, labium or scrotum (genitofemoral nerve).
• Blood supply runs longitudinally in the adventitia (from the medial side above the brim, lateral side in the pelvis); stripping it causes ischaemic stricture.

Renal masses and infections on imaging

Adrenal glands

• On CT: inverted Y or V shapes. Right adrenal lies directly behind the IVC, with liver lateral and the right crus medial; left lies anteromedial to the upper pole of the left kidney, behind pancreas and splenic vessels.
• Arteries: superior (inferior phrenic), middle (aorta), inferior (renal). Veins: right to the IVC directly (short, so tears easily at surgery), left to the left renal vein.
• Cortex from mesoderm (coelomic epithelium, beside the gonadal ridge, so rests occur in the broad ligament or testis); medulla from neural crest, supplied by preganglionic sympathetic fibres. The fetal zone is large and makes DHEA sulfate, which the placenta converts to oestriol; it involutes after birth.

• Lipid-rich adenoma: abundant intracellular lipid makes it under 10 HU unenhanced and causes signal loss on opposed-phase MRI (fat and water in the same voxel cancel). About a third of adenomas are lipid poor and need washout CT (adenomas wash out rapidly).
• Myelolipoma: macroscopic fat (strongly negative HU). Metastasis (lung commonest): over 10 HU, slow washout, often bilateral and heterogeneous; even in a cancer patient, a small nodule is still often an adenoma.
• Addison disease: autoimmune adrenalitis gives small, non-calcified glands; tuberculosis gives enlarged glands early and small, densely calcified glands late (common in Pakistan). Rifampicin speeds cortisol breakdown, so replacement doses must rise.

Remember Hilum front to back: vein, artery, pelvis. Upper-moiety ureter obstructs, lower-moiety ureter refluxes.

Exam trap The commonest site for a ureteric stone to lodge is the vesicoureteric junction, not the pelvic brim.

Exam trap Primary hyperaldosteronism has low renin; high renin with high aldosterone points to renal artery stenosis.

How FCPS asks about this

• Arteries arching over the bases of the pyramids: arcuate arteries, at the corticomedullary junction.
• Straight tubules running up into the cortex: medullary rays; cortex dipping between pyramids is a renal column.
• Embryological cause of a duplex kidney: split or double ureteric bud; upper moiety obstructs, lower refluxes.
• Adrenal adenoma under 10 HU with opposed-phase signal loss: high intracellular lipid.
• Conn adenoma zone: zona glomerulosa; renin suppressed, potassium low.
• Captopril renography change: loss of angiotensin II efferent arteriolar tone in the stenotic kidney.
• Hypertension after a subcapsular haematoma (Page kidney): renin from the compressed kidney.