Function of the endocrine system

Maintaining homeostasis within the body requires the coordination of many different systems and organs.

The endocrine system is one of  communication mechanisms  between neighboring cells, and between cells and tissues in distant parts of the body.

This communication occurs through the release of chemicals called hormones.

Hormones are released into body fluids, usually blood, which carries them to their target cells where they elicit a response.

The cells that secrete hormones are often located in specific organs, called endocrine glands.

The cells, tissues, and organs that secrete hormones make up the endocrine system.

Organs of the endocrine system

Some of the regulatory functions of the endocrine system include:

  • the control of heart rate,
  • the control of blood pressure,
  • immune response to infection,
  • reproduction,
  • emotional state,
  • the overall growth and development of the body.

Parts of the endocrine system

The endocrine system consists of

Many other organs, such as the liver, skin, kidney, and parts of the digestive and circulatory systems, produce hormones in addition to their other physiological functions.

Endocrine vs. exocrine glands

Endocrine and exocrine parts of the endocrine systemEndocrine glands are ductless glands that secrete hormones directly into the bloodstream, whereas exocrine glands release their secretions through ducts or tubes.

Examples of exocrine glands are sweat glands, salivary glands, and tear (lacrimal) glands.

Video: Endocrine glands and hormones review

Different types of hormones in human body - steroid vs. nonsteroid hormones and their mechanisms of action

The endocrine system produces two main types of hormones:

  1. steroid hormones
  2. nonsteroid hormones

Steroid hormones

Steroid hormones, such as cortisol, are manufactured from cholesterol.

Each type of steroid hormone is composed of a central structure of four carbon rings attached to distinctive side chains that determine the hormone’s specific and unique properties.

Within the endocrine cells, steroid hormones are synthesized in the smooth endoplasmic reticulum (ER). Cholesterol and steroid hormones 

Since steroid hormones are hydrophobic, they combine with a protein carrier that transports them through the bloodstream. 

Fat-soluble steroid hormones can pass through the membrane of a target cell.

Once inside the target cell, steroid hormones attach to a protein receptor molecule in the cytoplasm. This hormone- receptor complex then enters the nucleus, where it binds with and activates a specific gene on the cell’s DNA molecule.

The activated gene then produces an enzyme that initiates the desired chemical reaction within the cell.

Nonsteroid hormones

Nonsteroid hormones, such as adrenaline, are composed of either proteins, peptides, or amino acids.

These hormone molecules are not fat-soluble, so they usually do not enter cells to exert their effect. Instead, they bind to receptors on the surface of target cells.

This combination substance then triggers a specific chain of chemical reactions within the cell. 

Types of hormones and their actions (steroid and nonsteroid hormones)

Video: Endocrine system - Types of hormones

Organs of the endocrine system, their hormones and functions

Endocrine GlandAssociated HormonesEffect

Pituitary gland

Pituitary (anterior) growth hormone promotes growth of body tissues 
  prolactin promotes milk production
  thyroid-stimulating hormone stimulates thyroid hormone release
  adrenocorticotropic hormone stimulates hormone release by adrenal cortex 
  follicle-stimulating hormone  stimulates gamete production
  luteinizing hormone stimulates androgen production by gonads in males;
stimulates ovulation and production of estrogen and progesterone in females
Pituitary (posterior) antidiuretic hormone stimulates water reabsorption by kidneys
  oxytocin stimulates uterine contractions during childbirth

Thyroid and Parathyroid glands

Thyroid thyroxine,
stimulate metabolism 
  calcitonin reduces blood Ca2+ levels 
Parathyroid parathyroid hormone increases blood Ca2+ levels

Adrenal gland

aldosterone increases blood Na+ levels
increase blood-glucose levels
stimulate fight-or-flight response


Pancreas insulin  reduces blood-glucose levels
  glucagon  increases blood-glucose levels

Pineal gland

Pineal gland melatonin regulates circadian rhythms

Thymus gland

Thymus gland thymosin  stimulates the development of T cells