Endocrine System

The endocrine system is a system of ductless glands that release chemicals (hormones) into the blood to regulate body functions

  • A hormone is a chemical messenger that is transported via the bloodstream to act on distant target cells
  • Hormones are specific and will only activate cells or tissues that possess the appropriate target receptor
  • The endocrine system is slower to initiate, but has a more prolonged response when compared to the nervous system

Endocrine Signalling


Endocrine Glands

Endocrine glands secrete their product (hormones) directly into the bloodstream, rather than through a duct (e.g. exocrine gland)

  • Major endocrine glands include the pancreas, adrenal gland, thyroid gland, pineal gland and the gonads (ovaries and testes)
  • The hypothalamus and pituitary gland are neuroendocrine glands and function to link the nervous and endocrine systems
  • Some organs may also secrete hormones despite not being endocrine glands (e.g. adipose tissue secretes leptin)

Examples of Endocrine Glands

endocrine table


The hypothalamus is the section of the brain that links the nervous and endocrine systems in order to maintain homeostasis

  • It receives information from nerves throughout the body and other parts of the brain and initiates endocrine responses
  • It secretes certain neurochemicals (called releasing factors) into a portal system which stimulate or inhibit the pituitary gland
  • It also secretes certain hormones directly into the bloodstream via neurosecretory cells that extend into the pituitary gland

The pituitary gland lies adjacent to the hypothalamus and is in direct contact due to a portal blood system

  • The pituitary gland is often referred to as the ‘master gland’, as it controls the secretion of a number of other endocrine glands
  • The pituitary gland receives instructions from the hypothalamus and consists of two lobes (anterior and posterior lobe)
    • The anterior lobe (adenohypophysis) releases hormones in response to stimulation by hypothalamic releasing factors
    • The posterior lobe (neurohypophysis) releases hormones produced by the hypothalamus itself (via neurosecretory cells)

The Role of the Hypothalamus and Pituitary Gland in Endocrine Function