Understanding the actions of hormones is really fundamental to getting a grasp of what the endocrine system does. The relatively small volume of hormones secreted by endocrine glands into the large volume of the systemic circulation means that the actual concentrations of hormones reaching their target cells is quite low (in the order of 10-9 to 10-12 M). However, because of the high affinity that the receptor on the target tissue has for the hormone, even these low concentrations are more than enough to produce quite dramatic effects.
Some hormones appear not to produce direct effects on other physiological systems but to regulate the release of hormones by other endocrine organs and/or to control the development and growth of these endocrine organs. These hormones are normally referred to as stimulating or tropic hormones to reflect these functions.
Most hormones have multiple target tissues (or multiple effects on single target tissues) and some physiological variables are regulated by the coordinated action of a number of different hormones.
Following their release into the circulation, some hormones bind to carrier proteins and are transported in the blood as complexes. The hormone-carrier protein complex is inactive in the sense that whilst bound to a carrier protein the hormone cannot bind to its receptors. The hormone must dissociate from the carrier protein in order to produce effects on the target tissue. These important principles are illustrated in the animation on the right.
These complexes are useful because:
1. They provide a readily available supply of the hormone within the circulation. This is a bit liking having savings in the bank (i.e. if you need to use some you don’t have to go through the hassle of earning it first).
2. They act as a buffer and smooth out the spikes in hormone secretion that are characteristic of some endocrine organs.
3. They make the hormone less likely to be metabolised (broken-down) by enzymes in the blood, liver and kidneys.
Hormones that are bound to carrier proteins tend to be those involved in long-term actions such as growth. Hormones involved in shorter duration actions such as adrenaline tend to be found free in the circulation.
Hormones are classified according to their structure and the substances from which they are derived. On this basis hormones can be separated into three distinct classes. We will consider the features of each of these classes in subsequent sections