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Why can very high blood glucose cause dehydration and hyperglycemic coma?

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An anatomical illustration of the human pancreas shows its location near the stomach and intestines, a detailed cross-section highlighting ducts and the islet of Langerhans, and an inset of pancreatic tissue histology with labeled alpha, beta, and delta cells. — diabetes mellitus The islets of Langerhans are responsible for the endocrine function of the pancreas. Each islet contains beta, alpha, delta, and pancreatic polypeptide (PP) cells that are responsible for the secretion of pancreatic hormones. Beta cells secrete insulin, a well-characterized hormone that plays an important role in regulating glucose metabolism.

Very high blood glucose, or hyperglycemia, leads to dehydration and coma through the following physiological processes:

Dehydration

When blood glucose levels rise significantly, the kidneys cannot reabsorb all the filtered glucose. The excess glucose is excreted in the urine, where it creates an osmotic effect that prevents the reabsorption of water. This results in polyuria, or the excretion of large volumes of urine, which causes significant fluid loss and subsequent dehydration.123

Hyperglycemic Coma

The loss of large volumes of fluid reduces total blood volume and increases serum osmolality. In patients with type 2 diabetes, this severe and prolonged state can lead to vascular collapse and impaired central nervous system function, resulting in a hyperglycemic coma.13

Additionally, in type 1 diabetes, severe hyperglycemia is often accompanied by diabetic ketoacidosis. This occurs when the body breaks down fat for energy, releasing ketones that increase blood acidity, which further impairs nervous system function and can culminate in coma and death.14