The kidneys are bilateral bean-shaped organs, located in the posterior abdomen. They are reddish-brown in colour.
The main function of the kidneys 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 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. This can be used to gauge any changes in size when interpreting radiographs.
The adrenal glands sit immediately superiorly to the kidneys within 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 – Encloses the kidneys and the suprarenal glands.
- Pararenal fat – Mainly located on the posterolateral aspect of the kidney.
In addition to their external coverings, the kidneys have an intricate inner structure. Internally, 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 – 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.
Each renal artery enters the kidney via the renal hilum, dividing into segmental branches. These branches 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 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.
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.
Lymph from the kidney drains into the lateral aortic nodes, which are located at the origin of the renal arteries.
Clinical Relevance: Congenital Abnormalities of the Kidneys
The kidneys are the most common site of congenital abnormalities. They are of varying severity, ranging from asymptomatic to life-threatening. Here, we shall look at a few congenital defects that can occur:
Embyrologically, the kidneys develop in the pelvis, and ascend into the abdomen. Occasionally, one of the kidneys can fail to ascend, and remains in the pelvis, 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 from the pelvis to the abdomen – they become fused and consequently ‘stuck’ underneath the inferior mesenteric artery.
This type of kidney is still drained by two ureters, and is usually asymptomatic, although it can be prone to obstruction.
Renal dysgenesis is used to describe any underdevelopment of the kidneys. There are two main forms:
- Renal agenesis – complete failure of one or both kidneys to develop. It is most commonly unilateral.
- Renal hypoplasia – the kidneys develop with a normal architecture, but are of a smaller size.