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kidneys

The kidneys are paired retroperitoneal organs essential for filtration of blood, regulation of electrolytes, acid–base balance, and excretion of metabolic waste. They are located on either side of the vertebral column, typically extending from T12 to L3 vertebral levels, with the right kidney slightly lower due to the liver. Each kidney measures approximately 10–12 cm in length, 5–7 cm in width, and 3–4 cm in thickness.

Anatomically, each kidney has a convex lateral border and a concave medial border, where the renal hilum transmits the renal artery, renal vein, lymphatics, nerves, and ureter. Internally, the kidney is organized into:

  • Cortex: outer granular layer containing glomeruli and proximal/distal tubules.

  • Medulla: inner layer arranged into renal pyramids containing loops of Henle and collecting ducts.

  • Renal pelvis: funnel-shaped proximal ureter that collects urine from calyces.

The kidneys are surrounded by three protective layers: the renal capsule, perinephric fat, and renal fascia (Gerota’s fascia). They play a major role in urine formation, erythropoietin secretion, blood pressure regulation (via renin), and vitamin D metabolism.

Synonyms

  • Renes

  • Renal organs

Function

  • Filtration of blood to form urine

  • Regulation of fluid, electrolyte, and acid–base balance

  • Endocrine functions: secretion of erythropoietin, renin, and activation of vitamin D

  • Maintenance of systemic blood pressure and hematopoiesis support

Nerve Supply

  • Derived from the renal plexus, with sympathetic fibers from T10–L1 via the least splanchnic and lumbar splanchnic nerves

  • Parasympathetic supply from the vagus nerve (CN X)

  • Pain referred to flank, loin, or groin regions

Arterial Supply

  • Renal arteries (branches of abdominal aorta at L1–L2)

  • Segmental arteries → interlobar arteries → arcuate arteries → interlobular arteries → afferent arterioles

Venous Drainage

  • Renal veins drain into the inferior vena cava

  • Left renal vein crosses anterior to the aorta, receiving tributaries from the gonadal vein, left adrenal vein, and left inferior phrenic vein

MRI Appearance

T1-weighted images:

  • Cortex: slightly higher signal than medulla due to protein and vascular content

  • Medulla: lower signal intensity

  • Renal sinus fat: hyperintense, outlining collecting system

T2-weighted images:

  • Cortex: intermediate signal

  • Medulla: slightly hypointense compared to cortex

  • Collecting system/urine: bright hyperintense signal

STIR:

  • Suppresses sinus fat, highlighting renal parenchyma

  • Useful in detecting edema, inflammation, tumors, or trauma

T1 Fat-Saturated (Pre-contrast):

  • Renal parenchyma shows intermediate signal intensity

  • Helps in lesion detection against suppressed fat

T1 Fat-Saturated Post-Contrast (Gadolinium):

  • Strong homogeneous enhancement of cortex in arterial phase, with progressive medullary enhancement in venous phase

  • Detects tumors, cysts, scars, infarcts, or perfusion defects

MRI Non-Contrast 3D Imaging:

  • Provides high-resolution reconstructions of kidneys, renal arteries, and collecting systems

  • Useful for congenital anomalies, stones, and pre-surgical planning

CT Appearance

CT Pre-Contrast:

  • Kidney cortex and medulla distinguishable by density differences

  • Useful for detecting renal calculi, hemorrhage, and calcifications

CT Post-Contrast:

  • Dynamic phases:

    • Corticomedullary phase (25–40s): cortex enhances brightly, medulla hypodense

    • Nephrographic phase (80–120s): homogeneous parenchymal enhancement

    • Excretory phase (3–5 min): opacification of collecting system

  • Essential for tumor detection, trauma assessment, renal infarcts, and obstruction

MRI images

kidneys MRI axial image anatomy image -img-00000-00000

MRI images

kidneys MRI coronal image anatomy image -img-00000-00000

CT image

kidneys CT coronal image