Tissue Fluid Formation

Capillaries are the exchange vessels of the circulation, but the actual transfer of substances between blood and tissue cells occurs via an intermediate fluid — tissue fluid — that bathes the cells. Tissue fluid forms at the arterial end of capillaries through ultrafiltration and is partly reabsorbed at the venous end by osmosis, with any surplus collected by the lymphatic system. This lesson explains the competing roles of hydrostatic pressure and oncotic (colloid osmotic) pressure along a capillary bed and summarises the return pathway via lymph, matching OCR A-Level Biology A specification 3.1.2 (d).

Key Definitions:

• Tissue fluid — the watery fluid that surrounds cells, derived from blood plasma by ultrafiltration and providing the immediate environment for cellular exchange.
• Hydrostatic pressure — the physical pressure a fluid exerts against the walls of its container (here, capillary blood against vessel wall).
• Oncotic pressure (colloid osmotic pressure) — the osmotic pressure due to plasma proteins, which tends to draw water into the capillary.
• Lymph — excess tissue fluid that has drained into lymphatic vessels; slightly different in composition because it may contain fat droplets from the intestine.

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Composition of Blood, Plasma and Tissue Fluid

Before considering the dynamics, it is important to understand what each fluid contains:

Component	Plasma	Tissue fluid	Lymph
Water	Yes	Yes	Yes
Ions (Na⁺, Cl⁻, etc.)	Yes	Yes	Yes
Glucose, amino acids, O₂, CO₂	Yes	Yes	Yes
Plasma proteins (albumin, globulins, fibrinogen)	Yes	Little/none	Some
Red blood cells	Yes	No	No
White blood cells	Yes	Few	Yes (particularly lymphocytes)
Platelets	Yes	No	No

The critical point is that plasma proteins are too large to pass through the capillary wall under normal circumstances; they remain in the blood. Tissue fluid is therefore similar to plasma but depleted in protein.

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Forces Across a Capillary Wall

Two opposing forces determine whether fluid moves out of or into the capillary:

1. Blood hydrostatic pressure (HP) — generated by the heart pumping and by resistance in the downstream vessels. Pushes fluid out of the capillary through gaps in the endothelium.
2. Blood oncotic pressure (OP) — produced by plasma proteins (chiefly albumin). Because proteins remain in the capillary, they create a lower water potential inside than outside. Water therefore tends to move into the capillary by osmosis.

There are also small contributions from tissue fluid hydrostatic pressure and tissue fluid oncotic pressure, but these are usually minor in simple A-Level treatments.

flowchart LR
    subgraph Capillary
    A[Arterial end] --> V[Venous end]
    end
    A -- HP about 4.7 kPa > OP about 3.3 kPa --> OUT[Net OUT: water and small solutes leave]
    V -- HP about 1.7 kPa < OP about 3.3 kPa --> IN[Net IN: water returns by osmosis]

At the Arterial End

• Blood hydrostatic pressure is high (~4.6 kPa / 35 mmHg).
• Oncotic pressure is around 3.3 kPa / 25 mmHg.
• HP > OP, so the net force pushes fluid out of the capillary into the tissues.
• Water, ions, glucose, amino acids, O₂ and small molecules leave the capillary by ultrafiltration through the fenestrations between endothelial cells.
• Plasma proteins and red blood cells are too large and remain in the blood.

Along the Capillary

• Blood pressure falls progressively due to resistance.
• Oncotic pressure stays roughly the same (plasma proteins are not lost).
• At some point along the capillary, HP falls to equal OP; beyond this, the balance reverses.

At the Venous End