Re: why doesnt a paramecium or ameba swell up in distilled water?

Patrick:

That is a good question.  The main reason is that the amoeba has 
mechanisms regulate the water that would normally enter and leave via 
diffusion.  In distilled water, cells would tend to swell since the water 
moves from high concentration (outside) to low concentration (inside).
     In cells, this process is offset by osmosis, or active transport.  
This is a mechanism that cells can use to push water or solutes against a 
concentration gradient, much like a reverse osmosis filter works in a 
house.  Amoeba in particular possess a special organelle called a 
contractile vacuole which swells and contracts to release water into the 
cytoplasm as needed.  This also helps them regulate the amount of water 
inside so that they don't 'pop' or shrink down too much.  
     In the wild, this is an advantage for protists as they are likely to 
encounter many different types and purity of water and they need to be 
able to respond to changes in salinity or quality and not die.  Red blood 
cells, on the other hand have two things going for/against them that 
prevent them from responding like this:

#1, Red blood cells in humans are anuclear (frogs have them).  This means 
that the cells do not transcribe a lot of genes into proteins in response 
to the environment, so they are not likely to adapt to conditions with new 
gene products, unlike an amoeba.

#2, And probably the more important of the two.  Throughout the body a red 
blood cell lives in an extremely stable environment.  This includes 
temperature, pH, pressure, and salt and solute concentration.  It is 
evolved to be isotonic with respect to the blood, and doesn't need 
mechanisms to cope with changes in H2O concentration.  In fact, small 
changes in the balance of sodium and pH in your blood have serious and 
usually fatal consequences beause of the imbalance in the system.  Changes 
in these properties affect the blood cells, nerve and muscle function and 
in particular they alter the dynamics by which red blood cells are able to 
change O2 for CO2 with the lungs, resulting in suffocation from inside.

Hope this answers your question.  The short answer is they don't need to 
because the world in which they live doesn't change much.