Urinary System III
Control of Blood Composition by the
kidney
Blood Composition
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Blood composition depends on 3 major factors:
Diet
Cellular metabolism
Urine output
In general, the kidney have 4 major roles to play,
which help keep the blood composition relatively
constant:
1. Excretion of nitrogen-containing wastes
2. Maintaining water
3. Maintaining electrolyte balance
4. Ensuring proper blood pH
Excretion of Nitrogen-containing
Wastes
• Important nitrogenous
wastes found in blood: Urea,
Uric Acid and creatinine
- Urea: is formed by the liver
when amino acids are used to
produce energy; it is an end
product of protein
breakdown
Excretion of Nitrogen-containing
Wastes
- Uric Acid: is released
when nucleic acids are
metabolized
- Creatinine: is associated
creatine metabolism in
muscle tissue
Maintaining Water and Electrolyte
Balance of Blood
•
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Water weight:
Male: 60%
Female: 50%
Babies: 75%
This is because females have relatively less
muscle and a larger amount of body fat (fat
contains least water out of all body tissues)
Fluid compartment
•
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Water occupies 3 main locations within the body:
Intracellular fluid volume:
Fluid that is contained within the living cells
40% of body weight
Extracellular fluid volume:
includes all the body fluids located outside the
cells
 20% of body weight
a. Interstitial fluid: Fluid between cells and tissues;
80% of ECF
b. Blood plasma fluid: 20% of ECF
Electrolyte Balance
• The proper concentrations of the various
electrolytes must be present in both
intracellular and extracellular fluids:
- Most electrolytes enter the body in foods and
“hard” (mineral rich) water
- The major factor regulating the electrolyte
composition of body fluids is the kidneys
Hormones: ADH
• Reabsorption of water and electrolytes by the
kidneys is regulated by hormones:
- Antidiuretic hormone (ADH): Prevents excessive
water loss in the urine
a. Anti: against / Diuretic: flow of urine from the
kidney
b. Target: collecting ducts of the kidneys, where it
causes the duct cells to reabsorb more water
c. ADH is released more or less continuously
unless the solute concentration of the blood
drops too low
Hormone: Aldosterone
- Aldosterone: helps to
regulate blood
composition and blood
volume by preventing
excessive water loss in the
urine:
a. Stimulates kidney tubule
cells to reabsorb more
sodium ions
b. Water follows the salt
back into bloodstream
Maintaining acid-base balance
• For the cells of the body to function properly,
blood pH must be maintained between 7.35
and 7.45
• Alkalosis: When the pH of the blood rises
above 7.45
• Acidosis: when the pH of the blood fall below
7.35
Characteristics of Urine
• Freshly voided urine is generally clear and pale
to deep yellow
- The normal yellow color is due to urochrom, a
pigment that results from the body’s
destruction of hemoglobin
• When formed, urine is sterile and its odor is
slightly aromatic
- If it is allowed to stand, it takes on an
ammonia odor caused by bacterial action
Characteristics of Urine
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Urine pH is usually slightly acidic (around 6):
A diet rich in protein can cause acidic urine
A vegetarian diet can cause basic urine
Bacterial infection of the urinary tract also
may cause the urine to be basic
• Urine is more dense than distilled water