Chemical Coordination and Integration
Endocrine Glands And Hormones
The nervous system provides rapid point-to-point coordination. However, not all functions can be regulated by nerve impulses alone. The endocrine system provides chemical coordination by producing signalling molecules called hormones.
Endocrine Glands:
- These are ductless glands that secrete hormones directly into the bloodstream.
- The blood transports the hormones to target tissues or organs, which have specific receptors for that hormone.
- Examples: Pituitary gland, Thyroid gland, Adrenal gland, Pancreas (endocrine part), Gonads (Testis, Ovary), Pineal gland, Parathyroid gland, Thymus.
Exocrine Glands:
- These glands have ducts and secrete their products (e.g., enzymes, mucus, sweat, saliva) into ducts, which then carry the secretions to a specific location (surface or cavity).
- Examples: Salivary glands, Sweat glands, Gastric glands, Pancreas (exocrine part).
Hormones:
- Hormones are non-nutrient chemicals which act as intercellular messengers.
- They are produced in trace amounts.
- They are transported by the blood to their target organs.
- Target organs have specific receptors that bind to the hormone, triggering a specific physiological response.
Integrated Systems:
The neural system and the endocrine system together coordinate and integrate physiological functions in the body. This is often referred to as the neuroendocrine system. The hypothalamus, part of the brain, plays a key role in linking the nervous system to the endocrine system by controlling the pituitary gland.
Chemical Nature of Hormones:
Hormones are diverse in their chemical composition:
- Peptide, polypeptide, protein hormones: Composed of amino acids. Examples: Insulin, Glucagon, Pituitary hormones, Hypothalamic hormones.
- Steroid hormones: Lipid-soluble, derived from cholesterol. Examples: Cortisol, Aldosterone, Oestrogen, Progesterone, Testosterone.
- Iodothyronines: Amino acid derivatives containing iodine. Example: Thyroid hormones ($T_3, T_4$).
- Amino acid derivatives: Modified amino acids. Example: Adrenaline (Epinephrine), Noradrenaline (Norepinephrine).
The chemical nature of a hormone influences its mechanism of action on the target cell.
Human Endocrine System
The human endocrine system consists of various endocrine glands located in different parts of the body. These glands secrete hormones that regulate a wide range of physiological processes.
*(Image shows a diagram of the human body with the major endocrine glands (hypothalamus, pituitary, pineal, thyroid, parathyroid, thymus, adrenal, pancreas, testis/ovary) labelled)*
The Hypothalamus
- Location: Part of the forebrain, located below the thalamus.
- Plays a crucial role in regulating the release of hormones from the pituitary gland.
- Produces two types of hormones:
- Releasing hormones: Stimulate the secretion of pituitary hormones (e.g., GnRH - Gonadotropin Releasing Hormone, stimulates pituitary to release FSH and LH).
- Inhibiting hormones: Inhibit the secretion of pituitary hormones (e.g., Somatostatin, inhibits growth hormone release).
- These hypothalamic hormones are transported through a portal system to the anterior pituitary.
- The hypothalamus also produces two hormones, Oxytocin and Vasopressin (ADH), which are transported to and released by the posterior pituitary.
The Pituitary Gland
- Location: Located in a bony cavity called sella turcica, attached to the hypothalamus by a stalk.
- It is often called the 'master endocrine gland' because it regulates the activity of many other endocrine glands. However, it is itself controlled by the hypothalamus.
- The pituitary gland is divided into two parts:
- Adenohypophysis (Anterior Pituitary):
- Controlled by hypothalamic releasing and inhibiting hormones.
- Secretes several hormones:
- Growth Hormone (GH): Stimulates growth and development.
- Prolactin (PRL): Regulates mammary gland development and milk production.
- Thyroid Stimulating Hormone (TSH): Stimulates the thyroid gland.
- Adrenocorticotropic Hormone (ACTH): Stimulates the adrenal cortex.
- Luteinizing Hormone (LH): (in males) Stimulates synthesis and secretion of androgens from testis; (in females) induces ovulation and corpus luteum formation.
- Follicle Stimulating Hormone (FSH): (in males) Stimulates spermatogenesis; (in females) stimulates growth and development of ovarian follicles.
- Melanocyte Stimulating Hormone (MSH): Acts on melanocytes (pigment cells).
- Neurohypophysis (Posterior Pituitary):
- Stores and releases two hormones produced by the hypothalamus:
- Oxytocin: Stimulates uterine contractions during childbirth and milk ejection from mammary glands.
- Vasopressin (ADH - Antidiuretic Hormone): Acts on kidneys, stimulates reabsorption of water (reduces urine output), regulates blood pressure (vasoconstriction).
- Stores and releases two hormones produced by the hypothalamus:
- Adenohypophysis (Anterior Pituitary):
The Pineal Gland
- Location: Located on the dorsal side of the forebrain.
- Secretes the hormone Melatonin.
- Melatonin regulates the 24-hour rhythm of the body (circadian rhythms), such as sleep-wake cycle, body temperature, metabolism, pigmentation, and menstrual cycle.
