Chemical Coordination and Integration

1. Chapter at a Glance

The neural system provides a point-to-point rapid coordination among organs. This neural coordination is fast but short-lived. As the nerve fibres do not innervate all cells of the body, and the cellular functions need to be continuously regulated, a special kind of coordination and integration has to be provided. This function is carried out by hormones. The neural system and the endocrine system jointly coordinate and regulate the physiological functions in the body. This chapter covers endocrine glands and hormones, the human endocrine system (hypothalamus, pituitary, pineal, thyroid, parathyroid, thymus, adrenal, pancreas, testis and ovary), hormones of the heart, kidney and gastrointestinal tract, and the mechanism of hormone action. NEET asks about two questions every year, usually on the source and function of particular hormones and the disorders caused by their deficiency or excess.

2. Endocrine Glands and Hormones

• Endocrine glands lack ducts and are hence called ductless glands. Their secretions are called hormones.
• The classical definition of a hormone as a chemical produced by endocrine glands and released into the blood and transported to a distantly located target organ has current scope only for a limited number of hormones.
• A hormone is now defined as a non-nutrient chemical which acts as an intercellular messenger and is produced in trace amounts.
• The new definition covers a number of new molecules in addition to the hormones secreted by the organised endocrine glands. Invertebrates possess very simple endocrine systems with few hormones, whereas a large number of chemicals act as hormones and provide coordination in vertebrates.

3. Human Endocrine System

The endocrine glands and hormone-producing diffused tissues or cells located in different parts of our body constitute the endocrine system. Pituitary, pineal, thyroid, adrenal, pancreas, parathyroid, thymus and gonads (testis in males and ovary in females) are the organised endocrine bodies in our body. In addition, some other organs, e.g. the gastrointestinal tract, liver, kidney and heart, also produce hormones.

The hypothalamus

• The hypothalamus is the basal part of the diencephalon, forebrain, and it regulates a wide spectrum of body functions.
• It contains several groups of neurosecretory cells called nuclei, which produce hormones. These hormones regulate the synthesis and secretion of pituitary hormones.
• The hormones produced by the hypothalamus are of two types: the releasing hormones (which stimulate secretion of pituitary hormones) and the inhibiting hormones (which inhibit secretions of pituitary hormones). For example, gonadotrophin-releasing hormone (GnRH) stimulates the pituitary synthesis and release of gonadotrophins. On the other hand, somatostatin from the hypothalamus inhibits the release of growth hormone from the pituitary.
• These hormones originating in the hypothalamic neurons pass through axons and are released from their nerve endings. They reach the pituitary gland through a portal circulatory system and regulate the functions of the anterior pituitary. The posterior pituitary is under the direct neural regulation of the hypothalamus.

The pituitary gland

• The pituitary gland is located in a bony cavity called the sella turcica and is attached to the hypothalamus by a stalk.
• It is divided anatomically into an adenohypophysis and a neurohypophysis.
• The adenohypophysis consists of two portions, the pars distalis and the pars intermedia. The pars distalis region of the pituitary, commonly called the anterior pituitary, produces growth hormone (GH), prolactin (PRL), thyroid-stimulating hormone (TSH), adrenocorticotrophic hormone (ACTH), luteinizing hormone (LH) and follicle-stimulating hormone (FSH).
• The pars intermedia secretes only one hormone called melanocyte-stimulating hormone (MSH). However, in humans, the pars intermedia is almost merged with the pars distalis.
• The neurohypophysis (pars nervosa), also known as the posterior pituitary, stores and releases two hormones called oxytocin and vasopressin, which are actually synthesised by the hypothalamus and transported axonally to the neurohypophysis.

The pineal gland

The pineal gland is located on the dorsal side of the forebrain. It 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 the sleep–wake cycle and body temperature. In addition, melatonin also influences metabolism, pigmentation, the menstrual cycle and our defence capability.

