are the most important excretory organ
. Medical terms related to the kidneys either involve renal
is the study of the kidneys.
The major functions of the kidney are to filter wastes from the bloodstream and to regulate the pH, serum osmolality and blood pressure (and secondarily, the volume of the blood). The wastes are concentrated to form urine which is passed on to the bladder via the ureters.
The kidney also serves an endocrine function by secreting erythropoietin, a hormone that regulates red blood cell production in the bone marrow. Other hormones produced by the kidneys are renin, active form of vitamin D - 1,25-dihydroxycholecalciferol (calcitriol) and prostaglandins.
In humans the kidneys are the two organs that are located in the posterior part of the abdomen, on either side of the spine just below the liver and spleen on the right and left sides of the body respectively. Superior to each kidney is an adrenal gland (also called the suprarenal gland).
- Kidneys viewed from behind with spine removed
The kidneys are retroperitoneal, which means they lie behind the peritoneum
that surounds most of the other abdominal organs. They are approximately at the vertebral level T12 to L3, and the right kidney usually lies slightly lower than the left, due to the size of the liver
The upper parts of the kidneys are protected somewhat by the eleventh and twelfth ribs, and each whole kidney is surrounded by two layers of fat, the perirenal fat and the pararenal fat, which help to cushion it.
In a normal human adult, each kidney is about 11 cm long and about 5 cm thick, weighing 150 grams
. The kidneys are 'bean
-shaped' organs, and have a concave side facing inwards (medially). On this medial aspect of each kidney is an opening, called the hilus, which admits the renal artery
, the renal vein
, nerves, and the ureter
A kidney is divided into a renal cortex
, medulla and pelvis.
The basic functional unit of the kidney is the nephron, of which there are more than a million in each normal adult kidney. Nephrons regulate water and soluble substances (especially ions) in the body by filtering it all out first, reabsorbing what should be kept and excreting the rest. They use countercurrent exchange mechanisms.
A nephron consists of a Bowman's capsule, a proximal convoluted tubule, a loop of Henle and a distal convoluted tubule which empties the urine into a collecting duct.
- renal capsule: The membranous covering of the kidney.
- cortex: The outer layer over the internal medulla. It contains blood vessels, glomeruli (which are the kidneys' "filters") and urine tubes and is supported by a fibrous matrix.
- hilus: The opening in the middle of the concave medial border for nerves and blood vessels to pass into the renal sinus.
- renal column: The structures which support the cortex. They consist of lines of blood vessels and urinary tubes and a fibrous material.
- renal sinus: The cavity which houses the renal pyramid.
- calyces: The recesses in the internal medulla which hold the pyramids. They are used to subdivide the sections of the kidney. (singular - calyx)
- papillae: The small conical projections along the wall of the renal sinus. They have openings through which urine passes into the calyces. (singular - papilla)
- pyramids: The conical segments within the internal medulla. They contain the secreting apparatus and tubules and are also called malpighian pyramids.
- renal artery: Two renal arteries come from the aorta, each connecting to a kidney. The artery divides into five branches, each of which leads to a ball of capillaries. The arteries supply (unfiltered) blood to the kidneys. The left kidney receives about 60% of the renal bloodflow.
- renal vein: The filtered blood returns to circulation through the renal veins which join into the inferior vena cava.
- renal pelvis: Basically just a funnel, the renal pelvis accepts the urine and channels it out of the hilus into the ureter.
- ureter: A narrow tube 40 cm long and 4 mm in diameter. Passing from the renal pelvis out of the hilus and down to the bladder. The ureter carries urine from the kidneys to the bladder.
Diseases and disorders
Congenital diseases of the kidneys
- Congenital hydronephrosis
- renal dysplasia
- Congenital obstruction of urinary tract
- horseshoe kidney
- duplicated ureter
Acquired diseases of the kidneys
- Kidney stones are a relatively common and particularly painful disorder.
- Pyelonephritis is infection of the kidneys and is frequently caused by complication of a urinary tract infection.
- Azotemia is a toxic condition characterized by abnormal and dangerously high levels of urea, creatinine, various body waste compounds, and other nitrogen-rich compounds in the blood.
- Hydronephrosis is the enlargement of one or both of the kidneys caused by obstruction of the flow of urine.
- In nephrotic syndrome, the glomerulus has been damaged so that a large amount of protein in the blood enters the urine. Other frequent features of the nephrotic syndrome include swelling, low serum albumin, and high cholesterol.
- kidney tumors
- Renal failure (acute and chronic)
- Diabetic nephropathy
- Lupus nephritis
A simple means of estimating renal function is to measure serum urea, creatinine and basic electrolytes (sodium and potassium). As the kidney is the most important organ in controlling these values, any derangement in these values would suggest renal impairment.
A more formal test of renal function would be to measure the glomerular filtration rate; usually a creatinine clearance test is performed.
Dialysis and kidney transplants
Generally, one can live fine with just one kidney. If both kidneys don't function properly, dialysis is performed, where the blood is filtered outside of the body. Kidney transplants are now also quite common. The first successful such transplant was announced on March 4, 1954 by Peter Bent Brigham Hospital in Boston.