Thyroid Gland
- Location: Located on either side of the trachea in the neck, composed of two lobes connected by an isthmus.
- Composed of follicles and stromal tissues. Follicular cells secrete thyroid hormones.
- Secretes two main hormones: Thyroxine ($T_4$) and Triiodothyronine ($T_3$). These hormones require iodine for their synthesis.
- Secretes a protein hormone called Thyrocalcitonin (TCT), which regulates blood calcium levels.
- Functions of thyroid hormones ($T_3, T_4$): Regulate basal metabolic rate (BMR), control metabolism of carbohydrates, proteins, and fats, support RBC formation, influence growth and development (especially brain development).
- Disorders:
- Hypothyroidism (underactive thyroid): Causes simple goitre (enlargement of thyroid due to iodine deficiency), cretinism (stunted physical and mental development in infants due to deficiency during pregnancy), myxoedema (in adults, symptoms include fatigue, weight gain, swelling).
- Hyperthyroidism (overactive thyroid): Causes Graves' disease (exophthalmic goitre), symptoms include increased BMR, weight loss, bulging eyes.
Parathyroid Gland
- Location: Four small glands located on the back side of the thyroid gland (two on each lobe).
- Secretes the hormone Parathyroid Hormone (PTH).
- Functions of PTH: Increases blood calcium levels. It stimulates bone resorption (breakdown of bone to release calcium), increases calcium reabsorption by kidney tubules, and increases calcium absorption from digested food.
- PTH and TCT (Thyrocalcitonin) work antagonistically to regulate blood calcium levels.
Thymus
- Location: Located between the lungs, behind the sternum on the ventral side of the aorta.
- Secretes the hormone Thymosins.
- Functions of Thymosins: Play a role in the development of the immune system. Stimulate differentiation of T-lymphocytes.
- The thymus is large at birth and gradually reduces in size with age.
Adrenal Gland
- Location: A pair of glands located on the anterior part of each kidney.
- Composed of two parts:
- Adrenal Cortex: The outer part. Secretes corticosteroids (steroid hormones):
- Glucocorticoids: (e.g., Cortisol) Regulate carbohydrate metabolism (gluconeogenesis), suppress immune response, anti-inflammatory.
- Mineralocorticoids: (e.g., Aldosterone) Regulate water and electrolyte balance, act on kidney tubules to increase $Na^+$ and water reabsorption.
- Small amounts of adrenal androgens.
- Adrenal Medulla: The inner part. Secretes catecholamines (amino acid derivatives):
- Adrenaline (Epinephrine): 'Fight or flight' hormone. Increases heart rate, breathing rate, blood pressure, glucose level.
- Noradrenaline (Norepinephrine): Similar effects to adrenaline.
- Adrenal Cortex: The outer part. Secretes corticosteroids (steroid hormones):
- The adrenal medulla hormones are secreted rapidly in response to stress, preparing the body for action.
Pancreas
- Location: Located behind the stomach.
- It is a mixed gland, functioning as both exocrine (secreting digestive enzymes) and endocrine.
- The endocrine part is composed of islets of Langerhans.
- Islets of Langerhans contain different types of cells:
- Alpha ($\alpha$) cells: Secrete Glucagon. Glucagon increases blood glucose levels (hyperglycemic hormone) by stimulating glycogenolysis (breakdown of glycogen) and gluconeogenesis (synthesis of glucose).
- Beta ($\beta$) cells: Secrete Insulin. Insulin decreases blood glucose levels (hypoglycemic hormone) by stimulating glucose uptake by cells, promoting glycogenesis (synthesis of glycogen), and inhibiting gluconeogenesis.
- Insulin and Glucagon work antagonistically to regulate blood glucose homeostasis.
- Diabetes Mellitus: A condition caused by deficiency or impaired action of insulin, leading to high blood glucose levels.
Testis
- Location: Located in the scrotum in males.
- Primary male sex organ, producing sperms.
- Functions as an endocrine gland, secreting Androgens, primarily Testosterone.
- Functions of Androgens: Regulate the development, maturation, and functions of the male accessory sex organs, stimulate spermatogenesis, influence the development of male secondary sexual characters (e.g., beard, moustache, deep voice), muscle growth, aggressive behaviour.
Ovary
- Location: Located in the abdomen in females.
- Primary female sex organ, producing ova.
- Functions as an endocrine gland, producing two groups of steroid hormones: Oestrogen and Progesterone.
- Functions of Oestrogen: Stimulates the development of female secondary sexual characters, development of growing ovarian follicles, influence female sexual behaviour.
- Functions of Progesterone: Supports pregnancy, stimulates mammary gland development.
The gonads (testis and ovary) also secrete other hormones in smaller amounts. The hormones secreted by these various endocrine glands are crucial for regulating metabolism, growth, development, reproduction, and homeostasis in the human body.
