Human Endocrine System Practice Flashcards

Introduction to the Human Endocrine System

The human endocrine system is a complex network of glands and tissues that regulate vital bodily functions through the secretion of chemicals known as hormones. Together with the nervous system, the endocrine system allows animals to respond to external changes and maintain precise control over their internal conditions.

Hormones: These are chemical messengers produced by organs and glands. They are transported via the bloodstream and affect specific target cells or organs. Their role is to coordinate various bodily functions and maintain homeostasis. Hormones secreted directly into the bloodstream allow for faster physiological responses.

Classification and Function of Glands

The body utilizes two distinct types of glands to transport substances:

• Endocrine Glands: These are ductless glands that secrete hormones directly into the bloodstream. This method allows hormones to travel throughout the body to reach distant target organs and tissues.

• Exocrine Glands: These glands secrete substances through ducts onto body surfaces or into cavities. Examples of exocrine secretions include sweat, tears, milk, saliva, and digestive enzymes.

Note: The Pancreas is unique as it performs both exocrine and endocrine functions.

Principles of Homeostasis and Feedback Mechanisms

Homeostasis: This is the maintenance of a constant internal environment within the body, specifically regarding the tissue fluid and blood. Factors requiring constant regulation for cellular health include:

• Water concentration

• Electrolytes

• Temperature

• Carbon dioxide ($CO_2$) and Oxygen ($O_2$) levels

• Glucose concentration

• pH levels

External factors such as diseases, toxins, and pathogens can disrupt this balance. If homeostasis is not maintained, the body enters a state of disease (at ease being lost).

Feedback Mechanisms:

1. Negative Feedback: Occurs when a change in a variable triggers a response that reverses the initial change, aiming to return the system to its set point. Examples include blood glucose regulation and osmoregulation.

2. Positive Feedback: Occurs when a change in a variable triggers a response that causes further change in the same direction. Examples include lactation, blood clotting, and uterine contractions during labor.

Components of a Negative Feedback System:

1. Receptor: Detects the deviation from the set point and informs the control center.

2. Control Centre: (Usually the brain) Processes information and activates a corrective mechanism.

3. Effector: Responds to the corrective mechanism to return the variable to the set point.

The Hypothalamus and the Role of ADH

The hypothalamus is located in the brain and serves as the primary link between the nervous and endocrine systems via the pituitary gland.

Anti-Diuretic Hormone (ADH):

• Production and Storage: Produced by the hypothalamus and sent to the posterior lobe of the pituitary gland for storage.

• Osmoregulation: ADH increases the permeability of the collecting ducts of the nephrons in the kidneys. This increases water reabsorption into the blood.

• Urine Effects: Results in a smaller volume of highly concentrated urine.

• Blood Pressure: ADH causes vasoconstriction and increases blood volume (through water retention), thereby raising blood pressure ($BP$).

The Pituitary Gland (Hypophysis): Structure and Secretions

Known as the "Master Gland," the pituitary gland hangs from the hypothalamus by a thin stalk and consists of two distinct lobes: the anterior and the posterior.

The Posterior Lobe (Secretions):

1. ADH: Controls water balance.

2. Oxytocin: Stimulates uterine contractions during labor and triggers milk ejection from mammary glands (breasts).

The Anterior Lobe (Secretions):

1. TSH (Thyroid-stimulating hormone): Targets the thyroid to release thyroxin.

2. FSH (Follicle-stimulating hormone): Stimulates gamete development.

3. LH (Luteinising hormone): Triggers ovulation and stimulates testosterone production.

4. Growth Hormone (GH): Promotes growth and mitosis.

5. Prolactin: Stimulates milk production (lactation).

The Thyroid Gland: Thyroxin and Metabolic Homeostasis

The thyroid gland is a butterfly-shaped organ located in front of the trachea. It produces thyroxin, which requires iodine to become active.

Functions of Thyroxin:

• Targets all body cells to increase metabolic rate.

• Ensures normal operation of the nervous system.

• Increases heart rate.

• Essential for normal organ development and growth, particularly in children.

• Critical for fetal brain development.

Negative Feedback Control (TSH and Thyroxin): If thyroxin levels drop below the set point, the pituitary secretes more TSH, which stimulates the thyroid to produce more thyroxin. Conversely, high thyroxin levels inhibit the release of TSH.

Thyroid Abnormalities:

1. Hyperthyroidism (Overactive): Caused by tumors, Graves' Disease (autoimmune), or Goitre. Symptoms include weight loss, exophthalmos (protruding eyeballs), anxiety, restlessness, and hyperactivity.

2. Hypothyroidism (Underactive): Caused by iodine deficiency, surgery, or autoimmune attacks. Leads to Myxoedema in adults (fatigue, weight gain) or Cretinism in children (mental retardation, stunted growth, lack of sexual maturity).

Growth Hormone (GH) and Developmental Disorders

Growth Hormone promotes an increase in cell size and mitosis, and increases protein synthesis. It is primarily released during sleep and affects bone and muscle tissue.

GH Abnormalities:

• Dwarfism: Hyposecretion of GH in children; resulting in a small adult with proportional features.

• Gigantism: Hypersecretion of GH in children; resulting in extreme height and often a shorter lifespan.

• Acromegaly: Hypersecretion of GH in adults. Since epiphyses (bone ends) have fused, no height increase occurs, but bones thicken. Symptoms include enlarged hands, feet, jaw, and tongue. (e.g., The James Bond villain).

Bovine Growth Hormone: Bovine somatotropin ($BST$) or $BGH$ (and the genetically engineered version $rBST/rBGH$) is used in cows to increase milk production, though it is considered harmful to animals and potentially people.

