Peptide and catecholamine hormones are water-soluble. Therefore, with the exception of a few peptides, these hormones are transported simply dissolved in plasma (Table 10-2). In contrast, the steroid hormones and the thyroid hormones circulate in the blood largely bound to plasma proteins.
Even though the steroid and thyroid hormones exist in plasma mainly bound to large proteins, small concentrations of these hormones do exist dissolved in the plasma. The dissolved, or free, hormone is in equilibrium with the bound hormone:
Free hormone + Binding protein 3'
The total hormone concentration in plasma is the sum of the free and bound hormone. It is important to realize, however, that only the free hormone can diffuse across capillary walls and encounter its target cells. Accordingly, the concentration of the free hormone is what is physiologically important rather than the concentration of the total hormone, most of which is bound. As we shall see, the degree of protein binding also influences the rate of metabolism and the excretion of the hormone.
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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.