Accessory Organs - Kidneys Flashcards
What is the function of the kidneys?
Filter and excrete waste products from the blood.
They are also responsible for water and electrolyte balance in the body.
Anatomical position?
The kidneys lie retroperitoneally in the abdomen, on either side of the vertebral column.
They extend from T12-L3, although the right kidney is often situated lower due to the presence of the liver. Each kidney is approximately 3 vertebraes in length (about 10cm).
The renal gland sits immediately superior to the kidney within a separate envelope of renal fascia.
Kidney structure - external?
The kidneys are encased in complex layers of fascia and fat. They are arranged as follows (deep to superficial):
1) Renal capsule - tough fibrous capsule
2) Peri-renal fat - collection of extraperitoneal fat
3) Renal fascia (also known as Gerota’s fascia or perirenal fascia) - encloses the kidneys and the suprarenal glands.
4) Pararenal fat - mainly located in the posterolateral aspect of the kidney.
Kidney structure - internal?
The renal parenchyma can be divided into two areas: the medulla and cortex. The cortex extends into the medulla, and divides into triangular shapes - these are known as renal pyramids.
The apex of renal pyramids is known as a renal papilla. Each renal papilla is associated with a small structure known as the minor-calyx, which collects urine from the pyramids. Some minor calices merge together to from a major calyx. From the major calyx, urine passes into the renal pelvis, and then into the ureter.
The medial margin of each kidney is marked by a deep fissure, known as the renal hilum. From here renal vessels and the ureter enter/exit.
Anatomical relations - left kidney?
Anterior:
- Suprarenal gland
- Spleen
- Stomach
- Pancreas
- Left colic flexure
- Jejunum
Posterior:
- Diaphragm
- 11th and 12th rib
- Psoas major, quadratus lumborum, and transversus abdominis
- Subcostal, iliohypogastric, ilioinguinal
Anatomical relations - right kidney?
Anterior:
- Suprarenal gland
- Duodenum
- Liver
- Right colic flexure
Posterior:
- Diaphragm
- 12th rib
- Psoas major, quadratus lumborum and transversus abdominis
- Subcostal, ilioinguinal, and iliohypogastric nerves.
Arterial supply?
Renal arteries (arise from the aorta), immediately distal to the origin of the superior mesenteric artery.
Because the aorta is postioned slightly to the left, the right renal artery is slightly longer than the left and crosses the vena cava posteriorly.
The renal artery enters the kidney via the 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 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 arcuates arteries.
- At 90 degrees to the arcuate arteries, the interlobar arteries arise.
- The interlobar 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.
The outer 2/3 of the renal cortex, the efferent arterioles form what is a known as a peritubular network, supplying the nephron tubles 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 that reaches the kidney. If a supernumerary artery does not enter the kidney through the hilum, it is called aberrant.
Venous drainage?
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 veins is slightly longer, and travels anteriorly to the abdominal aorta below the origin of the mesenteric artery. The right renal artery lies posterior to the inferior vena cava.
Lymphatics?`
Lymph from the kidneys 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
Pelvic kidney - 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.
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 the 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 tumour development, most commonly renal cell carcinoma.
Due to the segmental vascular supply of the kidney ut us 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.
