Grade 12 Life Sciences: The Human Endocrine System and Homeostasis

Overview of Endocrine and Exocrine Glands

• The endocrine system is composed of several endocrine glands that produce and secrete hormones.

• Endocrine Glands: These glands are ductless. Their secretions (hormones) are released directly into the bloodstream to be transported to specific sites of action.

  • Example: The thyroid gland.

• Exocrine Glands: These glands possess ducts and do not secrete their substances into the blood. Instead, their secretions travel through ducts to reach the area where they are required. Their primary secretions are typically enzymes.

  • Examples: Salivary glands, the liver, and the pancreas (pancreatic juice secretion).

Characteristics and Properties of Hormones

• Definition: A hormone is an organic chemical substance produced by a ductless endocrine gland. It is transported via the bloodstream to one or more target organs to regulate specific metabolic reactions.

• Chemical Classification:

  • Most hormones, such as thyroxin and insulin, are proteins.

  • Some hormones, such as testosterone, are steroids. Steroids are synthesized from cholesterol.

• Potency and Concentration: While the concentration of hormones in the blood remains low, they exert powerful effects on the body.

• Lifespan and Breakdown: Many hormones have a very short lifespan. This allows the body to stop their effects rapidly. They are broken down by enzymes in the blood or cells, or they are excreted in urine.

• Specificity: Each hormone affects a specific group of cells known as target cells. These cells possess receptors that are specific to that particular hormone.

• Functional Role: Hormones generally stimulate their target cells. A single hormone may exert several specific effects on target organs, thereby controlling a variety of biological activities.

The Hypothalamus and Osmoregulation

• The hypothalamus is a part of the brain located above the pituitary gland.

• Primary Functions:

  • Controls the autonomic nervous system.

  • Regulates the release of hormones from the pituitary gland.

• Anti-Diuretic Hormone (ADH): The hypothalamus is considered a gland because it manufactures ADH. This hormone is then stored in and released from the posterior lobe of the pituitary gland.

• Osmoregulation Process:

  • ADH increases the permeability of the collecting ducts within the nephrons of the kidneys.

  • This allows more water to be reabsorbed back into the blood from the filtrate.

  • The result is decreased water loss in urine, leading to water conservation within the body.

• Disorders of ADH:

  • A lack of ADH secretion results in a condition called diabetes insipidus.

  • This condition is characterized by the excretion of large quantities of dilute urine, approximately $5-10\,\text{litres}$ per day.

• Releasing Factors: The hypothalamus secretes releasing factors that travel through the blood to the anterior lobe of the pituitary gland to stimulate hormone production.

  • Example: At the onset of puberty, the hypothalamus releases Gonadotropin-Releasing Hormone (GnRH), which triggers the anterior pituitary to secrete Luteinising Hormone (LH) and Follicle Stimulating Hormone (FSH).

The Pituitary Gland: The Master Gland

• The pituitary gland is known as the "master gland" because it acts as the chemical co-ordinator for most other endocrine glands.

• Structure: It is attached to the hypothalamus at the base of the brain by a short stalk and consists of two distinct lobes.

• The Anterior Lobe: This is the purely glandular section composed of secretory cells. It releases the following hormones:

  • Thyroid Stimulating Hormone (TSH): Stimulates the thyroid gland to secrete thyroxin and regulates thyroxin levels in the blood.

  • Growth Hormone (GH): Promotes skeletal and muscular growth by stimulating protein synthesis and affecting metabolism.

  • Follicle Stimulating Hormone (FSH): In females, it stimulates follicle development (oogenesis) in the ovaries. In males, it stimulates sperm development (spermatogenesis) in the testes.

  • Luteinising Hormone (LH): In females, it stimulates the secretion of oestrogen and progesterone, leading to ovulation. In males, it stimulates testosterone secretion by the testes.

  • Prolactin: Stimulates milk production (lactation) in mammary glands after childbirth, maintained by suckling. It is also linked to maternal instinct.

• The Posterior Lobe: This lobe is primarily composed of neurons and releases:

  • Oxytocin: Known as the "love hormone," it stimulates prostaglandin release to promote uterine contractions during labour and triggers milk release post-birth. It influences social bonding, trust, empathy, sexual arousal, and stress reduction in both sexes.

Hormonal Disorders of the Pituitary Gland

• Growth Hormone Under-secretion in Children: Results in pituitary dwarfism. The individual is small but perfectly in proportion. This can be corrected using commercially produced GH before puberty.

