Amino acids are often processed for energy and to build other organic compounds. The "Deamination" of amino acids results in the formation of an acetyl group that may enter the krebs cycle in the Mitochondria and used to make ATP. Ammonia (NH3) will also be produced. Ammonia can be toxic to the cell if too concentrated.

Organisms that are very thin and live in extremely moist environments (water is needed to dilute cell wastes) may have no specilaized excretory system (sponges, jelly fish, roundworms). Simple diffusion may be all that is needed. The more complex the tissues in an organism the more abundant cell wastes tend to become which would ultimately lead to large ammounts of ammonia being produced. This will require the presence of an excretory system.

In addition to an excretory system many multicellular organisms have evolved enzymes that bind the ammonia to other substances to decrease toxicity. The Urea cycle has been established in mammals as a means of dealing with the production of ammonia.

UREA CYCLE - Ammonia is bound to Ornitihine using CO2 and removing H2O (dehydration sysnthesis). The chemical formed is called Citruline. An additional amonia group is bound to the Citruline making a compound called Arginine. The Arginine has a Urea group (CN2H4O) removed converting back into ornithine to repeat the cycle. The beauty of this cycle is that Urea is formed which is much less toxic than ammonia and requires less water to dispose of.

Birds and reptiles have evolved the ability to encapsulate the uric acid crystals with a protein material that allows easy export from the body. As a result birds and reptiles do not have a bladder. They instead excrete uric acid out through a common cavity (Cloaca) shared with the rectum. Birds and reptiles do not urinate. When they have a bowl movement what passes from their body will be part feces (black - nondigestable material from the rectum) and uric acid (white - concentrated cell waste from the kidney). The excretion of waste in the form of uric acid requires hardly any water. Therfore birds and reptiles can manage a diet or an environment that has limited water available.

Imagine lunchtime with the seagulls if they had bladders...... umbrellas for everyone.

1- What is the main advantage for removing cell waste as uric acid?

Flatworms are the simplest multicellular organisms to have an excretory system. They posses a system of tubes that have specialized cells attached to them called "Flame Cells". The flame cells have flagella that establishes circulation of fluid out through a "nephridiapore" on the side of the body. The circulation insures the passage of wastes out of the body. This system is said to be composed of "Proto-nephridia" (which =s before true nephridia)

Earthworms have a system a bit more advanced. in almost every segment there exists "Meta-nephridia". These are surrounded by their own cappilary network. The nephrostome pumps fluid from the forward segment into the nephridium. Capillaries reclaim essential substances leaving only water and cell waste to pass out of the nephridiapore.

Insects like the grasshopper have an open circulatory system (no capillaries) and can not have an excretory system as complex as that seen in the earthworm. Insects possess a system of tubes that are attached to the digestive tract and extend into the body cavity. Wastes are actively transported from body fluids into the tubes which then pass them into the digestive tract to leave the body with nondigested food. These excretory tubes are called "malpighian tubules"

The excretory system of mammals is the most complex of all. Special organs called Kidneys contain millions of individual tubes called nephrons (diagram above and lower left) . Each nephron is serviced with two capillary beds. The "glomerulus" is the first capillary bed located in a bowl shaped depression (Bowmans Capsule) at the beginning of the nephron. The glomerulus raises BP enough to cause large amounts of the fluid (filtrate) that enters it to be forced into the nephron tube.

The second capillary bed surrounds a portion of the nephron that extends downward in the kidney to the inner medula.ggg

Filtrate includes water, urea, salts, ions, glucose, vitamins, and amino acids. The function of the nephron is to recover beneficial substances leaving only water and urea in the tube. Cells in the nephron actively transport sugar and amino acids back into the capillaries in the convoluted tubules. Salts, water, and useful ions will move back into the capillaries in the Loop of Henle. urea and water will be dumped into a collecting tubule and make it's way to the renal pelvis. From there it will move down the ureter to the bladder for temporary storage. When the time for removal is needed, a sphincter muscle at the base of the urethra will be relaxed and the smooth muscle around the bladder will force the contents out of the body. Strecth receptors imbedded in the walls of the bladder indicates when it is full.

Many animals are able to expel the urine in a precise direction. This can be done for marking a territory or to temporarily blind a would be predator.

View A Movie About Nephron Function