Antidiuretic hormone (also known as vasopressin) is a 9-peptide hormone secreted by nerve cells in the supraoptic and paraventricular nuclei of the hypothalamus and released after reaching the pituitary gland via the hypothalamic-pituitary bundle. Its main role is to increase water permeability of the distal convoluted tubules and collecting ducts, promote water absorption, and is a key regulatory hormone for urine concentration and dilution. In addition, the hormone also enhances the permeability of the collecting ducts of the inner medulla to urea. After drinking large amounts of water, blood is diluted, crystal osmolarity decreases, and antidiuretic hormone secretion is reduced. The main factors regulating antidiuretic hormone are plasma crystal osmolarity and circulating blood volume and arterial blood pressure. 1. Changes in plasma crystal osmolarity can significantly affect the secretion of antidiuretic hormone. Heavy sweating. When the body loses water due to severe vomiting or diarrhea, the plasma crystal osmolarity increases, which can cause an increase in the secretion of antidiuretic hormone, resulting in an obvious increase in the reabsorption of water by the kidneys, leading to a concentration of urine and a decrease in urine volume. On the contrary, after drinking large amounts of water, the urine is diluted and the urine volume increases, thus causing the excess water in the organism to be excreted from the body. For example, after a normal person drinks 100ml of clear water at a time, the urine volume starts to increase after about half an hour, and by the end of the first hour, the urine volume can reach the highest value; then the urine volume decreases, and after 2-3 hours, the urine volume returns to the original level. If isotonic saline (0.9 NaCI solution) is consumed, the urine output does not show the same changes as after drinking clear water. This phenomenon of increased urine volume caused by drinking large amounts of clear water is called hydronephrosis, and it is a common test used clinically to detect the dilution capacity of the kidney. 2. Changes in circulating blood volume can reflexively affect the release of antidiuretic hormone. When the blood volume is excessive, the left atrium is dilated, which stimulates the volume receptors, and the afferent impulses are transmitted to the center via the vagus nerve, which inhibits the release of antidiuretic hormone from the hypothalamic-posterior pituitary system, thus causing diuresis, and normal blood volume is thus restored due to the discharge of excess water. The opposite change occurs when blood volume is reduced. An increase in arterial blood pressure stimulates carotid sinus pressure receptors, which reflexively inhibit the release of antidiuretic hormone. In addition, atrial natriuretic peptide inhibits antidiuretic hormone secretion, while angiotensin II stimulates its secretion.