• Overall, the excretory system acts in order to maintain a healthy fluid environment within animals. It works in order to remove nitrogenous wastes such as ammonia (which is a highly toxic compound). Due to the fact that osmoregulation is structurally and functionally linked in many animals, the process of osmoregulation will also be included. Osmoregulation is the general process by which animals control solute concentrations and balance water gain and loss.

Diagram of Excretory System

  • Osmolarity is used in order to measure osmotic pressure in a solution and is expressed as moles of solute per liter of solution.
Animals maintain water in two ways, and are separated into two categories: osmoconformers and osmoregulators.
All osmoconformers are marine animals and most live in water with stable compositions, allowing them to have a constant internal osmolarity.

  • Osmoconformers are isoosmotic (the same osmolarity) with their surrounding.
  • Osmoregulators control internal osmolarity independant of environmental osmolarity


Goldfish (left) are stenohaline while barnacles (right) are euryhaline.
  • Most animals are stenohaline and cannot tolerate substantial changes in external osmolarity while others are euryhaline and can survive large fluctuations.


As shown in the above diagram, fish use their gills in order to perform osmoregulation. For saltwater fish, salt ions are excreted from the gills while for freshwater fish, salt is taken in through the gills. Saltwater fish also lose water through their gills while freshwater fish gain water through their gills.


Ammonia - extremely toxic, found in some marine animals
Urea - low toxicity, found in adult amphibians, sharks, and some marine bony fishes and turtles.
Uric Acid - relatively non-toxic, found in insects land snails, reptiles, and birds.


In the Excretive System, there are specific organs that are necessary to stay alive.


Dia. of Kidney & Nephron
Dia. of Kidney & Nephron

  • Kidneys are the main organ of the excretory system, conducting osmosis and filtering in general. Each kidney has a million nephrons, which filter out urea, and other solutes (salt, sugar...) These nephrons use the process of selective reabsorption, which allows water and valuable nutrients to be filtered back into the blood.
    • The kidney is also in charge of filtering blood. The blood begins from the arteries, goes to the glomerulus, which is a ball of capillaries on a nephron, and then continues to the Bowman's capsule, which is the blind end of the tubule that surrounds the glomerulus.
    • This capsule squeezes everything out from the blood (Filtration), reabsorbs all the nutrients it needs (Reabsorption), then pumps out all the excess waste such as urea (Secretion and Excretion).
  • So what's in blood?
blood cells
blood cells

  • <--Cells AND Plasma
  • <--H2O = want to keep
  • <--proteins = want to keep
  • <--glucose = want to keep
  • <--salts / ions = want to keep
  • -->urea = want to excrete

-The Role of Skin & Lungs

  • The lungs constantly expel gaseous wastes from the blood stream, but this is part of respiration.
  • Skin releases a fluid, called sweat, which is, in a sense, a metabolic waste, making it a minimal part of the excretory system.

urinar27.jpg (640×480)
urinar27.jpg (640×480)
-Urinary Bladder

  • This is your pee container! :-D
  • This bladder acts as any other bladder would. It collects urine from the kidneys, which is the result of the waste being filtered. The urine goes through from the Ureter and exits through the urethra. In men, the bladder is connected to the same canal as where sperm are released. In females, the bladder is not connected to the uterus, which means that they have no relationship.


  • The Urethra is simply a passage way for urine to get from inside the body, to outside. Again, the urethra also has a reproductive function for males, because this is also a passage for semen during sexual activity.


  • This is a muscle that actually forces the urine out from the kidneys into the urinary bladder.
  • This is also a site where kidney stones appear often...