The thyroid gland

• The thyroid gland is composed of two lobes, located on either side of the trachea. Both lobes are interconnected with a thin flap of connective tissue called the 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 synthesise 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 and so on. In adult women, hypothyroidism may cause the 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 thyroid hormones is increased to abnormally high levels, leading to a condition called hyperthyroidism, which adversely affects body physiology.
• Exophthalmic goitre (Graves' disease) is a form of hyperthyroidism characterised by enlargement of the thyroid gland, protrusion of the eyeballs, increased basal metabolic rate and weight loss.
• 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.
• The thyroid gland also secretes a protein hormone called thyrocalcitonin (TCT), which regulates the blood calcium levels.

The parathyroid gland

• 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.
• PTH increases the Ca²⁺ levels in the blood. It 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

• The thymus gland is a lobular structure located on the dorsal side of the heart and the aorta.
• The thymus plays a major role in the development of the immune system. This gland secretes 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.
• The thymus is degenerated in old individuals, resulting in a decreased production of thymosins. As a result, the immune responses of old persons become weak.

The adrenal gland

• 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 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 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, pupillary dilation, piloerection (raising of hairs), sweating and so on. Both the hormones increase the heart beat, 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 can be divided into three layers, called the inner zona reticularis, the middle zona fasciculata and the outer zona glomerulosa.
• The adrenal cortex secretes many hormones, commonly called corticoids. The corticoids 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 cardiovascular system as well as the kidney functions. Glucocorticoids, particularly cortisol, produce anti-inflammatory reactions and suppress the immune response. Cortisol stimulates 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.
• Underproduction of hormones by the adrenal cortex alters carbohydrate metabolism, causing acute weakness and fatigue, leading to a disease called Addison's disease.

The pancreas

• The pancreas is a composite gland which acts as both exocrine and endocrine gland.
• The endocrine pancreas consists of Islets of Langerhans. There are about 1 to 2 million Islets of Langerhans in a normal human pancreas, representing only 1 to 2 per cent of the pancreatic tissue.
• The two main types of cells in the Islet of Langerhans are called α-cells and β-cells. The α-cells secrete a hormone called glucagon, while the β-cells secrete insulin.
• Glucagon is a peptide hormone and plays an important role in maintaining normal blood glucose levels. Glucagon acts mainly on the liver cells (hepatocytes) and stimulates glycogenolysis, resulting in an increased blood sugar (hyperglycemia). In addition, this hormone stimulates the process of gluconeogenesis, which also contributes to hyperglycemia. Glucagon reduces cellular glucose uptake and utilisation. Thus, glucagon is a hyperglycemic hormone.
• Insulin is a peptide hormone which plays a major role in the regulation of glucose homeostasis. Insulin acts mainly on hepatocytes and adipocytes (cells of adipose tissue) and enhances cellular glucose uptake and utilisation. As a result, there is a rapid movement of glucose from blood to hepatocytes and adipocytes, resulting in decreased blood glucose levels (hypoglycemia). Insulin also stimulates conversion of glucose to glycogen (glycogenesis) in the target cells.
• The glucose homeostasis in blood is thus maintained jointly by the two hormones — insulin and glucagon.
• Prolonged hyperglycemia leads to a complex disorder called diabetes mellitus, which is associated with loss of glucose through urine and formation of harmful compounds known as ketone bodies. Diabetic patients are successfully treated with insulin therapy.

