Removing Nitrogenous Wastes

Nitrogenous wastes are produced when the amine group of amino acids are removed.  This process is called deamination and take place in the liver.  An example is below.

One of the problems is the toxicity of ammonia.  It needs to be either quickly converted or removed from the body.  Examples of how ammonia is treated by various animals is shown below 

As you can see, most fish don't bother converting the ammonia.  They just excrete it straight into the environment via the gills.  Sharks convert it to urea and use it to regulate their internal fluid levels (not needed for this course).  However, land based and egg laying animals are faced with the challenge of not poisoning themselves while retaining water.  As a result while benefits are gained, it comes at a cost in energy.

Mammals and amphibians use the urea molecule as their solution.  It is about 100 times less toxic than ammonia, and very water soluble.  As a result removal of urea can occur with far less fluid.  In mammals the main organ of excretion is the kidney.

The fluid from the blood is squeezed into part of the nephron called the Bowman's capsule.  From there, it travels along the tubule.  By a combination of active transport, osmosis and the selectively permeable nature of membranes, things that the body needs are returned to the blood and waste salts and urea are removed via the urine stream.

Birds and reptiles also have kidneys.  However the material they excrete is uric acid.  Uric acid has a 2 key characteristics.

1. It has very low toxicity
2. It is very insoluble. 

Uric acid causes a great deal of pain to gout sufferers because of its insolubility.  But it means that if you are developing in an egg for a few weeks, the material will solidify before it can poison you.  In addition, it means that very little water is needed to get rid of it as it can be fed into the fecal pellets and voided with that waste.  Pictured below are the wastes of lizard and bird. The uric acid is the white stuff.

Insects on land generally lay eggs and so use uric acid as a means disposing of nitrogen containing wastes.  It is collected by a series of tubules in their bodies called Malpighian tubes.  The uric acid is concentrated and is then fed into the digestive tract where it can be excreted as part of the feces.