What is a hormone

A hormone is a chemical messenger or regulator, secreted by an endocrine gland which reaches its destination by the bloodstream and has the power of influencing the activity of other organs. Some hormones have a slow action over a period of years such as the growth hormone from the anterior pituitary, while others have a quick action such as adrenaline from the adrenal medulla. Hormones, therefore, regulate and coordinate various functions in the body.

The endocrine glands are ductless glands, as the hormones they secrete pass directly into the bloodstream to influence the activity of another organ or gland. The main endocrine glands are as follows:

• pituitary gland

thyroid gland

• parathyroid glands

adrenal glands

• islets of Langerhans in the pancreas

• ovaries in the female

Parathyroid glands

Medulla

Anterior lobe

Posterior lobe

Pituitary

Anterior lobe

Posterior lobe

Pineal

Pituitary

Pineal

Parathyroid glands

Medulla

Testis

Fig 9.1 The endocrine glands

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