The endocrine system encompasses a variety of glands that are widely distributed throughout the body.
These glands produce an array of hormones that have many different functions. You have already learned in
Module 4 that hormones are bloodborne chemical messengers that move from the endocrine glands where they
are produced, to the target cells (or effector cells) upon which they act. In this module, you will study the basic
structure and function of the hormones secreted by the endocrine glands. We will also discuss how the hormones
are transported in the blood, their metabolism and excretion, and the mechanisms of hormone action.
Endocrinology: The study of hormones.
Hormone: A chemical secreted by a cell or group of cells into the blood for transport to a distant
target, where it exerts its effect at very low concentrations.
Mechanism of Action: How the hormone binds to the target cell receptor to initiate a biochemical response.
What Makes a Chemical a Hormone?
• Secreted by a cell or group of cells
• Secreted into the blood
• Transported to a distant target
• Exert their effect at very low concentrations
• Act by binding to receptor (cellular mechanism of action)
• Action must be terminated (broken down by enzymes according to halflife)
This unit begins with an ANATOMY SUMMARY of Hormones in Figure 72. Here you will find a listing of
the endocrine glands, the hormones they secrete, and the major functions that each hormone controls. This
figure is for reference and orientation only. You may want to refer to this table from time to time throughout the
remainder of the course. As you look at this table, note the variety of functions that the hormones control. Note
also that organs such as the heart and kidneys have endocrine cells that can produce hormones.
Although there are many different hormones with different functions, the hormones can be grouped into
three major chemical classes peptides, steroids, and amines. Table 71 is an excellent comparison. You should
carefully study this table.
3 Chemical Classes of Hormones:
• Peptide/protein hormones, steroid hormones & AAderived hormones
o Peptide/protein are composed of linked AA
o Steroid hormones are all derived from cholesterol
o AAderived hormones are modification of single AA (either tryptophan or tyrosine)
• If a hormone is not a steroid hormone or AAderivative, it must be a peptide or protein
The peptide hormones are secreted from secretory vesicles by exocytosis as depicted in Figure 73. They
dissolve in the plasma, and are therefore quickly cleared from the circulation by the liver and kidneys. The
receptors for these hormones are located on the cell membranes of the target tissue as seen in Figure 75.
By contrast, the steroid hormones can cross the hydrophobic plasma membrane by diffusion. They are
poorly soluble in plasma and circulate in the blood mainly bound to specific binding proteins. A small amount
1 Module V
can dissolve in the plasma, and this "free" amount is in equilibrium with bound hormones. Therefore, changes in
binding protein concentration can affect the level of free hormones. Free hormone molecules are the only ones
that are bioactive; they can cross the cell membrane and enter the cytoplasm of target cells. The receptors for
these hormones are located mainly on the inside of the target cell, usually in the nucleus. Figure 77 shows a
steroid hormone in action. Steroid hormones generally survive longer than peptide hormones before being
metabolized or excreted.
The amine hormones can be divided into the catecholamines, which function much like the peptide
hormones and thyroid hormone, whose action more closely resembles that of steroid hormones.