Introduction to the Endocrine System

The endocrine system is one of the primary control and communication systems of our body. It consists of ductless glands that secrete biochemical messengers within specific organs. These chemicals are commonly known as hormones and they are carried via the bloodstream to multiple other body organs.

The hormones act as messengers and guide critical processes such as growth, reproduction, sleep, emotions, metabolism, and digestion. The hormones also improve our ability to sustain homeostasis and react to external and internal stimuli.

Endocrine System Organs – The Glands and their Hormones

The endocrine glands are devoid of any duct and discharge their secretions straight into the intercellular fluid or into the blood. The collection of endocrine glands and their hormones makes up the endocrine system.

Most hormones work together in groups to form specific regulatory hormonal cascades. This hormonal cascade gets initiated via external or internal stimulus to the hypothalamus, releasing a hypothalamic hormone. This hypothalamic hormone affects one or more pituitary hormones, which in turn affects one or more target gland hormones.

The level of hormones in the bloodstream and other body organs is constantly regulated and maintained collectively by the endocrine organs, the immune system, and other control factors. Here is the list of Endocrine glands:

• Hypothalamus
• Adrenals
• Pituitary
• Thyroid and Parathyroid
• Pineal
• Thymus
• The Gonads or the Reproductive organs – Ovaries in females and testes in males.
• The pancreas also belongs to the endocrine system which not only produces hormones but also helps the digestion process.
• Placenta during pregnancy.

The Hypothalamus and Its Hormones

The hypothalamus is a small part of the brain which controls multiple body functions as below. These functions are carried out due to the electrochemical connection from the hypothalamus to all other body organs. Hypothalamic hormones control metabolism, growth and development, and reproduction.

• Eating and Drinking
• Blood Pressure and Heart rate
• Sexual functions
• Behaviors
• Body temperature
• Sleep-wake cycle
• Emotional states (e.g., fear, anger, pain, and pleasure).

The endocrine system is also responsible for the electrochemical connection from the hypothalamus of the brain to all the organs of the body which controls the two types of hormones secreted in the endocrine system. These are Steroidal and non-steroidal (or protein-based) hormones.

The Pituitary Gland and Its Hormones

The pituitary gland sometimes called the hypophysis or the Master gland is a cherry-sized structure. It is present in the brain straight below the hypothalamus. It secretes hormones that affect the growth and secretion of other endocrine glands.

The pituitary gland has two parts: the anterior pituitary and the posterior pituitary.

Anterior pituitary gland

It secretes important hormones that stimulate other endocrine system glands as below:

• Adrenocorticotropic hormone (ACTH) stimulates the adrenal glands
• Antidiuretic hormone (ADH) which decreases urine secretion
• Follicle-stimulating hormone (FSH) which affects the ovaries
• Thyroid-stimulating hormone (TSH) which controls thyroid activity

Other than stimulating hormones, the Pituitary gland also secrets the Prolactin hormone and Growth hormone (GH) which is important in regulating growth.

Posterior pituitary gland

It cannot produce its own hormones; rather, it stores two hormones —vasopressin (controls amount of water in our body) and oxytocin and these are produced by the neurons in the hypothalamus.

The Growth Hormone

Out of the different pituitary hormones, Growth Hormone (GH) is the most abundant. The GH levels in the blood are the highest in early childhood and during puberty and reduce after that. However, in the later stages of life, the lower GH levels still may be significant and its deficiency may cause the symptoms of aging.

• The growth hormone controls the body’s growth and development by:
  • Increases the linear growth of the bones
  • Stimulates the growth of internal organs, connective tissue, fat tissue, endocrine glands, and the muscles
  • Controls the growth of the reproductive organs.
• Besides its growth-stimulating role, GH affects the metabolism of carbohydrates, fats (i.e., lipid) and proteins. The Growth Hormone improves the levels of the sugar in the blood by:
  • Reducing glucose uptake by muscle cells and adipose tissue.
  • Stimulating glucose production (i.e., gluconeogenesis) from precursor molecules in the liver. (These actions are opposite to the activities of the hormone insulin).
• GH also increases the uptake of amino acids from the blood into cells, and also their integration into proteins, and promotes the breakdown of lipids in adipose tissue.

The Prolactin Hormone

The prolactin combined with other hormones helps develop the female breasts. It is also responsible for inducing lactation after childbirth. The function of prolactin in men is not well understood, but too much prolactin release can cause reduced sex drive (i.e., libido) and impotence.

In nursing women, prolactin is released by the suckling action of the infant. Additionally, the hypothalamus also promotes or inhibits prolactin levels through other hormones. For example, dopamine creates an inhibitory effect.

