In order for blood to perform its essential functions of bringing nutrients and oxygen to the cells of the body, and carrying waste materials away from those cells, the chemical composition of the blood must be carefully controlled. Blood contains particles of many different sizes and types, including cells, proteins, dissolved ions, and organic waste products. Some of these particles, such as proteins like hemoglobin, are essential for the body. Others, such as urea (a waste product from protein metabolism), must be removed from the blood or they will accumulate and interfere with normal metabolic processes. Still other particles, including many of the simple ions dissolved in the blood, are required by the body in certain concentrations that must be tightly regulated, especially when the intake of these chemicals varies. The body has many different means of controlling the chemical composition of the blood. The largest responsibility for maintaining the chemistry of the blood falls to the kidneys, a pair of organs located just behind the lining of the abdominal cavity. It is the job of the kidneys to remove the harmful particles from the blood and to regulate the blood's ionic concentrations, while keeping the essential particles in the blood. The kidneys meet these challenges through a remarkably elegant system. Essentially, kidneys act like dialysis units for blood, making use of the different sizes of the particles and specially-maintained concentration gradients. Blood passes through the membrane-lined tubules of the kidney, which are analogous to the dialysis bags used in this Experiment. Particles that can pass through the membrane pass out of the tubules by diffusion, thus separating the particles that remain in the blood from those that will be removed from the blood and excreted. When the kidneys do not function properly, dialysis must be performed artificially. Without this artificial kidney dialysis, toxic wastes build up in the blood and tissues, and cannot be filtered out by the ailing kidneys. This condition is known as uremia, which means literally "urine in the blood." Eventually this waste build-up leads to death. The artificial kidney uses cellulose membranes in place of the phospholipid-bilayer membranes used by real kidneys to separate the components of blood. Dr Andrew Levine is also the Director of the following Dialysis Clinics: DSI 411 Lindberg Mcallen Tx. 78501 956-687-6701 Fresenius Medical Care 901 Plaza Drive Mission Tx. 78572 Fresenius Medical Care 5406 S Jackson Rd. Edinburg Tx 78569 956-668-1205 Fresenius Medical Care 1653 Treasure Hills Blvd Harlingen Tx. 78550 956-412-1097 DSI 910 S, Utah Street Weslaco Tx. 78596