Blood cells, plasma and tissue fluid
Blood, plasma and tissue fluid
- Blood is mostly water carrying cells and dissolved substances.
- The liquid that leaks out to bathe your cells is tissue fluid.
- It is how every cell gets its oxygen and food.
Blood cells
- Red blood cells carry oxygen (packed with haemoglobin, no nucleus).
- White blood cells defend the body — you should recognise:
- monocytes, neutrophils (lobed nucleus), and lymphocytes.
Practice
The main job of a red blood cell is to:
Red blood cells are packed with haemoglobin to carry oxygen; white cells handle defence.
Practice
Which of these is a white blood cell?
Lymphocytes, neutrophils and monocytes are white blood cells (they have a nucleus and are larger).
Why water is ideal
- Water is a good solvent, so it carries dissolved substances (glucose, ions, gases).
- Water has a high specific heat capacity, so the blood's temperature stays steady.
Tissue fluid
- At the start of a capillary, the high blood pressure pushes liquid out of the plasma — but not the cells or large proteins.
- This liquid around the cells is tissue fluid: it supplies oxygen and glucose and carries waste away.
- Most of it returns to the capillary at the far (venous) end, where the pressure is lower.
Practice
Tissue fluid forms because:
High hydrostatic pressure forces liquid out of the plasma; cells and large proteins are too big to leave.
Practice
Most tissue fluid returns to the capillary:
By the venous end the pressure has dropped, so most of the fluid is drawn back into the capillary.
You've got it
Key idea
- red blood cells carry oxygen; white cells (monocytes, neutrophils, lymphocytes) defend
- water = good solvent + high specific heat capacity (steady temperature)
- tissue fluid = plasma pushed out by high pressure (no cells/large proteins) to bathe cells
- most tissue fluid returns at the low-pressure (venous) end