Body Fluid Compartments

Part 1 · Foundations · Chapter 4 of 47

Learning objectives

By the end of this page you should be able to:

• State the size of each body fluid compartment and how it varies with age, sex and body fat.
• Compare the ionic composition of intracellular and extracellular fluid.
• Explain how compartment volumes are measured by indicator dilution.
• Describe daily water balance.
• Explain fluid exchange at the capillary using both the traditional and the revised Starling principle.
• Classify the causes of oedema and match each to a physiotherapy approach.
• Apply fluid physiology to exercise, dehydration and training.

Why it matters

A swollen ankle after a sprain, the puffy arm after breast cancer surgery, the patient with heart failure whose legs fill with fluid by evening: all are problems of fluid in the wrong compartment.

So is the dizzy patient who has been sweating through a long session without drinking. Treating any of them well starts with knowing where the body’s water normally sits and what moves it.

How much water, and where

Chapter 1 introduced the 60–40–20 rule: total body water about 60% of body weight, intracellular fluid about 40% and extracellular fluid about 20%. Those are averages for a lean young man. The real figure depends mostly on body fat, because fat holds little water while muscle holds a lot.

The low water reserve of older adults matters clinically. A given fluid loss is a larger share of their total, and the sense of thirst also tends to weaken with age. Elderly patients can become dehydrated quickly in hot weather or during illness.

Composition: two very different fluids

The ICF and ECF have the same osmolality but very different contents. The extracellular fluid is salty: high in sodium and chloride. The intracellular fluid is high in potassium, phosphate and protein.

Figure 1 · Two very different fluids

Plasma and interstitial fluid have almost identical ion concentrations, because water and small ions cross the capillary wall freely. The difference between them is protein. Plasma carries about 70 g/L of protein, mostly albumin; interstitial fluid carries much less. That protein difference is what creates the oncotic pressure that holds water in the circulation.

Measuring the compartments

You cannot drain a compartment to measure it, so physiologists use indicator dilution. Inject a known amount of a marker that spreads only through the compartment of interest. Wait for it to mix, measure its concentration, and divide.

Figure 2 · Measuring a compartment by dilution

For example, a plasma volume of 3 L with a haematocrit of 0.45 gives a blood volume of about 3 ÷ 0.55, roughly 5.5 L. Textbooks round these volumes slightly differently; what matters is the method and the order of size.

Daily water balance

Water intake and output balance over a day. Thirst and the kidneys adjust both, and the details belong to the renal chapters.

Figure 3 · Daily water balance

Notice the “insensible” losses through skin and lungs. They continue whether or not the person sweats, and they rise with fever, rapid breathing and dry air. A patient on a ventilator or with a high respiratory rate loses more water than you might expect.

Fluid exchange at the capillary

Plasma and interstitial fluid exchange water across the capillary wall all the time. Two kinds of force act on it. Hydrostatic pressure, the pressure of the fluid itself, pushes water out of the capillary. Oncotic (colloid osmotic) pressure, created by plasma protein that cannot easily cross, pulls water back in.

Ernest Starling described this balance in the 1890s, and it has been taught ever since as a simple picture. Fluid filters out at the arterial end, where hydrostatic pressure is high, and is reabsorbed at the venous end, where it is lower.

Figure 4 · Fluid exchange at the capillary

Measurements over the past few decades have changed that picture. The main barrier to protein is now known to be the glycocalyx, a gel-like layer lining the inner surface of the capillary [1].

Levick and Michel showed that in most tissues, absorption back into the capillary is only temporary. A slight net filtration continues along most of the capillary, even in venules, and the lymphatic system returns that fluid to the blood [1].

This revised Starling principle has practical consequences. Woodcock and Woodcock argue that the oncotic pressure difference opposes filtration but does not reverse it. Infusing protein solutions therefore cannot be expected to pull oedema fluid back into the circulation [2].

The lymphatic system

If filtered fluid returns mainly as lymph, the lymphatics are not an overflow; they are the main return route. Lymph vessels collect interstitial fluid and protein, pass them through lymph nodes, and empty into the great veins in the neck.

Lymph moves because of valves in the vessels, rhythmic contraction of the vessel walls, and compression by surrounding muscle and breathing. Movement therefore helps lymph flow, which is one reason why exercise is part of lymphoedema treatment.

Oedema

Oedema is excess fluid in the interstitial space. It appears when filtration out of capillaries exceeds what the lymphatics can return. There are four broad mechanisms.

Figure 5 · Four causes of oedema

Fluid shifts in exercise and training

During exercise, the working muscles take in fluid from plasma as their metabolites draw water in and capillary pressure rises. Sweating then removes water from the extracellular fluid. Plasma volume therefore falls during hard or prolonged exercise.

The American College of Sports Medicine advises preventing a water deficit greater than about 2% of body weight, because larger losses impair performance. Sweat rates vary widely between people, so they recommend estimating each person’s loss by weighing before and after exercise [3].

Over weeks of endurance training, blood volume can increase. Mack and colleagues found that ten weeks of endurance training raised both blood volume and maximal oxygen uptake in men [4].

