Learning focus
Build precise biological explanations, interpret diagrams and data, and connect structure, process, health and practical evidence.
Formation of tissue fluid
At the arterial end of a capillary, blood pressure forces water and small dissolved substances through the thin wall. Blood cells and most plasma proteins remain in the vessel. The fluid around cells is called tissue fluid.
Supply to cells
Oxygen and glucose diffuse from capillary blood into tissue fluid and then into cells down concentration gradients. Ions and other nutrients also move according to gradients and membrane transport mechanisms.
Removal from cells
Carbon dioxide and other metabolic wastes diffuse from cells into tissue fluid and then into capillary blood. Continuous blood flow maintains concentration gradients.
Return of fluid
Much tissue fluid returns to capillaries, particularly toward the venous end. Some enters lymph vessels and is later returned to the blood. Detailed lymphatic anatomy is not required, but failure of fluid return can cause swelling.
Structure supports exchange
Capillary walls one cell thick, slow blood flow, narrow lumens and extensive networks create a large surface area and short diffusion distance.
Practical or data skill
Use a model with membrane tubing to distinguish substances small enough to pass from larger components retained inside. Explain the limitations of the model compared with a living capillary.
Examination tip
Tissue fluid is not the same as blood plasma because it normally contains no red cells and far less plasma protein.
Review questions and suggested answers
Question 1
What is tissue fluid?
Suggested answer
Fluid surrounding body cells, formed from blood plasma through capillary walls.
Question 2
Why do red cells remain in capillaries?
Suggested answer
They are too large to pass through the capillary wall.
Question 3
How does oxygen reach a body cell?
Suggested answer
It diffuses from blood into tissue fluid and then into the cell.
