Overview of the Endocrine System

Overview of the Endocrine System

The endocrine system is a system of ductless glands that secrete hormones—chemical messengers that are carried for long distances.

Learning Objectives

Produce a brief overview of the endocrine system

Key Takeaways

Key Points

  • The endocrine system is a system of ductless glands that secrete hormones directly into the circulatory system to be carried long distances to other target organs that regulate key body and organ functions.
  • The major endocrine glands include the pituitary, pineal, ovaries, testes, thyroid, hypothalamus, and adrenal glands.

Key Terms

  • hormone: A molecule released by a cell or a gland in one part of the body that sends out messages affecting cells in other parts of the organism.
  • endocrine system: The system of ductless glands that secretes hormones directly into the circulatory system.

The Endocrine System

The endocrine system is a system of ductless glands that secretes hormones directly into the circulatory system to be carried long distances to other target organs regulating key body and organ functions. For example, the pineal gland, located at the base of the brain, secretes the hormone melatonin, responsible for regulating sleep patterns.

Endocrine glands are typically well vascularized and the cells comprising the tissue are typically rich in intracellular vacuoles or granules that store hormones prior to release. Endocrine signaling is typically slow to initiate but is prolonged in response; this provides a counterpoint to the more rapid and short-lived nervous system signals.

The endocrine system is in contrast to the exocrine system, which features ducted glands that secrete substances onto an epithelial surface; for example, a sweat gland. Additionally the endocrine system is differentiated from shorter distance signaling such as autocrine (a cell affecting itself), juxtacrine (a cell affecting it’s direct neighbors), and paracrine (a cell affecting other nearby cells) signaling.

Key Endocrine Glands

The major endocrine glands include the pituitary, pineal, ovaries, testes, thyroid, hypothalamus and adrenal glands, additionally other tissues such as the kidney and liver also display secondary adrenal functions.

This is a drawing of the head and neck that shows the locations of the endocrine systems. The endocrine systems found in the head and neck include the hypothalamus, pineal, pituitary and thyroid glands.

Endocrine glands of the head and neck: The endocrine systems found in the head and neck include the hypothalamus, pineal, pituitary and thyroid glands.

Comparing the Nervous and Endocrine Systems

The nervous system and endocrine system both use chemical messengers to signal cells, but each has a different transmission speed.

Learning Objectives

Distinguish between the nervous system and the endocrine system

Key Takeaways

Key Points

  • The nervous system can respond quickly to stimuli, through the use of action potentials and neurotransmitters.
  • Responses to nervous system stimulation are typically quick but short lived.
  • The endocrine system responds to stimulation by secreting hormones into the circulatory system that travel to the target tissue. 
  • Responses to endocrine system stimulation are typically slow but long lasting.

Key Terms

  • hormone: A molecule released by a cell or a gland in one part of the body that sends out messages affecting cells in other parts of the organism.
  • neurotransmitters: Endogenous chemicals that transmit signals from a neuron to a target cell across a synapse.

The body must maintain a constant internal environment, through a process termed homeostasis, while also being able to respond and adapt to external events. The nervous and endocrine systems both work to bring about this adaptation, but their response patterns are different. The nervous system and the endocrine system use chemical messengers to signal cells, but the speed at which these messages are transmitted and the length of their effects differs.

Nervous System

The nervous system responds rapidly to stimuli by sending electrical action potentials along neurons, which in turn transmit these action potentials to their target cells using neurotransmitters, the chemical messenger of the nervous system. The response to stimuli by the nervous system is near instantaneous, although the effects are often short lived. An example is the recoil mechanism of an arm when touching something hot.

Endocrine System

The endocrine system relies on hormones to elicit responses from target cells. These hormones are synthesized in specialized glands at a distance from their target, and travel through the bloodstream or inter-cellular fluid. Upon reaching their target, hormones can induce cellular responses at a protein or genetic level.

This process takes significantly longer than that of the nervous system, as endocrine hormones must first be synthesized, transported to their target cell, and enter or signal the cell. However, although hormones act more slowly than a nervous impulse, their effects are typically longer lasting. 

Additionally, the target cells can respond to minute quantities of hormones and are sensitive to subtle changes in hormone concentration. For example, the growth hormones secreted by the pituitary gland are responsible for sustained growth during childhood.