  • Genetic dwarfism (achondroplasia) is different; it involves a defective gene causing a normal-sized torso and head but very short legs, and there is no cure.

  • Case Example: Shauna Rae's growth stopped at $1.17\,m$ following childhood brain cancer treatment.

• Growth Hormone Over-secretion in Children: Results in gigantism, where the skeleton, muscles, and organs grow abnormally large.

  • Case Example: Robert Wadlow, the tallest man in history, reached a height of $2.72\,m$ and a weight of $199\,kg$.

• Growth Hormone Over-secretion in Adults: Leads to acromegaly. This is characterized by the enlargement of bones in the face, hands, and feet, along with the thickening of soft tissues (e.g., enlarged tongue and facial features).

The Thyroid Gland: Metabolism and Development

• Anatomy: A butterfly-shaped gland with two lobes located on either side of the trachea, just below the larynx in the neck.

• Thyroxin: The primary hormone released by the thyroid. It requires iodine (found in seafood or iodised salt) for production.

• Basal Metabolic Rate (BMR): Thyroxin controls the BMR, which is the amount of energy the body needs at rest. It influences body heat, heart rate, cellular respiration, and the nervous system.

• Disorders of Hypothyroidism (Under-secretion):

  • In Children: Results in cretinism. Symptoms include stunted physical growth, immature sexual development, and intellectual disability. These conditions are irreversible.

  • In Adults: Results in myxoedema. Symptoms include mental and physical sluggishness, low blood pressure, and low heart and respiratory rates. It can be treated with iodine or synthetic thyroxin.

• Disorders of Hyperthyroidism (Over-secretion): Leads to a high metabolic rate, often accompanied by a goitre (enlarged thyroid).

The Pancreas: Endocrine Function and the Islets of Langerhans

• The pancreas is a dual-function gland (exocrine and endocrine).

• The endocrine portion consists of cell clusters called the Islets of Langerhans.

• Alpha (\alpha) cells: Secrete glucagon.

• Beta (\beta) cells: Secrete insulin.

• Function of Glucose: Glucose is the primary cellular energy source. It requires insulin to enter cells.

• Antagonistic Effects:

  • Insulin: Lowers blood glucose levels by instructing the liver to convert glucose (soluble) into glycogen (insoluble storage form).

  • Glucagon: Raises blood glucose levels by instructing the liver to convert stored glycogen back into glucose.

Homeostatic Control of Blood Glucose

• The body maintains blood glucose within a narrow limit of approximately $\pm 80-100\,mg$ per $100\,cm^3$ of blood.

• High Blood Glucose Risks: If levels rise too high, water potential in body fluids falls, causing cells to lose water to tissue fluid.

• Low Blood Glucose Risks: If levels fall too low, cells lack sufficient energy for functions.

• Role of the Liver: The liver acts as the effector in glucose homeostasis. It contains enzymes to catalyze the conversion between glucose and glycogen.

• Role of the Pancreas: Acts as both the receptor (detecting changes via Alpha and Beta cells) and the source of hormonal messengers.

Diabetes Mellitus and Hypoglycaemia

• Hyperglycaemia: High levels of blood glucose.

• Diabetes Mellitus: A chronic disease caused by hyperglycaemia.

  • Type 1 (Insulin Dependent): Usually starts in childhood. It is an autoimmune disease where the insulin-producing Beta cells are destroyed.

  • Type 2 (Non-insulin Dependent): Usually starts in adulthood. It is influenced by lifestyle factors (age, obesity, high-sugar/fat diet, lack of exercise). The body produces less insulin or cannot use it correctly. Treatment involves a balanced diet with low Glycaemic Index (GI) carbohydrates.

• Hypoglycaemia: Low blood glucose levels, often due to insulin over-secretion. Treatment involves consuming carbohydrates like sugars or starch.

The Adrenal Glands: Medulla and Cortex

• Two pyramid-shaped adrenal glands sit on top of each kidney. Each consists of an outer cortex and an inner medulla.

• Adrenal Cortex: Secretes steroid hormones, including:

  • Aldosterone: Regulates electrolyte balance by increasing sodium ion reabsorption in the nephrons.

  • Cortisone: Involved in the stress response.

  • Reproductive Hormones: Small quantities of sex hormones.

• Adrenal Medulla: Secretes adrenaline, the "fight or flight" hormone.

