Endocrine System

The endocrine system is one of the key regulatory systems within the body. It consists of a series of glands that interact with each other and external factors to control organs located far away from them by secreting hormones into the bloodstream. It works in concert with the nervous system, however, instead of short term rapid changes, it creates slower acting, longer-lasting changes in the body.

Types Of Cell Signalling

There are four main types of cell signalling used to convey messages within the body. These vary in the range and type of signalling molecule used. In increasing order of range, the four signalling strategies are:

• autocrine signalling,
• juxtacrine signalling,
• paracrine signalling, and
• endocrine signalling

The type of signalling used in the endocrine system is, as the name suggests, endocrine signalling. A brief overview of each of these is provided below.

Autocrine Signalling

Autocrine signalling is used by cells to effect changes within themselves. The cell releases a signalling molecule, such as a hormone or cytokine. This signalling molecule then binds to receptors on the same cell, triggering changes within that cell. This form of signalling is employed by cells of the immune system, amongst others.

Fig. 1 - A diagram showing the process of autocrine signalling

Juxtacrine Signalling

Juxtacrine signalling is used by cells to effect changes directly in neighbouring cells. In this mode of signalling, nothing is released from the signalling cell. Instead, proteins on the signalling cell interact with receptor proteins on or in the receiving cell.

Fig. 2 - An overview of juxtacrine signalling

Paracrine Signalling

Paracrine signalling is used by cells to effect changes within cells in the local area. The signalling cell releases paracrine factors into the surrounding area. These factors then spread into the surrounding area. Diffusion of the paracrine factors creates a gradient, with the concentration lowering the further from the signalling cell you get. The stronger the concentration of paracrine factors a cell is exposed to, the larger its response. This type of signalling is used for example within synapses.

Fig. 3 - An overview of paracrine signalling, showing the diffusion gradient that causes more distant cells to be exposed to lower concentrations of the signal molecule

Endocrine Signalling

Endocrine signalling is the type of cell signalling used by the endocrine system. Endocrine signalling relies on the use of chemical signalling molecules called hormones.

This form of signalling can act on cells throughout the body, outside of the local area, a significant increase in communication range when compared to the above signalling methods. This range is achieved by the gland secreting the hormone into the bloodstream, rather than the immediate extracellular area. The bloodstream then carries the hormone around the body, ready for it to interact with relevant receptors on its target.

Fig. 4 - A diagram showing how endocrine signalling works

The Function Of The Endocrine System

The endocrine system regulates virtually all factors within the body. Anything in which some degree of homeostasis is required is regulated by a part of the endocrine system such as blood glucose levels or metabolic rates. The endocrine system regulates factors within the body directly, through the action of hormones, or indirectly, through knock-on effects of hormone interactions.

Because of this wide range of impacts, the endocrine system is involved in many diseases, ranging from metabolic disorders to cancer. The study of the role the endocrine system plays in disease is known as endocrinology. Diseases caused by the endocrine system generally stem from altered levels of hormone release, inappropriately timed hormone release, the gland being missing or damaged, or the gland being overgrown and therefore overactive.

The Nervous System vs the Endocrine System

While the nervous system and the endocrine system both allow signals to be carried throughout the body, this is where their similarity ends. Nerve cells conduct electrical signals around the body. They do use chemical messengers, but only over incredibly short distances at special junctions between cells known as synapses, rather than long-distance via the bloodstream like the endocrine system does. They also generally act over a very short time period, but the signals travel quickly. This is the opposite of the endocrine signal, where due to the requirement for hormones to circulate in the blood, the response is much slower but more long-lasting as it acts until the hormones are broken down.

Diagram Of The Endocrine System

The diagram below shows the key glands of the endocrine system including the ovaries, testes, pineal gland, adrenal gland, thyroid, parathyroid, pituitary gland, hypothalamus and pancreas, each of which is discussed in depth after.

Fig. 5 - A diagram showing the key components of the endocrine system

The glands of the endocrine system differ in structure quite significantly from other glands, which make up the exocrine glands. Exocrine glands, such as those found in the digestive tract or the sweat glands, secrete their products outside of the body. In order to do this, they must have a duct to carry the product to the required area. As they don't secrete their product into the blood, they have few blood vessels. Endocrine glands in contrast generally do not have ducts as their product diffuses into the bloodstream. This need for access to the bloodstream also means they have a lot more blood vessels than exocrine glands.

