Endocrine system

The endocrine system is a method of communication in the body and it involves the secretion of hormones into the blood stream where they act on target tissues. The nervous system and the endocrine system are the two main communication pathways in the body and the endocrine system is a slower route of communication which reaches long distance organs.

Endocrine organs can be found throughout the body and are divided into two classes- the primary endocrine organs and the secondary endocrine organs. The primary organs release hormones and the secondary organs release hormones due to some other function.

Primary Endocrine Organs
There are a number of different primary endocrine organs in the body such as the hypothalamus and the pituitary glands. Other primary glands include the pineal glands, the thyroid gland, the parathyroid gland, the thymus, the adrenal gland, the pancreas and the ovaries in females and the testes in males. The placenta for a growing foetus is also a primary endocrine gland.

The Hypothalamus and Pituitary Gland
The hypothalamus and the pituitary gland are responsible for regulating every system in the body. The hypothalamus resides in the brain and it secretes hormones that predominantly act on the pituitary gland which is connected to the hypothalamus by the infundibulum which is a thin tissue structure. Once the hormones are released, they enter the hypothalamic pituitary portal system which is where the blood vessels are arranged in capillary beds. The hormones travel through the infundibulum in the portal vein and then the hormones can act on the anterior pituitary. The pituitary gland can be split into two different regions called the anterior lobe and the posterior lobe. The posterior lobe is the neural connection between the pituitary and the hypothalamus and it is in this lobe that the axon terminals of the neurons are located. The neural endings release vasopressin and oxytocin both of which are synthesised by the neurons. They are then transported to the axon terminals in the posterior pituitary by secretory vesicles. Their release is controlled by exocytosis when a signal comes in from the nerves and they are then released into the blood where they have various functions. Vasopressin regulates water absorption in the kidneys and oxytocin stimulates uterine contractions.

The anterior portion of the pituitary gland secrete trophic hormones which regulate secretion of other hormones in the body. The tropic hormones can be split into stimulating hormones which increase the secretion of certain hormones or the inhibiting hormones which has the opposite effect. There are a number of cells within the anterior pituitary region that synthesise the majority of hormones secreted by the gland. Each cell is responsible to secreting a different type of hormone. The neural input is the control mechanism that regulates the secretion of the anterior hormones. There are seven main hormones that are released by the anterior pituitary:

1. Growth hormone releasing hormone - this hormone stimulates the growth hormone secretion and the growth hormone regulates growth and energy metabolism as well as stimulating    the liver to release insulin-like growth factors.

2. Corticotrophin releasing hormone - this hormone causes the release adrenocorticotropic hormone which can then act on the adrenal cortex, stimulating the release of other hormones.

3. Prolactin releasing hormone - this hormone causes the release of prolactin from the anterior pituitary which stimulates mammary gland development.

4. Prolactin inhibiting hormone- this hormone inhibits the release of prolactin. Dopamine also has the same effect.

5. Thyrotropin releasing hormone - this causes the release of thyroid stimulating hormone which can stimulate the release of thyroid hormones from the thyroid glands.

6. Growth hormone inhibiting hormone - this hormone inhibits the release of the growth hormone.

7. Gonadotrophin releasing hormone- This hormone is responsible for stimulating the release of the follicle stimulating hormone and the luteinizing hormone in females. It also causes the release of sex hormones such as oestrogen, progesterone and androgens.

Pineal Gland
The pineal gland can be found in the brain and it is composed of glandular tissue. The main hormone secreted in melatonin which is important in the establishment and maintenance of circadian rhythms. These rhythms control the body in relation to night and day time. Melatonin levels rise at night and fall during the day which led to the theory that it was a sleep-inducing hormone. It can also affect immune function by boosting immunity and it can also suppress the reproductive system.

Thyroid and Parathyroid Glands
The thyroid gland can be found on the ventral surface of the trachea and resembles the shape of a butterfly. It releases tetraiodothyronine, thriiodothyronine and calcitonin in order to control the metabolic rate of the body and the calcium levels in the blood. The thyroid gland is weighs approximately one ounce but is heavier in females than males because it becomes larger during menstruation and pregnancy. The gland itself is surrounded by a layer of connective tissue and the inside of the gland is made up of a number of different vesicles.

There are four parathyroid glands which are found over the surface of the thyroid gland. These secrete the parathyroid hormone which helps regulate calcium levels.

The Thymus
The thymus can be found near the heart and it changes in shape throughout development. Nearing puberty, the thymus gland has almost disappeared. It is made up of two lobes which extend from the fourth costal cartilage. The sternum and muscles cover the gland and it sits upon the pericardium. There are a number of lobes situated on the outer surface and it is pink-grey in colour.

The thymus secretes a hormone called thymosin. It is essential for immune function because maturation of T cells occurs in the thymus and thymosin regulates their function.

Adrenal Glands
The adenal glands are found above the kidneys and they are also known as the suprarenal glands. The outer layer of each adrenal gland is called the cortex and is the main weight of the gland. The inner region of the gland is called the medulla. The cortex itself can be split into three different layers called the zona glomerulosa, the zona fasciculata and the zona reticularis. Within each layer is a different composition of enzymes which synthesise the hormones.

