The kidneys, along with the lungs are one of the two major sets of paired organs in the body. They are located in the lower back, just below and to either side of the vertebral column in an area known as the retroperitoneal space; the left kidney being situated slightly higher than the right due to its larger size. The main function of the kidneys is to clean and process the blood via the removal of waste products and excess water. Within the kidneys, a network of fine capillaries called the glomeruli act as filters. Compounds with a lower molecular weight than approximately 60,000 are able to pass through the glomeruli. Many of these molecules can then be reabsorbed into the bloodstream, these molecules can include water molecules, glucose molecules and nucleotides. This can be done by either passive movement of hydrophobic molecules through membranes or movement by transporters that have broad specificities. The kidneys also have an endocrine function, as they produce and secrete hormones such as erythropoietin.