The Digestive SystemRegulation of Extracellular Fluid Composition•The total amount of water and electrolytes in the body does not change unless the person is growing, gaining weight, or losing weight.ThirstoThe sensation of thirst increases if extracellular fluid becomes more concentrated or if blood pressure decreases.IonsoSodium ions are one of the dominant extracellular ions. Aldosterone increases sodium reabsorption from the filtrate. ADH increases water reabsorption from the nephron, and atrial natriuretic hormone increases sodium loss in the urine.oAldosterone increases potassium secretion in the urine. Increased blood levels of potassium stimulate, and decreased blood levels of potassium inhibit, aldosterone secretion.oParathyroid hormone secreted from the parathyroid glands increases extracellular calcium levels by causing bone resorption and increased calcium uptake in the kidney and small intestine. Parathyroid hormone increases vitamin D synthesis. Calcitonin, secreted by the thyroid gland,
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