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B3.2.12 (HL)—Exchange between tissue fluid and cells

Tissue fluid bathes cells and mediates diffusion of oxygen, nutrients, ions, carbon dioxide and wastes between blood and body tissues.

Syllabus
First assessment 2025
Objective
B3.2.12
Level
HL

Cells Exchange with Tissue Fluid by Short Diffusion Paths

HL only

Tissue fluid is the extracellular liquid that bathes body cells and provides the immediate route for exchange between blood plasma and cell membranes.

It forms from filtered plasma, so it contains water and small solutes such as oxygen, glucose, amino acids and ions, but normally lacks blood cells and contains far fewer large plasma proteins.

Oxygen and nutrients diffuse from tissue fluid into cells, while carbon dioxide and other metabolic wastes diffuse out. After passing cells, tissue fluid tends to contain less oxygen and more carbon dioxide than the plasma that initially supplied it.

A respiring muscle cell lowers the local oxygen concentration and raises carbon dioxide concentration, maintaining opposite diffusion gradients between the cell and surrounding tissue fluid.

Tissue fluid is not whole blood or plasma unchanged: cells and most large proteins remain in capillaries, and metabolism alters the solute composition as fluid contacts tissues.

Pressure, Heart, Lymph, And Phloem

HL only

HL transport adds pressure and route systems. Tissue fluid forms by capillary pressure and returns by osmotic pull or lymph. Double circulation separates pulmonary and systemic routes. The heart creates directional pressure with chambers, septum, valves, and cycle timing. Plants add root pressure and phloem pressure-flow translocation.

  • Tissue fluid: hydrostatic pressure out, osmotic pull back, lymph drains excess.
  • Heart: double circulation, one-way valves, thick left ventricle, cardiac cycle sequence.
  • Plant HL: root pressure by active ion loading and osmosis; phloem by source-to-sink pressure flow.

Concept essentials

  • Tissue fluid is the exchange medium between capillaries and body cells.
  • Oxygen and nutrients diffuse from blood to tissue fluid and then into cells.
  • Carbon dioxide and wastes diffuse from cells into tissue fluid.
  • Tissue fluid contains fewer large proteins than blood plasma.