Question

Give reasons always vasopressin is not secreted

Answer 1

Vasopressin is also called Anti diuretic hormone. It is secreted when:
1- Decrease in extra cellular fluid volume occurs..
2- If osmolality of the plasma is increased.
Basically , its main function is the retention of water. This means it converts diluted urine into concentrated urine. If a person is drinking plenty of water , a lot of re absorption occurs so body tries to remove excess water more rather producing and secreting this hormone. 

Answer 2

Vasopressin or antidiuretic hormone is one of the hormones that is used by the body to regulate the loss of water from the body. Vasopressin hormone inhibits diuresis, that is to say loss of urine. It is thus important for retention of water in the body. It is only used by  the homeostatic system when the body is experiencing dehydration. Dehydration also cause low blood pressure, this is why vasopressin which is a vasoconstrictor, is used by this system to correct the low blood pressure.