The endocrine system is a communication system made up of glands and endocrine cells that secrete hormones into the bloodstream. Hormones are chemical messengers that regulate target cells and coordinate processes such as metabolism, growth, hydration, reproduction, and responses to exercise.
How Hormonal Communication Works
- An internal or external stimulus, such as increased blood glucose or physical activity, is detected.
- An endocrine gland releases a hormone directly into the blood.
- The blood transports the hormone throughout the body.
- Only target cells with a complementary receptor bind the hormone.
- Hormone-receptor binding initiates a cellular response, such as increased glucose uptake or a change in enzyme activity.
For example, during exercise the adrenal glands release adrenaline. Adrenaline binds to receptors on specific target cells, increasing heart rate and helping mobilize glucose for respiration. Although the hormone circulates widely, cells without the appropriate receptor do not respond.
| Hormone type | Mechanism of action |
|---|---|
| Water-soluble hormones, such as adrenaline | Bind to receptors on the cell membrane and activate intracellular signalling pathways |
| Lipid-soluble hormones, such as testosterone | Pass through the cell membrane and bind to intracellular receptors, influencing gene expression |
Hormone secretion is commonly controlled through negative feedback, which reverses a deviation from a normal range. After blood glucose rises, the pancreas secretes insulin; insulin promotes glucose uptake and storage, causing blood glucose to fall. As the level returns toward its set range, insulin secretion decreases.
Compared with nervous communication, endocrine communication generally has a slower onset but produces longer-lasting and more widespread effects. The two systems interact to maintain homeostasis and coordinate exercise responses.
Exam Technique
For IB SEHS subtopic A.1.1, explain the complete communication pathway: stimulus, gland, hormone, bloodstream, receptor, target cell, and response. Avoid the common misconception that every cell responds to a circulating hormone; only cells with the correct receptor respond.
