On ascent to high altitude, there is a marked drop in Hb-O2 saturation, this drop can be explained by: (Select all that are correct. There may be one or more correct options)Group of answer choicesFall in temperature in the atmosphere creates a right ward shift of the Hb-O2 curveDecrease barometric air pressure means the air is less dense and thus the number of O2 molecules inspired with each breath is lessDecrease barometric air pressure decrease O2 partial pressure in the surrounding airPercentage composition of oxygen decreases with altitude, thus less O2 is inspired with each breath

The correct options that explain the marked drop in Hb-O2 saturation on ascent to high altitude are:

1. Decrease barometric air pressure means the air is less dense and thus the number of O2 molecules inspired with each breath is less: As we ascend to higher altitudes, the barometric pressure decreases. This means that the air is less dense, and therefore, the number of oxygen molecules that we inhale with each breath is reduced. This leads to a decrease in the amount of oxygen that can bind to hemoglobin, resulting in a drop in Hb-O2 saturation.

2. Decrease barometric air pressure decrease O2 partial pressure in the surrounding air: The partial pressure of a gas is the pressure that it would exert if it occupied the total volume of the mixture at the same temperature. At high altitudes, the decrease in barometric pressure leads to a decrease in the partial pressure of oxygen in the air. This means that there is less pressure to drive oxygen into the lungs and into the blood, leading to a decrease in Hb-O2 saturation.

The other options are incorrect:

• Fall in temperature in the atmosphere creates a right ward shift of the Hb-O2 curve: This statement is incorrect. A rightward shift in the Hb-O2 curve (also known as the oxygen dissociation curve) indicates that hemoglobin has a decreased affinity for oxygen, meaning it is more likely to release oxygen to the tissues. This is usually caused by factors such as an increase in temperature, not a decrease.

• Percentage composition of oxygen decreases with altitude, thus less O2 is inspired with each breath: This statement is also incorrect. The percentage composition of oxygen in the air remains approximately constant at different altitudes (about 21%). What changes is the density of the air and the partial pressure of the oxygen, not its percentage composition.