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Blood is a liquid tissue that contains cells suspended in a fluid. The four main components of blood are:
Blood consists of plasma, which is the liquid component, carrying various types of cells and cell fragments. Most of the cells carried in plasma are red blood cells, with a much smaller number of white blood cells. Platelets are small cell fragments formed from special cells in the bone marrow.
Red blood cells possess several unique structural features that make them highly efficient at transporting oxygen:
Absence of nucleus: Unlike most cells, red blood cells do not possess a nucleus or mitochondria. The lack of a nucleus means there is more space for packing in millions of molecules of haemoglobin.
Biconcave disc shape: Red blood cells are shaped like flat discs that have been pinched in on both sides. This shape, together with their small size, gives them a relatively large surface area compared with their volume. This high surface area to volume ratio speeds up the rate at which oxygen can diffuse in and out of the red blood cell.
Small size: The small size of red blood cells enables them to squeeze through even the tiniest capillaries, meaning oxygen can be taken very close to every cell in the body.
Red blood cells are red because they contain the pigment haemoglobin. This is a protein that contains iron, and it readily combines with oxygen.
Key Term: Haemoglobin is a red pigment found in red blood cells, which can combine reversibly with oxygen; it is a protein.
Where oxygen concentration is high (at the alveoli in the lungs):
Where oxygen concentration is low (at respiring cells):
The primary function of red blood cells is transporting oxygen from the lungs to all body tissues. The iron in haemoglobin combines with oxygen where the oxygen concentration is high (lungs), and releases oxygen where the oxygen supply is low (respiring tissues).
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