Overview
The endocrine system is made up of glands and scattered cells that secrete hormones into the blood to regulate distant target tissues. This topic covers the classification of endocrine organs and hormones, how hormones act and are regulated, the general anatomy and histology of the endocrine glands, and endocrine cell groups found inside non-endocrine organs.
The endocrine system is a system of biochemicals that regulate distant tissues by means of secretion into the circulatory system. This system, along with the nervous system, is responsible for homeostasis. Growth, development, reproduction, blood pressure, concentrations of ions and other substances in the blood, and even behavior are all regulated by the endocrine system. Endocrine physiology involves the secretion of hormones and their subsequent actions on target tissues.
Classification of endocrine organs and hormones
Classification of endocrine organs. Endocrine structures are grouped by how much of their activity is endocrine:
- Purely endocrine glands: pituitary gland (hypophysis), pineal gland, thyroid gland, parathyroid glands and adrenal (suprarenal) glands.
- Mixed glands (endocrine and exocrine or other functions): pancreas (islets of Langerhans), testis and ovary, placenta.
- Endocrine organoids and diffuse endocrine cells inside non-endocrine organs (see the last section).
- Neuroendocrine centres: the hypothalamus, whose neurons secrete releasing and inhibiting hormones, oxytocin and vasopressin.
A hormone is a chemical substance that is classified as a peptide, a steroid, or an amine. Hormones are secreted into the circulation in small amounts and delivered to target tissues, where they produce physiologic responses.
- Hormones are synthesized and secreted by endocrine cells usually found in endocrine glands.
- The classic endocrine glands are the hypothalamus, anterior and posterior lobes of the pituitary, thyroid, parathyroid, adrenal cortex, adrenal medulla, gonads, placenta, and pancreas.
- The kidney is also considered an endocrine gland, and endocrine cells are found throughout the gastrointestinal tract.
Hormones are categorized into three classes: peptides and proteins, steroids, or amines.
Peptide and Protein Hormones- largest group
- Most hormones are peptide or protein in nature.
- Peptide and protein hormones are synthesized from amino acids.
- Being hydrophilic and lipophobic (repelling fats), they cannot freely pass through the cell membrane. Their sizes vary widely and include hormones such as growth hormone, follicle-stimulating hormone (FSH), thyrotropin-releasing hormone (TRH), insulin, glucagon, leptin, ADH, and oxytocin.