Lesson 3 of 9 / Transport in humans
Exchange between blood and body cells
How does a substance in the blood actually reach a cell?
In this lesson: Explain exchange between capillaries, tissue fluid and cells.
About 5 min
The key ideaTissue fluid surrounds cells and forms the link between their membranes and nearby capillaries.
For urea, the source cell represents a liver cell. Red cells and most large plasma proteins (purple circles) remain in blood. Formation of tissue fluid by pressure is a different process from the diffusion arrow shown.
Explanation
Most body cells do not touch the blood directly. They are surrounded by tissue fluid. Near the arterial end of capillaries, blood pressure pushes water and small dissolved substances through the capillary wall, forming this fluid.
Red blood cells and most large plasma proteins remain in the capillary. Tissue fluid therefore resembles plasma but normally contains far fewer large proteins and no red blood cells. White blood cells can leave vessels during defence.
Oxygen and dissolved nutrients move from blood into tissue fluid and then into cells down their relevant concentration gradients. Respiration uses oxygen and produces carbon dioxide, helping maintain these differences.
Carbon dioxide and other metabolic wastes move from cells through tissue fluid into blood, which carries them away. Fluid is returned to circulation, including through lymph vessels. This prevents continuous accumulation around the cells.
Distinguish formation from exchange: pressure drives filtration of fluid; a concentration gradient drives diffusion of a particular dissolved substance. An arrow showing net diffusion does not mean every molecule moves in only one direction.
Step by step
- 1
Name the material
Oxygen and carbon dioxide do not necessarily move in the same direction.
- 2
Include the tissue fluid
Trace blood -> tissue fluid -> cell, or the reverse for a waste.
- 3
Use the gradient
Explain why net movement is down the concentration gradient of that substance.
Worked example
A working muscle
Why does carbon dioxide move from an actively respiring muscle cell towards a capillary?
One way to explain it
Respiration produces carbon dioxide in the muscle cell, raising its concentration relative to the surrounding fluid and blood. Carbon dioxide diffuses out of the cell, through tissue fluid and into the capillary down its concentration gradient.
Why this answer works
- Start with the cellular source of carbon dioxide.
- Name tissue fluid as the intermediate.
- Blood flow carries carbon dioxide away, helping maintain the gradient.
Is this true? "Red blood cells leave the capillaries to deliver oxygen."
Red blood cells normally stay inside capillaries. Oxygen leaves haemoglobin and diffuses through tissue fluid to the cells.