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 on the posterior abdominal wall, with one on either side of the vertebral column, in the perirenal space. The long axis of the kidney is parallel to the lateral border of the psoas muscle, and lie on the quadratus lumborum muscles. In addition, the kidneys lie at an oblique angle, that is the superior pole is more medial and anterior than the inferior renal pole. Due to the right lobe of the liver, the right kidney usually lies slightly lower than the left kidney.

Structure

The kidney is bean-shaped with a superior and an inferior pole. The midportion of the kidney is often called the midpole. In adults, each kidney is normally 10-15 cm in length, 3-5 cm in width and weighs 150-260 g. The left kidney is usually slightly larger than the right.

The kidney has a fibrous capsule, which is surrounded by pararenal 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 extension of 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 calyxes, 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

• remove excess water, salts and wastes of protein metabolism from the blood

Arterial supply

• renal arteries (from the abdominal aorta)

Venous drainage

• renal veins (to inferior vena cava)

Innervation

• sympathetic and parasympathetic renal nerve plexus, which is derived from abdominopelvic splanchnic nerves

Relations

Right kidney

• anteriorly
  • superiorly: right adrenal gland, liver
  • inferiorly: hepatic flexure, small intestine
• medially: duodenum
• posteriorly: diaphragm, posterior abdominal wall

Left kidney

• anteriorly
  • superiorly: left adrenal gland, stomach, spleen
  • inferiorly: pancreas
• laterally: splenic flexure, descending colon
• medially: jejunum
• posteriorly: diaphragm, posterior abdominal wall

Variant anatomy

See article: developmental anomalies of the kidney and ureter.

Radiographic appearance

Plain radiograph

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

CT

• on unenhanced CT, the renal pyramids can appear hyperdense ⁷ (see: white pyramid sign)

Ultrasound

Antenatally, fetal kidneys show varying texture depending on the gestation age. It is echogenic in the first trimester and the echogenicity decreases 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
• cortex is more echogenic than the medulla
• pyramids (if seen) are slightly less echogenic than the cortex
• 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

Development

The collecting system arisearises 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.

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.

Under complex signalling pathways, the ureteric bud incites the metanephric tissue to form small renal vesicles that eventually form primitive S-nephrons that are invaginated by endothelial cells from nearby angioblasts and go on to form the definitive nephron.

The kidney develops in the pelvis but takes its normal caudal 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 disease affecting the kidneys include: 

• renal cell carcinoma
• renal angiomyolipoma
• renal stones
• hydronephrosis
• pyelonephritis