H H H Oh H H H H H Oh

(e) Prostaglandin PGE2

(d) Oxytocin

Figure 13.3

(a and b) Structural formulas of a steroid hormone (cortisol) and an amine hormone (norepinephrine). (c and d) Amino acid sequences of a protein hormone (PTH) and a peptide hormone (oxytocin). (e) A prostaglandin (PGE2).

Table 13.1 lists the names and abbreviations of some of the hormones discussed in this chapter. Table 13.2 and figure 13.3 summarize the chemical composition of hormones. Other hormones related to specific organ systems are discussed in their appropriate chapters.

99 What is a hormone?

^9 How do endocrine glands and exocrine glands differ? ^9 How are hormones chemically classified?

Actions of Hormones

Hormones exert their effects by altering metabolic processes. For example, a hormone might change the activity of an enzyme necessary for synthesizing a particular substance or alter the rate at which particular chemicals are transported through cell membranes.

Hormones may reach all cells, but bind only those that have appropriate receptors. Each hormone receptor is a protein or glycoprotein molecule that has a binding site for a specific hormone. A hormone delivers its message to a cell by uniting with the binding site of its receptor. The

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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