Fluid /Electrolyte Balance  Elmhurst College
Fluid Exchange Processes Fluid Exchange kidneys Fluid Deficit  Chemistry Department
Fluid Exchange in Tissues Fluid Excess    Virtual ChemBook

Click for larger image 

Fluid Exchange Processes in the Kidneys


The kidney maintains the major control over both water and salt balance. The major objective in urine formation (diuresis) is to conserve base (bicarbonate) by excreting hydrogen ions and to excrete wastes and excess electrolytes such as potassium, phosphate, and urea. This process also requires some water loss. The body must conserve glucose, sodium and chloride ion, water, etc. To accomplish this objective it is necessary to excrete a urine of higher osmotic pressure than the blood. Waste products are more concentrated in the urine than the blood.

Blood pressure causes filtration of plasma fluid into the Bowman capsule #l - figure on left. Proteins normally do not pass out of the blood capillaries. These exert osmotic pressure on the developing urine causing water reabsorption #2. Salt, glucose, and other substances are also reabsorbed. The tubule membrane is not permeable to certain waste substances such as urea and potassium ions. At one point salt is carried out of the developing urine by an active transport process #3. This causes a final osmotic flow of water out of the urine which causes it to be quite concentrated in terms of waste products #4.

Click for larger image 


l. Blood Pressure: An increase causes a greater rate of filtration which causes more water to be present in the developing urine at # 1.

2. Increased Concentration of Salts or Glucose in Urine: These substances osmotically draw more water from blood into tubules and urine. This reverses or slows down osmosis at # 2 and # 4.

3. Decreased Plasma Protein: This leads to a failure to osmotically draw water out of tubules into the blood at # 2. Therefore, water loss from the kidneys increases.

4. Vasopressin Hormone: Urine volume varies inversely to the amount of this hormone from the posterior pituitary. The hormone may be stimulated by osmotic pressure in the plasma. Increasing osmotic pressure, increases vasopressin, which finally causes a decrease in urine volume.

5. Water Loss from Other Sources: Urine volume varies inversely with water lost from other systems such as perspiration or diarrhea.

6. Diuretics: These agents stimulate the production of more urine and
the excretion of salt.

Summary and Application: The chief natural diuretic is water itself. Ingestion of a large amount of water temporarily increases the plasma compartment volume. This causes a rise in blood pressure which increases the rate of filtration. In addition the plasma has been diluted, therefore, osmotic pressure is lowered. The lower osmotic pressure causes a decrease in vasopressin hormone. All of these factors lead to an increase in urine volume.

Click for larger image 


QUES. 22: In diabetes mellitus, glucose is present in urine when normally it is absent. At step # 2 the developing urine maybe 360 mmoles. Explain what happens to urine volume as a result at step # 2 in the figure?

QUES. 23: In shock or congestive heart failure the blood pressure falls to a value of maybe 28 mm. What happens to urine volume as a result at # 1?

QUES. During hot weather, perspiration losses increase, what happens to urine volume as a result?