Human Endocrine System

Chemical messages carried by the blood

The body's organs need to work together. The endocrine system coordinates and controls body functions using chemical messages called hormones. It consists of glands that secrete, or release, hormones directly into the bloodstream.

Hormones are large molecules. The blood carries them around the body, but a particular hormone produces its effect at a target organ. This is the organ that responds to that hormone. Hormonal control therefore connects a gland in one part of the body with an organ elsewhere: the gland releases the message, the blood transports it and the target organ responds.

The nervous system also coordinates responses, but it carries electrical impulses along nerve cells. Compared with nervous responses, hormonal responses are generally slower to take effect but last longer. Hormonal control is therefore useful for regulating processes over time, although some hormones also help the body respond to emergencies.

The pituitary gland coordinates other glands

The pituitary gland, located in the brain, is often called the master gland. It secretes several hormones, some of which act on other endocrine glands and stimulate them to release their own hormones.

This creates a chain of control: a pituitary hormone acts on another gland, which releases a hormone that affects the body. For example, the pituitary gland can stimulate the thyroid gland to release thyroxine.

Adrenaline prepares the body for action — Higher Tier only

Adrenaline is secreted by the adrenal glands, which lie just above the kidneys. Fear or stress can trigger its release, preparing the body for ‘flight or fight’: escaping from danger or confronting it.

Adrenaline increases heart rate and boosts the delivery of oxygen and glucose to the brain and muscles. Oxygen and glucose are needed for aerobic respiration, which releases energy. Increasing their delivery supports greater energy release for rapid action, such as running away. Adrenaline does not itself supply the energy; it helps cells obtain the substances needed to release it.

Thyroxine regulates activity at rest — Higher Tier only

The thyroid gland, in the neck, secretes thyroxine. Thyroxine stimulates the basal metabolic rate: the rate of chemical reactions in the body when it is at rest. Even when you are sitting still, your cells are carrying out reactions needed to keep you alive.

Thyroxine also plays an important role in proper growth and development. Its release needs to be regulated so that the amount in the blood stays around a normal level.

Negative feedback stabilises thyroxine levels — Higher Tier only

Negative feedback means that a change triggers a response which tends to reverse that change. If a level falls, the response raises it; if it rises too far, the response lowers it. This stabilises the system rather than allowing the original change to become larger.

Thyroxine control involves TSH, or thyroid-stimulating hormone, secreted by the pituitary gland. TSH stimulates the thyroid gland to release thyroxine.

• When thyroxine levels are too low, the pituitary secretes more TSH. The thyroid releases more thyroxine, raising its level towards normal.
• When thyroxine levels are too high, TSH secretion is inhibited, so less TSH is released. The thyroid releases less thyroxine, allowing its level to fall towards normal.

To interpret a negative feedback diagram, start with the change and follow the arrows through the response. In the low-thyroxine pathway, increased TSH leads to increased thyroxine release: the final rise opposes the initial fall. In the high-thyroxine pathway, reduced TSH leads to reduced thyroxine release: the final fall opposes the initial rise. In both cases, the outcome reverses the starting change. As thyroxine returns towards normal, the corrective response is reduced.

Endocrine control

• Endocrine glands secrete hormones directly into the blood.
• Hormones are large molecules carried to target organs, where they produce effects.
• Hormonal responses are slower but longer-lasting than nervous responses.
• The pituitary gland, in the brain, is the master gland: several of its hormones stimulate other glands to release hormones.

Adrenaline and thyroxine — Higher Tier only

• Adrenal glands → : boosts oxygen and glucose delivery to the brain and muscles for flight or fight. Supports respiration and energy release.