Blood is cells carried in plasma, and plasma is mainly water. Recognise red blood cells, neutrophils, monocytes and lymphocytes, explain why water suits transport, and see how tissue fluid forms and returns in a capillary bed.
Blood contains red blood cells (biconcave, no nucleus, full of haemoglobin) and white cells: neutrophils (lobed nucleus, granular cytoplasm, phagocytes), monocytes (the largest, with a kidney-shaped nucleus, becoming macrophages) and lymphocytes (small, with a large round nucleus filling most of the cell). Water is the main component of blood plasma and of tissue fluid. Its solvent action lets it carry glucose, ions, amino acids, urea and hormones in solution, and its high specific heat capacity lets blood carry heat without large temperature changes. Tissue fluid bathes the cells, supplying them with oxygen, glucose and ions and removing wastes such as carbon dioxide. It forms at the arterial end of a capillary, where hydrostatic pressure forces fluid and small solutes out; plasma proteins stay in, so at the venous end, where hydrostatic pressure has fallen, water returns by osmosis. Excess fluid drains into lymph vessels.
Click each cell in turn: the red blood cell, the neutrophil, the monocyte and the lymphocyte. Look at the shape of each nucleus, because that is how you tell white cells apart on a slide.
| Cell | Size and shape | Nucleus | Role |
|---|---|---|---|
| red blood cell | biconcave disc, about 7 μm across | none | carries oxygen bound to haemoglobin |
| neutrophil | larger than a red cell; granular cytoplasm | lobed (often 2 to 5 lobes) | phagocyte: engulfs and digests bacteria |
| monocyte | the largest white cell | kidney or bean shaped | leaves the blood and becomes a macrophage |
| lymphocyte | small, little cytoplasm | large, round, fills most of the cell | specific immune response (B and T cells) |
When you draw blood cells, draw the outline of each cell and its nucleus with clear lines, keep the relative sizes right (a red cell is smaller than every white cell) and do not shade.
Water and transport. Water is the main component of blood and tissue fluid. It is a good solvent because it is polar, so ions and polar molecules such as glucose, amino acids, urea and hormones dissolve in plasma and tissue fluid and are carried in solution. Its high specific heat capacity means a lot of energy is needed to change its temperature, so blood can carry heat from active organs such as muscles and the liver to the rest of the body while its own temperature stays fairly stable.
A lymphocyte's nucleus nearly fills the cell, leaving a thin rim of cytoplasm; a monocyte is much bigger with a bean-shaped nucleus. Do not call any cell with a large nucleus a monocyte.
Watch the blue water particles leave the capillary near the arterial end and return near the venous end. Then drag the red line or move the blood pressure slider and see where the two pressure lines cross.
If blood pressure is too high, more fluid leaves than can return and tissue fluid builds up: oedema.
Tissue fluid is not the same as plasma. It has almost no plasma proteins and no red blood cells, because these are too large to leave the capillary.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
In the blood cells tab, which cell has a lobed nucleus and granular cytoplasm?
According to the lab, what does a monocyte become after it leaves the blood?
In the tissue fluid model at the default blood pressure, where does water leave the capillary?
What does the lab show happening when the blood pressure slider is raised above about 1.3?
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Which property of water makes blood plasma suitable for carrying glucose, ions and urea?
Why does water return to the capillary at the venous end of a capillary bed?
Which component is found in blood plasma but is almost absent from tissue fluid?
A blood smear shows a cell about twice the diameter of a red blood cell with a large, kidney-shaped nucleus. What is it?
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