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By the end of this lesson you should be able to explain and apply each part of this topic — Composition of Blood, Plasma and Tissue Fluid, Forces Across a Capillary Wall, The Role of the Lymphatic System and Oedema — When the Balance Fails — and use these ideas accurately in exam-style questions.
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.
The Starling balance is best understood by actually working out the net pressure at each end, because a single subtraction reveals the direction of fluid movement. Consider first the arterial end. The blood hydrostatic pressure pushing fluid out is about 4.6 kilopascals, and the oncotic pressure of the plasma proteins pulling fluid in is about 3.3 kilopascals. The net pressure is simply the outward force minus the inward force, which is 4.6 minus 3.3, giving a net outward pressure of about 1.3 kilopascals. Because the answer is positive and outward, fluid leaves the capillary here by ultrafiltration. That single positive number is the whole reason tissue fluid forms.
Now do the same subtraction at the venous end. The oncotic pressure has not changed, because the plasma proteins are still trapped inside the capillary, so it remains at about 3.3 kilopascals pulling fluid in. But the hydrostatic pressure has fallen substantially, to about 1.6 kilopascals, because the blood has lost pressure to friction as it travelled along the capillary. The net pressure is now the inward force minus the outward force, which is 3.3 minus 1.6, giving a net inward pressure of about 1.7 kilopascals. Because the balance has reversed, fluid now moves back into the capillary by osmosis. The reason most of the filtered fluid is recovered is precisely that the inward pressure at the venous end, at 1.7 kilopascals, is slightly larger in magnitude than the outward pressure at the arterial end, at 1.3 kilopascals.
This calculation also lets us predict disease quantitatively, which is exactly what examiners reward. Suppose severe malnutrition halves the plasma protein concentration, dropping the oncotic pressure from 3.3 to about 1.7 kilopascals. Redo both subtractions. At the arterial end the net outward pressure becomes 4.6 minus 1.7, which is 2.9 kilopascals, so far more fluid is now forced out than before. At the venous end the net inward pressure becomes only 1.7 minus 1.6, which is a mere 0.1 kilopascals, so almost no fluid is reabsorbed. The result is a large net loss of fluid into the tissues that the lymphatic system cannot possibly keep pace with, and the tissues swell with the fluid accumulation we call oedema. Being able to put numbers to the mechanism, rather than simply asserting that low protein causes swelling, is a genuine top-band discriminator.
It is worth adding that the lymphatic system normally provides a substantial safety margin. Under ordinary conditions the lymphatics carry away only the small surplus that the venous end fails to reclaim, but they can increase their flow several-fold before they are overwhelmed. This reserve is why a healthy person does not develop visible swelling after standing still for a while, or after a mildly salty meal that transiently raises capillary pressure: the extra filtered fluid is simply cleared by a temporary rise in lymph flow. Oedema becomes visible only when the Starling imbalance is large enough, or sustained for long enough, that it exceeds even this expanded lymphatic drainage capacity. Understanding that oedema reflects the point at which one safety mechanism is finally exhausted, rather than any single pressure crossing a fixed threshold, is the mark of a genuinely mechanistic answer.
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.
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