The endocrine system

Glands and hormones

• Endocrine glands
• Produce and secrete hormones which regulate the activity of the cells or organs
• The endocrine system is regulated by feedback
• e.g. a signal is sent from the hypothalamus to the pituitary gland ('releasing hormone') which causes the pituitary to secrete a 'stimulating hormone' into bloodstream. This signals the target gland e.g. the adrenal to secrete its hormone. As levels of this hormone increases the hypothalamus shuts down the secretion of the stimulating hormone, thus slowing down the secretion of the targets hormone and resulting in stable concentration in the bloodstream
• Hormones
• A hormone usually only affects a limited number of cells - target cells, which respond as they have the receptor for that hormone. When enough receptor sites are stimulated by hormones, it results in a physiological reaction. Too much or too little at the wrong time can result in a dysfunction. E.g. too much cortisol lead to Cushing's syndrome.

Pituitary gland

• Hormones produced by the pituitary gland
• Has two main parts, which produce different hormones to act on different glands or cells:
• Anterior (front) - releases ACTH as a response to stress, which stimulates the adrenal glands to produce cortisol. It also produces luteinising hormone and follicle stimulating hormone which are important in controlling reproductive functions and sexual characteristics, like in females the ovaries are stimulated to produce oestrogen and progesterone and in males, the testes with testosterone and sperm
• Posterior (back) - releases oxytocin, which stimulates contractionof the uterus during childbirth and is important for mother-infant bonding
• It declines with age (Elabd et al, 2014)
• Produces hormones to influence the release of hormones from other glands and then regulate the body's functions.
• It is controlled by the hypothalamus which receives information about basic body functions which uses it to regulate them by e.g. the pituitary gland.
• The pituitary gland produces hormones that travel in the bloodstream to specific target
• These hormones either directly cause changes in physiological processes or stimulate other glands to produce hormones
• High levels of hormones can stop the hypothalamus and pituitary releasing their own hormones - negative feedback - that prevents hormone levels rising too high

The adrenal glands

• On top of the kidneys
• Outer part is called adrenal cortex and inner region is called adrenal medulla. The main difference is that adrenal cortex hormones are essential to life, those released by the adrenal medulla are not
• Hormones produced by the adrenal glands
• Adrenal cortex produces cortisol to support body functions like cardiovascular and anti-inflammatory. Cortisol production is increased in response to stress. If the cortisol level is low, there is low blood pressure, poor immune function and an inability to deal with stress
• Aldosterone, responsible for maintaining blood volume and blood pressure
• Adrenal medulla releases adrenaline and noradrenaline (fight or flight). Adrenaline helps deal with stressful situations by increasing heart rate and blood flow to muscles and brain and conversion of glycogen to glucogen for energy. Noradrenaline constrict blood vessels, causing blood pressure to increase
• Ovaries
• Female reproductive system - release oestrogen and progesterone
• Testes
• Male reproductive system - release testosterone