Blood as a tissue
Blood is a liquid you can pour, and it is a tissue. Both of those are true, and it takes the definition of connective tissue to see how.
Blood is a specialised connective tissue
It has cells, it has fibres, and it has a matrix — so it fits the definition. What makes it special is that its matrix, the blood plasma, is not secreted by the blood cells. In every other connective tissue, the cells make the matrix they sit in.
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The characteristics
- Blood tissue consists of a fluid matrix called blood plasma and of formed elements.
- Red blood cells, white blood cells and platelets are suspended in the plasma.
- Fibres are not always present in blood tissue — they arise only when the blood clots.
The fibres appear only when they are needed
A connective tissue is supposed to have fibres in its matrix. Blood keeps its in reserve: the fibres form at the moment the blood clots, making a mesh across the wound that traps the cells and seals it. A tissue that had its fibres permanently in place could not flow.
Plasma is not the same as plasma membrane
The two words appear in the same chapter and mean nothing alike. Blood plasma is the liquid matrix of blood tissue. The plasma membrane is the boundary of a single cell, met in grade 10. Nothing connects them but the name.