Endocrine system: Ductless glands, Types, Functions, Significance, Practice Problems and FAQs

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Similarly our body also has a system which releases certain chemicals that function according to the needs of the body. This system is the endocrine system which uses feedback loops to control distant target organs by controlling the release of these chemicals called hormones. Hormones are secreted by the endocrine glands directly into the blood. Let’s understand in detail about this system and how it functions in this article.

        Fig: The human endocrine system

Table of Contents

• Endocrine system
• Glands
• Endocrine glands
• Hypothalamus
• Pituitary gland
• Pineal gland
• Thyroid gland
• Parathyroid gland
• Thymus gland
• Adrenal gland
• Pancreas
• Testes
• Ovaries
• Significance of endocrine system
• Practice Problems
• FAQs

Endocrine system

It is a system of glands that secrete hormones to control and coordinate various physiological processes in the body. The metabolism, energy level, reproduction, growth, development, responses to injuries, stress, and mood are all regulated and controlled by the various hormones.

Glands

Glands are groups of cells or organs which produce and release substances that perform a specific function in the body. Glands are of two types such as exocrine glands and endocrine glands.

Exocrine glands

These glands possess ducts. They secrete the contents near its target or through duct into the target organ. Examples include sweat glands, sebaceous glands, mucous glands and digestive glands.

Endocrine glands

Endocrine glands are ductless glands. They release their secretions directly into the bloodstream.

The substances secreted by endocrine glands are called hormones. The major glands of an endocrine system are the pineal gland, pituitary gland, pancreas, ovaries, testes, thyroid gland, parathyroid gland, hypothalamus, and adrenal gland.

                                    Fig: Endocrine glands

Hypothalamus

It provides necessary inputs to the pituitary gland for the production of hormones and therefore it is also called the master control centre or supreme commander. The hypothalamus produces hormones that stimulate or inhibit hormone production in the anterior and median lobe of the pituitary gland. The hormones that stimulate the hormone secretion are called releasing hormones and the hormones that inhibit secretion are called inhibiting hormones. Releasing and inhibiting hormones reach the pituitary gland through the hypothalamo-hypophyseal portal system.

      Fig: Hypothalamo-hypophyseal portal system

Location of hypothalamus

It is found at the base of diencephalon in the brain. It is a part of the diencephalon.

         Fig: Location of hypothalamus

• Hormones of hypothalamus

• Hypothalamus secretes the following hormones:

• Antidiuretic hormone (ADH) or vasopressin
• Oxytocin
• Thyrotropin releasing hormone (TRH)
• Corticotropin releasing hormone (CRH) or Adrenocorticotropic releasing hormone (ARH)
• Gonadotropin releasing hormone (GnRH)
• Growth hormone releasing hormone (GHRH) or somatotropin releasing hormone (SRH)
• Growth hormone inhibiting hormone (GHIH)
• Prolactin releasing hormone (PRH)
• Prolactin inhibiting hormone (PIH)
• Melanocyte releasing hormone (MRH)
• Melanocyte inhibiting hormone (MIH)

Pituitary gland

Pituitary gland is called the master gland because it directs the secretion of hormones of all the other glands in the body. It is attached to the hypothalamus by a stalk. It is located in a bony cavity called sella turcica of the sphenoid bone in the skull. It is anatomically divided into an adenohypophysis (anterior lobe of pituitary), neurohypophysis (posterior lobe of pituitary) and pars intermedia (median lobe of pituitary).

                                Fig: Pituitary gland

Posterior lobe of pituitary

Posterior lobe of the pituitary, pars nervosa or neurohypophysis does not synthesise any hormones. It only stores and secretes the hormones synthesised by the hypothalamus. Oxytocin (birth hormone) and vasopressin (antidiuretic hormone) are the two important hormones released by the posterior lobe of the pituitary gland.

Median lobe of pituitary

It is also called the pars intermedia. It secretes melanocyte stimulating hormones.

Anterior lobe of pituitary

Anterior lobe of the pituitary gland is also called pars distalis or the adenohypophysis. The secretions of hormones from the anterior pituitary is regulated by two classes of hormones that are releasing and inhibiting hormones from the hypothalamus. Hormones secreted by anterior pituitary include prolactin, growth hormone (GH), luteinising hormone (LH), follicle stimulating hormone (FSH), thyroid stimulating hormone (TSH) and adrenocorticotropic hormone (ACTH).