The Endocrine System's Glands

There are nine main components that form the endocrine system, these are the ovaries, testes, pineal body, adrenal gland, thyroid, parathyroid, pituitary gland, pancreas, and hypothalamus. A brief overview of each of these is given below.

The Ovaries

The ovaries are the site of egg maturation and release, however, they also possess a significant role in hormonal control, both ovulation and many other systems. Hormones produced by the ovaries include oestrogen, inhibin and progesterone. Dysfunctions in the hormonal secretion of the ovaries can lead to several issues, one of the most common of which is Polycystic Ovary Syndrome.

Testes

The testes are the male equivalent to the ovaries, producing sperm cells instead of eggs. Like the ovaries, as well as producing gametes, they produce the hormone testosterone. Lowered hormone production by the testes, termed hypoandrogenism, leads to an array of symptoms such as genital shrinkage, low muscle mass, and frail bones amongst other symptoms.

Pineal Body

The pineal body, also known as the pineal gland, is a small gland located within the brain. It regulates the circadian rhythm via the production of melatonin, also known as the sleep hormone. Dysregulation of melatonin production can lead to a corresponding dysregulation of the circadian rhythm and reproductive cycles.

Adrenal Gland

The adrenal glands are located at the top of each kidney. Like most glands of the endocrine system, they produce an array of hormones including steroid hormones which regulate blood pressure and salt levels and the metabolism of many compounds like androgens. These are usually converted to testosterone and DHT or estrogens, depending on the gender of their owner. The other hormones produced by the adrenal glands, for which they are named, are adrenaline and noradrenaline. These are responsible for the fight or flight response and act to increase blood pressure, heart and breathing rate, and blood sugar levels.

Thyroid

The thyroid is located in front of the windpipe within the neck. It produces several hormones, collectively known as thyroid hormones. Two variants of thyroid hormones are produced, numbered according to the number of iodine within their structure. T3 is the active form, and T4 its prohormone, with these being secreted in a 1:14 ratio. T4 is converted to T3 by deiodinases located throughout the body. Thyroid hormones act to increase metabolism throughout the body, increase respiratory and heart rate, and regulate brain development and function.

Parathyroid

The parathyroid glands are small glands that sit behind the thyroid. They are responsible for the regulation of blood calcium levels through the production of parathyroid hormone. This induces calcium release from the bones, along with making the kidneys increase vitamin D production and calcium ion retention. Vitamin D acts to raise calcium levels by increasing absorption within the gut. Over or underproduction of the parathyroid hormones can lead to disordered calcium levels within the blood.

Pancreas

The pancreas is an organ within the digestive tract. It is responsible for regulating blood sugar levels by producing the hormones glucagon and insulin. Glucagon is released from alpha cells in the pancreas and raises blood sugar. Insulin is produced by beta cells in the pancreas and lowers blood sugar levels. These work together in a pair of negative feedback loops, which maintain a narrow range of blood sugar levels. Disorders of the pancreas most commonly lead to diabetes mellitus.

Hypothalamus

The hypothalamus is the control centre of the endocrine system. It takes input from around the body and triggers the release of hormones from the pituitary gland. It does this by producing a specialised kind of hormone known as a neurohormone. These then travel down the hypophyseal system to the pituitary where they control the release of hormones from the pituitary.

Pituitary Gland

The pituitary is a gland that sits at the base of the hypothalamus within the brain. It secretes a wide array of hormones, many of which regulate other glands around the body. Actions that are controlled by a series of glands and hormones are referred to as an axis, such as the hypothalamic-pituitary-adrenal axis, which regulates various systems throughout the body, including digestion, immunity, mood, libido and metabolism. Because of its role in regulating the activity of other glands, the pituitary is often referred to as the master gland. Hormones released by the pituitary include but are not limited to, human growth hormone, thyroid-stimulating hormone, luteinising hormone, follicle-stimulating hormone, anti-diuretic hormone and oxytocin. Due to the large number of systems impacted by the pituitary, many different diseases can arise from pituitary disorders.