Can anyone explain how the excretory system works step by step?
Excretory system is the bodily system (composed of organs) needed to regulate the chemical composition of the body, by removal of excess metabolic wastes in the bodily fluids. Prokaryotes have a relatively simpler excretion system and all the unwanted metabolites are simply excreted out of the cell. Eukaryotes, on the other hand, have a much more sophisticated system.
Note that excretory system refers to cleanup of bodily fluids and does not include removal of feces from our body.
In humans, excretory system mainly consists of kidneys, ureters, bladder and urethra; that is, it is in essence urinary system. The liver is also considered a part of our excretory system, since it helps in breakdown of complex and harmful molecules into the simpler ones. An example is breakdown of ammonia into urea, that can be more easily expelled from the body. Our two kidneys act as a continuous filter, without our control and constantly filter the blood to remove wastes in fluidic form. This filtered liquid is known as urine and is sent to our bladder using the ureters (one connected to each kidney). Our bladder thus receives a continuous influx of urine and informs our brain that the body needs to excrete the urine. The urine is excreted from our body using urethra.
Two other important parts of excretory system in human beings include the skin and lungs. Skin is used for removal of excess fluid (in the form of sweat) from our body and helps in maintaining body temperature and salt levels. Lungs are used for removal of carbon dioxide, generated during the cellular respiration, from our body (through the respiratory system).
Hope this helps.