K325 / 2027

Lesson 3 of 9 / Transport in humans

Exchange between blood and body cells

How does a substance in the blood actually reach a cell?

I can explain exchange between capillaries, tissue fluid and cells.

  • I can explain capillary and tissue-fluid exchange.Syllabus

    Syllabus K325, Topic 5(c). Explain capillary and tissue-fluid exchange.

About 5 min

Make a guess. You are not marked.

Red blood cells carry oxygen to a muscle cell. Do the red blood cells leave the capillary to drop it off?

  1. They squeeze out and touch the muscle cell.
  2. They burst and release oxygen into the tissue.
  3. Oxygen diffuses out; the red cells stay inside.
Show the answer

Oxygen diffuses out; the red cells stay inside.

Oxygen leaves haemoglobin and diffuses through the capillary wall and tissue fluid to the cell.

The key idea

Tissue fluid surrounds cells and forms the link between their membranes and nearby capillaries.

Net movement of oxygen from capillary to cell through tissue fluidCapillaryTissue fluidCelloxygen: arrow shows net diffusion
oxygen moves down its concentration gradient from blood, through tissue fluid, into a cell that uses it.

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. 1

    Name the material

    Oxygen and carbon dioxide do not necessarily move in the same direction.

  2. 2

    Include the tissue fluid

    Trace blood -> tissue fluid -> cell, or the reverse for a waste.

  3. 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.

Watch out for this

A student says: "Red blood cells leave the capillaries to deliver oxygen." What is wrong with this?

Show the answer

Red blood cells normally stay inside capillaries. Oxygen leaves haemoglobin and diffuses through tissue fluid to the cells.

Check your understanding

Which change forms tissue fluid around a typical body capillary?

  1. Red cells actively pump plasma proteins into every body cell.
  2. Red cells and plasma proteins squeeze through gaps in the wall along with water and glucose.
  3. Blood pressure pushes some water and small solutes out while red cells remain inside.
Show the answer

Blood pressure pushes some water and small solutes out while red cells remain inside.

Pressure-driven filtration produces tissue fluid; dissolved materials can then exchange with cells.

Quick recall

Card 1 of 3. Answer in your head, then check.

How does a substance in the blood actually reach a cell?

You can return to this lesson any time.