Human endocrine system

The endocrine system is a group of glands that release chemical messengers called hormones directly into the bloodstream. Blood carries these hormones around the body, allowing a gland to send a signal to another part of the body.

Although a hormone travels throughout the body, it produces an effect only in a target organ. Cells in that organ can respond because they have the appropriate receptors for the hormone. In this way, hormonal signals coordinate activity between glands and other parts of the body.

Locations of endocrine glands

• The pituitary gland is at the base of the brain.
• The thyroid gland is in the neck, in front of the trachea.
• The pancreas is in the upper abdomen, behind the stomach.
• The adrenal glands sit on top of the kidneys.
• The ovaries are in the pelvis, on either side of the uterus.
• The testes are in the scrotum.

Pituitary gland

The pituitary gland is a gland at the base of the brain. It releases chemical messengers called hormones into the blood. The blood carries these hormones to other parts of the body.

The pituitary is called a master gland because some of its hormones control other endocrine glands. An endocrine gland releases hormones into the blood.

How the pituitary coordinates a hormonal response

In response to conditions in the body, the pituitary can secrete a hormone that stimulates another endocrine gland. That gland then releases its own hormone into the blood, producing an effect in the body.

The response therefore follows a chain of signals: pituitary hormone → another endocrine gland → that gland’s hormone → an effect in the body. In this way, the pituitary helps coordinate hormonal responses through other glands.

Speed and duration of nervous and hormonal control

The body can coordinate activity using signals carried along nerves or chemical messengers carried in the blood. These two methods differ in how quickly they act and how long their effects generally last.

Nervous control acts rapidly because signals travel quickly along nerves to the parts of the body that respond. Its effects are generally short-lived: the response usually does not continue for long after the signalling stops.

Hormonal control uses chemical messengers called hormones. They travel in the blood to target organs, so their effects generally begin more slowly than those of nervous control. Hormonal effects also tend to last longer.

Speed and duration describe different features of a response: speed is how quickly an effect begins, whereas duration is how long the effect lasts. Nervous control is generally rapid and short-lived; hormonal control is generally slower and longer-lasting.