Content Reviewers:Viviana Popa, MD, Scott Caterine, BSc (Hons.), MSc, MB, BCh, BAO (Hons.)
Looking to learn about the kidneys and their associated organs? Well, “urine” the right place! The kidneys are a pair of bean-shaped abdominal organs, which filter metabolic waste out of the blood in the form of urine.
The urine then gets funneled into the urinary bladder by the ureters. Sitting on top of the kidneys, there are the suprarenal glands or adrenal glands.
These glands function as a part of the endocrine system by secreting adrenal hormones, which regulate various functions of the body.
Let's begin with the kidneys, which are retroperitoneal organs, meaning they lie posterior to the peritoneum. They lie on the posterior abdominal wall, at the level of the T12 to L3 vertebrae, on both the right and the left sides of the vertebral column.
The right kidney sits slightly lower than the left one, which helps make space to accommodate the large size of the liver located on that side of the abdomen.
Now, positioned superior to both kidneys are the suprarenal glands, but they also have a number of different relationships with surrounding organs.
Anterior to the left kidney is the spleen, stomach, pancreas, left colic flexure, and jejunum, while the liver, duodenum, right colic flexure and ascending colon lie anterior to the right kidney. The right kidney and liver are separated by the hepatorenal recess, also known as Morisons pouch.
Posteriorly, both kidneys are associated with the diaphragm, the psoas major, quadratus lumborum and transversus abdominis muscles as well as the subcostal nerve and vessels, and the iliohypogastric and ilioinguinal nerves.
The left kidney is also posteriorly associated with the 11th and 12th ribs, while the right is mainly associated with the 12th rib.
Each kidneys has a convex, lateral margin, and a concave, medial margin. On their concave margins, each kidney has a vertical cleft, called the renal hilum, through which renal vessels, nerves, and the ureters enter and exit each kidney.
At the renal hilum, the renal veins are positioned anteriorly and exit the hilum draining filtered blood away from the kidney.
Posterior to the renal veins are the renal arteries which supply blood to the kidney, and posterior to the renal arteries is the renal pelvis which is the superior portion of the ureters which go on to deliver urine to the bladder.
Now, the kidneys are surrounded by several layers of connective tissue and fat, which protect and cushion these delicate organs.
When looking at a transverse section of the abdomen, there is the paranephric fat, which lies superficial to the renal fascia.
And then deep the renal fascia, there is the perinephric fat. Finally, deep to the perinephric fat, is the renal capsule, which is a tough fibrous sheath that directly envelops the kidney.
Now, let’s take a look at the internal structure of a kidney in a coronal section. On the inside, each kidney consists of an outer renal cortex, and an inner renal medulla.
The functional units of the kidney are called nephrons and different portions of these microscopic nephrons are found in both the renal cortex and medulla.
Now, the renal cortex has extensions that project into the renal medulla, creating segments called renal columns. These columns divide the medulla into pyramidal-shaped sections known as renal pyramids.
The base of each renal pyramid is directed toward the cortex and the apex, called the renal papilla, is directed toward the renal hilum.
The renal papilla is where urine is excreted into a funnel-shaped structure called a minor calyx. Many minor calyces combine to form two or three major calyces, which in turn unite to form the renal pelvis which is the expansion of the superior end of the ureter. The apex of the renal pelvis is continuous with the ureter.
Ok, now remember that the main job of the kidneys is to filter the waste products out of our blood which are then eliminated through urine.
To do this, blood is delivered to the kidneys by the renal arteries which are direct branches of the abdominal aorta, emerging laterally at the level of the L1 to L2 vertebrae, just below the root of the superior mesenteric artery.
Now, because the abdominal aorta is located slightly to the left of the spine, the right renal artery is slightly longer than the left, crossing posteriorly to the inferior vena cava to reach the right kidney.
At the level of the hilum, each renal artery typically divides into a number of segmental arteries, each of which supplies a different renal segment.
Venous blood leaves the kidneys via the renal veins, which drain straight into the inferior vena cava. The right renal vein exclusively drains the right kidney, while the left renal vein receives additional tributaries such as the left inferior phrenic vein, left suprarenal vein, and the left gonadal vein.
As the left renal vein courses towards the inferior vena cava it passes anterior to the abdominal aorta and posterior to the superior mesenteric artery.
Now let’s hit the pause button, so we can have a quick quiz before we move on. From outside to inside, recall the layers of fat and fascia that surround the kidneys.
Ok, now, after urine is formed in the kidneys, it collects in the renal pelvis and leaves the kidneys through muscular tubes called the ureters, which carry the urine from the kidney to the urinary bladder.
The point where the ureters meet the renal pelvis is called the ureteropelvic junction. From there, the ureters then descend retroperitoneally in the abdomen anterior to the psoas major muscle.