The Excretory System
What is it?
As you've probably guessed, the excretory system is all about excreting. But why exactly is it important, besides relieving the desire to go to the bathroom? It's all very connected to water balance, because water balance depends on solute movement between internal fluids and external environment. Much of this movement is handled by the excretory system because it disposes of metabolic wastes (urine) and controls the amount of solute in the fluids by adjusting rates of solute loss. This makes it very important.
Organs involved in the human excretory system
How does it work?
To sum up how the exretory system works, first, blood is taken to the kidneys (bean-like organs in the diagram above) via the renal artery. Second, the blood is filtered in order to get rid of the "bad stuff" (nonessential solutes and wastes such as unwanted water, minerals, and a nitrogen-rich molecule called urea); this is the function of the kidneys - to filter. Third, with the "good stuff" left in the blood, it is taken back to the heart via the renal vein. Then, the unneeded fluid, which is now called urin, goes down the ureters from the kidneys to the urinary bladder, which stores the urin until a person has to "go." Next, the urin is finally released from the body through the urethra.
Other kinds of excretory systems
Now, that was the human excretory system. Although all excretory systems do the same job - excrete - they do not all work exactly the same way, such as there being different routes to the same destination. Different excretory systems have different organs involved too.
This is the class in which humans are included and have an excretory system that basically revolves around the kidneys - the "stars" of the process.
Flatworms have a type of excretory system called protonephridia, which are tubes (called tubules) that run across the entire body of the flatworm and have no internal openings. These tubules have small branches called a flame bulb, which absorb solutes into the tubule system and moves what is now urine outward through the tubules until they empty to the external environment through openings called nephridiopores.
Flatworm Flatworm excretory system
Earthworms have what is known as metanephridia excretory systems; each segment of the worm has a metonephrium. These contain tubes with internal openings that collect body fluid (from anterior segment), which becomes waste/urine, and then exits the worm's body.
Earthworm excretory system
Insects have malpighian tubules that remove nitrogenous wastes and also function in osmoregulation. The insect's digestive tract is connected to these tubes, through which salt, water, and nitrogenous wastes move; from the digestive tract the waste continues toward the anus where both urine (in the form of uric acid, not urea) and feces are released.
Insect excretory system