Unit 4: The circulatory and excretory systems                        1   2   3   4

4.3. Functions of the kidney

a) Urine formation

It is the main function of kidneys. Urine is formed by three processes:

      1^st) Glomerular filtration 

           Blood is under high pressure in the glomerulus. Blood plasma

           (except for plasma proteins) moves from glomerulus capillaries

           into the Bowman’s capsule.

           This fluid is called primary urine. It is  a clear liquid which contains

           many useful substances (aminoacids, glucose, ions, etc)

           and a large amount of water.

      2^nd) Tubular reabsorption

      As the filtrated liquid moves along the renal tubule most of the water

      and the other useful molecules are reabsorbed.

      These substances go through the tubule wall and pass to the

      blood capillaries that surround it. As a result the primary urine

      is progressively more and more concentrate.

     3^rd) Tubular secretion

      At the same time that reabsorption takes place, the cells of

      the renal tubule secrete some toxic substances which are added

      to the urine.

      Once the fluid moves into the collecting duct, it is called final urine.

      In the collecting duct, additional water is removed from the urine,

      concentrating more the wastes.

Urine is a liquid waste product which contains urea, uric acid, ions and water.

Urea and uric acid are very toxic and have to be excreted dissolved, so that it is absolutely necessary that urine contains some water.

Anyway, the most part of water is reabsorbed. For each millilitre of urine, kidneys filtrate 125 ml of blood plasma.

b) Maintenance of homeostasis

Kidneys have other important functions that help in the maintenance of homeostasis, such as:

- Regulate the blood plasma volume (the water that it contains) and,

  as a result, the blood pressure.

- Control the concentration of plasma ions: sodium (Na+), potassium (K+),

  chloride (Cl-), and nitrogenous wastes (especially urea).

   - Contribute to keep the pH of the plasma (concentration of H+ ions)

4.6. If the glomerular filtration speed is 125 ml/min, what volume

        of blood plasma is filtrated, and then reabsorbed, daily?

        Calculate the urine volume formed during a day.

4.7. Look at the picture about the nephron physiology.

        Where is reabsorbed the water? And the solutes?

        Where is it produced the tubular secretion?

4.8. If you eat a very salad meal, will you urinate more or less than usually?

        Why?

4.9. Listen and identify the phase of the urine formation that is described: