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The Kidney

The human kidneys:
Link to graphic showing the location of the kidneys and their blood supply (64K).

The Nephron

The nephron is a tube; closed at one end, open at the other. It consists of a:

Formation of Urine

The nephron makes urine by In 24 hours the kidneys reclaim The steps:
Composition of plasma, nephric filtrate, and urine (each in g/100 ml of fluid). These are representative values. The values for salts are especially variable, depending on salt and water intake.
ComponentPlasmaNephric FiltrateUrineConcentration% Reclaimed
Uric acid0.0040.0040.0512X91%
Amino acids0.050.05None-100%
Total inorganic salts0.90.9<0.9-3.6<1-4X99.5%
Proteins and other macromolecules8.0NoneNone--
As the fluid flows into the loop of Henle, it is approximately isotonic to the blood. Here more sodium ions are pumped out, but water does not follow them. So,

In the distal tubules, more sodium is reclaimed by active transport, and still more water follows by osmosis.

back up to diagram

Final adjustment of the sodium and water content of the body occurs in the collecting tubules.


Although 98% of the sodium has already been removed, it is the last 2% that determines the final balance of sodium - and hence water content and blood pressure - in the body. The reabsorption of sodium in the collecting tubules is closely regulated, chiefly by the action of the hormone aldosterone.


The release of ADH (from the posterior lobe of the pituitary gland) is regulated by the osmotic pressure of the blood.

Diabetes insipidus

This disorder is characterized by: It can have several causes:

Liddle's Syndrome

The most obvious effect of this rare, inherited disorder is extremely high blood pressure (hypertension). It is caused by a single mutant allele (therefore the syndrome is inherited as a dominant trait) encoding the aldosterone-activated sodium channel in the collecting tubules. The defective channel is always "on" so too much Na+ is reabsorbed and too little is excreted. The resulting elevated osmotic pressure of the blood produces hypertension.

Tubular Secretion

Although urine formation occurs primarily by the filtration-reabsorption mechanism described above, an auxiliary mechanism, called tubular secretion, is also involved.

The cells of the tubules remove certain molecules and ions from the blood and deposit these into the fluid within the tubules. Example: Both hydrogen ions (H+) and potassium ions (K+) are secreted directly into the fluid within the distal tubules. In each case the secretion is coupled to the ion-for-ion uptake of sodium ions (Na+).

Tubular secretion of H+ is important in maintaining control of the pH of the blood.

The Kidney and Homeostasis

While we think of the kidney as an organ of excretion, it is more than that. It does remove wastes, but it also removes normal components of the blood that are present in greater-than-normal concentrations. When excess water, sodium ions, calcium ions, and so on are present, the excess quickly passes out in the urine. On the other hand, the kidneys step up their reclamation of these same substances when they are present in the blood in less-than-normal amounts. Thus the kidney continuously regulates the chemical composition of the blood within narrow limits. The kidney is one of the major homeostatic devices of the body.

Hormones of the Kidneys

The human kidney is also an endocrine gland secreting three hormones:
Link to discussion of the hormones of the kidney.

The Artificial Kidney

The artificial kidney uses the principle of dialysis to purify the blood of patients whose own kidneys have failed.

The left portion of the figure ("Dialysis unit") shows the mechanism used today in artificial kidneys. Small molecules like urea are removed from the blood because they are free to diffuse between the blood and the bath fluid, whereas large molecules (e.g., plasma proteins) and cells remain confined to the blood. The bath fluid must already have had essential salts added to it to prevent the dangerous loss of these ions from the blood. Note that blood and bath fluid flow in opposite directions across the dialysis membrane. This "counter-current" exchange maintains a diffusion gradient through the entire length of the system. An anticoagulant is added to the blood so it will not clot while passing through the machine. The anticoagulant is neutralized as the blood is returned to the patient.

Artificial kidneys have proved of great benefit in helping patients of acute kidney malfunction survive the crisis until their own kidneys resume operation. They have also enabled people suffering from chronic kidney failure to remain alive, though at an enormous expense of time (often three sessions of 6 or more hours per week), money, and psychological well-being. Furthermore, although dialysis does a good job at removing wastes, it cannot perform the other functions of the kidney:

An artificial kidney of the future?

In an attempt to solve these problems, a research team at the University of Michigan is experimenting with adding a "Bioreactor unit" to the dialysis unit. The bioreactor consists of many hollow, porous tubes on the inner wall of which is attached a monolayer of proximal tubule cells (derived from pigs). The dialysis bath fluid passes through the lumen of the tubes where molecules and ions can be picked up by the apical surface of the cells. Discharge of essential molecules and ions (as well as hormones) at the basolateral surface of the cells places these materials back in the blood (just as the proximal tubule cells in the nephron normally do). So far, all the testing has been done using dogs, but the results seem promising.

The ideal alternative to long-term dialysis is transplantation of a new kidney. The operation is technically quite easy. The major problems are:
Link to discussion of organ transplants.
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2 March 2001