ABSTRACT
The thymus hormone is responsible for the development of thymic lymphocytes. These are part of the immune system and give rise to cellular immunity. The kidneys secrete three separate hormones. Renin is secreted in response to a decrease in blood pressure. The lowered blood pressure is detected by the decreased blood flow through the kidneys. Renin acts as an enzyme to turn a plasma protein into angiotensin. This causes vasoconstriction, thereby raising the blood pressure back towards its normal value. The pineal gland secretes melatonin almost exclusively at night. In humans it has a role in matching body rhythms to the day–night cycle. The gastrointestinal (GI) tract secretes a number of different hormones, most of which are involved in controlling digestion. Gastrin is secreted in response to certain foodstuffs, like proteins, being present in the stomach. It leads to an increase in the amount of acid being secreted from the stomach wall.
