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FACTORS CONTROLLING URINE VOLUME
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.
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