DIET AND RENAL DISEASE
The kidneys are intricate and efficient processing systems that excrete wastes, maintain volume and composition of body fluids, and secrete certain hormones. To accomplish these tasks, they filter the blood, cleansing it of waste products, and recycle other, usable, substances so that the necessary constituents of body fluids are constantly available (Figure 19-1).
Each kidney contains approximately 1 million working parts called nephrons. Each nephron contains a filtering unit, called a glomerulus, inwhich there is a cluster of specialized capillaries (tiny blood vessels connecting veins and arteries). Approximately 180 liters of ultrafiltrate is processed each day. As the filtrate passes through the nephrons, it is concentrated or diluted to meet the body’s needs. In this way, the kidneys help maintain both the composition and the volume of body fluids and, consequently, they maintain fluid balance, acid-base balance, and electrolyte balance.
The liquid waste is sent via two tubes called ureters from the kidneys to the urinary bladder, from which they are excreted in approximately 1.5 liters of urine per day. These waste materials include end products of protein metabo-lism (urea, uric acid, creatinine, ammonia, and sulfates), excess water and nutrients, dead renal cells, and toxic substances. When the urinary output is less than 500 ml/day, it is impossible for all the daily wastes to be eliminated. This condition is called oliguria. When the kidneys are unable to adequately eliminate nitrogenous waste (end products of protein metabolism), renal failurecan result. The recycled materials are reabsorbed (taken back) by the blood.
They include amino acids, glucose, minerals, vitamins, and water.
The kidneys synthesize and secrete certain hormones as needed. For example, it is the kidneys that make the final conversion of vitamin D. Active vitamin D promotes the absorption of calcium and the metabolism of calcium and phosphorus. The kidneys indirectly stimulate bone marrow to reproduce red blood cells by producing the hormone erythropoietin.