Blood is not a simple liquid. If you cut your finger, what comes out is a thick, slightly sticky fluid that eventually turns into a solid clot. That alone tells you it contains both a liquid part and solid parts that can change state. The intuition is simple: blood is a suspension — a liquid with things floating in it.
The precise biological statement is that blood is a fluid connective tissue composed of two main fractions: plasma (the liquid matrix) and formed elements (the cells and cell fragments suspended in it).
The Two Fractions
Plasma is the straw-coloured liquid that makes up about 55% of total blood volume. It is mostly water (about 92%), but the remaining 8% is a cocktail of proteins (albumin, globulins, fibrinogen), nutrients, gases, hormones, and waste products. Plasma is the transport medium — it carries everything from glucose to carbon dioxide.
Formed elements make up the remaining 45%. These are not all whole cells; some are fragments. They fall into three categories:
| Formed Element | Main Function | Key Fact |
|---|
| Red Blood Cells (Erythrocytes) | Oxygen transport | No nucleus in mammals; biconcave disc shape increases surface area |
| White Blood Cells (Leukocytes) | Immunity | Nucleated; five types (neutrophils, lymphocytes, etc.) |
| Platelets (Thrombocytes) | Blood clotting | Cell fragments, not whole cells; no nucleus |
The 55% plasma / 45% formed elements ratio is called the haematocrit. If you spin blood in a centrifuge, the heavier cells pack at the bottom and the plasma stays on top — a perfect visual of the composition.
Why "Connective Tissue"?
This is the part that surprises most students. Connective tissues typically have cells scattered in an extracellular matrix (e.g., bone has osteocytes in a mineralised matrix). Blood follows the same pattern: the formed elements are the cells, and the plasma is the extracellular matrix. The difference is that the matrix is liquid, so blood flows. That makes it a fluid connective tissue.
Blood is classified as connective tissue because it has a common embryonic origin (mesoderm) with other connective tissues, and it fits the structural definition: cells + extracellular matrix.
The Big Picture for Exams …