- 1 Anatomical Position
- 2 Kidney Structure
- 3 Anatomical Relations
- 4 Arterial Supply
- 5 Venous Drainage
- 6 Lymphatics
- 7 Clinical Relevance: Congenital Abnormalities of the Kidneys
- 8 Clinical Relevance – Renal Cell Carcinoma
The kidneys are bilateral bean-shaped organs, reddish-brown in colour and located in the posterior abdomen. Their main function is to filter and excrete waste products from the blood. They are also responsible for water and electrolyte balance in the body.
Metabolic waste and excess electrolytes are excreted by the kidneys to form urine. Urine is transported from the kidneys to the bladder by the ureters. It leaves the body via the urethra, which opens out into the perineum in the female and passes through the penis in the male.
In this article we shall look at the anatomy of the kidneys – their anatomical position, internal structure and vasculature.
The kidneys lie retroperitoneally (behind the peritoneum) in the abdomen, either side of the vertebral column.
They typically extend from T12 to L3, although the right kidney is often situated slightly lower due to the presence of the liver. Each kidney is approximately three vertebrae in length.
The adrenal glands sit immediately superior to the kidneys within a separate envelope of the renal fascia.
The kidneys are encased in complex layers of fascia and fat. They are arranged as follows (deep to superficial):
- Renal capsule – tough fibrous capsule.
- Perirenal fat – collection of extraperitoneal fat.
- Renal fascia (also known as Gerota’s fascia or perirenal fascia) – encloses the kidneys and the suprarenal glands.
- Pararenal fat – mainly located on the posterolateral aspect of the kidney.
Internally, the kidneys have an intricate and unique structure. The renal parenchyma can be divided into two main areas – the outer cortex and inner medulla. The cortex extends into the medulla, dividing it into triangular shapes – these are known as renal pyramids.
The apex of a renal pyramid is called a renal papilla. Each renal papilla is associated with a structure known as the minor calyx, which collects urine from the pyramids. Several minor calices merge to form a major calyx. Urine passes through the major calices into the renal pelvis, a flattened and funnel-shaped structure. From the renal pelvis, urine drains into the ureter, which transports it to the bladder for storage.
The medial margin of each kidney is marked by a deep fissure, known as the renal hilum. This acts as a gateway to the kidney – normally the renal vessels and ureter enter/exit the kidney via this structure.
The kidneys sit in close proximity to many other abdominal structures which are important to be aware of clinically:
The kidneys are supplied with blood via the renal arteries, which arise directly from the abdominal aorta, immediately distal to the origin of the superior mesenteric artery. Due to the anatomical position of the abdominal aorta (slightly to the left of the midline), the right renal artery is longer, and crosses the vena cava posteriorly.
The renal artery enters the kidney via the renal hilum. At the hilum level, the renal artery forms an anterior and a posterior division, which carry 75% and 25% of the blood supply to the kidney, respectively. Five segmental arteries originate from these two divisions.
The avascular plane of the kidney (line of Brodel) is an imaginary line along the lateral and slightly posterior border of the kidney, which delineates the segments of the kidney supplied by the anterior and posterior divisions. It is an important access route for both open and endoscopic surgical access of the kidney, as it minimises the risk of damage to major arterial branches.
Note: The renal artery branches are anatomical end arteries – there is no communication between vessels. This is of crucial importance; as trauma or obstruction in one arterial branch will eventually lead to ischaemia and necrosis of the renal parenchyma supplied by this vessel.
The segmental branches of the renal undergo further divisions to supply the renal parenchyma:
- Each segmental artery divides to form interlobar arteries. They are situated either side every renal pyramid.
- These interlobar arteries undergo further division to form the arcuate arteries.
- At 90 degrees to the arcuate arteries, the interlobular arteries arise.
- The interlobular arteries pass through the cortex, dividing one last time to form afferent arterioles.
- The afferent arterioles form a capillary network, the glomerulus, where filtration takes place. The capillaries come together to form the efferent arterioles.
In the outer two-thirds of the renal cortex, the efferent arterioles form what is a known as a peritubular network, supplying the nephron tubules with oxygen and nutrients. The inner third of the cortex and the medulla are supplied by long, straight arteries called vasa recta.
Clinical Relevance: Variation in Arterial Supply to the Kidney
The kidneys present a great variety in arterial supply; these variations may be explained by the ascending course of the kidney in the retroperitoneal space, from the original embryological site of formation (pelvis) to the final destination (lumbar area). During this course, the kidneys are supplied by consecutive branches of the iliac vessels and the aorta.
Usually the lower branches become atrophic and vanish while new, higher ones supply the kidney during its ascent. Accessory arteries are common (in about 25% of patients). An accessory artery is any supernumerary artery that reaches the kidney. If a supernumerary artery does not enter the kidney through the hilum, it is called aberrant.
The kidneys are drained of venous blood by the left and right renal veins. They leave the renal hilum anteriorly to the renal arteries, and empty directly into the inferior vena cava.
As the vena cava lies slightly to the right, the left renal vein is longer, and travels anteriorly to the abdominal aorta below the origin of the superior mesenteric artery. The right renal artery lies posterior to the inferior vena cava.
Lymph from the kidney drains into the lateral aortic (or para-aortic) lymph nodes, which are located at the origin of the renal arteries.
Clinical Relevance: Congenital Abnormalities of the Kidneys
In utero, the kidneys develop in the pelvic region and ascend to the lumbar retroperitoneal area. Occasionally, one of the kidneys can fail to ascend and remains in the pelvis – usually at the level of the common iliac artery.
A horseshoe kidney (also known as a cake kidney or fused kidney) is where the two developing kidneys fuse into a single horseshoe-shaped structure.
This occurs if the kidneys become too close together during their ascent and rotation from the pelvis to the abdomen – they become fused at their lower poles (the isthmus) and consequently become ‘stuck’ underneath the inferior mesenteric artery.
This type of kidney is still drained by two ureters (although the pelvices and ureters remain anteriorly due to incomplete rotation) and is usually asymptomatic, although it can be prone to obstruction.
Clinical Relevance – Renal Cell Carcinoma
The kidney is often the site of tumor development, most commonly renal cell carcinoma.
Due to the segmental vascular supply of the kidney it is often feasible to ligate the relative arteries and veins and remove the tumour with a safe zone of healthy surrounding parenchyma (partial nephrectomy) without removing the entire kidney or compromising its total vascular supply by ischaemia.