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Portrait of The kidney

Science · Medicine & the body

The kidney

Artificial version built 1943

Medicine — the renal system and dialysis

One of a pair of bean-shaped organs lying behind the abdominal cavity on either side of the spine, filtering the blood, balancing water and salts, controlling acidity and producing urine. Each contains about a million nephrons, microscopic filtration units that begin at a knot of capillaries in Bowman's capsule and run through the loop of Henle to a collecting duct. Between them the pair filter roughly 180 litres of fluid a day and return almost all of it, and they double as endocrine glands, secreting renin, erythropoietin and the active form of vitamin D. When they fail, dialysis or transplantation takes over.

How to spot them

  • ▸Bean-shaped, retroperitoneal, the right one lower because the liver sits above it
  • ▸About a million nephrons each; glomerulus, Bowman's capsule, loop of Henle, collecting duct
  • ▸Secretes renin, erythropoietin and calcitriol, so it is an endocrine organ too
  • ▸Willem Kolff's rotating drum of 1943 and Joseph Murray's twin transplant of 1954
  • ▸Receives about a fifth of the cardiac output for its size

Key facts

A pair of bean-shaped retroperitoneal organs, the right one sitting lower than the left

The liver pushes the right one down; they lie roughly between T12 and L3

About a million filtration units in each, beginning at Bowman's capsule

The nephron, the functional unit

The glomerulus, the loop of Henle and the collecting duct

Filtration, countercurrent concentration, and final water recovery

Willem Kolff's rotating drum machine, improvised in the occupied Netherlands in 1943 (1943)

The first workable artificial version of the organ; his first survivor came in 1945

The Herrick twins in Boston in 1954, and Joseph Murray's Nobel Prize of 1990 (1954)

The first successful transplant of the organ, between identical twins, so no rejection

It secretes renin, erythropoietin and the active form of vitamin D

Blood pressure, red cell production and calcium handling

Roughly 180 litres filtered a day, of which about 1.5 litres leaves the body

Over 99 per cent of the filtrate is reabsorbed

Belding Scribner's shunt of 1960, which made long-term treatment possible (1960)

Before it, the machine could only be used once or twice per patient

Key ideas

  • Filtration is indiscriminate and reabsorption is selective: the glomerulus pushes almost everything smaller than a protein out of the blood, and the tubule then works hard to take back the water, glucose and ions that were worth keeping.
  • The loop of Henle is a countercurrent multiplier — it builds a salt gradient deep in the medulla so that the collecting duct can draw water out of the filtrate and produce concentrated urine.
  • Dialysis is diffusion across a semipermeable membrane: wastes move down their concentration gradient into the dialysate, while fluid is removed by a pressure difference.

Quick-fire facts

  • •A pair of retroperitoneal organs about 11 cm long, lying roughly from T12 to L3, with the adrenal glands sitting on top
  • •Each contains around a million nephrons: renal corpuscle (glomerulus plus Bowman's capsule), proximal convoluted tubule, loop of Henle, distal convoluted tubule and collecting duct
  • •Structure from outside in: cortex, medulla with its renal pyramids, minor and major calyces, and the renal pelvis leading to the ureter
  • •The pair receive around 20–25 per cent of the cardiac output and filter about 180 litres a day, producing 1–2 litres of urine
  • •The juxtaglomerular apparatus secretes renin, triggering the renin–angiotensin–aldosterone system
  • •Erythropoietin from the kidney stimulates red cell production in the bone marrow; the kidney also converts vitamin D to calcitriol
  • •Antidiuretic hormone (vasopressin) acts on the collecting duct to conserve water; aldosterone acts on the distal tubule to conserve sodium
  • •Willem Kolff built the first practical dialysis machine, a rotating drum, in Kampen in the occupied Netherlands in 1943; his first surviving patient was treated in 1945
  • •Belding Scribner's arteriovenous shunt, devised in Seattle in 1960, made repeated long-term dialysis possible
  • •Peritoneal dialysis uses the patient's own peritoneum as the membrane instead of a machine
  • •The first successful transplant was performed by Joseph Murray in Boston in 1954 between the identical twins Ronald and Richard Herrick; Murray shared the 1990 Nobel Prize
  • •Renal calculi are stones; nephritis is inflammation; polycystic disease is the commonest inherited cause of failure

Drill it — 9 cards

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