Human Endocrine System Flashcards

Overview of the Human Endocrine System

• The human endocrine system consists of a network of glands and tissues that regulate vital bodily functions through the secretion of hormones.
• The nervous system and hormones act in conjunction to enable animals to respond to external changes and control internal conditions.
• Hormones: Chemical messengers that travel in the bloodstream and affect specific target cells. They are released from various organs to coordinate bodily functions and maintain homeostasis.
• Endocrine Glands: These glands secrete hormones directly into the bloodstream. This direct secretion allows for a faster distribution to target organs and tissues throughout the body.
• Exocrine Glands: These glands secrete substances (such as sweat, tears, milk, digestive enzymes, and saliva) into ducts that lead onto body surfaces or into body cavities.

Homeostasis and Feedback Mechanisms

• Homeostasis: The maintenance of a constant internal environment in the body. The internal environment specifically refers to the tissue fluid and the blood.
• For cells to function properly, several factors in the internal environment must remain constant:     - Water concentration.     - Electrolytes.     - Temperature.     - Carbon dioxide ($CO_2$) and Oxygen ($O_2$) levels.     - Glucose concentration.     - $pH$ levels.
• Disruption of Homeostasis: Many external factors can disrupt this balance, including diseases, toxins, and pathogens.
• Feedback Control:     - Negative Feedback: Occurs when a change in a variable triggers a response that reverses the initial change (e.g., blood glucose regulation). This is the primary method for maintaining homeostasis.     - Positive Feedback: Occurs when a change in a variable triggers a response that causes further change in the same direction (e.g., lactation, blood clotting, and uterine contractions during labor).
• Components of a Negative Feedback System:     1. Receptor: Detects the change from the set point and sends information to the control center.     2. Control Centre: Usually the brain; activates a corrective mechanism and sends instructions to effectors.     3. Effector: Responds to and corrects the change, returning the variable to the set point.
• Failure to maintain homeostasis results in the body no longer being "at ease" (balanced), leading to disease.

The Hypothalamus and Antidiuretic Hormone (ADH)

• The hypothalamus is located in the brain and acts as the link between the nervous and endocrine systems via the pituitary gland.
• It produces چندین (several) hormones, including Anti-Diuretic Hormone (ADH).
• ADH is produced in the hypothalamus but is sent to the posterior lobe of the pituitary gland for storage until it is needed.
• Functions of ADH (Osmoregulation):     - It increases the permeability of the collecting ducts of the nephrons in the kidney.     - This increases the reabsorption of water back into the blood.     - Consequently, urine becomes more concentrated, urine volume decreases, and the body conserves water.     - ADH also increases blood pressure ($BP$) by causing vasoconstriction (and the addition of water to the blood).

The Pituitary Gland (Hypophysis)

• Often called the "Master Gland," it is located hanging from the hypothalamus by a stalk.
• It consists of two distinct lobes: the Anterior Lobe and the Posterior Lobe.

Posterior Lobe of the Pituitary

• Releases hormones produced by the hypothalamus:     - ADH: Regulates water balance.     - Oxytocin: Causes uterine contractions during labor and stimulates milk ejection from mammary glands (breasts).

Anterior Lobe of the Pituitary

• Secretes its own hormones, including:     1. Thyroid-stimulating hormone (TSH): Targets the thyroid gland to release thyroxin.     2. Growth Hormone (GH): Stimulates growth and metabolism.     3. Follicle Stimulating Hormone (FSH): Involved in reproduction.     4. Luteinising Hormone (LH): Involved in reproduction.     5. Prolactin: Stimulates mammary glands to grow and produce milk (lactation).

The Thyroid Gland and Thyroxin

• The thyroid is a butterfly-shaped gland located in front of the trachea.
• It produces Thyroxin, which is produced in an inactive form and requires iodine to become active.
• Functions of Thyroxin:     - Targets all body cells to increase the metabolic rate.     - Increases heart rate.     - Increases the activity of the nervous system for normal functioning.     - Essential for normal organ growth and development, particularly in children.     - Critical for fetal brain development.

Abnormalities of Thyroid Secretion

1. Hyperthyroidism (Overactive thyroid):     - Causes: Goitre (enlarged thyroid), thyroid tumors, or Grave’s Disease (an autoimmune disorder where antibodies speed up hormone production).     - Symptoms: Restlessness, weight loss, increased appetite, excessive sweating, anxiety, hyperactivity, diarrhea, and exophthalmos (protruding eyeballs).
2. Hypothyroidism (Underactive thyroid):     - Causes: Improperly formed gland at birth, autoimmune diseases, lack of iodine in the diet, or surgical removal.     - Symptoms in Adults: Myxoedema — low metabolic rate, constant fatigue, weight gain, mental sluggishness, decreased heart rate, constipation, poor muscle tone, and goitre.     - Symptoms in Children: Cretinism — stunted physical growth, mental retardation, failure to reach sexual maturity, and an oversized tongue.

Growth Hormone (GH) and Growth Disorders

• GH exerts effects directly on almost all cells, especially bone and muscle.
• It promotes cell size increase, mitosis, and protein synthesis.
• Growth hormone is primarily released during sleep.
• Abnormalities in GH Secretion:     - Dwarfism: Under-secretion of GH in children; results in adults with small but proportionate stature.     - Gigantism: Over-secretion of GH in children; results in very tall adults, often with shorter lifespans.     - Acromegaly: Over-secretion of GH in adults (after bone epiphyses have fused). Height does not increase, but bones thicken. Symptoms include enlarged hands and feet, protruding jaw, enlarged tongue, and enlarged vertebrae (hunchback). A famous example is the character of the James Bond villain.
• Bovine Somatotropin (BST/BGH): Growth hormones produced in cow pituitaries. Genetically engineered versions ($rBST$ or $rBGH$) are used to increase milk production, though this is controversial due to potential harm to cows and humans.

