Endocrine system  (Page 12/9)

The specific character of the greater part of the toxins which are known to us (I need only instance such toxins as those of tetanus and diphtheria) would suggest that the substances produced for effecting the corrleation of organs within the body, through the intermediation of the blood stream, might also belong to this class, since here also specificity of action must be a distinguishing characteristic. These chemical messengers, however, or "hormones"(from the Greek ὁρμῶν, to excite or arouse), as we might call them, have to be carried from the organ where they are produced to the organ which they affect by means of the blood stream, and the continually recurring physiological needs of the organism must determine their repeated production and circulation throughout the body.

Hormones , as Starling noted, are produced by one organ and affect the activities of other organs. Unlike neurotransmitters , which you will learn about later in this module, hormones move via the bloodstream from the site of production to the site of action. But like neurotransmitters, hormones are key players in maintaining homeostasis. Before we discuss that, however, we need to review homeostasis and introduce the major classes of animal hormones.

Types of hormones

Maintaining homeostasis within the body requires the coordination of many different systems and organs. Communication between neighboring cells, and between cells and tissues in distant parts of the body, occurs through the release of chemicals called hormones. Hormones are chemicals that are released by cells into body fluids (usually blood) and which act on target cells at some distance from the cells that release the hormone. At the target cells , which are cells that have a receptor for the chemical, the hormones elicit a response. The cells, tissues, and organs that secrete hormones make up the endocrine system. Examples of glands of the endocrine system include the adrenal glands, which produce hormones such as epinephrine and norepinephrine that regulate responses to stress, and the thyroid gland, which produces thyroid hormones that regulate metabolic rates.

Although there are many different hormones in the human body, they can be divided into two general classes based on their chemical structure and water solubility: steroid hormones (most are derivatives of cholesterol), which are not soluble in water, and peptide (peptides and proteins) hormones, which are readily soluble in water. One of the key distinguishing features of lipid-derived hormones is that they can diffuse across plasma membranes whereas the peptide hormones cannot.

Lipid-derived hormones (or lipid-soluble hormones)

Most lipid hormones are derived from cholesterol and thus are structurally similar to it, as illustrated in [link] . The primary class of lipid hormones in humans is the steroid hormones. Examples of steroid hormones include estradiol, which is an estrogen , or female sex hormone, and testosterone, which is an androgen, or male sex hormone. These two hormones are released by the female and male reproductive organs, respectively. Other steroid hormones include aldosterone and cortisol, which are released by the adrenal glands along with some other types of androgens. Steroid hormones are insoluble in water, and need to be bound to transport proteins in order to be transported in the blood. As a result, they remain in the body longer than peptide hormones. For example, cortisol has a half-life of 60 to 90 minutes in humans, while epinephrine, an amino acid derived-hormone, has a half-life of approximately one minute.