Osmoregulation and excretion:
Basic idea is to keep a balance of salts in your body.
Osmosis review: definition - diffusion of water across a semi-permeable membrane.
diffusion - movement of a substance from a high concentration to a low
concentration.
Thus, a simple example:
- a beaker with 5% salt on one side, 0% salt on the other, divided by a
semi-permeable membrane (in this case salt can’t move across the
membrane).
- which way does water move?
- CONVERT to % water. So we have 100% water on one side, 95% water
on the other. Water will move from a high concentration to a low
concentration.
- lots of confusing terminology - hyperosmotic, hypoosmotic, hypotonic,
etc., the only one you need to know is isoosmotic (concentration is the
same). Otherwise, keep track of where the concentration of water is
higher or lower.
A problem for many animals: sometimes the concentration of salt (and therefore water) in
the surrounding environment is totally different from that inside the body.
Aquatic animals:
Types:
osmoconformer - animal has the same salt concentration as surrounding
water.
osmoregulator - animal needs to regulate salt concentration since salt
concentration in body is different that in the surrounding environment.
[Fig. 25.4A & B, p. 508]
Fresh water - problem is water will enter body. Animal must get rid of excess
water, or it will “explode”. How?
Fish - will eliminate water through kidneys (very dilute urine)
- absorbs extra salt ions through the kidneys.
Many other animals similarly will excrete large amounts of dilute urine.
Marine water - opposite problem. Water will leave body because salt
concentration outside the body is often higher than inside the body (water goes to
more concentrated salt area - the animal would shrivel like a raisin).
Fish (most)- will excrete highly concentrated urine, thus keeping water
inside the body.
- will also remove excess salt ions by excretion across gills.
Invertebrates - most are osmoconformers
(incidentally, in fresh water, many invertebrates can/must
pump out water).
Terrestrial animals:
The problem is water conservation. Water is lost due to respiration, evaporation
through surfaces (remember amphibians, for example), urine, etc.
- Water is restored by drinking, or conserved by behavioral adaptations
(active at night) or physiological adaptations (below).
- Features that prevent water loss are dead keratinized skin, exoskeleton,
and producing very concentrated urine.
- Excretion of metabolic wastes will now be examined in detail.
Excretion:
Substances excreted [Fig. 25.5, p. 509]:
- ammonia - this is the basic by-product of metabolism. Ideally, this is excreted
directly into the environment. Problem --> highly toxic. But many aquatic
organisms do this since they can easily replace water. [In fish, some of this is
often excreted across the gills].
- urea - for terrestrial animals ammonia is no good - they can’t get rid of it quickly
enough. Instead, they convert ammonia into urea (100,000 times less toxic than
ammonia) in the liver. This is then transported to the kidney and eliminated. This
is also used by some marine organisms that need to conserve water (since high
concentrations of urea are readily tolerated (e.g. sharks)).
- uric acid - many terrestrial organisms excrete uric acid (birds, many reptiles,
insects, etc.). Uric acid is not very water soluble, so it can be excreted with very
little loss of water. This is also good if you’re an egg-layer since the waste
material can be stored as a precipitate in the egg (the other two compounds would
remain dissolved, and even urea would eventually rise to toxic levels).
Mammalian kidneys:
- fairly complicated - here’s an overview:
[Fig. 25.6, p. 510].
- materials in blood are excreted through the glomerulus (highly coiled
capillaries) into Bowman’s capsule (a collecting area).
- important nutrients are then reabsorbed in the proximal tubule. In addition
water and salt are reabsorbed. The resulting fluid has about the same salt content
as the surrounding tissues, but there’s less of it. [Details: sodium is reabsorbed.
Chlorine and water then follow].
- in some nephrons there exists the “loop of Henle”. The main function of this
loop is to establish a concentration gradient that can be maintained. To do this,
salt is removed from the loop through passive and active transport. The
surrounding tissue is much saltier near the bottom of this loop.
- As the filtrate moves to the top of the loop, it again becomes more and more
dilute. This is because actual salt content of urine is controlled in the collecting
duct (i.e., you want to start with reasonably dilute material and then reabsorb as
necessary).
- at the distal tubule, concentrations of Potassium and Sodium ions are controlled,
and pH is buffered (using bicarbonate) - this is similar to what happens in the
proximal tubule.
- Finally, at the collecting duct, all that needs to be done is to change the
permeability of the membrane and urine can be as concentrated or dilute as
needed (within limits).
- This is much quicker than setting up the gradient every time. Note that
as one goes down collecting duct, surrounding tissue becomes more and
more salty. If membrane stays permeable, then a lot of water is
reabsorbed. If membrane is made impermeable then more water is
expelled.
- Details: go through [Fig. similar to 25.8, p. 512]
- note that water is removed as fluid descends loop. This is because loop
is water permeable, but not salt permeable.
- as one moves up ascending loop, salt is removed - loop is now permeable
to salt but not water, so salt will move from a high concentration within
loop to the outside.
- salt is still removed as fluid moves further up loop, but this time due to
active transport of molecules.
- note that all this salt removal establishes a concentration gradient, with
lots of salt at the bottom, and less salt at the top.
- after going through the distal tubule, fluid now moves down collecting
duct. Here the permeability of the membrane can be changed rapidly
depending on the need for water.
So how is all this controlled? Or, in other words, how is the permeability of the
collecting duct changed (remember - the gradient is maintained, all that changes is the
permeability of the collecting duct)?
Methods of controlling kidneys [Fig., not in book]:
- ADH pathway:
-ADH (anti-diuretic-hormone) is produced in the hypothalamus and stored
in pituitary. The hypothalamus monitors salt concentration of blood.
- If salt concentration rises, then => more water is needed in the body. So
ADH is released. ADH increases the permeability of the collecting duct
and so more water is reabsorbed.
- If the salt concentration drops, then the opposite happens. ADH is
retained, and collecting duct remains impermeable to water. More water
is expelled.
- [Alcohol disrupts this pathway some, causing more water to be expelled
than otherwise. Causes dehydration (and also the need to urinate more
often than normal)].
- RAAS pathway:
- responds to drops in blood pressure and/or blood volume (e.g., due to
bleeding).
- when pathway activates, overall urine volume is reduced. This raises
blood pressure and blood volume.
- Note: both pathways increase reabsorption of water. But ADH pathway directly
senses salt concentration. RAAS pathway senses changes in blood pressure and
volume.
- Why? Because blood loss can decrease blood pressure/volume without
changing salt concentration, so a mechanism has evolved that helps retain “body
fluids” during blood loss.