The Endocrine system sends information as chemical signals:

1. It’s also known as the hormonal system, it involves glands and hormones:

   • A gland is a group of cells that are specialised to secrete a useful substance, such as a hormone. For example, The pancreas secretes insulin

   • Hormones are ‘chemical messengers’. Many hormones are proteins or peptides, e.g. insulin. Some hormones are steroids, e.g. progesterone

2. Hormones are secreted when a gland is stimulated:

   • Glands can be stimulated by a change in concentration of a specific substance (sometimes another hormone)

   • They can also be stimulated by electrical impulses

3. Hormones diffuse directly into the blood, then they’re taken around the body by the circulatory system

4. They diffuse out of the blood all over the body, but each hormone will only bind to specific receptors for that hormone, found on the membranes of some cells (called target cells)

5. The hormones trigger a response in the target cell (the effectors)

   1. Stimulus: Low blood glucose concentration

   2. Receptors: Receptors on pancreas cells detect the low blood glucose concentration

   3. Hormone: The pancreas releases the hormone glucagon into the blood

   4. Effectors: Target cells in the liver detect glucagon and convert glycogen into glucose

   5. Response: Glucose is released into the blood, so glucose concentration increases

6. The endocrine system is responsible for regulating a large number of bodily functions, such as growth, metabolism, sleep, reproduction, etc. There are several major glands:

   • Hypothalamus: It produces hormones that control the pituitary gland

   • Pituitary gland: known as the ‘master gland’ because it releases hormones to control other glands in the endocrine system

   • Pineal gland: Responsible for the production of melatonin, which plays a role in the control of sleep patterns

   • Thyroid gland: Produces hormones such as thyroxine. The is responsible for controlling the body’s metabolic rate, as well as regulating growth and maturation

   • Parathyroid gland: Produces a hormone called the parathyroid hormone. This helps control the levels of minerals such as calcium within the body

   • Thymus gland: Regulates the immune system

   • Adrenal glands: Produce hormones such as adrenaline. Responsible for the ‘fight or flight’ response

   • Pancreas: Releases the hormones insulin and glucagon, which regulate blood sugar level

   • Gonads (ovaries and testes): Produce sex hormones, e.g. testosterone and oestrogen. These are important in reproduction and the development of sex organs and secondary sexual characteristics

Endocrine system communication is slower, long-lasting and widespread:

• Hormones aren’t released directly onto their target cells- they must travel in the blood to get there. This means that chemical communication (by hormones) is slower than electrical communication (by nerves

• They aren’t broken down as quickly as neurotransmitters, so the effects of hormones can last for much longer

• Hormones are transported all over the body, so the response may be widespread if the target cells are widespread

The Hypothalamus prepares you for ‘fight or flight’:

When the body is threatened it responds by preparing for action. This response is called the ‘fight or flight’ response. The hypothalamus helps coordinate this response:

The activation of the ‘fight or flight’ response:

1. In the initial shock response, the hypothalamus triggers activity in the sympathetic branch of the automatic nervous system (ANS).

2. This stimulates the adrenal medulla within the adrenal glands, which releases noradrenalin into the bloodstream.

3. These hormones affect the body in several ways, including:

   • Blood pressure and heart rate increase to get blood quickly to areas of the body where it’s needed for activity

   • Digestion decreases so that blood can be directed to the brain and muscles

   • Muscles become more tense so that the body is physically responsive

   • Perspiration increases so that the body can cool down

   • The breathing rate increases so that more oxygen can be sent to the muscles

   • Pupil size increases so more light can enter the eye to allow for clearer vision

   • Salvation decreases as the digestive system isn’t need

4. The result of these changes is that the body is ready to use energy to deal with the stressful situation.

   1. The hypothalamus triggers activity in the sympathetic branch of the ANS

   2. The ANS stimulates the adrenal medulla

   3. Adrenaline and noradrenaline released into the bloodstream cause physical effects on the body