WBCS Preliminary(Biology): Endocrine System

• Glands are of two types:

  1. Exocrine glands: Exocrine glands are those which pour their secretions into a duct. For example, sweat glands, tear glands, etc.
  2. Endocrine glands: Endocrine glands are those which are richly supplied with blood vessels and pour their secretions directly into the blood vessels. The secretions reach their target through blood. These glands are called the ductless glands as they do not have ducts. For example, thyroid, adrenal, etc.
• Hormones: The secretions of the endocrine glands are called hormones. Hormones are grouped into three classes based on their structure:
  1. Steroids: Steroids are lipids derived from cholesterol. e.g. Testosterone is the male sex hormone. Steroid hormones are secreted by the gonads, adrenal cortex, and placenta.
  2. Peptides: Peptides are short chains of amino acids; most hormones are peptides. They are secreted by the pituitary, parathyroid, heart, stomach, liver, and kidneys.
  3. Amines: Amines are derived from the amino acid tyrosine and are secreted from the thyroid and the adrenal medulla.
• The human endocrine system consists of the following glands:
  • Hypothalamus
  • Pineal
  • Thyroid
  • Parathyroid
  • Pituitary:
  • Thymus
  • Adrenal
  • Pancreas
  • Ovary in female
  • Testes in male
• Hypothalamus: Forms a part of fore - brain, secretes neurohormones, which effect the release of hormones from pituitary.
• Pituitary gland: Also called Master Gland. It is the smallest endocrine gland. It is a pea shaped gland that is located below the hypothalamus in the brain. It is under the control of the hypothalamus and in turn controls many functions in the body. Hormones Produced by Pituitary:
  • Thyroid stimulating hormone (TSH): Stimulates thyroid gland to produce thyroxine.
  • Growth Hormone (GH): Stimulates overall growth of the body. Its deficiency causes dwarfism and over-production causes gigantism.
  • Adrenocorticotrophic hormone (ACTH): Influences the secretion from the cortex of adrenal glands.
  • Follicle stimulating hormone (FSH): It is secreted in males and females both. In males, it stimulates spermatogenesis and development of seminiferous tubules. In females, it stimulates formation and growth of ovarian follicle in ovary.
  • Luteinizing hormone (LH): In females only. Final maturation of ovarian follicleand ovulation takes place by LH only.
  • Lactogenic Hormone : Initiates milk production in the pregnant females.
  • Prolactin maintains the pregnancy and stimulates the secretion of milk.
  • Oxytocin helps in contraction of uterus during delivery.
• Pineal : It is a small round gland in the brain. It secretes melatonin that regulates the sexual cycle.
• Thyroid Gland: It is the largest endocrine gland located in the neck between the trachea and larynx. It is a butterfly-shaped, bilobed gland that is situated at the base of the larynx. The two lobes are joined by an isthmus. The hormone secreted is thyroxine. Controls BMR (Basal Metabolic Rate).
  • Thyroxine : Regulates physical, mental and sexual development. A diet, poor in iodine, which is insufficient for the synthesis of thyroxin, leads to simple goitre. Oversecretion results in exophthalmic goitre in which the person shows a marked increase in metabolic rate, protrusion of eyes, rapid heart rate and shortness of breath.
  • Thyrocalcitonin : It controls the amount of calcium in the body.
• Parathyroid Gland: Secretes Parathormone, which is also known by the name of Collip's Hormone. Deficiency of parathormone causes brittle bones. Oversecretion of parathormone softens the teeth and bones.
• Thymus Gland: Situated near the heart. It secretes hormone called thymosin. Thymosin helps in the production of lymphocytes.
• Pancreas: It is a narrow gland present at the junction of stomach and duodenum. It is an exocrine as well as an endocrine gland. Its 3 types of cells secrete 3 different hormones. Beta cells secrete Insulin which controls the amount of sugar in the blood. Its hyposecretion leads to Diabetes Mellitus. Alpha cells secretes Glucagon which increases blood sugar level. The delta cells secrete somatostatin that inhibits the secretion of insulin and glucagon.
• Adrenal: The adrenal glands are present on top of the kidneys and appear cap-like on top of each kidney. Consists of 2 Distinct Parts : Outer cortex and inner medulla.
  • Adrenal Medulla secretes Adrenaline or epinephrine which effects liberation of glucose from glycogen stored in liver and increases the rate of metabolism. Its over - secretion leads to increased saliva flow, tears, bile and sweat, quickens heart beat, speeds up respiratory activities.
  • The adrenal cortex secretes hormones like the glucosteroids, mineralocorticoids and cortisones. Glucosteroids increase the blood sugar level in times of stress by converting protein into glucose.
• Testes produce testosterone which produces the secondary sexual characteristics like moustache and beard.
• Ovaries along with the egg-production, secrete oestrogen from the mature follicle that produces the secondary sexual characteristics like enlargement of breasts. After ovulation, another hormone, progesterone is produced from corpus luteum that maintains the pregnancy.
• Plant hormones or phytohormones: The phytohormones have been put in five different categories based on their actions.
  • Auxins: Auxins are phytohormones that are mainly concerned with cell enlargement. It is produced by shoot apex, young leaves and roots (to some extent). They only move in upward direction through phloem or xylem.
  • Gibberellins: Gibberellins are plant hormones that are mainly responsible for cell elongation. They cause the cells to grow in length. They are synthesised in embryos, young leaves, root tips, buds and seeds. They move up or down in the plant body through xylem or phloem.
  • Cytokinins: They are phytohormones that induce cell divisions even in mature tissues. There are many types of cytokinins present. For example, zeatin, a cytokinin present in maize grains. Cytokinins are synthesized in the fruits and seeds where rapid cell division takes place.
  • Abscissic Acid (ABA): It is a growth inhibitor that results in dormancy and abscission. It is synthesized in stem, leaves, fruits and seeds.
  • Ethylene:Ethylene is a gaseous growth regulator that speeds up the ripening process. It is a gas produced by most of the plant organs. Chemically, ethylene (ethene) is an unsaturated hydrocarbon