Physiology Flashcards

Which ion is primarily responsible for the depolarization phase of a cardiac action potential in ventricular myocytes?

Answer: Sodium (Na⁺)

Rapid Na⁺ influx through voltage-gated Na⁺ channels causes the sharp depolarization (phase 0) of ventricular action potentials.

The Frank-Starling mechanism states that cardiac output increases when:

Answer: Ventricular end-diastolic volume increases

Greater ventricular filling (preload) stretches sarcomeres, improving actin-myosin overlap and increasing stroke volume.

During exercise, which mechanism most directly increases venous return to the heart?

Answer: Skeletal muscle pump action

Rhythmic contraction of skeletal muscles compresses veins and, combined with venous valves, propels blood toward the heart.

Aldosterone primarily acts on which nephron segment to regulate sodium reabsorption?

Answer: Cortical collecting duct

Aldosterone binds mineralocorticoid receptors in principal cells of the cortical collecting duct, upregulating ENaC and Na⁺/K⁺-ATPase.

Which lung volume CANNOT be measured by spirometry alone?

Answer: Residual volume

Residual volume is the air remaining after maximal exhalation and requires body plethysmography or helium dilution because it cannot be exhaled into a spirometer.

The primary stimulus for erythropoietin (EPO) release from the kidneys is:

Answer: Hypoxia detected by peritubular cells

Low O₂ tension activates HIF-1α in peritubular interstitial cells, triggering EPO gene transcription to stimulate RBC production.

A patient has a plasma osmolality of 310 mOsm/kg. Which hormone is most likely elevated?

Answer: Antidiuretic hormone (ADH)

Hyperosmolality (>290 mOsm/kg) strongly stimulates hypothalamic osmoreceptors to trigger ADH release from the posterior pituitary.