Endocrine System

OVERVIEW

The endocrine system produces a variety of hormones that can coordinate homeostasis, metabolism, tissue function, growth development, sexual function, sleep and mode. Despite the tiny size of the endocrine organs compared to other organs, their role in human body is so crucial that any endocrine incapacity can develop problems in the growth during puberty, in the occurrence of pregnancy, and in the management of stress. Also with an unhealthy endocrine system, too much sugar, instead of being carried to cells to produce energy, are stored in the blood. It leads to overweight, bone weakness and lack of energy level. No specific region in our body is dedicated to the organs of endocrine system. Instead, they are distributed in different areas.

HORMONES

 Introduction

Each hormone has its own target cells or organs, which it affects only despite its circulation all over the body via blood. But what is the mechanism through which a target cell responds to a hormone? or why does a certain hormone affect a certain cell, not one else? It is done when the plasma membrane or the interior surface of this cell contains specific protein receptors to which that hormone can connect. Here the hormone can work effectively. Hormones function to stimulate or prevent cell division, excretion of a substance, and transcription of certain genes. In addition to its influence on the activity of enzymes, it opens or shuts ion channels to change plasma membrane permeability or membrane potential (Fig. 1).

The activity of the endocrine glands including the release of different hormones depends on a variety of stimuli which are classified into three major categories:  

 Hormonal Stimuli

It indicates the excitation of the endocrine organs in response to other hormones. For example, the anterior pituitary gland and other endocrine organs are spurred to release their hormones into the bloodstream by hormones of the hypothalamus and many anterior pituitary hormones, respectively. This mechanism of hormonal stimuli increases and reduces the hormone blood levels again and again in a rhythmic way. It is considered the most common exciter in human body (Fig. 2).

 Humoral Stimuli

It refers to the release of endocrine hormones in response to the altered blood levels of certain ions and nutrients. For example, the production of parathyroid hormone (PTH) is stimulated by the reduction of blood calcium ion level in the capillaries serving the parathyroid glands. However, this stimulus stops when the blood Ca2+ level rises again as PTH itself works to compensate that decline. In addition to PTH, insulin and calcitonin are prodded by humoral stimuli such as the pancreas and the thyroid gland, respectively (Fig. 3). 

 Neural Stimuli

It is defined as the response of endocrine cells to the nervous system, which sends a neural stimulus, so these cells release hormones. An example is the excretion of the catecholamines, norepinephrine and epinephrine by the adrenal medulla in response to a sympathetic nervous system stimulation (Fig. 4).  

ENDOCRINE GLANDS

 Introduction

The main function of the endocrine system is the release of hormones, and it is achieved through a variety of glands existed in different parts of the body. In addition to the hypothalamus, which is a major endocrine gland staying in the nervous system, the pituitary, thyroid, parathyroid, pineal, thymus, pancreas, adrenal glands, and gonads represent the other endocrine organs. These glands can be either endocrine only, releasing their hormones into the blood or lymph directly as they are ductless (as in the thyroid, parathyroid, anterior pituitary and adrenals) or mixed (endocrine and exocrine), using ducts to send their products to the body’s surface or into body cavities (as in pancreas and gonads).  Both types of glands are composed of an epithelial tissue. 

 Pituitary Gland

It is the major endocrine gland. Despite its tiny size (pea-sized organ), pituitary gland is considered as the body’s master controller. It is divided into two lobes: anterior pituitary (glandular tissue) and posterior pituitary (nervous tissue) (Fig. 5). The anterior pituitary gland is regarded as the master endocrine gland due to its domination over the activity of so many other endocrine glands. Its location below the hypothalamus at the base of your brain make its activity of releasing hormones highly controlled by this smart coordinating center. The function of the hypothalamus is to assure the accuracy of hormone secretions (Fig. 6). It can detect any rise or reduction in the hormone level, and consequently contacts the pituitary gland through the stalk to release hormones directed to the appropriate receptors with a message of adjusting tissue activities. A pituitary gland working perfectly means a healthy endocrine system. In case the pituitary gland is eradicated or even damaged, it can cause dramatic effects on the body.

Oxytocin and antidiuretic hormone are two extra hormones released by the hypothalamus. After travelling along the axons of the hypothalamic neurosecretory cells, they are stored in the posterior pituitary, waiting for nerve impulses to be released into the bloodstream (Fig. 7). Accordingly, the posterior pituitary is not classified as an endocrine gland; it acts as a store, not a maker. The pituitary hormones include thyroid stimulating hormone (TSH), follicle stimulating hormone (FSH) and luteinizing hormone (LH), as well as adrenocorticotrophic hormone (ACTH), which spur the thyroid gland, the gonads (to produce oestrogen, progesterone and testosterone), and the adrenal glands, respectively. Also, oxytocin, prolactin and antidiuretic hormone (ADH) are produced by the pituitary gland to prod the contractions of uterine muscles in childbirth, to secrete breast milk and to organize urine production, respectively. In addition, growth hormone affects and works on all the cells of the body (Fig. 8).

