Medical Physiology Paper II

## #### 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:** Rec