The kidneys are bilateral retroperitoneal organs that can be found in the upper left and right abdominal quadrants. They are shaped like large beans, with a major convexity and a minor concavity. Their main function is to eliminate excess bodily fluid, salts and the byproducts of protein metabolism. A microscopical approach reveals between one and four million nephrons within the renal tissue, which are composed of a renal corpuscle, a proximal convoluted tubule, the thin and thick limbs of the loop of Henle, the distal convoluted tubule and the collecting ducts respectively. This article will discuss the surrounding anatomical structures and theoretical borders, the macro anatomical parts of the kidney, the adjacent renal appendages, its various supplies and common pathology.
The kidney is a posterior abdominal internal organ that can be found to start at the level of the thoracic vertebra number twelve and finish at the third lumbar vertebra. The right kidney is situated slightly lower than the left due to the size and placement of the right lobe of the liver, in comparison to the much smaller left lobe.
Anatomically, its medial margin is concave and this is known as the hilum. It is here that the renal artery enters via five different segments and the renal vein exits, slightly in front of the latter structure. Also the renal pelvis leaves the kidney from this point, behind all of the vessels. The term for the entrance leading into the inner renal tissue is known as the renal sinus.
The adjacent organs include the liver, duodenum and ascending colon to the anterior right and the stomach, spleen, pancreas, jejunum and ascending colon to the anterior left. The quadratus lumborum can be found inferiorly and posteriorly while the diaphragm can be seen superoposteriorly.
Parts of the Kidney
As the hilum of the kidney has been discussed previously, with the main structures that enter and exit there already mentioned, the other aspects of the kidney will now be noted.
Each kidney has a superior and inferior pole which is quite apparent since the convex outer margin leaves the concave inner margin to point medially. In this case one can tell almost immediately the difference between the left and the right kidney.
The cortex is the outer layer of the kidney, while the medulla is the inner area of the renal tissues that is divided into medullary pyramids and both inner and outer layers contains the various structures that make up the nephrons.
Finally, the renal sinus is the initial widening of the ureter, which is separated into three large calices. There calices further divide into smaller ones which each encircle a renal papilla or tip of a medullary pyramid.
The Renal Fascia and Adjacent Adipose Tissues
There are two kinds of fat that can be found in the periphery of the kidneys. They are known as the pararenal fat and the perirenal fat.
The pararenal fat is the most lateral adipose tissue which surrounds the perirenal fat and the renal fascia. It is mostly seen from the posterior aspect of the kidney and acts as a cushion for the organ as well as to hold it in place.
The perirenal fat is connected with the fat that exists in the renal sinus and is connected to it at the hilum. It surrounds the kidney and the suprarenal gland completely, with pretty much the same function as the pararenal fat.
The renal fascia is the divide between the renal tissue and the perirenal fat and encompasses both the kidney and the suprarenal gland before continuing into the fascia of the diaphragm. As with the adjacent renal fatty tissue, it maintains the placement of the kidneys.
Innervation, Blood Supply and Lymphatic Drainage
The lymphatic drainage of the renal area follows the renal veins and drains into the lumbar aortic lymph nodes.
The renal veins are positioned in front of the renal arteries and the left renal vein can be seen to cross the aorta, just behind the superior mesenteric artery.
The renal arteries are branches of the abdominal aorta that branch off at the level of the first and second lumbar vertebrae. The right artery is seen to run behind the inferior vena cava and the insertion of both left and right renal arteries into the kidneys occurs anteriorly to the renal pelvis. Just before the main renal artery enters the renal tissue, its splits into four anterior and one single posterior segmental artery.
Finally, the nervous supply of the kidneys occurs via the renal plexus, which contains parasympathetic fibers from the vagus nerve and sympathetic fibers from the throacic splanchnic nerve.
Kidney or renal stones are clinically known as urolithiasis and are categorized as a disorder which obstructs the urinary outflow. This can be a partial or complete obstruction depending on the size of the stone and its constituents. The composition of a stone can include calcium complexes, struvite, uric acid, cystine and other unknown substances and their primary cause is an increased urinary concentration. Renal stones may be present without symptoms, no matter how large, especially if they are found to be in the renal pelvis. On the other hand, smaller stones can be detected if they dislodge themselves and end up blocking the ureter. The most common symptom is pain that radiates from the flank to the groin and is known as ureteral colic. Treatment nowadays usually consists of medication or ultrasonic vibrations, but most kidney stones are small enough to painlessly pass out with the urine.