USA Biology Olympiad (USABO) Practice Exam

Why do patients with sickle cell anemia struggle with breathing at high altitudes?

1. Red blood cells become rigid at high altitudes

2. Soluble oxygen diffuses rapidly through membranes

3. Elongation of red blood cells reduces surface to volume ratios

4. Self-associated hemoglobin proteins have altered cooperative binding affinities

The correct answer is: Elongation of red blood cells reduces surface to volume ratios

Patients with sickle cell anemia experience significant challenges at high altitudes primarily due to the unique properties of their red blood cells and how these interact with the reduced oxygen availability at increased elevations. In sickle cell anemia, the hemoglobin within red blood cells can polymerize under low oxygen conditions, causing cells to take on a rigid and elongated shape. This deformation impacts their function in several ways. Typically, red blood cells have a biconcave, disc-like shape that optimizes their flexibility and surface area for gas exchange and movement through small capillaries. When red blood cells become elongated, their surface area to volume ratio is negatively affected, limiting the efficiency of oxygen transport. At high altitudes, where oxygen levels are lower, the sickle-shaped cells are more likely to become rigid and create blockages in microcirculation, which can lead to reduced oxygen delivery to tissues. Consequently, this decrease in oxygen supply exacerbates breathing difficulties and can contribute to a range of symptoms associated with hypoxia, such as fatigue and shortness of breath. Thus, the elongated structure of the red blood cells significantly hinders their ability to adequately supply oxygen, which is critical for respiration and overall metabolic processes.