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Monitor internal and external environments	Ensure homeostasis
Allow appropriate adaptive changes
Communicate via chemical messengers

Figure 1.1 Chemical signalling in the endocrine and neural systems. (a) In endocrine communication, the producing cell secretes hormone into the blood vessel, where it is carried, potentially over large distances, to its target cell. (b) Sometimes hormones can act on the cell that produces them (autocrine, A) or nearby cells (paracrine, P) without the need for transport via the circulation. For instance, glucagon from α‐cells and somatostatin from δ‐cells can regulate insulin secretion by adjacent β‐cells within the pancreatic islet. (c) In neuroendocrine communication, neurons can secrete hormones into the surrounding blood vessels to reach a more distant target. A good example is hypothalamic regulation of the anterior pituitary. (d) In pure neural communication, neurons activate other neurons via neurotransmitters released from axonic terminals into the synaptic space. Conceptually, neurotransmitters are similar to hormones and in some instances, such as for norepinephrine/noradrenaline, can actually be the same chemical.

All organisms need to be able to analyze and respond to their surroundings in order to provide a constant internal environment. Maintaining this internal constancy is called homeostasis. For an organism comprised of a single or a few cells homeostasis is relatively easy as no cell is more than a short diffusion distance from the outside world or its neighbours. This simplicity has been lost with the evolution of more complex, larger, multicellular organisms. Diffusion alone is inadequate in larger animal species where discrete functions localize to specific organs. In humans, there are ∼10¹⁴ cells of more than 200 different types. With this compartmentalized function comes the need for effective communication to disseminate information throughout the whole organism – only a few cells face the outside world, yet all need to respond to it. Two communication systems facilitate this: the endocrine and nervous systems (Box 1.1).

Whereas gastrointestinal cells tend to secrete chemicals into ducts, the specialized cells that make up the glands and tissues of the endocrine system release their chemical messengers, called hormones, into the extracellular space, from where they enter the bloodstream. Historically, this blood‐borne transit of hormones was what defined ‘endocrinology’; however, the principle is identical for hormone action on a neighbouring cell (called a paracrine effect) or, indeed, the endocrine cell itself (an autocrine or intracrine effect) (Figure 1.1).

The nervous and endocrine systems interact. Endocrine glands can be under nervous control; the adrenal medulla is an excellent example (Chapter 6). Conversely, neural cells can themselves release hormones into the bloodstream. This is particularly relevant in the hypothalamus (Chapter 5). Indeed, this interplay between the body's two main communication systems has led to the composite specialty of ‘neuroendocrinology’ (Figure 1.1).