 Hormones of Anterior Pituitary Gland

 Growth Hormone 

Growth hormone (GH), as a vital hormone released by the anterior pituitary gland, is responsible for human development and the final body size as it controls the growth of skeletal muscles and long bones. Hence, it is referred to as a metabolic hormone. Also, GH is anabolic hormone stimulating the reproduction and regeneration of cells in human body. Furthermore, it converts amino acids into proteins, breaks down fats to be used for energy and keep blood sugar homeostasis (Fig. 8).

 Prolactin

It is similar to growth hormone for having the same protein structure. In females, its function comes to surface after childbirth targeting breasts to prod and keep milk production (Fig. 8). However, its function in males is unknown. 

 Gonadotropic Hormone 

The hormonal activity of the genital system in males and females depends upon the gonadotropic hormone. As the gonadotropin follicle-stimulating hormone (FSH) is responsible for stimulating follicle development of the ovaries in women, the follicles grow secreting estrogen, which allows for the ovulation. In men, FSH works on testes allowing them to develop sperms. Luteinizing hormone (LH) functions to prod the ovulation of an egg from the ovary and to spur testosterone released by the interstitial cells of the testes in women and men, respectively (Fig. 8).

 Thyrotropic Hormone

Thyrotropic hormone (TH) or thyroid-stimulating hormone (TSH) is released by the anterior pituitary gland. It has an important role in the growth and activity of the thyroid gland (Fig. 8). 

 Adrenocorticotropic hormone 

Adrenocorticotropic hormone (ACTH) is responsible for the production of cortisol from the cortex portion of the adrenal gland (Fig. 8).

 Hormones of Posterior Pituitary Gland

 Oxytocin 

This hormone is produced by the hypothalamus into the bloodstream in significant amounts only during childbirth and nursing in order to facilitate them. It works through causing the uterine muscles contracted strongly during sexual relations, labor, and breastfeeding. Also, it helps breastfeeding women to eject milk. Oxytocic drugs can be natural or synthetic; both are applied to stimulate labor or to make its course faster (Fig. 7).

Antidiuretic Hormone 

As diuresis refers to urine production, an antidiuretic substance implies to obstacle urine production. ADH is made in the hypothalamus, urging the reabsorption of more water by the kidneys from the forming urine. It leads to the decrease of urine volume, and the increase of blood volume. ADH release is hindered by drinking liquids such as water. Also, alcoholic drinks cause this inhibition leading to an output of large amounts of urine. It can explain the dehydration represented by the intense thirst and dry mouth experienced the next morning (Fig. 7).   

 Thyroid Gland

It is a vital endocrine organ due to its role in the regulation of metabolism, growth, and development of human body. It works through releasing constant amounts of hormones into the bloodstream. It is divided into two effective butterfly-shaped lobes located on both sides of the trachea (Fig. 9). Thyroxine (T4) and triiodothyronine (T3) are the two hormones execrated by the thyroid gland, with 3 & 4 indicating the content of iodine atoms in each hormone. They collectively form the thyroid hormone, which targets every cell in the body as it regulates the rate of glucose oxidization, turning it to heat and chemical energy (ATP) required by every cell in the body to accomplish their activities. Thyroid hormone is relevant to the reproductive and nervous systems because of its impact on the normal tissue growth and development (Fig. 10).

 Parathyroid Glands

They are four small pea-shaped glands surrounding the thyroid gland. Their function is crucial to the blood and bones for being in charge of adjusting levels of phosphorous, calcium (Ca+2), and magnesium through releasing parathyroid hormone (Fig. 11).

 Thymus Gland

The size of the thymus is changing. The older man is, the smaller in size the thymus. Infants are born with large thymus, which decreases in size till it is mostly formed of fibrous connective tissue and fat by old age. It urges a special group of white blood cells (T lymphocytes) and the immune response to develop normally through releasing a variety of hormones such as thymosin. Thymus is sited in the upper thorax, posterior to the sternum (Fig. 12).

 Adrenal Glands

There are two triangle-shaped adrenal glands located on the top of the kidneys. The main function of the adrenal glands is to deal with stress through two hormones, adrenaline (epinephrine) and noradrenaline (norepinephrine), released by a medulla existed in the center of the gland. A cortex surrounds the medulla, and it acts to execrate one of the sex hormones, which is androgen, in addition to the production of cortisone and aldosterone, two hormones helping in achieving fluid and electrolyte balance in the body (Fig. 13). 

 Pineal Gland

The pineal gland is located in the center of the brain foisted between the two hemispheres. It is responsible for the sleep/wake cycle through releasing melatonin, a hormone prodded by darkness and hindered by light. The highest level of melatonin happens at night making us sleepy while the lowest level happens during daylight around noon (Fig. 14). 

 Pancreas

The pancreas is a gland seen behind the stomach in the abdominal cavity. It has an exocrine and endocrine function in the digestive system and endocrine system, respectively. The exocrine function of the pancreas is achieved by a duct into the duodenum, which releases enzymes helping to complete the digestion of food in the small intestine by breaking down fats and carbohydrates and neutralizing stomach acids. The endocrine function of the pancreas is ductless, represented in the release of insulin from beta cells in the islets of Langerhans, and glucagon from alpha cells of the pancreas (Fig. 15). 

