5.1.4(a)-(b) - Endocrine communication and adrenal glands

Blood-Borne Signals

A hormone is a chemical messenger. It is produced by a gland or endocrine tissue, released into the blood, carried around the body in the blood plasma, and detected by particular target cells or target tissues.

The word directly matters. Endocrine glands are often described as ductless because their hormones enter the blood rather than travelling along a duct. This contrasts with an exocrine gland, which releases a product into a duct; digestive enzymes from the pancreas are a common example of exocrine secretion, but the pancreas also has endocrine tissue in a later lesson.

For this lesson, the core route is:

1. an endocrine gland secretes a hormone;
2. the hormone enters the blood;
3. the blood transports the hormone around the body;
4. target cells or tissues detect the hormone;
5. the target cells alter their activity.

Use that exact distinction in the definition check.

Target Cell Detection

A hormone may be carried all around the body, but it does not cause every cell to respond. A cell responds only if it can detect that hormone. This requires a specific receptor with a binding site complementary to the hormone and suitable intracellular signalling machinery.

[DIAGRAM: endocrine_signal_route: Lesson 106: Endocrine communication and adrenal glands - diagram 01; asset_slug: 106_m05_1_4_endocrine_communication_and_adrenal_glands__diagram_01; recommended_method: drawn_biology; description: A clean drawn biology flow diagram showing an endocrine gland secreting hormone molecules into a blood vessel, blood transporting the hormone, a target tissue with matching receptors responding, and a non-target tissue lacking matching receptors not responding.]

The diagram shows why blood transport and target-cell detection are separate ideas. The blood is the distribution route. Receptors explain specificity.

This is also why hormones can coordinate whole-body responses. One hormone can travel widely, but different tissues respond only if they are target tissues and if their receptors/signalling machinery lead to a response.

Peptide hormones cannot readily cross the phospholipid bilayer, so they bind cell-surface receptors and activate signalling inside the cell. Lipid-soluble steroid hormones can enter cells and bind intracellular receptors, often altering gene expression. Receptor location depends on hormone chemistry; all hormones do not share one receptor location.

Adrenal Gland Structure

The adrenal glands are a pair of endocrine glands. One adrenal gland is positioned above each kidney. Each gland has two main regions:

• the adrenal cortex, the outer region;
• the adrenal medulla, the inner region.

Both regions are endocrine tissue because they secrete hormones directly into the blood. They are not just two labels on the same tissue: the cortex and medulla secrete different hormones and have different functions.

The terms cortex and medulla appear in other organs too, so do not rely on memory tricks alone. In the adrenal gland, the cortex is outside and the medulla is inside.