 --- 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: Pituitary adenoma secreting excess ACTH - Frequency: Most common cause of Cushing's syndrome (80% of cases) - Treatment: Pituitary surgery or radiation --- Question 7: In the light pupillary reflex: a) Which type of receptor is stimulated? (1 Mark) - Photoreceptors in the retina (rods and cones) 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) - Pupillary constriction (miosis) - contraction of sphincter pupillae muscle --- Question 8: Define receptor adaptation and describe the mechanism using sound as an example. (5 Marks) Definition: Receptor adaptation is the gradual decrease in receptor sensitivity and response to a constant stimulus over time. Mechanism of Auditory Adaptation: 1. Hair Cell Adaptation: - Fast adaptation: Tip links adjust tension - Slow adaptation: Myosin motor proteins reset - Calcium-dependent process: Regulates mechanotransduction 2. Molecular Mechanism: - Initial stimulus opens mechanotransduction channels - Calcium influx activates adaptation motors - Tip links slide along stereocilia - Channels close despite continued stimulus 3. Temporal Characteristics: - Fast component: Milliseconds (10-100 ms) - Slow component: Seconds to minutes - Incomplete adaptation: Maintains some sensitivity 4. Physiological Significance: - Dynamic range: Allows detection of stimulus changes - Noise filtering: Reduces response to constant background sounds - Sensitivity maintenance: Prevents receptor fatigue Clinical Example: Adapting to constant background noise in a noisy environment. --- Question 9: Clearly explain the pathophysiology of Diabetic Ketoacidosis. (5 Marks) Pathophysiology of DKA: 1. Insulin Deficiency: - Absolute deficiency: Type 1 diabetes - Relative deficiency: Severe insulin resistance - Precipitating factors: Infection, stress, medication non-compliance 2. Metabolic Consequences: Hyperglycemia: - Decreased glucose uptake by tissues - Increased hepatic glucose production - Gluconeogenesis and glycogenolysis Lipolysis: - Activation of hormone-sensitive lipase - Increased free fatty acid release - Enhanced ketone body production 3. Ketogenesis: - Hepatic process: β-oxidation of fatty acids - Ketone bodies: Acetoacetate, β-hydroxybutyrate, acetone - Accumulation: Exceeds tissue utilization capacity 4. Metabolic Acidosis: - Ketoacids consume bicarbonate buffer - Decreased blood pH ( 4.0 Clinical Benefits: - Rapid symptom relief: Pain reduction within 24-48 hours - Accelerated healing: Faster ulcer healing rates - Recurrence prevention: Maintains healing environment - H. pylori eradication: Component of triple ther