In healthy adults, bone tissue is continually being remodeled and rebuilt. The kidneys play an important role in maintaining healthy bone mass and structure because one of their jobs is to balance calcium and phosphorus levels in the blood and ensure the vitamin D a person receives from sunlight and food becomes activated.Calcium is a mineral that builds and strengthens bones. Calcium is found in many foods, particularly milk and other dairy products. If calcium levels in the blood become too low, four small glands in the neck called the parathyroid glands release a hormone called parathyroid hormone (PTH). This hormone draws calcium from the bones to raise blood calcium levels. Too much PTH in the blood will remove too much calcium from the bones; over time, the constant removal of calcium weakens the bones.Phosphorus, an element found in most foods, also helps regulate calcium levels in the bones. Healthy kidneys remove excess phosphorus from the blood. When the kidneys stop working normally, phosphorus levels in the blood can become too high, leading to lower levels of calcium in the blood and resulting in higher PTH levels and the loss of calcium from the bones. Even before blood levels of phosphorus become elevated, the kidneys are forced to work harder to clear phosphorus from the body.Healthy kidneys produce calcitriol from vitamin D that is received from sunlight and food. Calcitriol helps the body absorb dietary calcium and phosphorus into the blood and bones. Calcitriol and PTH work together to keep calcium balance normal and bones healthy. If calcitriol levels drop too low, PTH levels increase and calcium is removed from the bones. In a person with kidney failure, the kidneys stop making calcitriol. The body then cannot absorb calcium from food, leading to increased PTH levels. The combination of decreased calcium absorption from food and PTH drawing calcium from bones makes the bones weak and brittle.
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