Kidneys

The kidneys are paired retroperitoneal organs that lie at the level of the T12 to L3 vertebral bodies.

Gross anatomy

Location

The kidneys are located to either side of the vertebral column in the perirenal space of the retroperitoneum, within the posterior abdominal wall. The long axis of the kidney is parallel to the lateral border of the psoas muscle and it lies anterior to the quadratus lumborum muscle. Being parallel to the psoas muscle, the kidneys lie at an oblique angle, with its superior pole more medial and posterior than its inferior pole. Due to the right lobe of the liver, the right kidney usually lies slightly lower than the left kidney ¹⁶.

Size

In adults, the normal kidney is 10-14 cm long in males and 9-13 cm long in females, 3-5 cm wide, 3 cm in antero-posterior thickness and weighs 150-260 g. The left kidney is usually slightly larger than the right.

• normal kidney size in adults

• normal kidney size in children

Structure

The kidney is bean-shaped with a superior and an inferior pole, anterior and posterior surfaces, and lateral and medial borders. The midportion of the kidney is often called the midpole.  

The kidney has a fibrous capsule, which is surrounded by perirenal fat. The kidney itself can be divided into renal parenchyma, consisting of renal cortex and medulla, and the renal sinus containing renal pelvis, calyces, renal vessels, nerves, lymphatics and perirenal fat.

The renal parenchyma has two layers: cortex and medulla. The renal cortex lies peripherally under the capsule while the renal medulla consists of 10-14 renal pyramids, which are separated from each other by an inward extension of the renal cortex called renal columns.

Urine is produced in the renal lobes, which consists of the renal pyramid with the associated overlying renal cortex and adjacent renal columns. Each renal lobe drains at a papilla into a minor calyx, four or five of these unite to form a major calyx. Each kidney normally has two or three major calyces, which unite to form the renal pelvis.

The renal hilum is the entry to the renal sinus and lies vertically at the anteromedial aspect of the kidney. It contains the renal vessels and nerves, fat and the renal pelvis, which typically emerges posterior to the renal vessels, with the renal vein being anterior to the renal artery.

Function

• filter the blood to remove excess water, minerals, and waste products of protein metabolism, producing urine in the process

• are involved in

  • blood pressure regulation

  • regulation of body fluid volume, osmolality and pH

  • vitamin D and red blood cell (RBC) production

Tests of renal function include:

• estimated glomerular filtration rate (eGFR)

• electrolytes

• blood urea nitrogen (BUN) or urea

• creatinine levels and creatinine clearance

• cystatin C levels

Relations

• posterior 

  • diaphragm and associated parietal pleura (upper pole of kidney)

  • 11th rib (left kidney only), 12th rib (both kidneys)

  • medial and lateral arcuate ligaments

  • psoas major, quadratus lumborum, transversus abdominis (medial to lateral)

  • subcostal nerve, iliohypogastric nerve

• anterior 

  • ​right kidney

    • right lobe of the liver

    • hepatorenal recess (Morison's pouch)

    • D2 duodenum (medial margin of the kidney)

    • right colic flexure (just above inferior pole)

  • left kidney

    • stomach

    • omental bursa/lesser sac (upper pole)

    • spleen (lateral margin)

    • tail of the pancreas (hilum)

    • left colic flexure (lateral to lower pole)

• superior

  • adrenal glands

Arterial supply

• renal arteries (from the abdominal aorta)

Venous drainage

• renal veins (to inferior vena cava)

Innervation

• sympathetic innervation: renal and aorticorenal plexuses, which is derived from lesser and least splanchnic nerves

• parasympathetic innervation: vagus nerve via the celiac, renal and aorticorenal plexuses

Variant anatomy

See article: developmental anomalies of the kidney and ureter.

Radiographic features

Plain radiograph

Kidney length should not be less than three vertebral body lengths, and no more than four vertebral body lengths ¹⁰. 

Ultrasound

Antenatally, fetal kidneys show varying texture depending on gestational age. It is echogenic in the first trimester, with decreasing echogenicity as the pregnancy progresses. Corticomedullary differentiation can be appreciated after 15 weeks of gestation but clear demarcation between cortex and medulla can be seen at 20 weeks. Renal echogenicity decreases compared to liver and spleen after 17 weeks ¹².

Normal kidney appearance in adult ¹¹:

• cortex is less echogenic than the liver

• medullary pyramids are slightly less echogenic than the cortex

• cortex thickness equals/is more than 6 mm ¹⁴

• if the pyramids are difficult to differentiate, the parenchymal thickness can be measured instead and should be 15-20 mm ¹¹

• central renal sinus, consisting of the calyces, renal pelvis and fat, is more echogenic than the cortex

• renal pelvis may appear as a central slit of anechoic fluid at the hilum

• normal ureters are generally not well seen on ultrasound

CT

On unenhanced CT the renal pyramids can appear hyperdense ⁷ (see: white pyramid sign).

MRI

Renal cortex has slightly higher signal than medulla on T1-weighted images. On T2-weighted images, medulla has slightly higher signal than renal cortex ¹⁶.

Development

The collecting system arises from the ureteric bud, which arises from the mesonephric duct in the fourth week of gestation. The renal parenchyma arises from the metanephros, which appears in the fifth week, a derivative of the intermediate mesoderm ¹⁵.

The ureteric bud penetrates the metanephric mesoderm, which forms as a cup-shaped tissue cap. The ureteric bud dilates and subdivides to form twelve or so generations of tubules with the first generations fusing to form the renal pelvis, major and minor calyces, and renal pyramids with the later generations forming approximately a million renal tubules ¹⁵.

Under the regulation of complex signaling pathways, the ureteric bud incites the metanephric tissue to form small renal vesicles that eventually form primitive S-nephrons which are invaginated by endothelial cells from nearby angioblasts; before going on to form the definitive nephron ¹⁵. Glomerular filtration begins at 9th week of gestation. Nephron development is complete at birth except in premature infants. The kidney matures functionally after birth ¹⁵.

The kidney develops in the pelvis but assumes its normal cranial abdominal location in adults due to disproportionate growth of the body in the lumbar and sacral regions ^8-9.

Related pathology

A discrepancy of >2 cm between renal lengths should be considered abnormal ¹⁰ and may indicate an underlying disease. Common diseases affecting the kidneys include: 

• gamuts

  • delayed nephrogram

  • persistent nephrogram

  • striated nephrogram

  • spotted nephrogram

  • renal emphysema

• neoplastic

  • benign renal neoplasm

  • malignant

    • renal cell carcinoma

• metabolic

  • renal hemosiderosis

  • renal cortical nephrocalcinosis

• infections

  • renal abscess

  • renal antibioma

  • perinephric abscess

  • pyelonephritis

  • xanthogranulomatous pyelonephritis (XGP)

  • emphysematous pyelonephritis

• others

  • renal pseudotumor

  • renal replacement lipomatosis

  • renal stones

  • hydronephrosis