The endocrine system and hormones (SACE Stage 2 Biology)

What this dot point is asking

You need to describe how hormones work, identify key glands and hormones, and compare the endocrine and nervous systems as the body's two coordination systems.

How the endocrine system works

The endocrine system is a collection of glands that secrete hormones directly into the bloodstream. A hormone is a chemical messenger carried in the blood to all parts of the body.

Although a hormone reaches every cell, it only affects target cells that have a specific receptor for it. The hormone binds to the receptor (like a key fitting a lock), triggering a response in that cell. Cells without the receptor are unaffected, which is how a blood-borne signal can still be specific.

Key glands and hormones

Examples that recur across Topic 3:

• Pancreas - releases insulin and glucagon to regulate blood glucose.
• Adrenal glands - release adrenaline for "fight or flight" responses.
• Thyroid gland - releases thyroxine, which controls metabolic rate.
• Pituitary gland - the "master gland", releasing hormones (such as ADH) that control other glands and processes.
• Hypothalamus - links the nervous and endocrine systems and controls the pituitary.

These hormones are central to the later dot points on blood glucose regulation, thermoregulation and osmoregulation.

How a hormone produces a response

The chemical nature of a hormone determines how it acts on its target cell, a distinction SACE sometimes probes. Steroid hormones (such as testosterone) are lipid-soluble, so they pass through the cell membrane and bind receptors inside the cell, often switching genes on or off directly. Peptide and protein hormones (such as insulin and ADH) are not lipid-soluble, so they bind receptors on the cell surface and trigger changes inside the cell through a second messenger, without entering the cell. In both cases the response is specific because only cells with the matching receptor can bind the hormone, and the size of the response depends on how much hormone is present and how many receptors the cell has.

Hormonal feedback and amplification

Many endocrine pathways are controlled by negative feedback, often through the hypothalamus and pituitary. For example, the hypothalamus stimulates the pituitary to release TSH, which stimulates the thyroid to release thyroxine; rising thyroxine then feeds back to inhibit the hypothalamus and pituitary, switching off further TSH. This keeps thyroxine within a narrow range. A single hormone molecule binding a receptor can also trigger many internal events, so a small hormone signal produces a large cellular response - a form of amplification that makes blood-borne signalling efficient despite the low concentrations of hormone involved.

Comparing the nervous and endocrine systems

Both systems coordinate the body and work together in homeostasis, but they differ:

In short, the nervous system handles rapid, short-term responses while the endocrine system handles slower, longer-lasting and more widespread responses.