Testis and ovary

• A pair of testes is present in the scrotal sac (outside the abdomen) of males. The testis performs dual functions as a primary sex organ as well as an endocrine gland.
• The testis is composed of seminiferous tubules and stromal or interstitial tissue. The Leydig cells or interstitial cells, which are present in the intertubular spaces, produce a group of hormones called androgens, mainly testosterone.
• Androgens regulate the development, maturation and functions of the male accessory sex organs like the epididymis, vas deferens, seminal vesicles, prostate gland, urethra and so on. These hormones stimulate muscular growth, growth of facial and axillary hair, aggressiveness and low pitch of voice. Androgens play a major stimulatory role in the process of spermatogenesis (formation of spermatozoa). Androgens act on the central neural system and influence male sexual behaviour (libido). These hormones produce anabolic (synthetic) effects on protein and carbohydrate metabolism.
• Females have a pair of ovaries located in the abdomen. The ovary is the primary female sex organ which produces one ovum during each menstrual cycle. In addition, the ovary also produces two groups of steroid hormones called estrogen and progesterone.
• The ovary is composed of ovarian follicles and stromal tissues. Estrogen is synthesised and secreted mainly by the growing ovarian follicles. After ovulation, the ruptured follicle is converted into a structure called the corpus luteum, which secretes mainly progesterone.
• Estrogens produce wide-ranging actions such as stimulation of growth and activities of female secondary sex organs, development of growing ovarian follicles, appearance of female secondary sex characters (e.g. high pitch of voice), mammary gland development. Estrogens also regulate female sexual behaviour.
• Progesterone supports pregnancy. Progesterone also acts on the mammary glands and stimulates the formation of alveoli (sac-like structures which store milk) and milk secretion.

4. Hormones of Heart, Kidney and Gastrointestinal Tract

• The atrial wall of our heart secretes a very important peptide hormone called atrial natriuretic factor (ANF), which decreases blood pressure. When blood pressure is increased, ANF is secreted, which causes dilation of the blood vessels. This reduces the blood pressure.
• The juxtaglomerular cells of the kidney produce a peptide hormone called erythropoietin, which stimulates erythropoiesis (formation of RBCs).
• Endocrine cells present in different parts of the gastrointestinal tract secrete four major peptide hormones, namely gastrin, secretin, cholecystokinin (CCK) and gastric inhibitory peptide (GIP).
• Gastrin acts on the gastric glands and stimulates the secretion of hydrochloric acid and pepsinogen. Secretin acts on the exocrine pancreas and stimulates secretion of water and bicarbonate ions. CCK acts on both pancreas and gall bladder and stimulates the secretion of pancreatic enzymes and bile juice, respectively. GIP inhibits gastric secretion and motility.
• Several other non-endocrine tissues secrete hormones called growth factors. These factors are essential for the normal growth of tissues and their repairing/regeneration.

5. Mechanism of Hormone Action

• Hormones produce their effects on target tissues by binding to specific proteins called hormone receptors located in the target tissues only.
• Hormone receptors present on the cell membrane of the target cells are called membrane-bound receptors, and the receptors present inside the target cell are called intracellular receptors, mostly nuclear receptors (present in the nucleus).
• Binding of a hormone to its receptor leads to the formation of a hormone–receptor complex. Each receptor is specific to one hormone only, and hence receptors are specific.
• Hormone–receptor complex formation leads to certain biochemical changes in the target tissue. Target tissue metabolism and hence physiological functions are regulated by hormones.

Most endocrine glands are regulated by feedback. For example, the hypothalamus releases a releasing hormone that stimulates the anterior pituitary to release a tropic hormone; this stimulates a target gland (e.g. thyroid), whose hormone, when it rises in blood, inhibits the hypothalamus and pituitary (negative feedback). This keeps hormone levels within a narrow range.

6. Master Summary Table

7. Mind Map

8. Tips and Tricks

9. Important One-liners

• Endocrine glands are ductless glands.
• The pituitary lies in the sella turcica.
• Somatostatin from the hypothalamus inhibits the release of growth hormone.
• In humans, the pars intermedia is almost merged with the pars distalis.
• Melatonin regulates the 24-hour (diurnal) rhythm.
• Iodine deficiency causes goitre.
• Graves' disease is a form of hyperthyroidism with protruding eyeballs.
• The thymus degenerates in old age, weakening immunity.
• Adrenaline and noradrenaline are called emergency hormones.
• Cortisol is the main glucocorticoid and aldosterone the main mineralocorticoid.
• Islets of Langerhans make up only 1–2% of pancreatic tissue.
• Leydig cells secrete testosterone; the corpus luteum secretes progesterone.

10. NEET-pattern Questions with Detailed Answers

These questions are original and based on concepts that NEET UG asks again and again.

11. Quick Revision Summary