Anatomy and Physiology 2e · Fluid, Electrolyte, and Acid-Base Balance
Body Fluids and Fluid Compartments
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In 30 seconds
Water is the solvent of life: it carries nutrients, gases, and wastes; it participates in chemical reactions; it cushions organs; and it distributes heat. In an average adult, Total body water All the water in the body, roughly 60% of body weight (commonly taught) Full entry → is commonly taught to be about 60% of body weight — a figure that varies with age, sex, and body composition, since adipose tissue holds much less water than lean tissue. That water is not one pool but several: about two thirds sits inside cells (the intracellular fluid, ICF) and one third lies outside cells (the extracellular fluid, ECF), which is further divided into the blood Plasma The fluid portion of blood (~1/4 of ECF) Full entry → and the Interstitial fluid ECF that bathes tissues but is not in blood vessels Full entry → bathing the tissues, plus small transcellular spaces. This topic maps the compartments, explains what is dissolved in each, and describes the forces that move water between them.
Why this matters
- Everything else in this chapter depends on it: Electrolyte balance, water balance, and acid–base balance are all statements about which compartment has which solute in what amount.
- Practical medicine and daily life (educational): Intravenous fluids are chosen to match or deliberately differ from plasma; a person who loses blood loses plasma, while a person with burns can lose large amounts of interstitial fluid; athletes replace both water and electrolytes, not water alone. No treatment guidance is given here.
- Exam value: The "which ion dominates which compartment" question — Na⁺ outside, K⁺ inside — is foundational, and the 60–40–20 rule of thumb is a favorite memory aid.
The college version
Core Concepts
Where the water is: the 60–40–20 rule of thumb
Using commonly taught approximations for an average adult:
- Total body water ≈ 60% of body weight. (Lean tissue is ~70%+ water; adipose tissue is much less, so the percentage is lower in people with more body fat and higher in lean individuals. Infants are proportionally wetter than adults; the percentage falls with age.)
- Intracellular fluid (ICF) Fluid inside cells (~2/3 of body water) Full entry → ≈ 40% of body weight — two thirds of total body water.
- Extracellular fluid (ECF) Fluid outside cells (~1/3 of body water) Full entry → ≈ 20% of body weight — one third of total body water.
- The ECF splits further: plasma ≈ 25% of the ECF (~5% of body weight) and interstitial fluid ≈ 75% of the ECF (~15% of body weight).
- A small transcellular component (cerebrospinal fluid, synovial fluid, peritoneal and pleural fluids, digestive secretions) is usually counted separately — tiny in volume but functionally important.
What is dissolved in each compartment
The composition of ICF and ECF differs dramatically, and that difference is actively maintained:
- ECF: sodium (Na⁺) is the dominant cation, with chloride (Cl⁻) and bicarbonate (HCO₃⁻) as the main anions. This is the fluid the body must defend most carefully, because it bathes the cells.
- ICF: potassium (K⁺) is the dominant cation, with phosphate and negatively charged proteins as major anions.
- The Na⁺/K⁺-ATPase Membrane pump moving Na⁺ out and K⁺ in, using ATP Full entry → pump in cell membranes maintains the gradient: it pumps Na⁺ out of cells and K⁺ in, using ATP. Without it, the concentration difference would collapse.
- The cell membrane is freely permeable to water but only selectively permeable to solutes, so water moves to equalize Osmolarity Total dissolved particle concentration (osmoles/L) Full entry → across membranes even though solute composition stays different.
Osmolarity and tonicity
- Osmolarity is the total concentration of dissolved particles in a fluid (osmoles per liter). Plasma osmolarity is commonly taught to be about 275–295 mOsm/L (often rounded to ~300 for teaching).
- Tonicity What a solution does to cell volume Full entry → describes what a solution does to cell volume, because it accounts for whether the solute can cross the membrane. A solute that cannot cross the membrane (like Na⁺ under normal conditions) is an effective osmole and drives water movement; a solute that crosses freely (like urea) does not create lasting osmotic pressure.
- Isotonic solution: cells keep their size. Hypertonic: water leaves cells, they shrink. Hypotonic: water enters cells, they swell.
The capillary exchange: how plasma and interstitial fluid trade
Fluid moves between plasma and interstitial fluid at the capillaries under the balance of four Starling forces (commonly taught):
- Capillary hydrostatic pressure pushes fluid out of the capillary (favoring filtration).
- Plasma oncotic (colloid osmotic) pressure, due mainly to plasma proteins like albumin, pulls fluid back in.
- At the arterial end of a capillary, filtration usually wins; at the venous end, reabsorption usually wins, with a small net excess of filtered fluid returned by the lymphatic system.
- If the balance tips — for example, low plasma protein or high capillary pressure — excess fluid accumulates in the tissues as edema (educational concept).
Water intake and output
Water balance means intake equals output (commonly taught approximations):
- Intake ≈ 2.5 L/day: drinking, water in food, and a small amount from metabolism.
- Output ≈ 2.5 L/day: urine, insensible losses (skin, lungs), sweat, and feces.
- The kidneys are the adjustable outlet; the regulation of that outlet (ADH, aldosterone, thirst) is the subject of the next topics in this chapter.
