Comprehensive study guide covering tubular secretion, steroid-induced infertility, Cushing's syndrome, and the pathophysiology of Diabetic Ketoacidosis.
SCHOOL OF MEDICINE DEPARTMENT OF MEDICAL PHYSIOLOGY --- --- INSTRUCTIONS: - This paper contains two sections . - Section A: Short Answer Questions — Answer all questions. - Section B: Structured Questions — Answer only TWO questions. --- SECTION A: SHORT ANSWER QUESTIONS (60 Marks) Answer all questions MEDICAL PHYSIOLOGY EXAM ANSWERS MBMP 2300 - Section A: Short Answer Questions --- Question 1: How does the tubular secretion of H⁺ occur, and how does it achieve HCO₃⁻ reabsorption? (5 Marks) Answer: Tubular secretion of H⁺ occurs through two primary mechanisms: Primary Active Transport: - H⁺-ATPase pumps in the apical membrane of intercalated cells actively transport H⁺ into the tubular lumen - This process requires ATP and creates an electrochemical gradient Secondary Active Transport: - Na⁺/H⁺ antiporter (exchanger) in proximal tubule cells exchanges intracellular H⁺ for luminal Na⁺ - Driven by the sodium gradient maintained by basolateral Na⁺/K⁺-ATPase HCO₃⁻ Reabsorption Mechanism: - Secreted H⁺ combines with filtered HCO₃⁻ in the lumen to form H₂CO₃ - Carbonic anhydrase on the brush border converts H₂CO₃ to H₂O and CO₂ - CO₂ diffuses into the cell and is converted back to HCO₃⁻ by intracellular carbonic anhydrase - HCO₃⁻ exits the cell via basolateral Na⁺/HCO₃⁻ cotransporter, achieving net reabsorption --- Question 2: Discuss potassium secretion by the nephron. (5 Marks) Answer: Location and Mechanism: - Primarily occurs in the principal cells of the collecting duct - K⁺ enters cells via basolateral Na⁺/K⁺-ATPase pump - K⁺ exits into tubular lumen through apical K⁺ channels (ROMK channels) Regulation: - Aldosterone: Increases K⁺ secretion by:Increasing Na⁺/K⁺-ATPase activity - Increasing apical K⁺ channel expression - Enhancing Na⁺ reabsorption, creating favorable electrical gradient - Plasma K⁺ concentration: High K⁺ levels directly stimulate aldosterone release and increase K⁺ secretion - Tubular flow rate: Increased flow enhances K⁺ secretion by maintaining concentration gradients - Acid-base status: Alkalosis promotes K⁺ secretion, while acidosis inhibits it Clinical Significance: - Essential for K⁺ homeostasis and prevention of hyperkalemia - Dysregulation can lead to dangerous cardiac arrhythmias --- Question 3: A male athlete taking large amounts of an androgenic steroid becomes sterile (unable to produce sperm capable of causing fertilization). Explain. (5 Marks) Answer: Mechanism of Steroid-Induced Infertility: Negative Feedback Suppression: - Exogenous androgens suppress hypothalamic GnRH release - This leads to decreased pituitary FSH and LH secretion - Without adequate LH, Leydig cells reduce endogenous testosterone production - Without adequate FSH, Sertoli cells cannot support spermatogenesis Direct Effects on Spermatogenesis: - High concentrations of synthetic androgens may have direct toxic effects on developing sperm cells - Altered testosterone/estrogen ratios affect normal sperm maturation - Disruption of the blood-testis barrier function Testicular Atrophy: - Prolonged suppression of gonadotropins leads to testicular shrinkage - Reduced seminiferous tubule diameter and function - Decreased sperm production and quality Reversibility: - Effects are typically reversible but may take 6-12 months after cessation - Some individuals may experience prolonged recovery or permanent damage --- Question 4: Describe the physiology of ejaculation. (5 Marks) Answer: Phases of Ejaculation: 1. Emission Phase: - Triggered by sympathetic stimulation (T12-L2) - Smooth muscle contraction of:Seminal vesicles - Prostate gland - Vas deferens - Semen is deposited into the posterior urethra - Internal urethral sphincter contracts to prevent retrograde ejaculation 2. Expulsion Phase: - Controlled by parasympathetic and somatic nervous systems - Rhythmic contractions of:Bulbospongiosus muscle - Ischiocavernosus muscle - Pelvic floor muscles - External