The endocrine system is a network of anatomically and embryologically discreet glands (organs and cells). Classically, endocrine glands are ductless and specialized in function.
However, hormones are produced and released from other tissues and organs: for example, atrial natriuretic peptide (ANP) from the heart and insulin-like growth factor 1 (ILG1) from the liver.
Thus, a hormone is a chemical released by a cell that has a biological action on a target cell.
Hormones released from the posterior pituitary and the hypothalamus are the product of nerve cells and not glandular tissue. Strictly speaking the term ‘neurocrine’ should be applied.
The active component of a hormone is that part which is unbound and thus free to interact with the target cell. For highly bound hormones, fluctuations of concentration of binding protein affect activity.
Because the endocrine system is intimately concerned with homeostasis, tight control is essential to prevent large oscillations in the system.
In broad terms, activity is dependent on three factors:
- The rate of production and release of active hormone
- The rate of its removal
- The end-organ sensitivity
Because the latter two factors vary with both physiological and pathological conditions, ultimately hormone activity is determined by the rate of production and release.
A trophic hormone may control the release of a second. Once appropriate activity of the second is attained it feeds back to inhibit release of the first.
Many systems do not involve hormone pairs. The inhibitory factor may be an ion, for example Ca2+ and parathormone, or a dependent substrate, e.g. glucose and glucagon or insulin.
Positive feedback may also occur but is less common. This occurs where secretion of a hormone is enhanced by an increase in the substrate targeted. Other regulatory mechanisms must intervene at some stage in this process once the desired biological effect is achieved. In the menstrual cycle, LH stimulates oestrogen release which further stimulates LH release.
The close integration between the ANS and the endocrine system means that some systems have significant neuronal control.
The response is usually consequent upon external rather than internal influences.
Catecholamine release is a classic example.