Endocrine system

• Introduction to the endocrine system
• The pituitary gland
• The thyroid gland
• The parathyroid glands
• The thymus
• The adrenal glands
• The pancreas
• The pineal gland

Introduction to the endocrine system

The role of the endocrine system is to maintain the body in balance through the release of hormones (chemical signals) directly into the bloodstream. Hormones transfer information and instructions from one set of cells to another. Many different hormones move through the bloodstream, but each type of hormone is designed to affect only certain cells.
A gland is a group of cells that produces and secretes chemicals. A gland selects and removes materials from the blood, processes them, and secretes the finished chemical product for use somewhere in the body. The endocrine gland cells release a hormone into the blood stream for distribution throughout the entire body. These hormones act as chemical messengers and can alter the activity of many organs at once.
The parts of the endocrine system are grouped together because they release hormones into the blood without going through a duct (which is basically a tube) first. This is different to an exocrine gland, which releases what it creates through a tube to somewhere other than the blood.
Hormones can act on some specific cells because they themselves do not actually cause an effect. It is only through binding with a receptor (part of the cell specifically designed to recognize the hormone) like a key into a lock – that causes a chain reaction to occur, changing the activity of the cells. If a cell does not have a receptor for a hormone then there will be no effect. Also, there can be different receptors for the same hormone, and so the same hormone can have different effects on different cells.

The pituitary gland

The pituitary gland is a small, oval gland lying at the base of the brain. It is divided into two sections – an anterior (meaning front) and posterior (meaning back) part because they are formed in different ways. The anterior pituitary is a collection of hormone-producing cells. The release of the hormones by these cells is controlled by a region of the brain called the hypothalamus
The posterior pituitary is made up of around 50,000 nerve endings. These nerves release their hormones straight into the blood.
Numerous hormones are released from the anterior pituitary. They are:

• Thyroid-stimulating hormone (TSH): This stimulates the thyroid gland.
• Adrenocorticotrophic hormone (ACTH): This stimulates the adrenal glands.
• Follicle-stimulating hormone (FSH): Promotes development of eggs within the ovaries and stimulates the secretion of oestroges (the primary female hormone). In men, FSH is important for sperm production in the testes.
• Luteinizing hormone (LH): Causes ovulation in women and prepares the uterus for pregnancy.
• Prolactin (PRL): Prolactin causes the development of breast tissue and the production of milk.
• Growth hormone (GH): Also called Somatotropin, it causes growth in almost all tissues in the body that are capable of growing. It promotes both an increase in cell size, and cell number.

Those hormones released from the posterior pituitary are:

• Anti-diuretic hormone (ADH): ADH causes the kidneys to keep more water in the body.
• Oxytocin: This causes contractions in the uterus of a pregnant woman and also causes the release of milk from the breast

The thyroid gland

The thyroid (meaning ‘shield-shaped’) gland sits in the centre of the neck, a the front, below the Adam’s apple. It is made of two lobes joined in the centre. At 15 to 20 grams it is one of the largest of the endocrine glands.
The thyroid secretes two major hormones called thyroxine (T₄) and triiodothyronine (T₃). They cause lots of things, but mostly they increase the rate of metabolism in the body. Metabolism is the amount of energy used by the body. An increase means more energy sources like fats and sugars are being broken down, and the body is using more energy to grow. The thyroid is controlled mainly by the release of Thyroid Stimulating Hormone (TSH) from the pituitary gland. The thyroid also secretes a hormone called calcitonin, important in keeping calcium levels in the body normal.
To create the thyroid hormones, the body needs a substance called iodine, which is found mainly in salt.
Low levels of thyroid hormone can result in feelings of tiredness, excessive sleep, loss of sex-drive, and smaller, less frequent periods in a woman.
Calcitonin is another thyroid hormone and this assists in the regulation of calcium concentration in body. Calcitonin lowers plasma calcium levels by inhibiting the cells which break down bone, and stimulating calcium excretion by the kidneys.

The parathyroid glands

The parathyroid glands are small, ovoid, and lie on the back of the thyroid gland. Most people have four parathyroid glands, two at the top, and two at the bottom.
There are two types of cell within the parathyroid gland. While calcitonin is released from the thyroid when calcium levels are too high, the parathyroids release their hormone when calcium levels are too low.