Common Confusions
| Do Not Confuse | With | Difference |
|---|---|---|
| ICF major cation | ECF major cation | K⁺ dominates inside cells; Na⁺ dominates outside — the pump maintains both |
| Osmolarity | Tonicity | Osmolarity is total particle concentration; tonicity is the effective water-moving power, which depends on membrane permeability to the solute |
| Plasma | Interstitial fluid | Both are ECF, but plasma is inside vessels and contains proteins; interstitial fluid is outside vessels and has far less protein |
| Plasma | Serum | Serum is plasma minus the clotting proteins — a laboratory distinction, not a body compartment |
| "Body water is 60%" | A fixed universal number | It is an approximation for an average adult; it varies with body fat, age, and sex |
| Total body water | ECF volume | Body water includes ICF (2/3); ECF volume is only the outside-third and is what blood pressure depends on most |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Your body is like a giant water balloon made of billions of tiny water balloons (cells). Most of the water is inside the tiny balloons — that's the inside water. Some is outside them, swimming around — that's the outside water. The water outside has lots of salt, and the water inside has lots of a different mineral, potassium. The balloon walls have little doors that let water pass freely, so water always moves to balance things out. If you drink a lot of plain water, the outside gets watery, and water moves inside the balloons to even things up.
Worked example
A classic thought experiment (commonly taught; no laboratory procedure is described):
- Isotonic bath: A red blood cell is placed in a solution matching plasma osmolarity (~300 mOsm/L, commonly taught). Water moves in and out at equal rates; the cell keeps its normal biconcave shape.
- Hypotonic bath: The cell is placed in a more dilute solution. Water enters faster than it leaves, the cell swells, and if the swelling is severe the membrane can rupture — a commonly taught phenomenon called lysis.
- Hypertonic bath: The cell is placed in a more concentrated solution. Water leaves, the cell shrinks and wrinkles — commonly taught as crenation.
Now translate the thought experiment to the whole body: the plasma that bathes all cells must be kept isotonic. That is why drinking pure water in huge volumes temporarily dilutes the ECF (cells swell slightly until the kidneys excrete the excess water), and why losing water without salt concentrates the ECF (cells shrink until thirst and ADH restore balance). The compartments are always trying to equalize osmolarity — the cells cannot choose; the kidneys and hormones decide.
Key takeaways
- 60–40–20 rule of thumb: Total body water ≈ 60% of body weight; ICF ≈ 40%; ECF ≈ 20%; plasma ≈ 5% (commonly taught approximations — varies with body composition).
- Na⁺ rules the ECF; K⁺ rules the ICF. The Na⁺/K⁺-ATPase pump maintains this difference at the cost of ATP.
- Water crosses cell membranes freely, so osmolarity equalizes across compartments even though solute composition stays different.
- Plasma osmolarity ≈ 275–295 mOsm/L (commonly taught, often rounded to ~300 for teaching).
- Osmolarity ≠ tonicity: tonicity accounts for whether a solute actually drives water movement across a membrane.
- Starling forces at capillaries (hydrostatic vs. oncotic pressure) move fluid between plasma and interstitial space; imbalance → edema (educational).
- The percentage of body water is not fixed: it falls as body fat rises and with aging; infants are proportionally wetter.
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
State the 60–40–20 rule of thumb for body water and name the compartments it describes.
Show answer
Total body water ≈ 60% of body weight; intracellular fluid ≈ 40%; extracellular fluid ≈ 20% (with plasma ≈ 5% and interstitial fluid ≈ 15% of body weight). These are commonly taught approximations.
Which ion dominates the ECF, which dominates the ICF, and what pump maintains the difference?
Show answer
Na⁺ dominates the ECF; K⁺ dominates the ICF. The Na⁺/K⁺-ATPase pump uses ATP to pump Na⁺ out of cells and K⁺ into cells, maintaining the gradients.
Why does water move between compartments even though solute composition stays different?
Show answer
Because cell membranes are freely permeable to water, water moves down its own concentration gradient to equalize osmolarity across the membrane — the solute composition stays different, but the total particle concentration equalizes.
What is the difference between osmolarity and tonicity? Give an example of a solute that is counted in osmolarity but is not an effective osmole for tonicity.
Show answer
Osmolarity is the total concentration of dissolved particles; tonicity is the effect on cell volume, which depends on whether the solute can cross the membrane. Urea is the classic example: it contributes to measured osmolarity but crosses membranes freely, so it does not create lasting osmotic water movement.
Name the four Starling forces that move fluid across capillaries, and what happens when filtration exceeds reabsorption.
Show answer
Capillary hydrostatic pressure (pushes fluid out), plasma oncotic pressure (pulls fluid in), interstitial hydrostatic pressure, and interstitial oncotic pressure. When outward forces exceed inward forces, fluid accumulates in the tissues — edema.
Why is the percentage of body water lower in a person with more body fat?
Show answer
Adipose tissue contains much less water than lean tissue, so a higher proportion of body fat means a lower fraction of body weight is water.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Total body water
- All the water in the body, roughly 60% of body weight (commonly taught)
- Intracellular fluid (ICF)
- Fluid inside cells (~2/3 of body water)
- Extracellular fluid (ECF)
- Fluid outside cells (~1/3 of body water)
- Interstitial fluid
- ECF that bathes tissues but is not in blood vessels
- Plasma
- The fluid portion of blood (~1/4 of ECF)
- Transcellular fluid
- Small special fluids: CSF, synovial, pleural, peritoneal, digestive
- Na⁺/K⁺-ATPase
- Membrane pump moving Na⁺ out and K⁺ in, using ATP
- Osmolarity
- Total dissolved particle concentration (osmoles/L)
- Tonicity
- What a solution does to cell volume
- Oncotic pressure
- Osmotic pull of plasma proteins (mainly albumin)
Sources & references
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