Functions of the Endocrine System
The endocrine system is concerned with controlling the body by means of glands that secrete hormones into the blood. These hormones can affect a wide range of functions such as growth, development, sex characteristics, heart rate, breathing rate, mood, blood sugar, digestion, and so on. A principal role of the endocrine system is to provide stability and balance, or homeostasis, to the internal environment. In order to achieve this it must work closely with the nervous system that we have already explored.
Homeostasis is maintained by the nervous and endocrine systems via a control process called a ‘negative feedback loop’. For example, if blood sugar increases, this is sensed by cells in the brain (feedback) which then triggers a release of hormones (in this case, insulin) to bring blood sugar back down. As another example, if core body temperature increases, this is sensed by thermoreceptors which send this feedback to the brain, which in turn triggers a sweating response to help bring the temperature back down. Or if core temperature falls, then the brain will invoke a shiver response to generate muscular heat and raise the temperature again.
Negative feedback loop: A change in the parameter (e.g. increased body temperature) creates feedback (e.g. thermoreceptors inform the brain) leading to a reaction to bring that parameter back to normal (e.g. body begins to sweat).
Another example of a negative feedback loop is as follows: Blood calcium levels fall. The brain senses this and stimulates the parathyroid gland to release parathyroid hormone. This has the effect of releasing calcium from bone in order to raise blood calcium levels and return them to normal.
In contrast, a ‘positive feedback loop’ will cause a parameter to move even further away from normal (e.g. in a football crowd, one fan starts chanting and is then quickly joined by another, then another in exponential fashion, until the whole crowd is involved). However, this positive feedback loop is rarely appropriate in the body, where the goal is to keep functions within quite narrow ranges, rather than to rapidly increase to one extreme.
The endocrine system is composed of a number of glands, known as ‘ductless glands’ that secrete hormones directly into the bloodstream. The major endocrine glands include the:
- Pituitary gland
- Thyroid gland
- Parathyroid gland
- Adrenal glands
The location of each endocrine gland is shown in the diagram that follows.
Each gland produces specific hormones. Although these hormones are produced in relatively small quantities, they have profound effects on growth, metabolism, energy supply, reproduction, and the body’s response to stress.
Major Endocrine Hormones Produced by Each Gland
This is known as the ‘master gland’. It is situated at the base of the brain and controls activity of the nearby pituitary gland.
The pituitary situated at the base of the brain, just beneath the hypothalamus. The anterior part of the pituitary gland secretes growth hormoneunder the ultimate control of the hypothalamus. Growth hormone is important for the stimulation of tissue growth and mobilisation of fats for energy (lipolysis).
The pituitary gland also secretes a range of different hormones that act as messengers and controllers of the other endocrine glands like the adrenals, testes and ovaries.
The thyroid gland is located in the neck, just below the larynx. It secretes thyroxinwhich stimulates metabolism in all cells of the body. Low levels of thyroxin can lead to reduced basal metabolism and rapid weight gain. This is a condition that affects some people as they age and is relatively easy to diagnose with a blood test. It does not, however, account for the majority of cases of obesity.
Humans usually have four parathyroid glands, located on the rear surface of the thyroid gland. They secrete parathyroid hormonewhich regulates the levels of calcium in the blood. When blood calcium levels fall, then parathyroid hormone is released. This has the effect of increasing calcium absorption from the gut, and also of breaking down bone in order to release calcium. Both these effects will serve to boost blood calcium levels again, but if there is insufficient calcium in the diet then this can be at the expense of bone mineral density.
The pancreas is located in the abdominal cavity, just beneath the stomach. It has a dual role: Firstly as a digestive gland, producing enzymes that help to break down fats, carbohydrates and proteins, and secondly as an endocrine gland producing the hormones insulinand glucagon. Insulin is released into the blood in response to rising blood sugar levels (i.e. after a meal). Its action is to promote transport of blood sugar (glucose) into the cells to be used for energy or to be stored as glycogen. Thus insulin reduces blood sugar. In contrast, when blood sugar levels fall (i.e. when someone has not eaten for several hours, or they have been exercising for a long time) then the hormone glucagon will break down stored glycogen to release glucose, thus increasing blood sugar levels. So the two pancreatic hormones, insulin and glucagon, work antagonistically to keep blood sugar levels within a healthy range – an example of homeostasis.
Note: Glycogen (pronounced gly-ko-jen) = stored carbohydrate
Glucagon (pronounced glu-ka-gon) = a hormone that releases glucose into the blood
The adrenal glands are situated in the abdominal cavity, just on top of each kidney. They secrete powerful hormones in response to stress, fear and excitement (sympathetic activity of the autonomic nervous system).
The core of each adrenal gland is called the medulla, and this releases catecholamines – adrenaline andnoradrenaline(also known as epinephrine and norepinephrine). These catecholamines act quickly and prepare the body for ‘fight or flight’ by increasing heart rate, breathing, muscle tone, alertness, etc. Blood flow is directed to the working muscles and the digestive system is practically shut down whilst these hormones dominate. Fat is mobilised from adipose tissue, and glycogen is broken down to provide glucose for fuel. Adrenaline and noradrenaline will normally be released before and during intense exercise.
In more relaxed circumstances then levels of catecholamines fall, which restores blood flow to the digestive system, decreases heart rate and breathing rate, and generally facilitates recovery (parasympathetic activity of the autonomic nervous system). This is sometimes referred to as ‘rest and digest’.
The outer layers of each adrenal gland are known as the cortex. Here, corticosteroid hormones, such as cortisol, are made and released into the blood. A regular release ofcorticosteroid hormones is necessary for everyday function. However, levels rise under conditions of chronic stress. This can have several negative consequences including:
- Reduced immune function
- Catabolism (utilising muscle tissue for energy)
Stress has many potential causes, such as bereavement, worry about work, money, relationships, family problems, excessive exercising, excessive dieting, chronic pain from arthritic joints, and so on.
One interesting side-effect of cortisol is that it reduces inflammation (probably linked with its immuno-suppression). Hence it is the basis of many anti-inflammatory skin creams.
The testes are located outside of the male abdominal cavity – outside rather than inside to keep them cool enough for effective sperm production. As an endocrine gland, the testes secrete testosterone, a powerfully anabolic hormone that promotes male sex characteristics, increased muscle size, decreased body fat, increased red blood cell count, and an inexplicable fascination with football. The testes also secrete a number of other androgensthat promote male characteristics.
The ovaries are located inside the female abdominal cavity. As well as their role in producing an ovum each month as part of the menstrual cycle, they also have an endocrine function, secreting oestrogen andprogesterone. Oestrogen in particular promotes female sex characteristics, controls the menstrual cycle, and tends to increase fat deposition – particularly around the hips, thighs and backs of the arms.
Endocrine System Summary
The table on the following page summarises the main glands, hormones and effects of the endocrine system:
(McArdle, Katch, & Katch, 2006)