75 Additional activities of the kidneys
Learning Objectives
After reading this chapter, you should be able to-
- Describe the role of the kidney in vitamin D activation.
- Describe the role of the kidney in regulating erythropoiesis.
- Describe the effect of parathyroid hormone (PTH) on renal handling of calcium and phosphate.
All systems of the body are interrelated. A change in one system may affect all other systems in the body, with mild to devastating effects. A failure of urinary continence can be embarrassing and inconvenient, but is not life threatening. The loss of other urinary functions may prove fatal. A failure to synthesize vitamin D is one such example.
Vitamin D Synthesis
In order for vitamin D to become active, it must undergo a hydroxylation reaction in the kidney, that is, an –OH group must be added to calcidiol to make calcitriol (1,25-dihydroxycholecalciferol). Activated vitamin D is important for absorption of Ca++ in the digestive tract, its reabsorption in the kidney, and the maintenance of normal serum concentrations of Ca++ and phosphate. Calcium is vitally important in bone health, muscle contraction, hormone secretion, and neurotransmitter release. Inadequate Ca++ leads to disorders like osteoporosis and osteomalacia in adults and rickets in children. Deficits may also result in problems with cell proliferation, neuromuscular function, blood clotting, and the inflammatory response. Recent research has confirmed that vitamin D receptors are present in most, if not all, cells of the body, reflecting the systemic importance of vitamin D. Many scientists have suggested it be referred to as a hormone rather than a vitamin.
Erythropoiesis
Erythropoetin (EPO) is a hormone produced by the kidney that stimulates the formation of red blood cells in the bone marrow. The kidney produces 85 percent of circulating EPO; the liver, the remainder. If you move to a higher altitude, the partial pressure of oxygen is lower, meaning there is less pressure to push oxygen across the alveolar membrane and into the red blood cell. One way the body compensates is to manufacture more red blood cells by increasing EPO production. If you start an aerobic exercise program, your tissues will need more oxygen to cope, and the kidney will respond with more EPO. If erythrocytes are lost due to severe or prolonged bleeding, or under produced due to disease or severe malnutrition, the kidneys come to the rescue by producing more EPO. Renal failure (loss of EPO production) is associated with anemia, which makes it difficult for the body to cope with increased oxygen demands or to supply oxygen adequately even under normal conditions. Anemia diminishes performance and can be life threatening.
Parathyroid Hormone (PTH) and Renal Handling of Calcium and Phosphate
Parathyroid hormone (PTH) plays a crucial role in regulating calcium and phosphate levels in the blood. When calcium levels drop, the parathyroid glands release PTH. PTH acts on the kidneys by increasing calcium reabsorption in the distal tubules, thereby reducing the amount of calcium excreted in the urine. This helps to maintain or raise blood calcium levels. Simultaneously, PTH decreases phosphate reabsorption in the proximal tubules, leading to increased phosphate excretion. This dual action helps to keep calcium and phosphate levels in balance, which is essential for many physiological processes, including bone mineralization and neuromuscular function.
Adapted from Anatomy & Physiology by Lindsay M. Biga et al, shared under a Creative Commons Attribution-ShareAlike 4.0 International License, chapter 25
disorder in which the bone softens
hormone produced by the kidney that stimulates the formation of red blood cells in the bone marrow
hormone produced by the parathyroid gland responsible for regulating calcium reabsorption
