FROM STAYING HEALTHY WITH KIDNEY DISEASE in press (Springer-Nature)
The kidneys have several jobs:
1. They eliminate waste products from protein breakdown. As muscles and other body tissues are repaired, the excess proteins are recycled. Any excess proteins are broken down and eliminated. Excess proteins can be broken down and then reassembled as glucose or ketones. Also, excess dietary protein must be broken down to ammonia and then converted to urea for elimination. When the kidneys fail, urea and other waste products accumulate.
2. Many medications are eliminated by the kidney. If the kidneys fail, these medications can accumulate. It is necessary for physicians to adjust the dose of medications in patients with CKD or avoid them altogether.
3. The kidneys make the hormone erythropoietin (EPO). EPO stimulates the precursor red blood cells in the bone marrow to mature. As the kidneys fail, the production of EPO decreases, and without their stimulus effect, kidney patients develop anemia.
4. Vitamin D is a hormone that absorbs calcium from the gastrointestinal tract and helps the bone cells make healthy bone. It has several additional effects and is involved in cell differentiation, blood pressure control, and the immune response. When the kidney function deteriorates the lack of active vitamin D leads to impaired calcium reabsorption and defective bone synthesis. Parathyroid hormone is synthesized and released in excess amounts from glands in the neck. This hormone stimulates the breakdown of bone as an adaptive response in an effort to maintain calcium levels.
5. Foods contain acid content that must be buffered and eliminated by the kidneys. As kidney function declines the bones first try to buffer the acids, exchanging calcium (Ca++) for hydrogen (H+) ions. As kidney function worsens acid levels build up. They cause cellular deterioration, particularly muscle weakness (sarcopenia). Metabolic acidosis is independently associated with mortality in CKD. Luckily nutritional adaptation and bicarbonate supplements can control metabolic acidosis and help preserve kidney function.
6. Potassium secretion is an important function of the kidneys. Most foods have some potassium in their cells, and many are loaded with potassium (bananas and oranges). Our cells depend upon a balance between potassium inside and outside of the cells. If there is too much potassium outside of the cell, the cell cannot function. Cramps, paralysis, and even death can occur. If the total body potassium levels are too low, heart cells cannot conduct regularly and develop arrhythmias. Potassium regulation by the kidney is critical for maintaining potassium in proper balance. Kidney disease may lead to increased serum potassium. This can be compounded by medications that are used to help protect the kidneys and preserve function.
7. The kidney tubules balance sodium so that it is effectively recycled. Sodium balance helps control blood pressure and body volume. The kidneys make it possible to go for extended periods of time without drinking fluids. They keep fluids in balance by diluting or concentrating the urine, based upon our fluid consumption, how we quenched our thirst, and weather conditions. When the kidneys fail, the kidneys are unable to get rid of excess fluids, nor can they concentrate and dilute the urine. Fluid overload and even congestive heart failure may result.