Reproductive Hormones and the Gonads

Follicle Stimulating Hormone (FSH):

• Males: Stimulates Sertoli cells in the germinal epithelium of the seminiferous tubules to produce sperm (spermatogenesis).

• Females: Stimulates maturation of Graafian follicles in the ovaries to form ova and produce oestrogen.

Luteinising Hormone (LH):

• Females: Triggers ovulation and forms the corpus luteum (which secretes progesterone).

• Males: Stimulates Leydig cells in the testes to produce testosterone.

Gonadal Hormones:

1. Oestrogen: Produced by follicles; develops secondary sexual characteristics and thickens the endometrium.

2. Progesterone: Produced by the corpus luteum; maintains the pregnancy by preparing the uterine lining and stopping uterine contractions.

3. Testosterone: Produced in much higher quantities in males ($40 - 60 \times$ higher than females); aids muscle growth and bone density.

The Pancreas: Endocrine and Exocrine Functions

The pancreas functions as both an exocrine gland (producing digestive enzymes for the small intestine via ducts) and an endocrine gland (secreting hormones from the Islets of Langerhans directly into the blood).

Hormones of the Islets of Langerhans:

• Alpha Cells: Produce Glucagon to raise blood sugar levels.

• Beta Cells: Produce Insulin to lower blood sugar levels.

Blood Glucose Regulation and Antagonistic Control

Normal blood glucose levels are between $3.5 - 5.5\,mmol/litre$ (or $80 - 100\,mg/dL$). Levels are maintained through the antagonistic relationship between insulin and glucagon.

Mechanism for High Blood Glucose (after a meal):

1. Stimulus: Glucose levels rise.

2. Receptor: Pancreas (Beta cells) secretes Insulin.

3. Effectors: Liver, muscles, and tissue cells.

4. Response: Insulin stimulates glucose uptake for respiration and converts glucose to glycogen (glycogenesis) for storage.

5. Result: Blood glucose levels return to normal.

Mechanism for Low Blood Glucose (fasting/exercise):

1. Stimulus: Glucose levels fall (Hypoglycaemia occurs at approximately $70\,mg/dL$).

2. Receptor: Pancreas (Alpha cells) secretes Glucagon.

3. Effectors: Liver and muscle tissue.

4. Response: Glucagon stimulates glycogenolysis (breakdown of glycogen to glucose).

5. Result: Blood glucose levels increase back to normal.

Diabetes Mellitus

Type I Diabetes:

• Nature: Autoimmune disease mostly diagnosed in youth.

• Cause: Immune system destroys beta cells, leading to a total lack of insulin.

• Treatment: Insulin therapy, monitoring, and diet.

Type II Diabetes:

• Nature: Lifestyle disease, often linked to age and obesity.

• Cause: Body cells become resistant to insulin or do not respond properly.

• Treatment: Healthy lifestyle, low sugar/fat diet, exercise, and medications to increase insulin sensitivity. Insulin injections are often ineffective for Type II patients.

The Adrenal Glands: Stress Response and Metabolism

Adrenaline (Epinephrine): Secreted from the adrenal medulla to prepare the "fight or flight" response. Its effects include:

1. Increased mental alertness.

2. Increased heart rate and blood volume to skeletal muscles.

3. Reduced blood flow to the skin and digestive tract.

4. Increased depth of breathing.

5. Glycogen conversion to glucose for energy.

6. Pupil dilation.

7. Inhibition of peristalsis and saliva production.

8. Piloerection (hair standing up).

Cortisol: A steroid hormone released in response to chronic stress.

• Functions: Increases blood sugar via gluconeogenesis, suppresses the immune system, and aids in fat/protein/carbohydrate metabolism.

• Negative effect: Decreases bone formation.

Infertility: Causes and Medical Treatments

Infertility is defined as the inability to achieve pregnancy after $12$ months of unprotected intercourse.

Causes:

• Female: Ovulation issues, endometriosis, blocked fallopian tubes, uterine fibroids, advanced maternal age.

• Male: Low sperm count, poor sperm quality, ejaculatory problems.

Treatments:

1. Medical: Drugs targeting the pituitary to stimulate FSH and LH for ovulation.

2. Surgical: Correcting blocked tubes or womb issues.

3. Assisted Conception: In-vitro fertilisation (IVF) and surrogacy.

Hormones in Sports Medicine

Hormones are often used or monitored in athletes for performance enhancement and recovery:

• Anabolic Steroids: Artificially produced hormones similar to testosterone. They promote muscle growth (anabolic effects), bone density, and recovery. Side effects include androgenic effects (masculine traits like voice deepening).

• Insulin and GH: Also used as performance-enhancing drugs.

• Cortisol and Overtraining: High cortisol levels from intense training can lead to chronic muscle breakdown and suppressed immunity. Athletes aim to manage cortisol to achieve positive tissue adaptations.

Practical Application: Case Studies and Data Interpretation

Liver Tissue Analysis: Micrographs show that healthy liver tissue stores glycogen as dark granules. In patients with metabolic disorders, an enlarged liver may be present despite low blood glucose if they cannot successfully mobilize those glycogen stores.

Banting Diet Experiment: Diabetics may follow low-carbohydrate/high-fat diets to manage insulin resistance. When designing such an experiment, fixed variables must include age range, starting health status, and duration of study ($3$ months). Validity is increased by larger sample sizes and randomized groups.

Thyroid Tissue Observation: Thyroid sacs are made of cuboidal cells. In sick cats with non-functioning thyroids, radioactive iodine tests may show that iodine is not being absorbed from the blood to manufacture thyroxin, confirming the diagnosis through a lack of glandular uptake compared to a healthy subject.