Reproductive Hormones (Gonads)

Follicle Stimulating Hormone (FSH)

• In Males: Stimulates Sertoli cells in the testes to facilitate sperm production (spermatogenesis).
• In Females: Stimulates the maturation of germ cells (Graafian follicle) in the ovaries to form ova (eggs) and stimulates the production of oestrogen.

Luteinising Hormone (LH)

• In Females: Triggered by rising oestrogen levels; causes ovulation (release of the egg) and stimulates the formation of the corpus luteum.
• In Males: Stimulates the Leydig cells in the testes to produce testosterone.

Female Gonadal Hormones

• Oestrogen: Released by the developing follicle; responsible for secondary sexual characteristics, thickening the uterine lining (endometrium), and regulating the menstrual cycle.
• Progesterone: Released by the corpus luteum; prepares the uterus for pregnancy, prevents uterine contractions during pregnancy, and inhibits prolactin. If no pregnancy occurs, levels drop, causing menstruation.

Male Gonadal Hormones

• Testosterone: Secreted by both sexes, though males secrete $40 - 60 \times$ more. It promotes muscle growth, bone density, and male secondary sexual characteristics.

The Pancreas and Blood Glucose Regulation

• The pancreas has both exocrine functions (digestive enzymes via ducts) and endocrine functions.
• Endocrine Tissue: The Islets of Langerhans, consisting of:     - Alpha cells: Produce Glucagon.     - Beta cells: Produce Insulin.

Functions of Insulin

• Released when blood glucose levels are high (e.g., after a carbohydrate-rich meal).
• Promotes glucose uptake by cells for cellular respiration.
• Stimulates the liver and muscles to convert glucose into glycogen for storage.
• Promotes fat deposition in adipose tissue and protein formation for growth.

Functions of Glucagon

• Released when blood glucose levels are low (e.g., during fasting).
• Stimulates the liver to convert stored glycogen back into glucose (glycogenolysis).
• Insulin and glucagon work antagonistically (in opposite directions).

Control Mechanism

• Normal blood glucose concentration: $3,5 - 5,5 \, mmol/litre$.
• Correction of Increase: Pancreas secretes insulin -> Liver converts glucose to glycogen -> Glucose levels lower -> Negative feedback inhibits further insulin release.
• Correction of Decrease: Pancreas secretes glucagon -> Liver converts glycogen to glucose -> Glucose levels rise -> Negative feedback inhibits further glucagon release.

Diabetes Mellitus

• Type I Diabetes: Autoimmune disease (immune system destroys beta cells). Diagnosed mostly in children/young adults. Treatment involves regular insulin therapy.
• Type II Diabetes: Lifestyle disease. Body cells resist insulin or don't respond to it. Treatment involves a low-fat, low-sugar diet and exercise. Insulin injections are often ineffective for Type II.
• Symptoms: Increased thirst, frequent urination, hunger, fatigue, and blurred vision.

Adrenal Glands and Stress Response

Adrenalin (Epinephrine)

• Secreted from the adrenal medulla.
• Triggers the "Fight or Flight" response.
• Effects:     1. Increases mental alertness.     2. Increases heart rate and blood volume pumped.     3. Redirects blood flow from the skin and digestive tract to the brain and skeletal muscles.     4. Increases breathing rate and depth.     5. Stimulates conversion of glycogen to glucose for energy.     6. Dilates pupils.     7. Inhibits peristalsis and saliva production.     8. Causes body hair to stand up.

Cortisol

• A steroid hormone released by the adrenal cortex in response to stress.
• Functions:     - Increases blood sugar through gluconeogenesis.     - Suppresses the immune system.     - Aids in metabolism of fats, proteins, and carbohydrates.     - Decreases bone formation.

Infertility

• Defined as the inability to achieve pregnancy after $12$ months of regular, unprotected intercourse.
• Female Factors: Ovulation problems, endometriosis, blocked fallopian tubes, uterine issues (fibroids), hormonal imbalances, and advanced maternal age.
• Male Factors: Low sperm count/quality, ejaculatory problems, testicular issues, and hormonal imbalances.
• Treatments:     1. Medicines: Target the pituitary to stimulate FSH and LH for ovulation.     2. Surgical procedures: Correct tubule or uterine problems.     3. Assisted Conception: In-vitro fertilization (IVF) and surrogacy.

Sports and Hormones

• Cortisol and Adrenaline: Intense exercise increases cortisol. While moderate levels are beneficial, chronic high levels lead to muscle breakdown, suppressed immunity, and lower testosterone.
• Steroids: Anabolic steroids are synthetic versions of androgens (like testosterone).     - Anabolic Effects: Build muscle, increase bone density, and speed up injury recovery.     - Androgenic Effects: Promote masculine traits (deeper voice, body hair).
• Growth Hormone and Insulin: Both have been used illegally as performance-enhancing drugs in sports.

Tutorial Data and Statistics Summary

• Hypoglycaemia: Blood glucose levels fall below the healthy range, approximately $70 \, mg/dL$.
• Target Range: Approximately $100 \, mg/dL$ or a range between $80 - 100 \, mg/dL$.
• Hypoglycaemia Symptoms: Weakness, shakiness, lightheadedness, headaches, seizures.
• Islet Cell Transplantation: A potential treatment for Type 1 diabetes (not Type 2 because Type 2 involves cell resistance, not a lack of insulin production).
• Fixed Variables in Diabetic Studies: Number of patients, duration of study (e.g., $3$ months), age of participants.
• Scientific Validity: Can be improved by increasing sample size or using a broader demographic.