KIDNEY

The kidneys are paired organs of the urinary system that have many homeostatic functions, including regulation of electrolytes, acid-base balance, and blood pressure; excretion of wastes such as urea and ammonium; reabsorption of glucose and amino acids; and production of hormones including vitamin D and erythropoietin.
Located behind the abdominal cavity in the retroperitoneum, the kidneys receive blood from the paired renal arteries, and drain into the paired renal veins. Each kidney excretes its waste product—urine—into a ureter, itself a paired structure that empties into the urinary bladder.The medical field that studies the kidneys and diseases of the kidney is called nephrology. The prefix nephro- meaning kidney is from the Ancient Greek word nephros (νεφρός); the adjective renal meaning related to the kidney is from Latin rēnēs, meaning kidneys.

In humans, the kidneys are located behind the abdominal cavity, in a space called the retroperitoneum. There are two, one on each side of the spine; they are approximately at the vertebral level T12 to L3. The right kidney sits just below the diaphragm and posterior to the liver, the left below the diaphragm and posterior to the spleen.

Above each kidney is an adrenal gland (also called the suprarenal gland). The asymmetry within the abdominal cavity caused by the liver typically results in the right kidney being slightly lower than the left, and left kidney being located slightly more medial than the right.] The upper (cranial) parts of the kidneys are partially protected by the eleventh and twelfth ribs, and each whole kidney and adrenal gland are surrounded by two layers of fat (the perirenal and pararenal fat) and the renal fascia. Each adult kidney weighs between 125 and 170 g in males and between 115 and 155 g in females. The left kidney is typically slightly larger than the right.

The kidneys are bean-shaped structures that have concave and convex surfaces. The concave surface, the renal hilum, is the point at which the renal artery enters the organ, and the renal vein and ureter leaves. The substance of the kidney is divided into two major structures: superficial is the renal cortex and deep is the renal medulla. They are positioned in the lower back and usually the left kidney is a bit higher than the right kidney. About 20% of the body's blood flows through the kidney thus giving it a dark red appearance.