The kidneys are a pair of bilateral bean-shaped organs located in the posterior abdomen. Their reddish-brown colouring gives them an unmistakable appearance and helps to identify their location. Their main function is to filter and excrete waste products from the blood. The kidneys are also responsible for water and electrolyte balance in the body. Metabolic waste and excess electrolytes are removed from the body by the kidneys to create urine.
In this article, we shall examine the anatomy of the kidneys, including their anatomical position, internal structure, and vasculature. We shall explore how these features are put together to form a complex organ that is vital for life.
The kidneys are situated retroperitoneally in the abdomen, both sides of the vertebral column. Generally, they are located from T12 to L3, however the right kidney may be slightly lower due to the presence of the liver. Each kidney is approximately three vertebrae in length, and the adrenal glands are located immediately superior to the kidneys, within a separate envelope of renal fascia.
The kidneys have a complex structure which includes multiple layers of fascia and fat. The arrangement of these layers, from deep to superficial, is the renal capsule, the perirenal fat, the renal fascia (also known as Gerota’s fascia or perirenal fascia), and the pararenal fat. The pararenal fat is mainly found on the posterolateral aspect of the kidney.
The internal structure of the kidneys is also intricate and unique. 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 which are known as renal pyramids. The apex of each renal pyramid is known as the renal papilla, which is associated with a structure called the minor calyx. Several minor calices join together to form a major calyx, and urine passes through this into the renal pelvis. From the renal pelvis, urine travels along the ureter before entering the bladder for storage.
Located on the medial margin of each kidney is a deep fissure known as the renal hilum. This is a gateway for the entry and exit of the renal vessels and the ureter.
The kidneys are supplied with blood via the renal arteries. These arise directly from the abdominal aorta, immediately distal to the origin of the superior mesenteric artery. The right renal artery is generally longer due to the positioning of the abdominal aorta slightly to the left of the midline. The renal artery enters the kidney via the renal hilum and divides into two vessels - the anterior division, which carries 75% of the blood supply, and the posterior division, which carries 25%. Further divisions of these two vessels will form five segmental arteries, each of which supplies a portion of the kidney.
The avascular plane of the kidney (or line of Brodel) is a line along the lateral and slightly posterior border of the kidney which delineates the segments supplied by the anterior and posterior divisions. This line is significant for both open and endoscopic surgical access of the kidney, as damage to major arterial branches can be minimized. It is important to note that the renal artery branches are anatomical end arteries, which means that a 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 artery undergo further divisions to supply the renal parenchyma. The segmental arteries divide into interlobar arteries, which are situated on either side of every renal pyramid. These interlobar arteries then divide further to form arcuate arteries. The arcuate arteries are at a 90-degree angle to the interlobular arteries, which are the next division of the vessels. The interlobular arteries then pass through the cortex, dividing one more time to form afferent arterioles. At this point, the afferent arterioles form a capillary network known as the glomerulus, where filtration takes place. The efferent arterioles, which are the next part of the network, are formed by the capillaries which come together. In the outer two-thirds of the renal cortex, the efferent arterioles form a structure known as the peritubular network, which supplies the nephron tubules with oxygen and nutrients. Meanwhile, 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 via the left and right renal veins.
The kidneys are bilateral bean-shaped organs, reddish-brown in colour and located in the posterior abdomen. They perform the crucial functions of filtering and excreting metabolic waste and excess electrolytes - forming urine - and maintaining water and electrolyte balances in the body. Urine is transported from the kidneys to the bladder by the ureters and then leaves the body via the urethra. In this article, we shall look at the anatomy of the kidneys – their anatomical position, internal structure, vasculature and clinical relevance.
The kidneys typically extend from T12 to L3, with the right kidney often situated slightly lower due to the presence of the liver. Each kidney is about three vertebrae in length and lies retroperitoneally (behind the peritoneum) in the abdomen, either side of the vertebral column. 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, arranged from deep to superficial as follows: renal capsule, a tough fibrous capsule; perirenal fat, a collection of extraperitoneal fat; renal fascia (also known as Gerota’s fascia or perirenal fascia); and pararenal fat, mainly located on the posterolateral aspect of the kidney.
Internally, the kidneys possess 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 - 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, from which urine passes into the renal pelvis – a flattened and funnel-shaped structure. From here, urine drains into the ureter, which in turn 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 and exit through this structure. The renal hilum leaves the renal parenchyma anteriorly to the renal arteries and empties directly into the inferior vena cava. The left renal vein is typically longer than the right due to the position of the vena cava, which lies slightly to the right, 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 artery.
Congenital Abnormalities of the Kidneys
Renal Cell Carcinoma: The kidney is often the site of tumor development, and the most common type of tumour is renal cell carcinoma.
The kidneys are situated close to many other important abdominal structures, making them important to consider in clinical scenarios:
The kidneys have their own unique arterial supply, originating from the abdominal aorta directly below the superior mesenteric artery. Due to the abdominal aorta being slightly on the left side of the midline, the right renal artery is longer than the left, and crosses the vena cava posteriorly. The renal artery then comes into the kidney at its hilum, forming two divisions – an anterior and a posterior division. The anterior division carries 75% of the blood to the kidney, while the posterior division carries 25%. Further division of these main arteries leads to five segmental arteries.
It is also very important to be aware of the avascular plane of the kidney – the line of Brodel – which separates the two segments of the kidney that are supplied via the anterior and posterior divisions. This plane, located along the lateral and slightly posterior border of the kidney, is an important surgical access route for both open and endoscopic access, since it helps to minimize the risk of any damage to major arterial branches. It is also important to note that the renal artery branches are anatomical end arteries, meaning that there is no communication between vessels. If there is trauma or obstruction in one arterial branch, it will eventually cause ischaemia and necrosis of the renal parenchyma supplied by that vessel.
From the segmental branches, the arteries undergo further divisions to supply the renal parenchyma: each segmental artery divides to form the interlobar arteries, situated either side every renal pyramid; the interlobar arteries further divide to create arcuate arteries; at a 90 degree angle to the arcuate arteries, the interlobular arteries arise; the interlobular arteries then pass through the cortex, dividing just one more time to form afferent arterioles. The afferent arterioles then create a capillary network known as a glomerulus, which is where the actual filtration takes place. The capillaries then come together to create the efferent arterioles. The efferent arterioles form what is known as a peritubular network in the outer two-thirds of the renal cortex, supplying oxygen and nutrients to the nephron tubules. In the inner third of the cortex and medulla, the efferent arterioles are called vasa recta.
The renal veins are responsible for the drainage of venous blood from the kidneys. These veins leave the hilum anteriorly, and empty directly into the inferior vena cava. The left renal vein is longer due to the vena cava being on the right side of the midline, and travels anteriorly to the abdominal aorta below the origin of the superior mesenteric artery. The right renal artery, meanwhile, lies posterior to the inferior vena cava.
The lymph from the kidney drains into the lateral aortic (or para-aortic) lymph nodes, located at the origin of the renal arteries.
The kidney is a common site of tumor development, such as renal cell carcinoma. Due to the segmental vascular supply of the kidney, it is possible to ligate the relative arteries and veins and remove the tumour with a safety buffer of healthy surrounding parenchyma (partial nephrectomy) without having to remove the entire kidney or compromising its total vascular supply. This is because the damage is done to only one specific branch, thus preserving the other arteries and veins.
