Hormonal signalling

A hormone is a chemical substance produced by an endocrine gland and carried in the blood to one or more specific target organs. At a target organ, the hormone alters the organ’s activity. The blood carries the hormone around the body, but it acts only on its specific targets.

Hormone-producing glands and their locations

• The adrenal glands lie above the kidneys and secrete adrenaline.
• The pancreas lies in the upper abdomen and secretes insulin.
• The testes lie in the scrotum and secrete testosterone.
• The ovaries lie on either side of the uterus and secrete oestrogen.

Adrenaline in the fight-or-flight response

When a situation is threatening or stressful, the body prepares to respond quickly. The adrenal glands secrete the hormone adrenaline into the bloodstream. Adrenaline causes several changes in body activity as part of the fight-or-flight response:

• Breathing rate increases, bringing more oxygen into the body.
• Heart rate increases, helping to deliver oxygen to body tissues more quickly.
• Pupil diameter increases, allowing more light to enter the eyes.

These changes are effects of the same hormonal response, preparing the body to react to the situation.

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.

Glucagon secretion by the pancreas

The pancreas releases glucagon into the blood. Glucagon is a hormone: a chemical signal that travels in the blood to reach other parts of the body.

The pancreas also secretes insulin. Although glucagon and insulin are both hormones carried in the blood, they are different chemicals and act as distinct signals.

Adrenaline and blood glucose concentration

In a threatening or stressful situation, the hormone adrenaline helps prepare the body for a fight-or-flight response. One of its effects is to increase blood glucose concentration. This makes more glucose available in the blood as the body responds.

Adrenaline also increases heart rate. The increase in heart rate and the rise in blood glucose concentration are separate effects of the same hormone: the rise in blood glucose is not caused by the faster heart rate.