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Red blood cell facts for kids

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Red blood cell
Scanning electron micrograph of human red blood cells (ca. 6–8 μm in diameter)

Red blood cells (also known as RBCs, red blood corpuscles or erythrocytes) are cells in the blood which transport oxygen. In women, there are about 4.8 million red blood cells per microliter of blood. In men, there are 5.4 million red blood cells per microliter of blood. Red blood cells are red because they have hemoglobin in them.


The most important function of red blood cells is the transport of oxygen (O2) to the tissues. The hemoglobin absorbs oxygen in the lungs. Then it travels through blood vessels and brings oxygen to all other cells via the heart. The blood cells go through the lungs (to collect oxygen), through the heart (to give all cells oxygen). They go back to the heart to be re-pumped to the lungs (to again collect oxygen), so the blood in your body travels in a double circuit, going through your heart twice before it completes one full circulation of the body.

A fact which makes mammalian red blood cells different to all other cells is that, when they are mature, red blood cells do not have a nucleus. All other vertebrates have red cells with nuclei.

Red blood cells are doughnut-shaped, but without the hole. This shape is called a bi-concave disc. However, hereditary diseases such as sickle-cell disease can cause them to change shapes and stop blood flow in capillaries and veins. Plasma is got from whole blood. To prevent clotting, an anticoagulant (such as citrate) is added to the blood immediately after it is taken.

Transport of CO2 in the blood

Carbon dioxide (CO2) is carried in blood in three different ways. The exact percentages vary depending whether it is arterial or venous blood.

  • Most of it (about 68% to 83%) is converted to bicarbonate ions HCO
    by the enzyme carbonic anhydrase in the red blood cells. by the reaction CO2 + H2O → H2CO3 → H+ + HCO
  • 5% – 10% is dissolved in the blood plasma.
  • 5% – 10% is bound to haemoglobin as carbamino compounds.

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