The Endocrine System

The endocrine system is a collection of glands that release hormones which have an influence on almost every cell, organ, and function of the body. Hormones are chemicals that carry messages from one cell to another through the bloodstream. The endocrine system regulates our mood, growth and development, the function of tissues, as well as metabolism and sexual function.

The endocrine system is in charge of body processes such as breathing and cell growth, which are controlled by hormones that transfer information and instructions from one set of cells to another. Many different hormones circulate throughout the bloodstream, and each one has a specific role. The levels of hormones circulating can be influenced by factors such as stress, infection, and changes in the balance of fluid and minerals in blood. In summary, the endocrine system produce’s chemicals called hormones, which are secreted directly into the blood stream, where they are then carried to their target organ.

The major glands that make up the human endocrine system include the:

  • Hypothalamus
  • Pituitary gland
  • Thyroid
  • Parathyroid’s
  • Thymus
  • Adrenal glands
  • Pineal body
  • Reproductive glands
  • Pancreas

A gland has more than one function. It produces and secretes chemicals, but it also selects and removes materials from the blood, processes them, and then secretes the finished chemical product to be used in a specific area in the body.

There are two different types of glands. Exocrine glands have ducts that carry their secretory product to a surface. Such glands include the sweat, sebaceous, and mammary glands. Endocrine glands release hormones directly into the blood stream for transportation around the body. Some other organs in the body, but not part of the endocrine system, also release hormones, such as the brain and heart.

We are going to look at each one in turn:

The hypothalamus

The hypothalamus is a small cone shaped structure of specialized cells that is located in the lower central part of the brain just above the brain stem. It acts as the primary link between the endocrine and nervous systems through the pituitary gland. The hypothalamus has to respond to many different signals and has the following functions:

  • Controls the autonomic functions
  • Controls emotions
  • Plays a significant role in homeostasis
  • Controls motor functions
  • Regulates food and water intake
  • Regulates the sleep and wake cycle

The hypothalamus controls the pituitary gland, which is sometimes referred to as the master gland, and sends hormones down to the pituitary gland.

The Pituitary Gland

Around the size of a pea and located just below the hypothalamus, this tiny gland has a powerful effect on the body. This master gland makes hormones that control other endocrine glands, such as the thyroid and adrenals, and receives its information from the hypothalamus, for example a change in temperature.

The pituitary is divided into two parts:

  • The anterior lobe regulates the thyroid, adrenals and reproductive glands.
  • The posterior stores and releases the hormones from the hypothalamus without making any hormones itself. For example, the antidiuretic hormone is released as it prevents excess water being excreted by the kidneys.

The pituitary gland also secretes endorphins that act on the nervous system to respond to pain as well as secreting the hormone FSH and luteinizing hormone that are vital for reproduction.

Thyroid Gland and Parathyroid

This gland is located in the front part of the lower neck and is shaped like a butterfly. It produces the hormone Thyroxine and triiodothyronine, which controls the rate at which cells burn fuels from food to produce energy. This phrase is called metabolism and deals with body temperature and weight. Iodine is found in the thyroid hormones which the thyroid needs to make the hormones.

If there is a lack of iodine in the diet, the thyroid cannot make the hormones. Hormones produced by the thyroid also aid in the development of the brain and nervous system in children. The release of these hormones is controlled by thyrotopin, secreted by the pituitary gland. Attached to the pituitary are four small glands, called the parathyroid’s which release the parathyroid hormone that regulates the levels of calcium in the blood.

Thymus

The thymus is a small gland that is situated behind the top of the breastbone in front of the trachea and plays an important role in immunity. The thymus increases in size and activity until puberty then begins to shrink. The thymus secretes several hormones which help develop the immune system, one in particular called thymosin are produced that stimulate the development of antibodies as well as producing T-lymphocytes which are white blood cells that fight infection and destroy any abnormal cells.

The Adrenal Glands

The body has two triangular adrenal glands, one on top of each kidney. The adrenals work with the hypothalamus and the pituitary gland by stimulating the adrenals to produce corticosteroid hormones. The adrenal glands consist of two parts, each one producing a set of hormones which has a different function. The adrenal cortex, which is the outer part.

produces hormones called corticosteroids directly into the blood stream which help regulate the salt and water balance in the body. The cortex also controls the body’s use of fats, proteins, and carbohydrates. The adrenal medulla, which is the inner part of the gland, is not essential for life but secretes epinephrine which increases blood pressure and the heart rate when under stress – also called adrenaline.

