Endocrine system

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.