V. URINARY SYSTEM
The waste in your blood is filtered and diffused into your kidneys. The red renal arteries bring the contaminated blood to the kidneys, while blue deoxygenated renal veins carry the clean blood back to the circulatory system. In the kidneys, water and useful components, such as amino acids, glucose (sugar) and some other nutrients are reabsorbed into the blood stream. Excess water and waste, in the form of urine, are transported through the ureters to the muscular bladder for storage. At the bottom of the bladder is a sphincter muscle, which is tightly contracted to prevent the urine from leaking out. However, when the urine reaches the volume of approximately 320 ml, the bladder is uncomfortably distended. If your bladder is full, nerve endings in the bladder wall will be touched off.  They then send impulses to the brain, which sends back messages to trigger your bladder to contract and the sphincter to relax, forcing out urine out of your body through the urethra.