This disorder is when uric acid builds up in the body as the kidneys are not able to get rid of it. The buildup settles in joints therefore causing joint problem and pains, and may also result in kidney stones. There are many types of kidney stones and the conditions in which they are formed determines which category they fall in. For the United States, 80% are calcium oxalate, or calcium-based, 5% are uric acid, 2% are cystine, and the remaining 13% are due to magnesium ammonium phosphate or other causes. Stones are often moved down the ureter where it causes much pain to the person. The kidney affected could end up being useless for the time being. Ultrasound can break smaller stones down so that it could be passed out of the body more easily if not small enough to pass naturally. Larger stones have to be removed through surgery. The cause of the stone(s) should also be taken into view to prevent further production of them. Body problems that could be the reason stones are produced are parathyroid glands to too much vitamin D to forms of cancer to genetics, but dehydration would be the number one factor. Symptoms would be severe pain in stomach or back, abnormal urine color, blood in urine, fever, chills, nausea, and vomiting. Most of the time, they do not appear until the stone moves from the kidney to the bladder. Drinking more water would help assist the moving of stones more easily.
external image footgout.jpg
gout in the joints (note the yellow colors which is build up of uric acid)

external image kidney-stones2.jpg
removed kidney stones

-Kidney Failure

This is the sudden loss of the ability of the kidneys to remove waste and concentrate urine without losing electrolytes. Many causes can be the reason as to why the kidney cannot perform its normal functions such as decreased blood flow which then results in burns, injury, and hemorrhage. Symptoms of kidney failure are bruising easily, loss of appetites, pain between the ribs and hips, hand tremor, metallic taste in mouth, urination changes, and much more. Tests can be taken to see if there is kidney failure. Ultrasound is preferred, but other options are available like x-ray, CT scan or MRI. After detecting the cause, treatment is taken, and an overnight stay in the hospital is common. Doctors have to restore the function of the kidney while preventing blockage of wastes as it heals. A diet will be given as toxins cannot be treated due to the kidney condition. Calcium or glucose will be given through the vein to make sure that spikes in blood potassium levels will not be achieved.


This causes inflammation of the nephrons through a variety of causes such as strep throat. There are different kinds of nephritis like glomerulnephritis which is inflammation in the glomeruli. Symptoms include bloody urine, odor of the urine, and edema which is swelling. There might be a possibility where there is an autoimmune attack on the glomeruli which is more severe. It can cause inflammations to one or both of the kidneys, and caused by infection, but also commonly autoimmune problems. People with lupus have more chances of getting nephritis. Nephritis can also be inherited genetically which is very rare. Nephritis is the ninth highest cause of human death as it causes the loss of needed proteins. One protein lost could be one that keeps blood from clotting together, but without it, it would result in strokes.


Urinary tract infections that have gotten to the pyelum of the kidney are pyelonephritis. Bacteria causing this are normally introduced to the body through catheter use, urinary tract surgery, kidney stones, enlarged prostate, or some sort of abnormality in the urinary tract that block the flow of urine. Symptoms include back pain, side pain, or groin pain, frequent urination, burning during urination, fever, nausea, vomiting, and blood in urine. To diagnosis this problem, a urine sample will be needed. If doubtful, blood samples will be needed also. E. coli, one of the many organisms to cause pyelonephritis, is the most common one and they all enter through the urinary tract. Antibiotics can be taken to treat this disorder as it is caused by bacteria. This disease is common as there are 12-13 cases every year for every 10,000 population in women, but 3-4 cases for every 10,000 men.

-Urinary Tract Infections

Bacteria infects the urethra and bladder and is caused by gram negative bacteria, an example being E. coli. This disorder is more common in women than men being said that the urethra of the women is much closer to the anus. After using the restroom, direction of wiping is crucial to the chances of contracting infections. It would be a common mistake to wipe from back to front and if girls are taught this way, they get this feeling and it becomes a habit. It should be the other way, from front to back. Another factor would be that the urethra of men are much longer than that of women, making it more difficult to infect the bladder. A way to solve this problem of infections would be catheterization if there is an obstruction of the urethra. However, the has a possibility of allowing more pathogens to invade. It would be more commonly treated with series of antibiotics which are usually successful. There have been severe cases where it has gotten to the kidneys and need more intense care. Symptoms would be strong urge to urinate, a burning feeling during urination, cloudy urine, blood in the urine, strong odor of urine, and pelvic pains.

external image 08032880.jpg
one of the many bacteria to cause urinary tract infections

5. External Links

Very helpful powerpoint! *download* --> ""
More on the Kidney --> ""
About the excretory system's protections --> ""

*What the Excretory System Helps Us Accomplish*

And even further help --> ""