Adrenocorticoids are the hormones released by the cortex and can be divided into three different classes. The mineralocorticoids are secreted by cells in the zone glomerulosa and they regulate sodium reabsorption and potassium release in the kidneys. The glucocorticoids are released by the zona fasciculata and the zona reticularis. The most common hormone is cortisol and these hormones predominantly regulate the stress response of the body and the metabolism of protein, carbohydrates and lipids. The final class of hormones is the sex hormones such as androgens which are secreted from the zona fasiculata and the zone reticularis. The sex hormones regulate reproductive function.

The adrenal medulla portion of the adrenal glands contain chromaffin cells and they secrete catecholamines. The majority of the catecholamines secreted is the epinephrine and the rest is made up of norepinephrine and dopamine. Epinephrine increases heart rate during times of stress and also controls the mobilisation of energy stores.

Pancreas
The pancreas is both an endocrine and exocrine gland. The exocrine portion of the pancreas os made up of acinar cells which secrete fluid into the gastrointestinal tract and the endocrine region of the pancreas contains cells in clusters called the islets of langerhams. These cells are the major source of hormones. Insulin is secreted by B cells and glucagon is secreted by the A cells also known as alpha and beta cells. These two hormones are crucial to the regulation of energy metabolism and the level of glucose found in the blood. There are also two other cells present in the islets of langerham and these include the D and F cells. D cells or delta cells as they are also referred to, secrete somatostatin which is involved in regulating the digestive system. F cells secrete a pancreatic peptide.

Gonads
The gonads are the sex organs in males and females and they are responsible for producing gametes. The testes in the male and the ovaries in the female have endocrine functions. Men secrete sex hormones such as androgens and testosterone whereas the female gonads secrete oestrogen and progesterone. These hormones are important in the development and maturation of the reproductive tract. Androgens are involved in the development of sperm and levels of oestrogen and progesterone fluctuate throughout the menstrual cycle.

Placenta
The placenta is also an endocrine gland and it secretes oestrogens and progesterones. These are critical for the development of a healthy foetus.

Secondary Endocrine Organs
There are many organs in the body that also secrete hormones. As this secretion is usually not their primary function, these organs are referred to as the secondary endocrine organs. The main organs that secrete hormones are the kidneys, heart, liver, skin and digestive organs.

The Heart
The heart is the pump of the body and is responsible for supplying organs and tissue with a constant blood supply. It also has an endocrine role as it secretes a hormone called atrial natriuretic peptide which regulates the reabsorption of sodium in the kidneys. When the atrial wall is distended, ANP is released and it increases the filtration rate of the kidneys by acting on the arterioles supplying the kidneys and causing them to constrict or dialate accordingly.

The Kidneys
The kidneys are the main filter of the body and are a pair of organs situated in the abdominal cavity. They are responsible for filtering the blood and removing toxic waste such as urea. They also secrete a hormone called erythropoietin which is important in red blood cell production. Erythropoietin travels in the blood to the bone marrow and stimulates the pluripotent cells to differentiate into red blood cells. The secretion of erythropoietin is usually high under low oxygen conditions.

The Gastrointestinal Tract
The digestive tract is responsible for breaking down food products and there are a number of different organs involved including the stomach and the intestines. However, some of the organs have a secondary endocrine function. The stomach releases gastrin which can act on the stomach and increase the level of acid secretion. The small intestine releases secretin which can affect the secretion of the gastrointestinal tract and it also releases glucose-dependent insulinotropic peptide which can regulate the liver and the pancreas.

The Liver
The liver processes metabolic nutrients and removes waste from the blood. It also secretes a hormone called Insulin-like growth factors. There are a number of different types of IGF and they are important for tissue growth. IGF-2 is important during the growth and development of a foetus whereas IGF-1 comes into play after birth.

The Skin
The skin is a large surface area that protects the body from unwanted toxins and infections. It also is involved in producing 1,25-dihydroxy vitamin D3. The main role of this hormone is to regulate the blood calcium levels.

Hormones
Hormones are circulated in the blood and act as messengers, communicating between different organs. The amount of hormones released depends on a number of different factors, the first of which is the rate of secretion. The cells that release the hormones act upon signals that they receive in order to increase or decrease the level of hormones in the blood. Sometimes the cells release a constant stream of hormones so that the rate remains steady. The amount of hormones in the blood also depends on whether they are bound to carrier proteins. If they are bound to carrier proteins, then they have an increased half life and will be in the blood longer. The metabolism of hormones also affects the amount of hormones in the blood. It depends on the type of hormone as to how long the hormone will be in the blood as steroids have a longer half life than amines.

Abnormal Secretion
The levels of hormones in the body are important and a slight imbalance can cause a number of different conditions. Sometimes disease can occur because of hypersecretion where there is an excess of hormones and this can cause acromegaly where the bones thicken and the organs enlarge. The opposite effect is hyposecretion where too little of a certain hormone is secreted. This can cause serious problems such as insulin-dependent diabetes mellitus where not enough insulin is released from the pancreas.

Diseases linked to the endocrine system can be categorised into two major groups. The first is called a primary secretion disorder where the gland that produces the hormone is affected. A secondary secretion disorder occurs when there is a problem with either the anterior pituitary or the hypothalamus.