Structure
Each kidney has upper and lower poles, anterior and posterior surfaces, a convex lateral border and a concave medial border with a vertical slit, the hilum. The hilum opens into the renal sinus, a space inside the kidney containing the renal pelvis, calyces, vessels and fat.
Coverings
From inside out, each kidney is wrapped in:
- a thin fibrous capsule;
- perirenal (perinephric) fat;
- renal fascia (Gerota's fascia), which also encloses the suprarenal gland in a separate compartment;
- pararenal fat, outside the fascia and thickest behind.
Internal structure
A cut kidney shows an outer cortex and an inner medulla made of renal pyramids. The cortex dips between the pyramids as renal columns. The apex of each pyramid, the renal papilla, drains into a minor calyx. Minor calyces join to form two or three major calyces, which join to form the renal pelvis. The pelvis narrows into the ureter at the pelviureteric junction.
Order at the hilum
From front to back the structures are the renal vein, the renal artery and the renal pelvis. Branches of the artery commonly lie both in front of and behind the pelvis.
Relations
The two kidneys share their posterior relations but differ in front, because different organs lie on each side.
| Surface | Right kidney | Left kidney |
|---|---|---|
| Anterior | Suprarenal gland, liver (across the hepatorenal pouch), second part of duodenum, right colic flexure, small intestine | Suprarenal gland, stomach, spleen, tail of pancreas with splenic vessels, left colic flexure, jejunum |
| Posterior | Diaphragm, 12th rib, psoas major, quadratus lumborum, transversus abdominis aponeurosis | Diaphragm, 11th and 12th ribs, psoas major, quadratus lumborum, transversus abdominis aponeurosis |
The subcostal (T12), iliohypogastric and ilioinguinal (L1) nerves run down and laterally behind each kidney. Above the twelfth rib the diaphragm separates the upper pole from the costodiaphragmatic recess of the pleura.
The right kidney lies a little lower than the left because of the bulk of the liver above it. Both move down a few centimetres with deep inspiration.
Blood supply and innervation
Each kidney is supplied by a renal artery from the side of the aorta at about L1–L2, just below the origin of the superior mesenteric artery. The kidneys receive roughly a fifth of the resting cardiac output.
- The right renal artery is longer, because the aorta is left of the midline. It passes behind the inferior vena cava.
- Near the hilum each artery divides into anterior and posterior divisions and then into segmental arteries. These are end arteries, so blocking one infarcts a segment.
- Accessory renal arteries are common, arising from the aorta above or below the main artery and often entering a pole directly.
The left renal vein is long. It passes in front of the aorta, just below the superior mesenteric artery, to reach the inferior vena cava, and it receives the left suprarenal and left gonadal veins. The right renal vein is short and drains directly into the cava.
Lymph drains to the lateral aortic nodes around the origins of the renal arteries. Sympathetic fibres reach the kidney through the renal plexus around the artery, from the lesser, least and first lumbar splanchnic nerves. Renal and ureteric pain is felt in the loin and can radiate to the groin.
Clinical relevance
The position and vascular pattern of the kidney explain several classic clinical findings.
- Percutaneous access: nephrostomy is placed below the twelfth rib where possible, to avoid the pleura in the costodiaphragmatic recess.
- Nutcracker phenomenon: compression of the left renal vein between the aorta and superior mesenteric artery, causing haematuria or a left varicocele.
- Left varicocele: a new left-sided varicocele in an adult can signal a left renal tumour obstructing the renal vein, since the left gonadal vein drains into it.
- Horseshoe kidney: the fused lower poles are caught under the inferior mesenteric artery during ascent, so the kidney lies low.
- Simple cysts are common in older people and usually harmless, but a complex cystic lesion needs imaging to exclude carcinoma.
On the specimen
This station shows both kidneys in situ on the posterior abdominal wall with the aorta and its upper visceral branches exposed.
- Right or left kidney: on the specimen the right kidney is usually lower. Relations help if the viscera are still present: the duodenum and liver on the right, the pancreatic tail and spleen on the left.
- Coeliac trunk: a short, stubby vessel from the front of the aorta just below the diaphragm, dividing almost at once into three.
- Superior mesenteric artery: arises about a centimetre lower and runs down and forward into the root of the mesentery, crossing in front of the left renal vein.
- Right renal artery: runs horizontally to the right behind the inferior vena cava. At the hilum it lies behind the renal vein and in front of the pelvis.
- Abdominal aorta: the thick-walled vessel just left of the midline on the vertebral bodies.