What is the chemical messenger released by the endocrine system into the blood stream?

What is a hormone?

A hormone is a chemical that is made by specialist cells, usually within an endocrine gland, and it is released into the bloodstream to send a message to another part of the body. It is often referred to as a ‘chemical messenger’. Hormones are found in all multicellular organisms and their role is to provide an internal communication system between cells located in distant parts of the body.

In the human body, hormones are used for two types of communication. The first is for communication between two endocrine glands, where one gland releases a hormone which stimulates another target gland to change the levels of hormones that it is releasing. The second is between an endocrine gland and a target organ, for example when the pancreas releases insulin which causes muscle and fat cells to take up glucose from the bloodstream.

Since hormones are released into the bloodstream and can therefore be carried around the entire body, they can perform both of these actions on many different targets. The complex interplay between the glands, hormones and other target organs is referred to as the endocrine system. Hormones affect many physiological activities including growth, metabolism, appetite, puberty and fertility.

The endocrine system, along with the nervous system, functions in the regulation of body activities. The nervous system acts through electrical impulses and neurotransmitters to cause muscle contraction and glandular secretion. The effect is of short duration, measured in seconds, and localized. The endocrine system acts through chemical messengers called hormones that influence growth, development, and metabolic activities. The action of the endocrine system is measured in minutes, hours, or weeks and is more generalized than the action of the nervous system.

There are two major categories of glands in the body - exocrine and endocrine.

Exocrine Glands

Exocrine glands have ducts that carry their secretory product to a surface. These glands include the sweat, sebaceous, and mammary glands and, the glands that secrete digestive enzymes.

Endocrine Glands

The endocrine glands do not have ducts to carry their product to a surface. They are called ductless glands. The word endocrine is derived from the Greek terms "endo," meaning within, and "krine," meaning to separate or secrete. The secretory products of endocrine glands are called hormones and are secreted directly into the blood and then carried throughout the body where they influence only those cells that have receptor sites for that hormone.

Hormones regulate internal functions from metabolism and growth to sexual development and the induction of birth. They circulate through the bloodstream, bind to target cells, and adjust the function of whole tissues and organs. It all starts with the hypothalamus and the pituitary gland, the masters of the endocrine system. The hormones they release control the secretions of the other endocrine glands and most endocrine functions. Throughout the body, hormones enable reactions to stress and other outside changes and keep regular processes running smoothly.

1. Circulation: How Hormones Reach and Target Particular Cells

Endocrine glands secrete hormones directly into the bloodstream. The hormones dissolve in plasma and travel the circulatory pathways through various body tissues. So why do hormones affect only their target cells in particular tissues? Because only those target cells have receptors for that particular hormone. Some hormones bind to receptors on the surface of target cells. Others enter the cells and bind to receptors in the cytoplasm or nucleus. The binding triggers a series of actions that change the physiologic behavior of the cell. In this way hormones can adjust the functions of whole organs and regulate total body processes such as metabolism and growth.

2. Directing from the Top: The Hypothalamus Sends Hormones to the Pituitary Gland to Control the Endocrine System

The hypothalamus and the pituitary gland (which is also called the hypophysis) are part of the diencephalon region of the brain. The hypothalamus receives nervous signals, processes them, and secretes hormones. These hormones enter the hypophyseal portal system, a network of capillaries that connect the hypothalamus to the pituitary. Capillaries from the superior hypophyseal artery surround the hypophysis and collect hypothalamic hormones that are carried to the anterior lobe of the pituitary via the portal system, where they stimulate or inhibit the release of pituitary hormones. Axons from the hypothalamus reach into the posterior pituitary where they release two hormones, oxytocin (OXT) and antidiuretic hormone (ADH or vasopressin), for storage and later release. The hormones generated by the hypothalamus and the pituitary control other endocrine glands and all major internal functions.

3. Anterior Pituitary Hormones Stimulate Growth and Control Glands throughout the Body

Hormones sent from the hypothalamus to the anterior lobe of the pituitary gland function as signals. They stimulate or inhibit the release of anterior pituitary hormones, which regulate endocrine glands and control a range of body functions. Human growth hormone (hGH) travels to skeletal muscles, bones, and the liver to promote overall growth and development. Thyroid-stimulating hormone (TSH) and adrenocorticotropic hormone (ACTH) target the thyroid and adrenal glands, two primary endocrine glands that regulate metabolism for temperature regulation, growth, and stress resistance. Follicle-stimulating hormone (FSH) and luteinizing hormone (LH) stimulate sex cell production and reproductive processes in the gonads, and prolactin (PRL) induces milk production in mammary glands.

4. Posterior Pituitary Hormones Regulate Water Levels and Induce Labor

Most hormones secreted by the hypothalamus travel to the anterior lobe of the pituitary, where they stimulate or inhibit the release of other hormones. But two, antidiuretic hormone (ADH or vasopressin) and oxytocin (OXT), are secreted into the posterior pituitary lobe by axonal extensions from the hypothalamus. The posterior pituitary stores ADH and OXT and releases them directly into the bloodstream when needed. ADH acts on the kidneys, blood vessels, and sweat glands in the skin to reduce water loss throughout the body. OXT factors into pregnancy and nurturing. It causes smooth muscle contractions of the uterus to induce birth. Later it stimulates milk ejection from the mammary glands and promotes bonding between mother and child.

5. Hormones Fuel the Body’s Response to New Stimuli and Stress

Hormones control ongoing internal functions. They also enable our body’s reactions to changes in the environment — for example, when we perceive a sudden threat or find ourselves under stress. In this case, the hypothalamus commands the adrenal glands directly (via nervous signals) to ramp up production of epinephrine and norepinephrine. These hormones promote the “fight-or-flight” response: breathing and heart rate increase and our muscles get a burst of energy. If the situation continues, the endocrine system kicks into the “resistance phase”: The hypothalamus directs the pituitary to release adrenocorticotropic hormone (ACTH). The ACTH stimulates the adrenal glands to release mineralocorticoids and glucocorticoids, and the pancreas secretes glucagon. These hormones increase blood sugar and sustain elevated blood flow and energy levels for prolonged stress.

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