6.3 - Endocrine System: Glands & Hormones
Definitions of glands and hormones
The endocrine system, sometimes called the hormonal system, consists of glands that produce hormones to send chemical signals throughout the body.
Glands are groups of specialised cells that secrete useful substances, including hormones. Hormones are chemical messengers that carry information to regulate bodily processes.
Types of hormones:
- Many hormones are proteins or peptides.
- Some hormones are steroids, such as progesterone.
How hormones are secreted and transported
Hormones are released in response to specific triggers and travel via the bloodstream to affect distant parts of the body.
Processes involved in hormone secretion and transport:
- Stimulation of glands - Glands release hormones when triggered by changes in the concentration of certain substances, including other hormones, or by electrical impulses.
- Movement through the body - Hormones diffuse directly into the bloodstream and are carried by the circulatory system.
- Targeting specific cells - Hormones leave the blood and bind only to receptors on target cells, known as effectors, where they initiate a response.
An example of hormone action
Hormones respond to changes in the body's internal environment, such as temperature, by triggering adjustments in target cells to restore balance.
Regulation of body temperature by thyroxine:
- Temperature receptors in the hypothalamus detect a drop in body temperature.
- The hypothalamus stimulates the thyroid gland to release thyroxine into the bloodstream.
- Thyroxine reaches target cells across the body.
- These cells increase their metabolic rate, producing heat as a byproduct.
- This process raises the overall body temperature back to normal levels.
Functions and major glands of the endocrine system
The endocrine system controls essential processes like growth, metabolism, sleep, and reproduction through a network of glands that release specific hormones.
Roles of the major glands:
| Gland | Key hormones | Main functions |
|---|---|---|
| Hypothalamus | Hormones that regulate the pituitary | Controls the pituitary gland and links the nervous and endocrine systems. |
| Pituitary gland | Various hormones | Acts as the 'master gland' by releasing hormones that influence other glands. |
| Pineal gland | Melatonin | Regulates sleep patterns by controlling the sleep-wake cycle. |
| Thyroid gland | Thyroxine | Influences metabolic rate, growth, and maturation. |
| Parathyroid glands | Parathyroid hormone | Manages calcium levels in the blood and bones. |
| Thymus gland | Thymosin (and others) | Supports the immune system by aiding T-cell development. |
| Adrenal glands | Adrenaline (and others) | Triggers the 'fight or flight' response to stress. |
| Pancreas | Insulin and glucagon | Maintains stable blood sugar levels. |
| Gonads (ovaries and testes) | Oestrogen, progesterone, testosterone | Involved in reproduction, development of sex organs, and secondary sexual characteristics. |
Characteristics of endocrine communication
Endocrine communication differs from nervous system signalling by being slower but more sustained and widespread in its effects.
Features of hormone-based communication:
- Speed and duration - Hormone effects are slower than electrical signals from nerves because hormones travel through the blood rather than being released directly onto target cells; however, hormones persist longer as they are not broken down as quickly as neurotransmitters.
- Scope of response - Hormones circulate throughout the body, leading to widespread effects if target cells are distributed widely.