**Question:** During a routine checkup of a middle-aged man's blood sample shows hematocrit of 45% and biconcave red blood cells. Biconcavity of RBCs is a function of which of the following?

A. Hemoglobin concentration
B. Hematocrit
C. Blood volume
D. Body temperature

**Correct Answer:**

**Core Concept:** Biconcave shape of red blood cells (RBCs) is a characteristic feature that contributes to their efficient oxygen transport and release.

**Why the Correct Answer is Right:** Biconcave shape of RBCs is a function of their **Hematocrit (B)**. Hematocrit is the ratio of red blood cells (RBCs) to the total blood volume. As the hematocrit increases, the RBCs become more biconcave due to increased plasma volume and reduced RBC volume. This enhances their surface area to volume ratio, allowing for better oxygen exchange.

**Why Each Wrong Option is Incorrect:**

**A.** Hemoglobin concentration: Although hemoglobin concentration is important for oxygen carrying capacity, it is not directly related to the biconcave shape of RBCs.

**C.** Blood volume: While blood volume affects hematocrit, the biconcave shape of RBCs is primarily determined by hematocrit.

**D.** Body temperature: Although body temperature influences RBC biophysical properties, it is not the primary determinant of their biconcave shape. Hematocrit plays a more significant role in shaping RBCs.

**Clinical Pearl:** A high hematocrit (45% in the given scenario) indicates polycythemia, which could be due to various causes like high altitude, polycythemia vera, or excessive erythropoietin production. In such cases, biconcave RBCs are beneficial for efficient oxygen transport, but inappropriately high hematocrit values can lead to complications like increased blood viscosity, reduced cardiac output, and increased risk of stroke.

Medical students should be familiar with the relationship between hematocrit, RBC shape, and oxygen transport, as well as the potential complications of high hematocrit values.