The Pineal Gland

This gland is situated in the middle of the brain, around a quarter or an inch long and secretes a hormone called melatonin when it’s dark that regulates the sleep – wake cycle. This is why some people feel depressed in the dark winter months when they may be producing too little melatonin. The pineal gland is shaped like a pine cone and helps to promote sleep as it is sensitive to light. But can also affect reproduction by depressing the activity of the gonads.

The ovaries secrete two female hormones; estrogen and progesterone, with the former being involved in the development of features such as breast growth and the accumulation of body fat around the hips and thighs. Progesterone causes the uterine lining to thicken in preparation for pregnancy.

The Pancreas

This organ is situated in the abdominal cavity behind the stomach, with the right side being the widest part and being attached to the duodenum. This organ is classed as a compound gland as it works as both an exocrine and endocrine gland, with the exocrine part secreting digestive hormones and the endocrine part producing two important hormones; insulin and glucagon.

A part of the pancreas, called the Islets of Langerhans secretes glucagon which tells the liver to take carbohydrate out of storage to raise a low blood sugar level if there is one. If the blood sugar level is too high the islet cells secrete insulin to tell the liver to take excess glucose out of circulation to lower it. Both hormones insulin and glucagon therefore work together to maintain a steady level of glucose, or sugar, in the blood and to keep the body supplied with fuel to produce and maintain stores of energy. If there is not enough insulin made by the body, the blood sugar will rise and become diabetes mellitus.

The exocrine part makes pancreatic juices, and as they are made, they flow into the main pancreatic duct which joins the common bile duct. This connects the pancreas to the liver and also the gallbladder.

The workings of the endocrine system

Once a hormone is secreted, it travels from the endocrine gland through the bloodstream to target cells designed to receive the message. During the transit to the cells, the hormones have special proteins bound to them. These proteins act as carriers that control the amount of hormones that are available to interact with and affect the target cells. Once at the target cells, receptors within the cells attach themselves to specific hormones so that only those hormones communicate with the cells. The hormone locks onto the cells receptors and chemical instructions are transmitted to inside the cell. Once the hormone level reaches the required amount, any further secretions are controlled by mechanisms to maintain it.

Gland/OrganHormoneProcess
Pituitary GlandTrophic hormones, Growth hormones, luteinising hormone (LH), Follicle stimulating hormone (FSH)Stimulates production of hormones from other glands Milk production
HypothalamusHormone releasing factors, anti-diuretic hormones, OxytocinStimulates pituitary gland to produce hormones, control of water Helps uterine contraction in childbirth and stimulates the letdown reflex for breastfeeding
ThyroidThyroxineControls rate of body processes and heat production and energy production from food
Parathyroid glandsParathormone or parathyroid hormoneControls the amount of calcium in blood and hormones
PancreasInsulinControls blood sugar
Adrenal glandsAdrenaline Cortisol Aldosterone AndrogensControls emergency action,response to stress Stress control, conversion of fats, proteins and carbohydrates to glucose. Acts on the kidneys to control salt and water balance
TestesTestosteroneControl of sperm, growth and development of male features at puberty, beard growth
OvariesProgesterone Estrogen Placental hormone (pregnancy only)Helps control normal progress of pregnancy. Interacts with FSH and LH and estrogen to control the menstrual cycle
Stomach wallGastrinStarts acid production by stomach
Small intestineSecretinTriggers release of digestive enzymes from pancreas

Pathologies of the Endocrine System

DiseaseMeaning
AcromegalyToo much growth hormone, causing body tissues to gradually enlarge.
ThyrotoxicosisWhen there is too much thyroid hormone in the body
Addison’s SyndromeDisorder of the adrenal glands affecting the production of adrenaline and cortisol.
Cushing’s syndromeA range of symptoms if there is too much cortisol in the blood.
GoitreAn abnormal swelling of the thyroid gland.
DiabetesThe amount of glucose in the blood is too high because the body cannot use it properly.
Polycystic Ovary Syndrome Condition which affects the workings of the ovaries, causing cysts to form around the edge of the ovaries.