PCAT Physiology 3 — Questions and Answers
Question 1: Which portion of the nephron is responsible for creating the medullary osmotic gradient via the countercurrent multiplier?
- Proximal convoluted tubule
- Loop of Henle (Correct answer)
- Distal convoluted tubule
- Cortical collecting duct
Correct answer: Loop of Henle
The descending limb is water-permeable and the ascending limb actively pumps NaCl, generating a hypertonic medullary interstitium via countercurrent multiplication.
Question 2: During the plateau phase (phase 2) of the cardiac ventricular action potential, which ion movement predominates?
- Na⁺ influx balancing K⁺ efflux
- Ca²⁺ influx through L-type channels balancing K⁺ efflux (Correct answer)
- Cl⁻ efflux through CFTR channels
- HCO₃⁻ influx via bicarbonate exchangers
Correct answer: Ca²⁺ influx through L-type channels balancing K⁺ efflux
Slow Ca²⁺ entry through L-type voltage-gated channels counteracts K⁺ outflow, maintaining the prolonged plateau that prevents tetanic contraction.
Question 3: Surfactant produced by type II pneumocytes primarily functions to:
- Increase surface tension in alveoli
- Decrease alveolar surface tension to prevent collapse (Correct answer)
- Transport oxygen across the alveolar membrane
- Stimulate mast cell degranulation
Correct answer: Decrease alveolar surface tension to prevent collapse
Dipalmitoylphosphatidylcholine (DPPC) in surfactant reduces surface tension, keeping small alveoli from collapsing per Laplace's law.
Question 4: Which receptor type mediates the baroreceptor reflex increase in heart rate when blood pressure drops?
- α₁-adrenergic receptors in the SA node
- β₁-adrenergic receptors in the SA node (Correct answer)
- M₂ muscarinic receptors in the SA node
- Nicotinic receptors in the SA node
Correct answer: β₁-adrenergic receptors in the SA node
Sympathetic activation during hypotension releases norepinephrine that binds β₁ receptors on the SA node, increasing cAMP and heart rate.
Question 5: The oxygen-hemoglobin dissociation curve shifts to the RIGHT in which condition?
- Decreased body temperature
- Alkalosis (high pH)
- Elevated 2,3-bisphosphoglycerate (2,3-BPG) (Correct answer)
- Decreased partial pressure of CO₂
Correct answer: Elevated 2,3-bisphosphoglycerate (2,3-BPG)
Increased 2,3-BPG (produced in exercising muscle) stabilizes deoxyhemoglobin, reducing O₂ affinity and promoting O₂ delivery to tissues.
Question 6: Glomerular filtration rate (GFR) is best clinically estimated using:
- Serum albumin concentration
- Plasma creatinine concentration and equations like CKD-EPI (Correct answer)
- Urine specific gravity alone
- Blood urea nitrogen (BUN) alone
Correct answer: Plasma creatinine concentration and equations like CKD-EPI
Creatinine is freely filtered and minimally secreted, so serum creatinine-based equations (CKD-EPI, MDRD) provide the standard GFR estimate in clinical practice.
Question 7: Which neurotransmitter is released at the neuromuscular junction to initiate skeletal muscle contraction?
- Norepinephrine
- Dopamine
- Acetylcholine (Correct answer)
- Serotonin
Correct answer: Acetylcholine
Acetylcholine released from motor neurons binds nicotinic receptors on the motor end plate, triggering an end-plate potential that leads to muscle contraction.
PCAT Physiology 3
Which portion of the nephron is responsible for creating the medullary osmotic gradient via the countercurrent multiplier?