Kidney

From ArticleWorld

The kidney is one of a pair of organs, each the size of a human fist, that sit in the back of the midportion of the abdominal area. The kidneys filter waste, especially urea, metabolites, salts and water from the blood in order to make urine. Technically the kidneys are not part of the abdominal cavity but are “retroperitoneal”, lying behind the lining of the abdominal cavity.

Anatomy

Each kidney is a bean-shaped organ weighing only 150 grams. They lie behind the abdominal contents at the level of the T12 to L3 vertebrae. The right kidney is slightly lower than the left due to the size of the liver in the right upper abdomen. A capsule covers the internal aspect of the kidneys. Beneath that is the renal cortex or fleshy part of the kidney and the medulla that contains approximately 10-20 pyramids. The pyramids are projections out into separate calyces that ultimately empty urine out into a medial funnel-shaped tube that marks the end of the kidney.

The kidneys are supplied by two large renal arteries that enter the kidney just above the renal pelvis. Two renal veins exit the kidneys and dump blood into the inferior vena cava.

The microstructure of the kidneys is complex. The nephron is a single filtering unit of the kidney. There are over one million nephrons per kidney. Nephrons filter blood via capillaries that surround the glomerulus, a ball-shaped filtering system.

The pre-urine filtered by the glomerulus travels through a complex system of tubules where excess water, salts and nutrients are reabsorbed. The fluid finally enters the collecting duct system of the kidneys, where it is called urine. The pyramids dump the urine into the larger collecting system of the kidney.

Function

The kidneys have multiple complex functions. A properly running kidney both filters water into the pre-urine and, under the influence of antidiuretic hormone (ADH), reabsorbs some of the water that the body still needs. The acidity or alkalinity of the blood, the mineral content of the body and the salts in the body are all regulated by the kidneys.

The kidneys are responsible for ridding the body of uric acid and urea, both byproducts of metabolism. Other products of metabolism and toxins are carried out of the blood and into the urine.

The kidneys secrete several hormones as well. Erythropoietin is secreted by the kidneys and is a stimulant for red blood cell production. Renin is another hormone the kidney secretes.

Conditions

The kidneys can be malformed or fail to function from birth. Depending on an individual’s water intake, diet and heredity, kidney stones can precipitate in the kidneys. A kidney infection is called pyelonephritis. Multiple inflammatory conditions can cause kidney dysfunction. Parts of the filtering system can fail, resulting in a condition known as nephrotic syndrome. The kidney can also become cancerous. Azotemia is a condition resulting from kidney failure and a build-up of nitrogen products in the blood.