Kidneys are the vital organs which are involved in the excretion process. But there are also other organs which remove the waste from the body. These organs together constitute the excretory system as
All the above organs are involved in elimination. Elimination involves removal of waste from the body.
Kidneys:
These organs are located on the dorsal side of the body. Their more physiology involves purification of the blood. All the blood flows through the nephrons in the kidneys. During this flow, most of the waste is removed from the blood into the nephron tube. This waste is expelled as urine. Most drugs, electrolytes, uric acid, urea, etc. are expelled from the body.
When blood flows into kidneys, it reaches the nephrons. There it undergoes processes like glomerular filtration, active tubular secretion, and passive tubular reabsorption. The waster formed are expelled in urine.
Skin: Skin is one of the minor organs of excretion. It helps to remove the waste in the form of sweat. It especially helps in excretion of sodium chloride, urea and aromatic substances like spices, etc.
During high temperature, the body tries to regulate the internal heat by expulsion of sweat.
Hence, sweating is a natural physiological process to regulate body temperature. If we tried to avoid sweating by use of an air-conditioned room or other cooling methods, the burden of waste removal lies on the kidney. So, the filtration and elimination increase the load on the kidneys which lead to their damage on the long run.
Even, in case of poisoning, the blood is decontaminated by the expulsion of toxin through urine. Hence the toxin elimination is hastened by either alkalinization or acidification of the urine.
Lungs: These are also the organs through which waste is removed in the form of gases. Lungs readily eliminate lipid-soluble compounds faster by breath. They help in the expulsion of carbon dioxide, ammonia, ethyl alcohol, etc. So, we can identify if a person has consumed alcohol. Since ethanol is expelled from the lungs, we can quickly determine if the person is drunk.
Liver: It is another major organ of excretion. It metabolizes an insoluble substance, biological molecules like the hormones, enzymes, etc. The water-insoluble forms are converted to water-soluble forms. These then released into the blood for expulsion through kidneys are into the intestine for expulsion through feces.
All the food and drugs we take enter the liver from the intestine. Here, they undergo first pass metabolism which could lead to identification of toxins and waste. Also, the waste in blood also is metabolized in liver for expulsion.
Intestine: As such, the intestine is not involved in excretion mechanism. However, intestine help to eliminate waste by not absorbing them back into the blood.
Besides the above, saliva and tears are also involved in excretion. Hence, you might have heard that teeth are damaged in few patients due to drug treatment. These drugs are released into saliva which degrades the teeth.