How the human endocrine system develops and controls the human body!
The glands in the endocrine system use chemicals called hormones to communicate with and control the cells and organs in our bodies. They are ductless glands that secrete different types of hormone directly into the bloodstream and target specific organs.
The target organs contain hormone receptors that respond to the chemical instructions supplied by the hormone. There are 50 different types of hormone in the body and they consist of three basic types: peptides, amines and steroids.
Steroids include the testosterone hormone. This is secreted by the cortex of the adrenal gland, the male and female reproductive organs and by the placenta in pregnant women. The majority of hormones are peptides that consist of short chains of amino acids. They are secreted by the pituitary and parathyroid glands. Amine hormones are secreted by the thyroid and adrenal medulla and are related to the fight or flight response.
The changes that are caused by the endocrine system act more slowly than the nervous system as they regulate growth, moods, metabolism, reproductive processes and a relatively constant stable internal environment for the body (homeostasis!. The pituitary, thyroid and adrenal glands combine to form the major elements of the endocrine system along with other elements such as the male testes, the female ovaries and the pancreas.
We have two adrenal glands that are positioned on top of both kidneys. The triangular-shaped glands each consist of a two-centimetre thick outer cortex that produces steroid hormones, which include testosterone, Cortisol and aldosterone.
The ellipsoid shaped, inner part of the gland is known as the medulla, which produces noradrenaline and adrenaline. These hormones increase the heart rate, and the body’s levels of oxygen and glucose while reducing non-essential body functions.
The adrenal gland is known as the ‘fight or flight’ gland as it controls how we respond to stressful situations, and prepares the body for the demands of either fighting or running away as fast as you can. Prolonged stress over-loads this gland and causes illness.
The pea-sized pituitary gland is a major endocrine gland that works under the control of the hypothalamus. The two organs inside the brain work in concert and mediate feedback loops in the endocrine system to maintain control and stability within the body.
The pituitary gland features an anterior (front) lobe and a posterior (rear| lobe. The anterior lobe secretes growth hormones that stimulate the development of the muscles and bones; it also stimulates the development of ovarian follicles in the female ovary. In males, it stimulates the production of sperm cells. The posterior lobe stores vasopressin and oxytocin that is supplied by the hypothalamus. Vasopressin allows the retention of water in the kidneys and suppresses the need to excrete urine. It also raises blood pressure by contracting the blood vessels in the heart and lungs.
Oxytocin influences the dilation of the cervix before giving birth and the contraction of the uterus after birth. The lactation of the mammary glands are stimulated by oxytocin when mothers begin to breastfeed.
Thyroid and parathyroids
The two lobes of the thyroid sit on each side of the windpipe and are linked together by the isthmus that runs in front of the windpipe. It stimulates the amount of body oxygen and energy consumption, thereby keeping the metabolic rate of the body at the current levels to keep you healthy and active.
The hypothalamus and the anterior pituitary gland are in overall control of the thyroid and they respond to changes in the body by either suppressing or increasing thyroid stimulating hormones. Overactive thyroids cause excessive sweating, weight loss and sensitivity to heat, whereas underactive thyroids cause sensitivity to hot and cold, baldness and weight gain. The thyroid can swell during puberty and pregnancy or due to viral infections or lack of iodine in a person’s diet.
The four small parathyroids regulate the calcium levels in the body; it releases hormones when calcium levels are low. If the level of calcium is too high the thyroid releases calcitonin to reduce it. Therefore, the thyroid and parathyroids work in tandem.
The pancreas is positioned in the abdominal cavity above the small intestine. It consists of two types of cell, the exocrine cells that do not secrete their output into the bloodstream but the endocrine cells do.
The endocrine cells are contained in clusters called the islets of Langerhans. They number approximately 1 million cells and are only one or two per cent of the total number of cells in the pancreas. There are four types of endocrine cells in the pancreas. The beta cells secrete insulin and the alpha cells secrete glucagon, both of which stimulate the production of blood sugar (glucose) in the body. If the beta cells die or are destroyed it causes type 1 diabetes, which is fatal unless treated with insulin injections.
The other two cells are the gamma and delta cells. The former reduces appetite while the latter reduces the absorption of food in the intestine.