The endocrine glands produce chemicals and pass them directly into the bloodstream.
Hormones can be thought of as chemical messages .
From the bloodstream, hormones communicate with the body by targeting their target cell to cause a particular change or effect in that cell.
The hormone can also create changes in the cells of the surrounding tissues (paracrine effect). The endocrine system works with the nervous system and the immune system to help the body cope with different events and stresses.
This branch of medicine, related to the study of the endocrine system, is called endocrinology and is practiced by endocrinologists. The field is expanding rapidly due to the understanding of the cellular pathways that hormones stimulate and the discovery of new hormones and their actions.
An exocrine gland, unlike an endocrine gland, is a gland that secretes substances (electrolytes, proteins, or enzymes) directly to a target site through ducts or tubes.
Some examples include:
- Salivary glands.
- Sweat glands.
- Sebaceous glands.
- The pancreas.
The pancreas is an endocrine and exocrine organ. It releases certain enzymes to aid in digestion delivered to the intestine through the pancreatic duct .
The endocrine pancreas also releases hormones such as insulin and glucagon, which are hormones predominantly related to glucose metabolism, into the bloodstream.
Endocrine system functions
Some of the roles of the endocrine system include:
- Sexual reproduction
- Homeostasis (constant internal balance).
How do hormones work?
A hormone will only act on one part of the body if it ‘fits in’. A hormone can be thought of as a key, and its target site (such as an organ) has specially formed blockages in cell walls. If the hormone adapts to the cell wall, then it will work.
Hormones can trigger a cascade of other signaling pathways in the cell to cause an immediate effect (for example, insulin signaling leads to rapid glucose uptake in muscle cells).
This process also happens due to a later effect (glucocorticoids bind to DNA elements in a cell to activate the production of certain proteins, which takes time to produce).
The endocrine system is a strictly regulated system that keeps hormones and their effects at the correct level. One way to achieve this is through ‘feedback loops’. Hormone release is regulated by other hormones, proteins, or neuronal signals.
The released hormone has its effect on other organs. This effect in the organ feeds back to the original signal to control any new hormone release. The pituitary gland is well known for its feedback loops.
Endocrine glands and organs
The main glands and organs of the endocrine system include:
Pituitary gland : it is inside the brain. It monitors the other glands and keeps hormone levels in check. It can cause a change in hormone production elsewhere in the system by releasing its own “stimulating” hormones.
The pituitary gland is also connected to the nervous system through a part of the brain called the hypothalamus .
The hormones released by the pituitary gland are:
- Gonadotropins (LH and FSH).
- Growth hormone (GH).
- Thyroid-stimulating hormone (TSH).
- Adrenocorticotropic hormone (ACTH).
- Antidiuretic hormone.
Thyroid gland – sits on the neck at the front of the windpipe. It releases thyroid hormone (T4 and T3) which is necessary for metabolism and body homeostasis. It is controlled by TSH, which is produced by the pituitary gland through a feedback loop.
Parathyroid gland : There are usually four parathyroid glands that are located next to the thyroid gland. The parathyroid gland is involved in the regulation of calcium, phosphate, and vitamin D.
Adrenal glands : There are two adrenal glands that are located at the top of each kidney.
They make several different hormones. The outer part of the gland (adrenal cortex) produces cortisol, aldosterone, and sex hormones. The center of the adrenal gland (adrenal medulla) produces adrenaline. Adrenaline is an example of a hormone that is under the control of the nervous system.
The Pancreas : it is a digestive organ that is inside the abdomen. It makes insulin, which controls the amount of sugar in the bloodstream. It also produces other hormones, such as glucagon and somatostatin.
Ovaries : they are inside the female pelvis. They create female sex hormones like estrogen.
Testicles : hang in the male scrotal sac. They create male sex hormones like testosterone.
Other lesser known endocrine organs include:
Adipose tissue (fatty tissue) is known to be metabolically important. It releases hormones like leptin, which affect appetite, and it is also a site of estrogen production.
Insulin also acts on adipose tissue.
Kidneys : produce erythropoietin (EPO) which stimulates the production of red blood cells, produces renin which is necessary for the regulation of blood pressure and produces the active form of vitamin D (1-25 dihydroxy vitamin D3)
Gut : An increasing number of hormones in the gut are being researched and understood to affect metabolism and appetite. They include glucagon-like peptide 1 (GLP-1), ghrelin, which stimulates appetite, and somatostatin.
Endocrine system problems
Numerous problems can occur in the endocrine system. These can be considered as excessive or poor hormone production.
The endocrine organs are also prone to tumors (adenomas) that can produce excess hormones.
Some endocrine system problems include:
- Diabetes – Too much sugar in the blood caused by problems with insulin production. This includes type 1 diabetes (insulin deficiency) and type 2 diabetes (initially excess, then deficiency, of insulin).
- Menstruation abnormalities : irregular menstruation or lack of menstruation. Some of the causes of this include polycystic ovary syndrome (PCOS), pituitary adenoma, or primary ovarian failure (PFO).
- Thyroid problems : when the gland is overactive (hyperthyroidism) or underactive (hypothyroidism). Thyroid nodules are common, but thyroid cancers are rare.
- Parathyroid problems : An enlarged or one of more of the parathyroid glands can lead to high levels of calcium in the blood (hypercalcemia).
- Pituitary adenomas : These are tumors of the pituitary gland that can produce too much of a certain hormone or cause hormone deficiencies. These tumors can be small (microadenomas) or large (macroadenomas).
- Neuroendocrine tumors : These are rare for tumors of certain endocrine glands (usually the adrenal gland, pancreas, or small intestine).
These can include too much adrenaline released by the adrenal gland (pheochromocytoma), or too much 5-HIAA hormone from a carcinoid tumor causing diarrhea and redness.