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Spec Mapping — OCR H420 Module 3.1.2 — Transport in animals, content statements covering the formation of tissue fluid by ultrafiltration at the arterial end of the capillary, the reabsorption of tissue fluid by osmosis at the venous end, the role of plasma proteins in maintaining oncotic pressure, and the function of the lymphatic system in returning excess tissue fluid to the circulation (refer to the official OCR H420 specification document for exact wording). This lesson applies Starling's framework — one of physiology's most elegant balance arguments — at A-Level depth.
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.
The intellectual debt is to Ernest Starling (1866–1927), professor of physiology at University College London, who in his 1896 Croonian lecture and subsequent papers established the four-pressure framework that bears his name. Starling realised — paraphrasing his school of thought — that the same plasma protein that creates oncotic pressure inside the capillary also fails to escape it, providing the return-driving force that pulls fluid back in at the venous end. The result is a continual cycle of filtration and reabsorption, with the lymphatic system mopping up any net excess. Starling's framework remains the foundation of clinical fluid management to this day.
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.
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.
Two opposing forces determine whether fluid moves out of or into the capillary:
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.
About 10% of the tissue fluid is not reabsorbed at the venous end. This excess, together with any plasma proteins that have escaped into the tissues, would quickly build up and cause oedema (swelling) if it were not removed. The lymphatic system provides the drainage route:
In the small intestine, specialised lymph capillaries called lacteals absorb the products of lipid digestion, giving lymph a milky appearance in that region (called chyle).
| Location | Net hydrostatic vs oncotic pressure | Direction of net movement |
|---|---|---|
| Arterial end of capillary | HP > OP | Out (filtration) |
| Middle of capillary | HP ≈ OP | Little net movement |
| Venous end of capillary | HP < OP | In (reabsorption) |
| Excess fluid and protein | — | Removed by lymph capillaries |
If any factor disturbs the balance, tissue fluid accumulates and causes swelling (oedema). Typical causes include:
| Condition | Mechanism | Where the oedema appears |
|---|---|---|
| Right heart failure | High systemic venous pressure → raised HP_c at venous end | Dependent oedema in ankles and lower legs |
| Left heart failure | High pulmonary venous pressure → raised pulmonary capillary HP | Pulmonary oedema (alveolar flooding) |
| Kwashiorkor (protein malnutrition) | Low albumin synthesis → reduced OP_c | Generalised oedema; characteristic distended abdomen |
| Liver cirrhosis | Failure to synthesise plasma proteins; portal hypertension | Ascites (oedema in peritoneal cavity) |
| Nephrotic syndrome | Glomerular damage → albuminuria → low plasma albumin → low OP_c | Periorbital and generalised oedema |
| Filariasis (elephantiasis) | Wuchereria bancrofti lymphatic-vessel infection blocks drainage | Massive limb swelling (lower limb, scrotum) |
| Cellulitis / inflammation | Capillary leakiness; raised Kf and protein leak | Local hot, red, swollen tissue |
| Pre-eclampsia | Endothelial dysfunction in pregnancy | Generalised oedema with hypertension |
Each example reduces to a Starling-balance perturbation. The clinical art lies in identifying which limb of the balance is disturbed and intervening accordingly: diuretics for heart failure, albumin infusion for severe hypoalbuminaemia, compression bandaging for lymphoedema, antibiotics for cellulitis.
A useful intuition: the same plasma proteins create the oncotic pull all along the capillary. The hydrostatic pressure, by contrast, falls with distance because of frictional resistance. So the two forces have different distance dependencies. At the arterial end, HP is high and pushes water out; OP is steady and tries to pull water in but is overpowered. Along the capillary, HP falls. At some point — the "transition point" — HP equals OP, and there is no net flux. Past that point, the still-steady OP wins, and water re-enters by osmosis. The net result is a small outward surplus (~10% of filtered volume), which the lymphatics handle. This elegant balance, with the right ratio of HP to OP, is what makes the system self-regulating: small fluctuations in HP shift the transition point along the capillary without disturbing the overall budget.
Exam Tip: In exam answers, always use the phrases "water moves out by filtration" and "water moves in by osmosis due to the water potential gradient created by plasma proteins". These exact terms are rewarded by OCR mark schemes.
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