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Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.
Key concept
At capillaries, blood pressure forces plasma out through capillary walls to form tissue fluid that bathes cells. Tissue fluid allows exchange of substances between blood and cells. Excess tissue fluid drains into lymph vessels, becoming lymph, and returns to the blood near the heart.
Tissue fluid formation is a Supplement topic that connects capillary structure to substance exchange. Examiners want you to explain why plasma leaks out (pressure), what stays behind (blood cells and large proteins) and how excess fluid is returned.
How tissue fluid forms
At the arterial end of a capillary bed, blood pressure forces small molecules out of the blood through the thin capillary walls. Water, glucose, amino acids, mineral ions and oxygen leave the blood to form tissue fluid. Blood cells and large plasma proteins such as albumin are too large to pass through and remain in the blood.
Tissue fluid surrounds and bathes every cell. Substances diffuse between tissue fluid and cells: oxygen and glucose move into cells for respiration; carbon dioxide and other wastes move out into tissue fluid and then back into the capillary.
Return of tissue fluid to the blood
Most tissue fluid re-enters the capillaries at the venous end, where blood pressure is lower and the osmotic effect of plasma proteins draws water back in. However, some tissue fluid does not return directly. This excess drains into blind-ended lymph capillaries and becomes lymph.
Lymph travels through lymph vessels and passes through lymph nodes, where lymphocytes filter the fluid and can mount immune responses against any pathogens present. Eventually, lymph drains back into the blood via the subclavian veins near the heart.
Comparing blood, tissue fluid and lymph
Feature
Blood
Tissue fluid
Lymph
Red blood cells
Yes
No
No
Large plasma proteins
Yes
No
Very few
Glucose and amino acids
Yes
Yes
Yes (lower levels)
Where found
Inside blood vessels
Between cells
Inside lymph vessels
Try a focused question (4 marks)
Explain how tissue fluid is formed and how excess tissue fluid is returned to the blood.
Show the answer and marking guidance
Blood pressure at the arterial end of capillaries forces plasma through the capillary walls.
Small molecules leave but blood cells and large proteins remain, forming tissue fluid.
Excess tissue fluid that does not re-enter capillaries drains into lymph capillaries.
Lymph travels through lymph vessels and nodes and is returned to the blood near the heart.
Original Cambridge-style practice written for this site. It is not an official Cambridge past-paper question.
Common mix-ups to avoid
Check these points against your own answer before moving on.
Saying tissue fluid contains red blood cells: only plasma and dissolved substances leave the capillary.
Forgetting that large proteins stay in the blood because they cannot pass through capillary walls.
Confusing lymph with blood: lymph has no red blood cells.
Not explaining the pressure difference that drives fluid out of capillaries.