Excretion in Humans PDF

EXCRETION IN HUMANS Excretion of Carbon Dioxide & Urea Unlike plants, humans have organs which are specialised for the removal of certain excretory products They include the lungs and kidneys The liver also has a vital role in excretion Excretion is the removal of the...

EXCRETION IN HUMANS Excretion of Carbon Dioxide & Urea Unlike plants, humans have organs which are specialised for the removal of certain excretory products They include the lungs and kidneys The liver also has a vital role in excretion Excretion is the removal of the waste substances of metabolic reactions (the chemical reactions that take place inside cells), toxic materials and substances in excess of requirements Carbon dioxide must be excreted as it dissolves in water easily to form an acidic solution which can lower the pH of cells This can reduce the activity of enzymes in the body which are essential for controlling the rate of metabolic reactions For this reason, too much carbon dioxide in the body is toxic Urea is also toxic to the body in higher concentrations and so must be excreted Excreting Urea When we eat food, any proteins in the food are broken down to smaller molecules inside the digestive system. The small molecules go into the blood. The blood transport them to the liver. If we have more protein than we need, the liver changes the smaller molecules onto urea. Urea is a poisonous substance. If it builds up in the body, it makes a person ill. As soon as urea is made in a liver cell, it is taken away from the liver in the blood. The urea is removed from the blood by the kidneys in the excretory system. The system is also known as the renal system.  As the blood flows through the kidneys, the kidneys filter the blood. They remove all the urea from it. The kidneys also remove excess water from the blood. The urea dissolves in the excess water. The solution made of urea dissolved in water is called urine The urine made in each kidney flows down a tube called a ureter. This carries it to the bladder, which can store it for a while. The urine can flow out of the bladder to the outside world through another tube, called the urethra.