Mature human red blood cells - 'erythrocytes' are small,round and biconcave ...option D

The correct answer is option 'D', which is erythrocyte. Erythrocytes, commonly known as red blood cells, are the only blood cells that have a distinct biconcave disc shape.

Explanation:
Erythrocytes are one of the major components of blood and are responsible for carrying oxygen to tissues and removing carbon dioxide from the body. They are specialized cells that have unique features that enable them to efficiently perform their functions.

Biconcave Disc Shape:
Erythrocytes are described as biconcave discs because of their unique shape. The cells have a flattened circular disc shape with a concave depression on both sides, resembling a donut or a biconcave lens. This shape provides them with several advantages, including a large surface area for gas exchange and flexibility to deform and squeeze through narrow blood vessels.

Key Features of Erythrocytes:
1. Hemoglobin: Erythrocytes contain a protein called hemoglobin, which binds to oxygen in the lungs and carries it to various tissues and organs. Hemoglobin also helps in the transport of carbon dioxide back to the lungs for elimination.

2. Lack of Nucleus: Unlike most other cells in the body, erythrocytes do not have a nucleus. This absence of a nucleus allows more space for hemoglobin, increasing the oxygen-carrying capacity of these cells.

3. Lifespan: Erythrocytes have a limited lifespan of about 120 days. After this period, they are removed from circulation and broken down in the spleen and liver. The continuous production of new erythrocytes occurs in the bone marrow to maintain a steady supply in the bloodstream.

4. Red Blood Cell Count: The number of erythrocytes in the blood is measured as the red blood cell count. This count is an essential component of a complete blood count (CBC) test and is used to evaluate conditions such as anemia or polycythemia.

In conclusion, erythrocytes, or red blood cells, are correctly described as biconcave discs. Their unique shape and features allow them to efficiently carry out their vital role in oxygen transport and carbon dioxide removal throughout the body.