The corticomedullary gradient is largely responsible for developing the gradients that are needed to concentrate urine (more solutes and less water). The ability of the kidneys to produce concentrated urine is a major determinant of the ability to survive the warm weather. When temperatures are high, we lose water through sweat, and so the kidneys retain water to maintain fluidity in the blood. The maintenance of a sodium (salt) gradient is required for urine concentration because increased medullary sodium concentration increases the reabsorption of water into the kidney, to be redistributed in the blood. The purpose of this study is to know if the corticomedullary gradient is altered in patients across a wide spectrum of kidney disease using sodium Magnetic Resonance Imaging (MRI), a machine that takes pictures and measures the salt content in the kidneys. 23Na kidney MRI, will provide functional MR of the kidney as a non-invasive tool to describe medullary function to improve management of chronic and kidney disease.