                           Fig: Parts of pituitary gland

Pineal gland

The pineal gland in human beings is a tiny pea sized endocrine gland located in the dorsal (back) side of the forebrain. It secretes the hormone melatonin. The natural 24 - hour cycle or rhythm of the body is controlled by melatonin. It keeps the body temperature and sleep/wake cycle in check. It is also in charge of defence capacity, menstrual cycle, and pigmentation.

                                                     Fig: Pineal gland

Thyroid gland

It is present on the neck, in front of the trachea or windpipe. This gland consists of 2 lobes, one on each side of the trachea. The two lobes are connected by a thin band of connective tissue called the isthmus. The thyroid gland is made of follicles, which are spaces lined by follicular cells and stromal tissues. The follicular cells synthesise two thyroid hormones triiodothyronine (T₃) and tetraiodothyronine (T₄) or thyroxine. The thyroid hormones increase the rate of metabolism including the metabolism of carbohydrates, proteins and fats. When less thyroid hormones are produced than required by the body, it results in hypothyroidism commonly known as goitre. When more thyroid hormones are produced than required by the body, results in hyperthyroidism. Thyroid glands also produce a protein hormone called calcitonin which helps to regulate the blood calcium level in the body.

                             Fig: Thyroid gland

Parathyroid gland

There are four parathyroid glands in the body and they are located on the back side or dorsal side of the thyroid gland. The parathyroid glands secrete parathyroid hormone, parathormone or PTH. It is a hypercalcemic hormone and raises the blood calcium levels. Whenever blood calcium concentrations are low, parathormone stimulates the reabsorption of calcium ions from bones into the blood.

                                 Fig: Parathyroid gland

Thymus

The thymus is not only an endocrine gland, it also plays a very important role in the immune system. The thymus secretes a peptide hormone called thymosin. Thymosin plays a major role in the differentiation of T-lymphocytes and plays a role in the production of antibodies. The thymus gradually reduces in size as a person grows older. This naturally results in a weaker immune system in the elderly and they fall sick easily.

         Fig: Thymus gland

Adrenal glands

Adrenal glands are also called glands of emergency. Body has one pair of adrenal glands, one above each kidney. The adrenal gland is consists of two distinct regions such as outer cortex and inner medulla.

Adrenal cortex

The adrenal cortex is divided into three zones such as zona glomerulosa, zona fasciculata and zona reticularis. Hormones secreted by adrenal cortex are called corticoids. Zona glomerulosa is the outer zone which secretes mineralocorticoids. Zona fasciculata is the middle zone which secretes glucocorticoids. Zona reticularis is the inner zone which secretes gonadocorticoids.

Adrenal medulla

The hormones released by adrenal medulla are known as catecholamines. It includes adrenaline or epinephrine and noradrenaline or norepinephrine. Adrenaline and noradrenaline are known as emergency hormones or 3F hormones (flight, fright and fight). These are secreted in responses to stress and emergency situations.

                            Fig: Adrenal gland

Pancreas

The pancreas is present in the bend of the duodenum. It is a composite gland which acts as both exocrine and endocrine gland. The endocrine part of pancreas consists of ⍺/alpha cells, ꞵ/beta cells and ẟ/delta cells. They are collectively termed as islets of Langerhans. There are about 1 to 2 million islets of Langerhans in a normal human pancreas. The alpha-cells secrete a peptide hormone called glucagon, the beta-cells secrete a peptide hormone insulin, and the delta cells secrete somatostatin.

Glucagon stimulates conversion of glycogen to glucose in the liver and synthesis of glucose both of these lead to hyperglycemia (increase blood glucose level). Insulin stimulates glycogenesis i.e, conversion of glucose to glycogen. It increases cellular glucose uptake and utilisation leading to hypoglycemia and decreased blood glucose levels. If a person has a high blood glucose level then the person suffers from the disease called diabetes mellitus. This is normally treated by insulin therapy and medicines.

                         Fig: Pancreas

Testes

Testes are part of the male reproductive system. A pair of testis is present in the scrotal sac outside the abdomen of male individuals. Testis performs dual functions. It act as a primary sex organ as well as an endocrine gland. Testis is composed of seminiferous tubules, sertoli cells and the Leydig cells or interstitial cells. The Leydig cells or interstitial cells,  produce a group of hormones called androgens mainly testosterone. Androgens play a major stimulatory role in the process of spermatogenesis which is formation of spermatozoa. Androgen also helps in the development of secondary sexual characters in males such as maturation of male accessory sex organs, growth of facial hair, low pitch of voice etc.

