Introduction
Q. What are Glands?
Ans: A gland is an organ which produces and releases substances that perform a specific function in the body. There are two types of glands. Endocrine glands are ductless glands and release the substances that they make (hormones) directly into the bloodstream. These glands form part of the endocrine
Glands
- The pituitary gland is located in a bony cavity called sella tursica and is attached to hypothalamus by a stalk.
- It is divided anatomically into an adenohypophysis and a neurohypophysis.
- Adenohypophysis consists of two portions, pars distalis, and pars intermedia.
- The pars distalis region of pituitary, commonly called anterior pituitary, produces
- 1. Growth Hormone (GH),
2. Prolactin (PRL),
3. Thyroid Stimulating Hormone (TSH),
4. Adrenocorticotrophic Hormone (ACTH),
5. Luteinizing Hormone (LH) and
6. Follicle Stimulating Hormone (FSH).
- Pars intermedia secretes only one hormone called Melanocyte Stimulating Hormone (MSH).
- However, in humans, the pars intermedia is almost merged with pars distalis.
- Neurohypophysis (pars nervosa) also known as posterior pituitary, stores, and releases two hormones called oxytocin and vasopressin, which are actually synthesised by the hypothalamus and are transported axonally to neurohypophysis.
- Over-secretion of GH stimulates abnormal growth of the body leading to gigantism and low secretion of GH results in stunted growth resulting in pituitary dwarfism.
- Prolactin regulates the growth of the mammary glands and formation of milk in them.
- TSH stimulates the synthesis and secretion of thyroid hormones from the thyroid gland.
- ACTH stimulates the synthesis and secretion of steroid hormones called glucocorticoids from the adrenal cortex.
- LH and FSH stimulate gonadal activity and hence are called gonadotrophins.
- In males, LH stimulates the synthesis and secretion of hormones called androgens from testis. In males, FSH and androgens regulate spermatogenesis.
- In females, LH induces ovulation of fully mature follicles (graafian follicles) and maintains the corpus luteum, formed from the remnants of the graafian follicles after ovulation. FSH stimulates growth and development of the ovarian follicles in females.
- MSH acts on the melanocytes (melanin containing cells) and regulates pigmentation of the skin.
- Oxytocin acts on the smooth muscles of our body and stimulates their contraction. In females, it stimulates a vigorous contraction of uterus at the time of childbirth, and milk ejection from the mammary gland.
- Vasopressin acts mainly at the kidney and stimulates resorption of water and electrolytes by the distal tubules and thereby reduces loss of water through urine (diuresis). Hence, it is also called as Anti-Diuretic Hormone (ADH).
- The pineal gland is located on the dorsal side of forebrain.
- Pineal secretes a hormone called melatonin.
- Melatonin plays a very important role in the regulation of a 24-hour (diurnal) rhythm of our body.
- For example, it helps in maintaining the normal rhythms of sleep-wake cycle, body temperature.
- In addition, melatonin also influences metabolism, pigmentation, the menstrual cycle as well as our defense capability.
- The thyroid gland is composed of two lobes which are located on either side of the trachea.
- Both the lobes are interconnected with a thin flap of connective tissue called isthmus.
- The thyroid gland is composed of follicles and stromal tissues. Each thyroid follicle is composed of follicular cells, enclosing a cavity. These follicular cells synthesize two hormones, tetraiodothyronine or thyroxine (T4) and triiodothyronine (T3).
- Iodine is essential for the normal rate of hormone synthesis in the thyroid. Deficiency of iodine in our diet results in hypothyroidism and enlargement of the thyroid gland, commonly called goitre.
- Hypothyroidism during pregnancy causes defective development and maturation of the growing baby leading to stunted growth (cretinism), mental retardation, low intelligence quotient, abnormal skin, deaf-mutism, etc.
- In adult women, hypothyroidism may cause menstrual cycle to become irregular.
- Due to cancer of the thyroid gland or due to development of nodules of the thyroid glands, the rate of synthesis and secretion of the thyroid hormones is increased to abnormal high levels leading to a condition called hyperthyroidism which adversely affects the body physiology.
- Thyroid hormones play an important role in the regulation of the basal metabolic rate.
- These hormones also support the process of red blood cell formation.
- Thyroid hormones control the metabolism of carbohydrates, proteins and fats.
- Maintenance of water and electrolyte balance is also influenced by thyroid hormones.
- Thyroid gland also secretes a protein hormone called Thyrocalcitonin (TCT) which regulates the blood calcium levels.
- In humans, four parathyroid glands are present on the back side of the thyroid gland, one pair each in the two lobes of the thyroid gland.
- The parathyroid glands secrete a peptide hormone called Parathyroid Hormone (PTH). The secretion of PTH is regulated by the circulating levels of calcium ions. Parathyroid hormone (PTH) increases the Ca2+ levels in the blood.
- PTH acts on bones and stimulates the process of bone resorption (dissolution/ demineralisation).
- PTH also stimulates reabsorption of Ca2+ by the renal tubules and increases Ca2+ absorption from the digested food.
- It is, thus, clear that PTH is a Hypercalcemic Hormone, i.e., it increases the blood Ca2+ levels.
- Along with TCT, it plays a significant role in calcium balance in the body.
- The thymus gland is a lobular structure located between lungs behind sternum on the ventral side of aorta.
- The thymus plays a major role in the development of the immune system.
- This gland secretes the peptide hormones called Thymosins.
- Thymosins play a major role in the differentiation of T-lymphocytes, which provide cell-mediated immunity.
- In addition, thymosins also promote production of antibodies to provide humoral immunity.
- Thymus is degenerated in old individuals resulting in a decreased production of thymosins. As a result, the immune responses of old persons become weak.
- Our body has one pair of adrenal glands, one at the anterior part of each kidney. The gland is composed of two types of tissues. The centrally located tissue is called the adrenal medulla, and outside this lies the adrenal cortex.
- The adrenal medulla secretes two hormones called adrenaline or epinephrine and noradrenaline or norepinephrine. These are commonly called as catecholamines.
- Adrenaline and noradrenaline are rapidly secreted in response to stress of any kind and during emergency situations and are called emergency hormones or hormones of Fight or Flight.
- These hormones increase alertness, pupilary dilation, piloerection (raising of hairs), sweating etc.
- Both the hormones increase the heartbeat, the strength of heart contraction and the rate of respiration.
- Catecholamines also stimulate the breakdown of glycogen resulting in an increased concentration of glucose in blood.
- In addition, they also stimulate the breakdown of lipids and proteins.
- The adrenal cortex secretes many hormones, commonly called as corticoids. The corticoids, which are involved in carbohydrate metabolism are called glucocorticoids. In our body, cortisol is the main glucocorticoid.
- Corticoids, which regulate the balance of water and electrolytes in our body are called mineralocorticoids. Aldosterone is the main mineralocorticoid in our body.
- Glucocorticoids stimulate gluconeogenesis, lipolysis and proteolysis; and inhibit cellular uptake and utilisation of amino acids.
- Cortisol is also involved in maintaining the cardio-vascular system as well as the kidney functions.
- Glucocorticoids, particularly cortisol, produces anti-inflammatory reactions and suppresses the immune response.
- Cortisol stimulates the RBC production.
- Aldosterone acts mainly at the renal tubules and stimulates the reabsorption of Na+ and water and excretion of K+ and phosphate ions. Thus, aldosterone helps in the maintenance of electrolytes, body fluid volume, osmotic pressure and blood pressure.
- Small amounts of androgenic steroids are also secreted by the adrenal cortex which play a role in the growth of axial hair, pubic hair and facial hair during puberty.