The endocrine system is a system of ductless glands that secrete hormones within specific organs. Hormones are chemical messengers that are carried by the bloodstream to different cells in the body. Hormones act at a distance from their place of production directly on the target cell, inciting it to increase or decrease the expression of specific genes.
TYPES OF HORMONES
Hormones can be either lipid-based, also known as steroidal, or protein-based, known as non-steroidal. These two types of hormones act on their target cells in different ways. Lipid-based hormones enter the target cell directly through the cell membrane, after which they travel to the nucleus and directly affect expression of their target genes.
Protein-based hormones cannot enter the cell, so they bind to receptors on the cell membrane, triggering a secondary messenger system within the cell. Hormone release by the endocrine system is regulated largely through negative feedback loops, and rarely through positive feedback loops. In negative feedback, increases in hormone activity lead to a decrease in the production of that hormone. In positive feedback, increases in hormone activity lead to an increase in production of that hormone.
Hormones are secreted by two types of glands: exocrine glands and endocrine glands. Exocrine glands include certain sweat glands, salivary, pancreatic and mammary glands. They are characterized by the secretion through a duct which empties to the outside or into the lumen of an organ. In contrast, endocrine glands have no duct and release their secretions directly into the intercellular fluid or into the blood. The main endocrine glands are the pituitary (anterior and posterior lobes), thyroid, parathyroid, adrenal (cortex and medulla), pancreas and gonads.