Endocrine system
- Is one of the regulating and integrating systems of the body, ensuring the constancy of its internal environment (homeostasis).
- Includes endocrine glands, clusters of endocrine cells in not endocrine organs, single endocrine cells, as well as neurosecretory nuclei of hypothalamus and neurohemal formations.
- Its products are released into the internal environment (tissue fluid, blood, lymph, cerebrospinal fluid) hormones, involved in the humoral regulation of vital activity.
Endocrine glands (glands of internal secretion):
- are independent compactly organized structures;
- do not have excretory ducts;
- are intensely supplied with blood, and contain fenestrated capillaries (often with a wide lumen of sinusoids and extensive pericapillary spaces);
- these include the epiphysis, pituitary gland, thyroid gland, parathyroid glands, adrenal glands.
Clusters of endocrine cells in not endocrine organs:
- are found in organs belonging to other organ systems and performing other specialized functions (digestion, urinary excretion, reproduction, etc.);
- are structurally similar to endocrine glands;
- are found in the pancreas, male and female gonads, kidneys, heart, thymus, placenta (endocrine formations of these organs are considered in the relevant sections of special histology).
Single endocrine cells:
- are scattered in the superficial epithelium of organs and in the associated exocrine glands;
- collectively form the diffuse (dispersed) endocrine system (DES) of the body.
Neurosecretory nuclei of the hypothalamus — are clusters of neurons that synthesize and release hormones into the blood and are therefore called neuroendocrine (neurosecretory) cells.
Neurohemal formations:
- do not produce their own hormones and serve as a place of accumulation and release of hormones of neuroendocrine cells into the blood;
- contain axons of neuroendocrine cells, the endings of which form neurohemal (axovasal) synapses with the walls of blood capillaries;
- these include the median eminence of the hypothalamus and the neurohypophysis.
Hormones:
- are substances with high biological activity — cause their effects in small quantities;
- have a selective effect due to the presence of recptors in the plasma membrane or in the cytoplasm of target cells: specific receptors to hormones;
- belong to different classes of chemicals: amino acid derivatives, peptides, steroids, biogenic amines;
- their effects can be a result of various types of action on target cells:
- endocrine — blood-mediated effects on cells of other organs (distant);
- paracrine — effects on cells of another type within the same tissue or organ (local);
- autocrine — impact on adjacent cells of the same type (local).
In the endocrine system, there are central and peripheral organs.