Urine is a liquid created by the body in the kidneys. It is stored in the bladder and comes out of the human body through the urethra, a tube which leads from the bladder to the urethral meatus ("meatus" is an opening in the body), at the tip of the penis (men) or the vulva (women). Urinating is how the body gets rid of unwanted or unneeded substances in the blood. Urine is the liquid that these substances are put in. After the blood has circulated through the body, its composition has changed, because some substances in the blood have been absorbed by the body, and because the body has released other substances into the blood. The blood has also absorbed substances produced from our food. The liver removes many toxins (harmful substances) and adds other substances that the blood needs (amino acids, proteins, coagulation factors, hormones, etc.). Then the kidneys remove excess amounts of urea, salts, etc. by putting them into the urine. After the blood has passed through the liver and the kidneys, it has the correct composition that the body needs.