The Sympathetic Response: Physiological Effects of Adrenaline

The Gonads: Male and Female Reproductive Hormones

• Ovaries (Females):

  • Oestrogen: Responsible for the puberty growth spurt, secondary sex characteristics (e.g., breast development), maturation of reproductive organs, and menstrual cycle changes in the endometrium.

  • Progesterone: Promotes cyclic changes in the endometrium, maintains the endometrium during pregnancy, and inhibits milk production during pregnancy.

• Testes (Males):

  • Testosterone: Known as the "impulsive hormone." Responsible for the puberty growth spurt, secondary sex characteristics (e.g., deepening voice, body hair), maturation of reproductive organs, and sex drive.

• Disorders:

  • Hypersecretion can lead to premature puberty or virilism (masculine features in women).

  • Reproductive issues include endometriosis, ectopic pregnancies, cysts, and problems with spermatogenesis or ovulation leading to infertility.

Homeostasis and Negative Feedback Systems

• Homeostasis: The maintenance of a constant internal environment (blood, tissue fluid, cytoplasm) despite changes in the external environment.

• Key Factors Regulated: Temperature, pH, glucose concentration, oxygen, carbon dioxide, and salts.

• Negative Feedback Mechanism: A process where a change in a factor triggers a corrective response to restore that factor to its normal level (the norm).

  • Receptor: Detects the change.

  • Control Centre: Processes the information.

  • Effector: Executes the change (e.g., the liver in glucose regulation).

Anabolic Steroids and Hormone Abuse in Sports

• Anabolic Steroids: Artificially produced hormones similar to androgens (male sex hormones like testosterone). There are over $100$ variations.

• Mechanism: They mimic testosterone to stimulate muscle tissue growth and "bulk up" the body in response to training. They can remain in the system for several days up to a year.

• Steroidal Supplements: Substances like DHEA and androstenedione. DHEA remains legal over-the-counter in some places; most others require a prescription. They are weaker and often marketed with false claims.

• Misconceptions: Research indicates steroids improve muscle mass and endurance but NOT skill, agility, or overall athletic performance.

Risks and Dangers of Steroid Use

• General Side Effects: Dizziness, premature balding, mood swings, nausea, trembling, high blood pressure, joint aching, and increased risk of heart disease, stroke, and cancer.

• Psychological Effects: Delusions, paranoia, depression, and "roid rage" (extreme, uncontrolled anger).

• Female-Specific Risks: Facial hair growth, deepening voice, breast shrinking, clitoral enlargement, and menstrual irregularites.

• Male-Specific Risks: Testicular shrinkage, breast development, pain during urination, impotence, and sterility.

• Infection Risks: Injecting steroids with shared needles increases the risk of HIV/AIDS, Hepatitis, and bacterial endocarditis (heart lining infection).

Human Growth Hormone (hGH) Abuse

• Usage: Legal only when prescribed for growth hormone deficiency. Misused as an athletic supplement based on myths of performance enhancement.

• Risks and Realities:

  • No evidence of performance improvement.

  • Unregulated products bought online are often just amino acids.

  • Cost: Extremely expensive, with street values ranging from $5000$ to $10000$ per month.

  • Injection risks include HIV and Hepatitis.

The Stress System: Adrenaline and Cortisol

• The adrenal glands produce adrenaline and cortisol to manage daily stressors (physical, emotional, or illness).

• Adrenaline (Acute Stress):

  • Acts first and rapidly for short-term stressors.

  • Prepares "fight-or-flight" response by increasing alertness, heart rate, and redirection of blood to muscles.

• Cortisol (Chronic Stress):

  • Released shortly after adrenaline; acts more slowly but for a longer duration.

  • Increases blood glucose by promoting the breakdown of fats and proteins.

  • Inhibits non-essential systems like digestion and the immune response to conserve energy for physical activity.

  • Follows a daily rhythm: highest in the early morning and lowest at midnight.

Cortisol: Effects on Body Composition and Energy Metabolism

• Negative Impact of High Cortisol: Prolonged high levels lead to muscle breakdown, delayed recovery, weakened immunity, and reduced bone formation (increasing osteoporosis risk). In students, it impairs memory and concentration.

• Weight Gain: Cortisol promotes fat storage, particularly in the abdominal (belly) area.

• Catabolic State: Intense strength training without sufficient carbohydrates puts the body in a catabolic state, where cortisol and ACTH (Adrenocorticotropic hormone) break down muscle proteins for fuel.

• ACTH Function: Stimulates the adrenal cortex to secrete natural steroids like cortisone and aldosterone.