urethral sphincter relaxes - Semen is forcefully expelled through the urethra Neural Control: - Spinal ejaculatory center located at L1-L2 - Requires integration of sympathetic, parasympathetic, and somatic pathways - Coordinated by higher brain centers including the hypothalamus Refractory Period: - Follows ejaculation - Prevents immediate re-ejaculation - Duration varies with age and individual factors --- Question 5: Where are thyroid hormone receptors located? How does thyroid hormone transduce its signal in the target cell? (5 Marks) Answer: Receptor Location: - Nuclear receptors: Located in the cell nucleus bound to DNA - Mitochondrial receptors: Present in mitochondria - Cytoplasmic receptors: Some receptors exist in cytoplasm before nuclear translocation - Plasma membrane receptors: Recently discovered membrane-bound receptors Signal Transduction Mechanism: Nuclear Pathway (Primary): - T₃ (active form) enters the cell via membrane transporters - T₃ binds to thyroid hormone receptors (TRα and TRβ) in the nucleus - Receptor-hormone complex binds to thyroid response elements (TREs) on DNA - Acts as transcription factor, increasing or decreasing gene expression - Results in synthesis of new proteins affecting cellular metabolism Non-genomic Effects: - Rapid effects (minutes) through membrane receptors - Activation of second messenger systems (cAMP, IP₃/DAG) - Direct effects on mitochondrial metabolism - Modulation of ion channels and transporters Biological Outcomes: - Increased metabolic rate - Enhanced protein synthesis - Cardiovascular effects - Thermogenesis - Growth and development --- Question 6: What are the symptoms of Cushing's syndrome? What is the difference between Cushing's syndrome and Cushing's disease? (5 Marks) Answer: Symptoms of Cushing's Syndrome: - Physical appearance: Central obesity, moon face, buffalo hump, purple striae - Metabolic effects: Hyperglycemia, diabetes mellitus, hypertension - Musculoskeletal: Muscle weakness, osteoporosis, pathological fractures - Skin changes: Thin skin, easy bruising, poor wound healing - Psychological: Depression, anxiety, mood swings, cognitive impairment - Reproductive: Menstrual irregularities, decreased libido, infertility Difference between Cushing's Syndrome and Cushing's Disease: Cushing's Syndrome: - Definition: General term for any condition causing excess cortisol - Causes: Pituitary adenoma (Cushing's disease) - Adrenal adenoma/carcinoma - Ectopic ACTH syndrome - Exogenous steroid administration - Broader category encompassing all causes of hypercortisolism Cushing's Disease: - Definition: Specific form of Cushing's syndrome caused by pituitary ACTH excess - Cause: Pituitary adenoma secreting ACTH - Represents: Most common cause of endogenous Cushing's syndrome (~70%) - Treatment: Typically requires pituitary surgery --- Question 7: In the light pupillary reflex: (5 Marks) Answer: a) Which type of receptor is stimulated? (1 Mark) - Photoreceptors (rods and cones in the retina) b) Which nerve transmits sensory information to the brain? (1 Mark) - Optic nerve (CN II) c) Which brain region serves as the integration center? (1 Mark) - Pretectal nucleus in the midbrain d) Which nerve transmits motor information to the effector? (1 Mark) - Oculomotor nerve (CN III) e) What is the motor response? (1 Mark) - Constriction of the pupil (miosis) through contraction of the sphincter pupillae muscle --- Question 8: Define receptor adaptation and describe the mechanism using sound as an example. (5 Marks) Answer: Definition of Receptor Adaptation: Receptor adaptation is the decreased responsiveness of sensory receptors to sustained or repeated stimuli, resulting in reduced action potential frequency despite continued stimulus presence. Mechanism of Adaptation: Types: - Rapid adaptation (phasic): Quick decrease in response - Slow adaptation (tonic): Gradual decrease in response Using Sound as an Example: Auditory System Adaptation: - Peripheral adaptation: Hair cells in the cochlea show decreased response to continuous sound - Central adaptation: Au