The effect is not universal. In a randomised trial of previously sedentary women, twelve weeks of training at either high or low intensity did not significantly change plasma volume [5]. The response probably depends on the person, the programme and the starting fitness.

Clinical relevance for the physiotherapist

• Post-injury swelling. Inflammatory leak dominates early; elevation, gentle movement and graded loading follow.
• Lymphoedema. Lymph is the main return route for filtered fluid, so its loss produces chronic swelling [1]. Compression, exercise, skin care and decongestive techniques are core physiotherapy treatments.
• Heart failure. Raised venous pressure drives oedema. Compression may overload the heart, so check with the medical team first.
• Deep vein thrombosis. New unilateral calf swelling needs medical assessment before treatment.
• Dehydration in exercise. Weigh patients or athletes before and after long sessions in heat; plan fluid to keep losses under about 2% [3].
• Older adults. Lower body water and blunted thirst make dehydration more likely during illness or heat.

Assessment and application

Common mistakes

• Treating 60% total body water as true for everyone. Women, older adults and people with obesity have less.
• Assuming plasma and interstitial fluid differ in ions. They differ mainly in protein.
• Describing venous-end reabsorption as the main way filtered fluid returns. In most tissues the lymphatics do most of it [1].
• Compressing every swollen limb. Rule out DVT and uncontrolled heart failure first.
• Ignoring insensible losses in breathless or febrile patients.
• Assuming every trainee will expand plasma volume [5].

Exam preparation

Long answer

Describe the body fluid compartments and their composition. Explain the forces governing fluid exchange across the capillary, and the causes of oedema.

• Compartment volumes and the 60–40–20 rule, with variation by age, sex and fat.
• Composition: Na⁺/Cl⁻ outside, K⁺/phosphate/protein inside (Figure 1).
• Measurement by indicator dilution (Figure 2, table).
• Starling forces, then the revised principle with glycocalyx and lymph return (Figure 4) [1, 2].
• Four causes of oedema with examples (Figure 5).

Short notes

• Indicator-dilution principle.
• Revised Starling principle and the glycocalyx [1, 2].
• Lymphatic system and lymphoedema [1].
• Dehydration and fluid replacement in exercise [3].
• Daily water balance (Figure 3).

Viva and practical

• “How would you measure interstitial fluid volume?” — measure ECF and plasma volume separately and subtract.
• “Why does albumin infusion not simply reverse oedema?” — the oncotic difference opposes filtration but does not reverse it [2].
• “How do you estimate an athlete’s sweat loss?” — weigh before and after exercise, allowing for fluid drunk [3].
• Practical: measure and compare limb circumferences for suspected lymphoedema.

Current evidence

Exam and traditional teaching

Fluid filters out at the arterial end of the capillary and is reabsorbed at the venous end, with the lymphatics collecting a small remainder.

What the evidence adds

The glycocalyx is the main protein barrier, and sustained absorption is uncommon in most tissues. Slight filtration continues along the capillary and lymph returns the fluid [1].

This changes how fluid therapy is understood: protein solutions cannot be relied on to pull oedema back into the vessels [2].

What the student should understand

Write the classic Starling balance for the exam, then add the revised view in two sentences. For practice, treat the lymphatics as the main route home for tissue fluid, and movement as one of the things that drives them.

Check yourself

10 questions on this page. Tap one to see the answer and the reasoning.

Quick review

Further reading

References

1. Levick JR, Michel CC. Microvascular fluid exchange and the revised Starling principle. Cardiovascular Research. 2010;87(2):198–210. doi:10.1093/cvr/cvq062 PMID 20200043 Review
2. Woodcock TE, Woodcock TM. Revised Starling equation and the glycocalyx model of transvascular fluid exchange: an improved paradigm for prescribing intravenous fluid therapy. British Journal of Anaesthesia. 2012;108(3):384–394. doi:10.1093/bja/aer515 PMID 22290457 Review
3. American College of Sports Medicine; Sawka MN, Burke LM, Eichner ER, Maughan RJ, Montain SJ, Stachenfeld NS. American College of Sports Medicine position stand. Exercise and fluid replacement. Medicine and Science in Sports and Exercise. 2007 Feb;39(2):377–390. doi:10.1249/mss.0b013e31802ca597 PMID 17277604 Position stand
4. Mack GW, Convertino VA, Nadel ER. Effect of exercise training on cardiopulmonary baroreflex control of forearm vascular resistance in humans. Medicine and Science in Sports and Exercise. 1993 Jun;25(6):722–726. PMID 8321110 Experimental study
5. Branch JD, Pate RR, Bourque SP, Convertino VA, Durstine JL, Ward DS. Exercise training and intensity does not alter vascular volume responses in women. Aviation, Space, and Environmental Medicine. 1999 Nov;70(11):1070–1076. PMID 10608603 Randomised trial

Related topics

• Previous: Chapter 3 — Transport Across the Cell Membrane
• Next: Chapter 5 — The Resting Membrane Potential
• Chapter 1 — Homeostasis and the Internal Environment
• Lymphoedema physiotherapy

Reviewed by the Physiotherapist India Team. · Human Physiology contents