Oxygen content in arterial blood comes from two sources: oxygen bound to hemoglobin and dissolved oxygen in plasma. The amount bound to Hb is Hb × 1.34 × SaO2 (with SaO2 as a decimal), and the dissolved portion is PaO2 × 0.003. Add them to get CaO2. Compute the bound portion: 14 g/dL × 1.34 × 0.98 ≈ 18.38 mL/dL. Compute the dissolved portion: 95 mmHg × 0.003 ≈ 0.285 mL/dL. Sum: 18.38 + 0.285 ≈ 18.67 mL/dL, which rounds to about 18.69 mL/dL. So the arterial oxygen content is approximately 18.7 mL/dL.

In Venturi calculations, oxygen from the source mixes with room air to achieve a target FiO2. When the desired FiO2 is higher than about 35%, precise math becomes unwieldy due to entrainment dynamics, so a conventional substitution is used to keep calculations practical and consistent. The standard substitute for the portion of gas that comes from entrained air is 20%, reflecting the approximate oxygen content of the air-entraining mix in this high-FiO2 range. This makes the math workable and aligns with common teaching tables and rules of thumb. Using 20% as the substituted oxygen concentration helps simplify the calculation without greatly distorting the result. The other options don’t fit this convention: using 100% would ignore air entrainment, using 35% would simply mirror a mid-range FiO2 rather than a calculation shortcut, and using 21% would approximate room air but isn’t the standard substitute adopted for high FiO2 Venturi calculations.

The idea being tested is how expiratory time relates to the total cycle time and the inspiratory time. The total cycle time (TCT) is made up of two phases: inspiration (Ti) and expiration (Te). Therefore Te is what’s left after you subtract Ti from TCT, so Te = TCT − Ti. This captures the fact that the breathing cycle is partitioned into a inhalation portion and an exhalation portion that together fill the entire cycle. If you tried other forms, they wouldn’t match how the cycle is composed: Te would not equal Ti, division of TCT by Ti changes the meaning and units, and adding Ti to TCT would exceed the total cycle time. The correct expression is Te = TCT − Ti.

The A-a gradient measures how well oxygen moves from the alveoli into the arterial blood. It’s simply the difference between alveolar oxygen tension (PAO2) and arterial oxygen tension (PaO2). Here, PAO2 is 365 mmHg and PaO2 is 100 mmHg. Subtracting gives 365 − 100 = 265 mmHg. So, the A-a gradient is 265 mmHg. That’s a markedly elevated gradient (normal is roughly 5–15 mmHg in young adults, rising a bit with age), indicating impaired transfer of oxygen from the alveoli to the blood, which can reflect diffusion limitations, V/Q mismatch, or shunt, especially in the context of high PAO2.