# Chapter 12: When the environment changes our hormones

For the human body to function properly, its various parts and organs must communicate to ensure the maintenance of a constant internal environment (homeostasis). Examples of homeostasis include the maintenance of body temperature and preserving the level of salts and minerals in the blood, which must not fluctuate outside established limits. Communication between the different parts of the body is necessary for the organism to respond appropriately to changes in the internal and external environment. Two systems contribute to this communication: the nervous and endocrine systems, which form the neuroendocrine system together. The nervous system generally enables the rapid transmission of information between different body parts, taking only fractions of a second. In contrast, hormonal communication, which is based on the production and release of hormones from various glands around the body and the transport of these hormones through the bloodstream, is more suitable for situations that require more extensive and prolonged regulatory actions. These two communication systems complement each other but also interact with each other so that stimuli from the nervous system can influence the release of certain hormones and vice versa.

Hormones are small molecules produced by endocrine glands. These include the hypothalamus, pituitary gland, thyroid gland, parathyroid glands, pancreas, adrenal glands, and sex glands (testes and ovaries) (Figure 12.1). The term "endocrine" means that the products of these glands, hormones, are released into the bloodstream in response to specific stimuli. The hormones are then transported through the blood to target cells; some hormones have only a few specific target cells, while others affect several types of cells in the body. The target cells for each hormone are characterized by the presence of specific intermediate molecules (called receptors) for the hormone, located either on the cell surface or inside the cell. The interaction between the hormone and its receptor triggers a cascade of biochemical reactions in the target cell that ultimately alter its function or activity. Thus, the binding of the hormone to the receptor is crucial for the initiation of individual regulatory steps that ultimately influence the behaviour of the cell. In general, however, hormones control the growth, development and metabolism of the body, the electrolyte composition of body fluids and reproduction.