                         Fig: Testes

Ovaries

Ovaries are part of the female reproductive system. Females have a pair of ovaries located in the pelvic cavity. It also has dual functions. It acts as a primary female sex organ that produces secondary occyte during menstrual cycle and also acts as an endocrine gland and produces hormones. Ovary is composed of ovarian follicles and stromal tissues. Oestrogen and progesterone are produced by the ovaries. Estrogen regulates the growth and development of the female secondary sex organs. Progesterone helps in pregnancy. This hormone acts on the mammary gland and leads to the formation of alveoli.

                                      Fig: Ovaries

Significance of endocrine system

The following are the major significances of endocrine system:

• It controls growth and development especially during childhood.
• It regulates body functions in adulthood.
• It also regulates the reproductive process.
• It controls and regulates the major functions and processes in the body.
• It helps to control mood, sleep/wake cycle and metabolism.

Practice Problems

1. Which of the following statements is correct about endocrine glands?

a. They produce enzymes
b. They have duct
c. They release their secretions directly into the blood
d. Salivary glands are endocrine glands

Solution: Endocrine glands are those glands which do not have ducts and are hence called ductless glands. They pour their secretions directly into the blood. Their secretions are called hormones. Hormones in the blood are transported to a far away located target organ. Salivary glands are exocrine glands as they have ducts and their secretions possess enzymes. Hence the correct option is c.

2. Identify the parts marked as ‘a’ and ‘b’ in the figure given below.

a. ‘a’- Pineal gland, ‘b’- Hypothalamus
b. ‘a’- Pineal gland, ‘b’- Pituitary gland
c. ‘a’- Hypothalamus, ‘b’- Pituitary gland
d. ‘a’- Pituitary gland, ‘b’- Hypothalamus

Solution: Hypothalamus provides necessary inputs to the pituitary gland for the production of hormones and therefore it is also called the master control centre or supreme commander. The hypothalamus produces hormones that stimulate or inhibit hormone production in the anterior and median lobe of the pituitary gland. Hence the correct option is c.

3. Which one of the following hormones, though synthesised elsewhere, is stored and released by the master gland?

a. Antidiuretic hormone
b. Luteinizing hormone
c. Prolactin
d. Growth hormone

Solution: Vasopressin, also known as the antidiuretic hormone, is synthesised by hypothalamus. It is stored and released by the posterior lobe of the pituitary gland (master gland). Luteinizing hormone is synthesised and released by the anterior lobe of the pituitary gland under the influence of GnRh secreted by hypothalamus. Prolactin and growth hormones are synthesised and released by the anterior lobe of the pituitary. Hence the correct option is a.

4. Hormones released by hypothalamus regulate the functioning of _______________.

a. anterior lobe of the pituitary gland
b. posterior lobe of the pituitary gland
c. both anterior and posterior lobes of the pituitary gland
d. posterior and median lobe of the pineal gland

Solution: Hormones released by hypothalamus are transferred to the anterior lobe of the pituitary gland through the hypothalamo-hypophyseal portal system and regulate the functioning of the anterior lobe of the pituitary gland. The posterior lobe of the pituitary gland is under the direct neural regulation of the hypothalamus. Hence the correct option is a.

                                   Fig: Hypothalamo-hypophyseal portal system

FAQs

1. Which hormone’s secretion is raised during stressful situations?
Answer: During stressful situations, more CRH (corticotropin-releasing hormone) is produced by the hypothalamus. CRH stimulates the release of ACTH (adrenocorticotropic hormone) by the anterior pituitary. The ACTH stimulates the adrenal cortex to release glucocorticoids such as cortisol, which are also called stress hormones.

2. What are hormones?
Answer: Hormones are chemical messengers produced in response to a specific stimulus. They are non-nutrient chemical messengers transported to target cells and are produced in trace amounts.

3. What is the precursor of triiodothyronine?
Answer: The amino acid tyrosine is a precursor of the thyroid hormone triiodothyronine or T3 and tetraiodothyronine or thyroxine or T4.

4. Which hormone stops period?
Answer: The hormone progesterone secreted by the corpus luteum stops menstruation and helps in maintaining pregnancy.