 Gonads

The gonads refer to the genital glands which are ovaries in females and testes in males (Fig. 16). The ovaries, located above the fallopian tubes on either side of the uterus, are responsible for releasing eggs monthly for fertilization (Fig. 17). Also, they are in charge of giving the typical female characteristics such as the development of breasts and reproductive organs through producing two categories of hormones, called oestrogen and progesterone. These hormones respond to the pituitary gland to prepare the body for pregnancy or for the menstrual cycle if fertilization does not happen. At the time of menopause, levels of oestrogen go low rapidly. In males, the testes, situated in a sac named the scrotum behind the penis, are in charge of releasing sperm as well as the male hormone testosterone which gives the typical male characteristics, such as body and facial hair (Fig. 18).

CLINICAL NOTES

 Acromegaly

It is a rare condition, taking place when growth hormone is produced with quantities bigger than usual after puberty. This excess release is usually due to an adenoma of the pituitary gland. The symptoms include the abnormally large growth of bones in hands, feet, face and jaw. This condition coincides with visceromegaly. Acromegaly can develop to death. Resection of the tumour is the highly recommended treatment in conjunction with medication (Fig. 19).

 Dwarfism

It is a condition caused either by a congenital aplasia meaning no tissue, or hypoplasia meaning under tissue of the pituitary gland. Because of the lack of growth hormone production, bones are badly affected remaining small and underdeveloped, and consequently the person is small all over. However, this condition has no effect on mental abilities or intelligence. Treatment aims at activating bone growth through exciting growth hormone. This case is called proportionate dwarfism, which differs from achrondroplastic dwarfism with a genetic aetiology (Fig. 20). 

 Goiter

It is a generic term indicating a swollen thyroid due to a tumor, or iodine deficiency (standing behind 90% cases of goiter worldwide). This enlargement is observed at the front of the neck affecting the larynx causing it swollen. Any thyroid dysfunction can lead to this condition (Fig. 21). 

Graves’ Disease

It refers to a disorder in the immune system occurring as a result of the overproduction of the thyroid hormone. As the metabolic rate of cells is high, thyrotoxic happens. Weight loss, sweating and rapid pulse are among the symptoms (Fig. 22).

 Hyperinsulinism

It takes place when the pancreas gives an abnormal overproduction of insulin, causing the withdrawal of glucose out of the bloodstream. Here occurs hypoglycaemia which can result in loss of consciousness and convulsions. There are two pathogens of hyperinsulinism: tumor such as carcinoma of the pancreas or an adenoma, and an excess intake of insulin (Fig. 23).

 Addison’s Disease

It is a rare condition happening with the underproduction of glucocorticoids and mineralocorticoids by the adrenal glands. The symptoms include weight loss, weakness, melanin pigmentation of the skin and excessive secretion of water and salts. Also, hypoglycaemia and hypotension can result. An infection and autoimmune adrenalitis are thought to be the pathogens (Fig. 24).

 Diabetes Mellitus

Glucose is one of the main sources of energy used in the cells to form tissues and muscles. When the body fails to metabolize it, it stays in the bloodstream without taking its normal course. Here diabetes mellitus occurs. Also, this disease appears with the insufficient production of insulin by the pancreas. The symptoms vary including overweight or loss of weight, ketonuria, polyuria, glycosuria and hyperglycaemia. Type 1 and type 2 of diabetes mellitus appear in children because of the insufficient secretion of insulin, and in the adults with unhealthy lifestyle including overweight, high cholesterol and high blood pressure (Fig. 25). 

 Hyperthyroidism

It happens when the thyroid gland secretes much more thyroxine than required because of a tumor in the gland. As a result, metabolism is accelerated, and patients lose their ability to endure heat. Symptoms include unintentional weight loss, rapid heartbeat, tense mood with an inability to relax. In Graves’ disease, which is one form of hyperthyroidism, patients have the thyroid gland swollen with an anterior protrusion of the eyes.  A part of the thyroid can be removed surgically or destroyed chemically using thyroid-blocking drugs or radioactive iodine (Fig. 26). 

 Hyperparathyroidism

When the parathyroid gland secretes plus quantities of parathyroid hormone, hyperparathyroidism results.  This condition leads to bones decalcification which occurs when the bloodstream receive excrescent amounts of calcium leaving the bones. Accordingly, osteoporosis, fractures and cysts happen.  A tumour in one of the parathyroid glands is the cause behind this disease, which can be treated through the eradication of the tumor (Fig. 27). 

 Syndrome of Inappropriate Antidiuretic Hormone (SIADH)

It is a disorder of damaged water excretion occurring because of the inability of the body to quell the production of antidiuretic hormone (ADH). If the water intake exceeds urine output, water retention results. Head injury, drug reaction or tumor can be the causes of this condition, which can be cured through dietary water restriction (Fig. 28). 

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