The Adrenal Glands and Its Hormones

The Adrenal Glands are small structures situated on top of the kidneys. The Adrenals get stimulated in stressful situations via the Pituitary gland and it secretes hormones that allow our body to adapt and respond.

The Adrenal glands have two parts, an outer layer – The Cortex and an inner layer – The Medulla. The adrenal medulla produces epinephrine and norepinephrine, also known as adrenaline hormone.

The adrenal cortex produces different hormones like glucocorticoids and mineralocorticoids. The primary glucocorticoid in humans is cortisol, also called hydrocortisone. The cortex also produces sex hormones, although much smaller in amount than the ovaries and testes.

The Adrenaline Hormone

In stressful situations, the Adrenaline causes blood sugar to rise by inhibiting insulin secretion from the pancreas. It also stimulates the processes of glycogenolysis (conversion of Glucagon to Glucose) and glycolysis (conversion of Glucose to energy).

The Cortisol Hormone

In stressful situations, Cortisol turns the immune system off, increase blood pressure, and increases the heart rate to push blood circulation in the body and hence energy release to the organs in need.

• It helps to control carbohydrate, protein, and lipid metabolism. For example, cortisol can increase as well as decrease blood glucose levels.
• In addition, cortisol promotes protein and lipid breakdown into products in various tissues, (i.e., amino acids and glycerol, respectively) which can be used for gluconeogenesis.
• Furthermore, cortisol protects the body against the harmful effects of various stress factors, major surgery, acute trauma, severe infections, pain, blood loss, hypoglycemia, and emotional stress.
• High doses of cortisol and other corticosteroids can be used medically to suppress tissue inflammation of injuries and also to reduce the immune response to foreign molecules.

The Pineal Gland

The Pineal Gland is a pinecone shaped small organ that is located nearly at the center of the human brain. It is the control center of our biological clock or the Circadian Rhythm.

It secrets hormones when triggered by the cyclic nervous action, which in turn responds to the external light sources. The pineal gland produces melatonin hormone that has the following functions:

• Regulates reproductive hormones via the sex glands
• Controls menstrual cyclicity
• Controls Sleep Cycle
• Mood Swings and changes
• Maintains circadian rhythm

The Gonads and Their Hormones

The gonads are found in the pelvic cavity. The gonads (i.e., the ovaries and testes) perform two major functions. Firstly, they produce the germ cells (i.e., ova in the ovaries and spermatozoa in the testes), which are necessary to produce the offspring.

Secondly, the gonads produce steroid sex hormones i.e. estrogen and progesterone in females and testosterone in males. The sex hormones are vital for the following functions in the body.

• Development and function of both female and male reproductive organs.
• Secondary sex characteristics (e.g., the presence of body hair, facial hair in men).
• Pregnancy, childbirth, menstrual cycle, and lactation in females.

The Estrogen Hormone

The major estrogen is estradiol, and with small amounts of estrone and estriol, it is produced mainly in the ovaries. In men and postmenopausal women, most estrogens present in the circulation are brought from the conversion of adrenal, testicular, and ovarian androgens. The conversion arises in peripheral tissues, primarily adipose tissue and skin.

The Thyroid Gland

It consists of two lobes and is often termed as a butterfly gland due to its unique shape. It is present in front of the windpipe (i.e., trachea), just below the voice box larynx.

The gland creates two structurally related hormones, thyroxine (T4) and triiodothyronine (T3), which regulates the metabolic rate. Both hormones are together referred to as thyroid hormone. It also produces calcitonin hormone, which regulates the amount of calcium in the blood plasma.

The Parathyroid Glands

These are four pea-sized bodies situated at the back of the thyroid gland that produces Para Thyroid Hormone PTH. This hormone enhances calcium levels in the blood and also helps to maintain bone quality.

• PTH promotes inhibits loss of Calcium in the urine and promotes excretion of phosphate instead.
• PTH helps to release stored calcium from the bones to augment calcium levels in the blood when it goes below the required levels.
• PTH promotes the absorption of calcium from the food in the gastrointestinal tract.

The Pancreas

It is situated in the abdomen and carries out two different functions. Firstly, it acts as an exocrine organ. Most of the pancreatic cells generate various digestive enzymes that are secreted in the gut and these are vital for the effective digestion of food.

Secondly, the pancreas serves as an endocrine organ as specific cell clusters (i.e., the Islets of Langerhans) produce two hormones—insulin and glucagon— that are released into the blood and play an essential role in glucose metabolism and to regulate our blood sugar levels.

References – The Endocrine System

Human Physiology / The endocrine body system

Anatomy of the Endocrine Body System

The Endocrine Body System