Red Blood Cells — Built for Oxygen Transport

Challenges to clear

• Match every feature to its meaning.
• Connect haemoglobin joined to carbon monoxide to why that compound is so dangerous.

The task

The red blood cell has thrown away parts most cells cannot live without. Match each feature to what it means.

What this lesson covers

• Diameter of a red blood cell — About 7 micron across (One micron is a thousandth of a millimetre. At this size the cells can pass through the finest capillaries in single file.)
• Shape of the mature cell — A biconcave disc, thin in the centre and thick at the rim (The two concavities give a large surface area for the volume, so oxygen is absorbed and released faster than a sphere would allow.)
• Average lifespan — About 120 days (Roughly 2 million are destroyed every second — about 1 per cent of all the erythrocytes in the body each day — so the marrow must replace them constantly.)
• Count in an adult male — About 5 million per cubic millimetre (An adult female has slightly fewer, about 4.5 million, and a newborn has 6 to 7 million. Learn the male figure as your anchor.)
• Organelle lost so the cell becomes biconcave — The nucleus (Losing the nucleus flattens the centre. It raises the surface area to volume ratio and lets more cells pack into the same space.)
• Organelle lost so none of the cargo is consumed — The mitochondria (Cellular respiration happens in mitochondria. Without them the cell cannot burn the oxygen it is carrying, so all of it reaches the tissues unused — and the plasma glucose is spared too.)
• Organelle lost so the cell stays flexible — The endoplasmic reticulum (An empty, pliable cell can bend and squeeze through capillaries only about 8 micrometres wide.)
• Haemoglobin joined to oxygen — Oxyhaemoglobin, an unstable compound (It must be unstable — it has to break apart readily at the tissues to hand the oxygen over.)
• Haemoglobin joined to carbon monoxide — Carboxyhaemoglobin, a stable compound (Being stable is the danger. It locks up the haemoglobin permanently, cutting the oxygen-carrying capacity of the blood, and can kill.)
• Where worn-out red cells are broken down — The spleen, liver and bone marrow (The iron is kept back in the liver and the rest leaves as the bile pigment bilirubin — nothing useful is wasted.)

Hint

The memory line is deficient, but more efficient. Three organelles are missing — nucleus, mitochondria, endoplasmic reticulum — and each loss buys a different advantage: shape, unconsumed cargo, flexibility. If you are asked why mammalian red cells are efficient, give all three reasons, not just the nucleus.