Hormones Of Heart, Kidney And Gastrointestinal Tract
Besides the major endocrine glands, some other organs in the body that are primarily associated with different systems (like circulatory, excretory, digestive) also produce hormones.
1. Heart:
- The atrial wall of the heart secretes a peptide hormone called Atrial Natriuretic Factor (ANF).
- Release is stimulated by increased blood pressure (due to increased blood volume).
- Function: Decreases blood pressure by causing vasodilation (widening of blood vessels) and inhibiting the release of renin and aldosterone (which reduces $Na^+$ and water reabsorption by kidneys). ANF is antagonistic to the RAAS system.
2. Kidney:
- The juxtaglomerular apparatus (JGA) in the kidney secretes a protein hormone called Renin.
- Release is stimulated by a decrease in GFR/blood pressure/blood volume.
- Function: Renin initiates the Renin-Angiotensin-Aldosterone System (RAAS), which ultimately leads to an increase in blood pressure and blood volume.
- Kidney cells also produce Erythropoietin, a peptide hormone.
- Function of Erythropoietin: Stimulates erythropoiesis (production of red blood cells) in the bone marrow.
3. Gastrointestinal Tract:
Endocrine cells in the wall of the gastrointestinal (GI) tract secrete several peptide hormones that regulate the activities of the digestive system.
- Gastrin: Secreted by the gastric mucosa. Stimulates the secretion of HCl and pepsinogen by the stomach.
- Secretin: Secreted by the duodenal mucosa. Stimulates the secretion of water and bicarbonate ions from the pancreas and bile from the liver. Acts exocrine pancreas.
- Cholecystokinin (CCK): Secreted by the duodenal mucosa. Stimulates the secretion of pancreatic enzymes from the pancreas and bile from the gall bladder. Acts on both pancreas and gall bladder.
- Gastric Inhibitory Peptide (GIP): Secreted by the duodenal mucosa. Inhibits gastric secretion and motility.
These GI hormones work together to coordinate the digestive process.
These examples show that endocrine function is not limited to the classic endocrine glands but is also carried out by cells in other organs, highlighting the widespread nature of chemical coordination in the body.
Mechanism Of Hormone Action
Hormones exert their effects by binding to specific receptor proteins on or in their target cells. The mechanism of action depends largely on the chemical nature of the hormone.
Hormone receptors are proteins that are specific to each hormone. Receptors are located either on the cell membrane (for hormones that cannot cross the membrane) or inside the cell (for hormones that can cross the membrane).
Mechanism of Action of Peptide, Polypeptide, Protein, and Amino Acid Derivative Hormones (e.g., Insulin, Glucagon, Pituitary hormones, Adrenaline):
- These hormones are water-soluble and generally cannot cross the cell membrane (which is lipid-based).
- Their receptors are located on the cell surface (membrane-bound receptors).
- Binding of the hormone to its receptor on the cell surface leads to the generation of a second messenger inside the target cell.
- Common second messengers include cyclic AMP (cAMP), $IP_3$ (Inositol triphosphate), $Ca^{2+}$ ions.
- The second messenger then triggers a cascade of biochemical reactions within the cell, leading to the specific physiological response. For example, cAMP activates various protein kinases that modify cellular proteins, altering cell activity.
- This mechanism often involves rapid, but sometimes short-lived, changes in cellular function.
*(Image shows a target cell membrane with a hormone binding to a receptor, triggering the activation of an enzyme (like adenylyl cyclase) that produces a second messenger (like cAMP) inside the cell, leading to a biochemical response)*
Mechanism of Action of Steroid Hormones and Iodothyronines (e.g., Cortisol, Oestrogen, Thyroid hormones):
- These hormones are lipid-soluble and can easily cross the cell membrane.
- Their receptors are located inside the target cell (intracellular receptors), either in the cytoplasm or the nucleus.
- Binding of the hormone to its intracellular receptor forms a hormone-receptor complex.
- The hormone-receptor complex then enters the nucleus and binds to specific regions on the DNA (hormone response elements).
- This binding regulates gene expression, either activating or inhibiting the transcription of specific genes.
- Changes in gene transcription lead to changes in the synthesis of specific proteins (enzymes or structural proteins).
- These newly synthesised proteins bring about the physiological effects.
- This mechanism typically results in slower, but more prolonged, changes in cellular function compared to mechanisms involving second messengers.
*(Image shows a target cell with a steroid/thyroid hormone passing through the membrane, binding to an intracellular receptor, forming a complex that enters the nucleus and binds to DNA, affecting gene transcription and protein synthesis)*
Hormone-Receptor Complex:
The binding of a hormone to its specific receptor forms a hormone-receptor complex. This complex is crucial for initiating the hormone's action.
The formation of this complex leads to specific biochemical and physiological changes in the target tissue. The number of receptors on a target cell can be regulated (upregulation or downregulation) in response to hormone concentration or other signals, influencing the cell's sensitivity to the hormone.
Understanding the mechanism of hormone action helps explain how these chemical messengers coordinate diverse processes in the body and how disruptions in these pathways can lead to various endocrine disorders.