The kidneys are bean-shaped organs weighing anywhere from 150 to 200 g in males and about 120 to 135 g in females. The dimensions are usually a length of 10 to 12 cm, a width of 5 to 7 cm, and the thickness varies from 3 to 5 cm. Each kidney is about the size of a closed fist. They are located retroperitoneally in the posterior abdominal wall and are found between the transverse processes of T12 and L3. Both of the upper poles are usually oriented slightly medially, and posteriorly to the lower poles. If the upper renal poles are oriented laterally, this could suggest a horseshoe kidney or a superior pole renal mass. Further, the left kidney is usually slightly more superior in position than the right kidney.
The following are the kidneys in relation to surrounding organs:
Posteriorly, the diaphragm rests over the upper third of each kidney with the 12th rib frequently passing over (anteriorly) the upper pole. The kidneys usually sit located over the medial aspect of the psoas muscle and the lateral aspect of the quadratus lumborum. The proximal ureters will typically pass over the psoas muscle on their way to the bony pelvis.
At the medial margin of each kidney lies the renal hilum, where the renal artery enters, and the renal pelvis and vein leave the renal sinus. The renal vein is found anterior to the renal artery, which is anterior to the renal pelvis. The renal pelvis is the flattened, superior end of the ureter. It receives 2 or 3 major calyces, each of which divides into 2 or 3 minor calyces. The minor calyces are indented by the renal papillae, which are the apices of the renal pyramids.
Each kidney has a capsule and is surrounded by fat, Gerota's fascia, and more fat. The entire area immediately involving the kidneys is considered the retroperitoneum.
The kidneys perform several important functions including excretion of waste products such as ammonia and urea, electrolyte regulation, and acid-base balance. They play a vital role in the control of blood pressure and the maintenance of intravascular volume via the renin-angiotensin-aldosterone system. They are responsible for the reabsorption of amino acids, electrolytes, calcium, phosphate, water, and glucose, as well as the secretion of the hormones calcitriol and erythropoietin.
The mammalian kidney develops from the intermediate mesoderm. Kidney development (nephrogenesis) develops in three successive phases the pronephros, mesonephros, and metanephros. The metanephros are primordia of the permanent kidney.
About 25% of the total cardiac output goes to the kidneys. These highly vascular organs are supplied via the renal arteries, which enter the hilum of the kidney at L2. The longer right renal artery passes posterior to the inferior vena cava (IVC). Both of the renal arteries divide close to the renal hilum giving off five segmental arteries. The first branch is the posterior segmental artery, and it supplies the posterior segment of the kidney. The remaining 4 main segmental arteries all arise from the anterior branch of the renal artery and are named according to the segment of kidney they supply: the superior segmental artery, the anterosuperior segmental artery, the anteroinferior segmental artery, and the inferior segmental artery.
The renal veins follow the path of the arteries. Of note is that the left renal vein is a few centimeters longer than the right vein as it has to cross the midline from the left side to reach the inferior vena cava. Thus for transplants, the left kidney is usually selected as a graft as it has a longer length vein. In most cases, the left gonadal vein drains into the left renal vein inferiorly. The left renal vein passes just below the origin of the superior mesenteric artery to reach the inferior vena cava. Conversely, on the right side, the gonadal and renal veins separately drain into the inferior vena cava. Finally, the kidneys are drained by lymphatics which enter the aortic lymph nodes on the left and the right lateral inferior caval lymph nodes on the right.
Innervation to the kidneys is communicated via the renal nerve plexus consisting of sympathetic and parasympathetic fibers. This plexus is usually supplied by fibers from the least splanchnic nerve. Visceral afferent fibers in a retrograde fashion follow the sympathetic fibers to the spinal ganglia and cord segments T10 to T11. Pain is often sensed in the corresponding dermatome; thus, flank pain can be referred pain from the corresponding kidney. Renal pain is typically caused by obstruction and dilation, inflammation (infection) or ischemia. Non-obstructing stones in the kidney are generally not the cause of any renal pain or colic.
All of the arteries in the kidneys are end arteries which means there are no renal arterial collaterals. This also means it is necessary to protect any branches or accessory arteries to the kidneys to avoid loss of parenchymal function.
Brodel's line is the relatively avascular line between the renal anterior and posterior end arteries. It runs longitudinally from superior to inferior and is located just posterior to the lateral convex border of each kidney. Its importance is that this is the ideal location to place a nephrostomy or to perform a renal incision to minimize blood loss.
The retroperitoneum is an enclosed space separate from the peritoneal cavity. When the kidney is injured or traumatized, it is usually wise not to open the retroperitoneum as this will cause a loss of the tamponade effect and may ultimately result in the surgical removal of a kidney that could otherwise have been saved.
The retroperitoneal space is located between the posterior parietal peritoneum medially and the transversalis fascia laterally. It is sometimes divided into 3 spaces (the anterior pararenal space, the perirenal space, and the posterior pararenal space) by the perirenal fascia.
Kidney disease is defined as the presence of an abnormal process or function in the kidney(s).
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