What is the primary fate of damaged or senescent erythrocytes (red blood cells) in the human body?

• A. They are filtered intact by the kidneys and excreted in the urine as whole cells under normal conditions.
• B. They are phagocytosed mainly by macrophages in the spleen and liver, and their hemoglobin is broken down for iron recycling and bilirubin production.
• C. They are converted directly into platelets within the bone marrow to support clotting.
• D. They divide by mitosis in the bloodstream to replace damaged components and continue functioning.

Answer: (B)

Verified step by step guidance

1. Understand that erythrocytes (red blood cells) have a limited lifespan, typically around 120 days, after which they become senescent or damaged and need to be removed from circulation.
2. Recognize that the kidneys do not filter intact erythrocytes under normal conditions because red blood cells are too large to pass through the glomerular filtration barrier; thus, they are not excreted in urine as whole cells.
3. Identify the primary organs responsible for removing damaged erythrocytes: the spleen and liver, where specialized immune cells called macrophages phagocytose (engulf and digest) these cells.
4. Know that during phagocytosis, hemoglobin from erythrocytes is broken down into its components: iron is recycled for new red blood cell production, and the heme portion is converted into bilirubin, which is processed by the liver and eventually excreted.
5. Understand that erythrocytes do not divide or transform into other cell types like platelets; their removal and recycling is a critical part of maintaining healthy blood composition.