Excretion & Water Balance
Dying of Thirst #1
- If you were dying of thirst in a life boat, should you drink sea water?
- Why or why not?
Relevant information
- Sea water is 3% salt, 97% water.
- Human body is 70-80% water overall.
- Interstitial fluids are ?
- IV saline solution is ?
Osmosis
- The movement of water molecules from areas of high water concentration to areas of low water concentration
OR
- from areas of low salt concentration to areas of high salt concentration.
What happens in gut?
Dying of Thirst #2
- If dying of thirst in the desert, should you drink your own urine?
Excretion
- Means ridding the body of nitrogenous waste products of protein metabolism
- Does not include "defecation."
- Proteins are chains of amino acids.
- Each amino acid contains an amine group
(-NH2 )
- -NH2 group converts to ammonia (NH3)
Excretion strategies
- Dilute with water (aquatic animals)
- Convert to less toxic form (terrestrial)
= uric acid in insects & birds
(crystaline = very dry)
= urea in mammals (by liver,
NH3 + CO2 --ATP--> urea)
- Then reabsorb the water (by kidney)
Mammalian kidney
- Filters urea from blood, losing as little water as possible in the process.
- 160 quarts (liters) of "filtrate" (blood plasma) removed each day, but only 1.5 quarts of urine (urine = urea + water)
- Kidney reabsorbs 99% of the water in
the filtrate.
Drink your own urine?
Drink urine? It depends!
- If the urine is dilute (light yellow),
drinking it can help, because excess water in the urine can be reabsorbed into blood.
- If the urine is concentrated (dark yellow),
any water in the swallowed urine will be needed to excrete the swallowed urea.
So no net gain of water.
Drink sea water?
Why not to drink sea water
- If you drink salt water:
- Kidney filters out excess salt in the blood, but water follows the salt into the urine.
- Blood volume decreases.
- Replacement water pulled by osmosis from interstitial fluid and cells.
What happens if drink alcohol?
- What happens to your urine volume?
- Why?
ADH decreases urine volume
- Pituitary gland secretes ADH (anti-diuretic hormone)
- This increases the water permeability of the distal tubule and collecting duct.
- More water leaves the tubule and reenters the blood stream, so less water leaves the body.
Alcohol increases urine volume
- Alcohol inhibits the pituitary gland’s production of ADH, so ...
- Permeability of distal tubule and collecting duct decreases, so ...
- More water stays inside the duct, so more urine (urea = water) leaves the body.
Caffeine
- Increases the amount of filtrate leaving the glomerulus and entering the capsule, so ...
Diuretics ("water pills")
- Decrease reabsorption of sodium ions
by the proximal tubule, so…
- More sodium ions remain inside the
tubule, so...
Kidney failure
- High blood pressure in glomerulus increases the amount of filtrate
- Overworked nephrons "harden"
- Toxins (urea) builds up in blood
- Edema (water retention)
Artificial kidney
- Kidney dialysis machine:
-- Few hours, twice a week
-- Blood passed by semipermeable tube
through a balanced salt "bath".
-- Toxins diffuse out of blood into bath.
- Peritoneal dialysis: Balanced salt solution put into abdominal cavity, suctioned out half an hour later.
Key points
- Terrestrial animals must conserve water while excreting nitrogenous wastes.
- To conserve water, the nephron employs both active and passive mechanisms.
- Urine-altering drugs affect different parts
of the nephron.