1. Which hormone is primarily responsible for the regulation of plasma calcium concentration? A. Calcitonin B. Parathyroid hormone C. Thyroid-stimulating hormone D. Cortisol Answer: B. Parathyroid hormone Rationale: Parathyroid hormone is the principal regulator of plasma calcium concentration. It increases blood calcium levels by stimulating osteoclasts to break down bone matrix and release calcium. 2. During the cardiac cycle, the "lub" sound of the heart corresponds to which event? A. Closure of the semilunar valves B. Closure of the atrioventricular valves C. Opening of the atrioventricular valves D. Opening of the semilunar valves Answer: B. Closure of the atrio