Medical Physiology Section A - Complete OmpathStudy

SECTION A: SHORT ANSWER QUESTIONS - 60 MARKS (ANSWER ALL QUESTIONS) --- Question 1: How does the tubular secretion of H+ occur, and how does it achieve HCO₃⁻ reabsorption? (5 Marks) Mechanism of H+ Secretion: Primary Process: Active transport via H+-ATPase pumps and Na+/H+ exchangers in proximal tubules, distal tubules, and collecting ducts Proximal Tubule: Na+/H+ antiporter secretes H+ into tubular lumen Distal Tubule: H+-ATPase pump actively transports H+ Collecting Duct: Type A intercalated cells secrete H+ via H+-ATPase HCO₃⁻ Reabsorption Process: Luminal Reaction: H+ + HCO₃⁻ → H₂CO₃ (catalyzed by carbonic anhydrase) Dehydration: H₂CO₃ → H₂O + CO₂ Cellular Entry: CO₂ diffuses into tubular cells Intracellular Reaction: CO₂ + H₂O → H₂CO₃ → H+ + HCO₃⁻ Basolateral Transport: HCO₃⁻ exits cell via Na+/HCO₃⁻ cotransporter Clinical Significance: Essential for acid-base balance and pH homeostasis. --- Question 2: Discuss potassium secretion by the nephron (5 Marks) Primary Sites of K+ Secretion: Main Location: Distal convoluted tubule and collecting duct (principal cells) Mechanism: Basolateral Uptake: Na+/K+-ATPase pump brings K+ into cell Apical Secretion: K+ channels allow K+ efflux into tubular lumen Driving Force: Electrochemical gradient created by Na+ reabsorption Regulation Factors: Aldosterone: Increases K+ secretion by upregulating Na+/K+-ATPase Plasma K+ levels: High K+ stimulates secretion Distal tubular flow rate: Increased flow enhances secretion Acid-base status: Alkalosis increases K+ secretion Clinical Relevance: Crucial for maintaining potassium balance and preventing hyperkalemia. --- Question 3: Describe the physiology of ejaculation (5 Marks) Phases of Ejaculation: 1. Arousal Phase: Sympathetic stimulation increases blood flow to penis Parasympathetic activity maintains erection 2. Emission Phase: Sympathetic stimulation (T12-L2) causes: Contraction of seminal vesicles Prostate gland secretion Vas deferens contraction Sperm and seminal fluid accumulate in urethral bulb 3. Expulsion Phase: Rhythmic contractions of bulbospongiosus and ischiocavernosus muscles Parasympathetic stimulation via pudendal nerve Forceful ejection of semen through urethra Neural Control: Sympathetic: Emission phase Parasympathetic: Expulsion phase Somatic: Muscle contractions --- Question 4: A male athlete taking large amounts of an androgenic steroid becomes sterile (unable to produce sperm capable of causing fertilization) explain. (5 Marks) Mechanism of Steroid-Induced Sterility: 1. Negative Feedback Suppression: Exogenous androgens suppress hypothalamic GnRH release Reduced pituitary FSH and LH secretion Decreased testosterone production by Leydig cells 2. Spermatogenesis Disruption: FSH deficiency: Impairs Sertoli cell function LH deficiency: Reduces intratesticular testosterone Direct toxic effects: High steroid levels damage germinal epithelium 3. Hormonal Imbalance: Altered testosterone/estrogen ratio Disrupted feedback mechanisms Testicular atrophy 4. Sperm Quality Effects: Decreased sperm count (oligospermia) Abnormal sperm morphology Reduced sperm motility Recovery: Often reversible after discontinuation, but may take months to years. --- Question 5: Where are thyroid hormone receptors located? How does thyroid hormone transduce its signals in its target cell? (5 Marks) Receptor Location: Nuclear receptors: Located in cell nucleus Cytoplasmic receptors: Some in cytoplasm, translocate to nucleus Target tissues: Most body cells, especially liver, heart, brain, skeletal muscle Signal Transduction Mechanism: 1. Hormone Entry: T3 and T4 enter cells via specific transporters T4 converted to active T3 by deiodinases 2. Nuclear Binding: T3 binds to thyroid hormone receptors (TR-α and TR-β) Forms hormone-receptor complex 3. DNA Interaction: Complex binds to thyroid response elements (TREs) Located in promoter regions of target genes 4. Gene Expression: Recruitment of coactivators/corepressors Modification of chromatin structure Increased/decreased mRNA transcription 5. Protein Synthesis: New protein production Metabolic enzyme activation Cellular response manifestation --- Question 6: What are the symptoms of Cushing's? What is the difference between Cushing's syndrome and Cushing's disease? (5 Marks) Symptoms of Cushing's: Physical Manifestations: Central obesity with buffalo hump Moon face and supraclavicular fat pads Purple striae (stretch marks) Easy bruising and poor wound healing Muscle weakness and fatigue Metabolic Effects: Hyperglycemia/diabetes mellitus Hypertension Osteoporosis Electrolyte imbalances Psychological Symptoms: Depression and mood changes Cognitive impairment Anxiety and irritability Differences: Cushing's Syndrome: Definition: Clinical condition caused by prolonged exposure to excess cortisol Causes: Any source of excess cortisol (pituitary, adrenal, ectopic, iatrogenic) Scope: Broader term encompassing all causes Cushing's Disease: Definition: Specific type of Cushing's syndrome Cause: Pitu