The thymus

The thymus is located in the lower part of the neck, and the front part of the upper chest. After puberty it is mostly replaced by fat.
While the thymus does not play a big role, it does produce several hormones important in the development and maintenance of a normal immune system.

The adrenal glands

The adrenal glands (also known as the suprarenal glands) are yellow, pyramid-shaped glands located at the top of the kidneys. They usually weigh roughly 7.5g and are heavier in men than women. Each adrenal gland has two parts: an adrenal medulla (inside), and an adrenal cortex (outside).
The adrenal cortex is the outer layer and secretes corticosteroids and male sex hormones which are derived from cholesterol and various other fats, hence their yellowish colour. It is divided into three distinct zones, each producing different hormones.
The adrenal medulla is reddish-brown and the cells here are like nerve cells and are activated by the nervous system. The cell types of this region are are known as pheochromocytes, or chromaffin cells.
Three major types of hormones are released from the adrenal cortex. These are mineralocorticoids, glucocorticoids and a small amount of sex hormones.
Mineralocorticoids are called as such due to their effects on the electrolytes (or minerals) of the body, as well as the level of water. Glucocorticoids control sugar (glucose) levels.
There are two horomes produced in the cortex of great importance and they are aldosterone, the major mineralocorticoid and cortisol, the major glucocorticoid.

Cortisol
Cortisol is a ‘stress hormone’ and is released in times when the body needs increased energy. It is stimulated for release by Adrenocorticotrophic hormone (ACTH), mentioned earlier as being released from the pituitary gland which can be caused by any stressful event.
Cortisol causes the liver to release more sugar, causes breakdown of muscle and fat for energy and also lowers the amount of energy used by the cells of the body. It is also anti-inflammatory and lowers the body’s ability to protect itself.

Aldosterone
Aldosterone causes the body to try and keep water and sodium in the body by acting on the kidney.
The adrenal medulla (the centre) secretes adrenaline, and noradrenaline. The secretion of these hormones is because of the need for quick bursts of energy. Their secretion triggers cellular energy use and allows access to the body’s energy reserves. These effects are very rapid and occur within roughly thirty seconds, and staying there for several minutes. The circulating adrenaline also causes constriction of virtually every vessel in the body (causing your hands to go pale), increased activity of the heart (making it beat faster), inhibition of the gastrointestinal tract (giving you butterflies) and dilation of the pupils of the eyes.

The pancreas

The pancreas is a pinkish-grey organ that lies behind to the stomach. The organ is approximately 15cm in length with a long, slender body connecting the head and tail segments.
The endocrine pancreas is separate from the exocrine pancreas which is discussed under the gastrointestinal section. The endocrine pancreas is made up of small clumps of cells within the pancreas, called pancreatic islets, or the islets of Langerhans. These account for only 1% of the pancreatic mass. It is composed of three distinct cell types each producing a different hormone. The two important hormones are:

Glucagon
Secretion of glucagon is controlled by the level of blood sugar, being released when levels are too low. This greatly increases the output of sugar from the liver and returns blood sugar levels to normal.

Insulin
Insulin is designed to lower blood sugar levels when they become too high and is released in periods when there is a lot of sugar available, like after a meal. A lack of insulin means the body has to use fat for metabolism rather than sugar and can lead to a condition known as ketoacidosis.

The pineal gland

The pineal gland is a small, red, pinecone-shaped structure in the brain.
The pineal gland secretes a substance called melatonin. Melatonin slows the maturation of sperm, eggs and reproductive organs by stopping the production of FSH and LH (mentioned earlier). Melatonin also appears to play a role in regulating the ‘circadian rhythms’ of the body, which influence the day-night cycle. It is also a powerful antioxidant and protects the brain from toxins.

References

1. Guyton AC, Hall JE. Textbook of Medical Physiology. Philidelphia: Harcourt Health Sciences; 2000.
2. Martin EA (ed). Concise Medical Dictionary. Oxford: Corgi Books; 1982.
3. Martini FH. Human Anatomy (third edition). New Jersey: Prentice-Hall; 2000.
4. Moore KL, Dalley AF. Clinically Oriented Anatomy (fourth edition). Baltimore: Lippincott Williams & Wilkins; 1999.
5. Nolte J. The Human Brain: An Introduction to its Functional Anatomy (fifth edition). St Louis: Mosby; 2002.