(2) If A is the target organ of antidiuretic hormone, then hormone X is antidiuretic hormone, and antidiuretic hormone can promote the reabsorption of water by renal tubules and collecting ducts, so A stands for renal tubules and collecting ducts. After someone drinks a lot of water at one time, the osmotic pressure of extracellular fluid decreases and the antidiuretic hormone secreted by hypothalamus and pituitary gland decreases.
(3) After entering the lower temperature environment, the skin capillaries contract, and the heat dissipation decreases, while the secretion of thyroid hormone and adrenaline increases, which promotes metabolism and increases heat production, thus maintaining body temperature balance.
(4) Insulin is secreted by islet B cells, which can only bind to the receptor on the target cell, thus regulating the blood sugar concentration. The function of B organ (islet) in diabetic patients is normal, which may be caused by abnormal insulin receptor on target cells.
So the answer is:
(1) vasopressin neurotransmitter
(2) The number of renal tubules and collecting ducts decreased.
(3) shrinkage increases
(4) The insulin receptor on the target cell is abnormal.