4.1 about 18 minDeep Learning viewL4 p.2–4, 6–8

Body water compartments and tonicity

Water crosses cell membranes freely, so it goes wherever effective solutes pull it. Sodium concentration tells you about water; the amount of sodium tells you about extracellular volume.

Where body water lives

L4 p.2

Water makes up about 60% of body weight in an adult man (about 42 L at 70 kg), less in women and older people (more fat, which holds little water) and more in infants. Two barriers divide it. The cell membrane separates intracellular fluid (ICF, two-thirds) from extracellular fluid (ECF, one-third). Inside the ECF, the capillary wall separates the interstitial fluid (three-quarters of ECF) from plasma (one-quarter of ECF).

Teaching schematic

Intracellular fluidExtracellular fluid 1/3 ≈ 14 L

Cell membrane (solid blue line)

Water crosses freely; Na⁺ is pumped out, K⁺ in. Effective osmoles decide which way water moves: osmosis.

Capillary wall (dashed pink line)

Water and small solutes cross freely; albumin stays in plasma. Hydrostatic vs oncotic pressure: Starling forces.

70-kg man, total body water ≈ 42 L (60% of body weight). Teaching schematic.

Total body water in a 70-kg man. Each barrier lets water through but holds back something different: the cell membrane holds sodium outside (the Na⁺/K⁺-ATPase pumps it out), the capillary wall holds protein inside the vessel.L4 p.2

What each barrier lets through

  • Cell membrane (ICF ↔ ECF)

    Water
    Crosses freely (aquaporins)
    Na⁺, Cl⁻, glucose, urea
    Na⁺ kept out by the pump; glucose needs transporters; urea crosses
    Plasma protein (albumin)
    Not relevant
    Force that moves water across it
    Osmosis driven by effective solutes

  • Capillary wall (plasma ↔ interstitium)

    Water
    Crosses freely
    Na⁺, Cl⁻, glucose, urea
    All cross freely
    Plasma protein (albumin)
    Held in plasma
    Force that moves water across it
    Starling forces: hydrostatic vs oncotic pressure (4.5)

Lecture slide

Lecture diagram: total body water divided into intracellular fluid (two-thirds) and extracellular fluid (one-third), with interstitial fluid three-quarters and plasma one-quarter of the extracellular fluid, separated by the plasma membrane and capillary endothelium.
The lecture's compartment diagram: plasma membrane between ICF and ECF, capillary endothelium between interstitial fluid and plasma.L4 p.2

Osmolality versus tonicity

L4 p.3–4

Osmosis is the movement of water across a semipermeable membrane toward the side with the higher solute concentration, until the concentrations are equal. Plasma osmolality counts every dissolved particle per kilogram of water; the normal range is 275–295 mOsm/kg.

Counting particles is not enough to predict water movement. A solute moves water across the cell membrane only if it stays on one side. Sodium (held out by the pump), glucose when insulin is lacking, and mannitol are effective osmoles. Urea crosses cell membranes and distributes equally inside and outside cells, so it raises osmolality without creating a gradient: it is an ineffective osmole. Ethanol behaves the same way. Tonicity (effective osmolality) counts only the effective osmoles, and it is tonicity that decides whether cells swell or shrink.

Teaching schematic

  • Na⁺ (and Cl⁻)

    Pumped out of cells, so it stays in the ECF. Water moves toward it: effective.

  • Glucose without insulin

    Cannot enter muscle and fat cells. Water leaves cells: effective (so is mannitol).

  • Urea

    Crosses the membrane (dashed) and equalizes. Osmolality rises, but no water moves: ineffective.

Left chamber = cell, right chamber = ECF, blue line = cell membrane. Teaching schematic.

An effective osmole stays on one side of the membrane and draws water toward itself; urea crosses and leaves cell volume unchanged.L4 p.3–4

Worked calculation · Calculated plasma osmolality

Serum Na⁺
140 mEq/L
Glucose
90 mg/dL
BUN
14 mg/dL

Formula

Posm = 2 × Na⁺ + glucose/18 + BUN/2.8

L4 p.3

Worked calculation · High osmolality, normal tonicity (uremia)

Serum Na⁺
140 mEq/L
Glucose
90 mg/dL
BUN
112 mg/dL (renal failure)

Formula

Posm = 2 × Na⁺ + glucose/18 + BUN/2.8; effective osmolality = 2 × Na⁺ + glucose/18

L4 p.3

Sodium concentration reports water; total sodium sets ECF volume

L4 p.5, 8–11, 13–15

Sodium is the main effective solute of the ECF, and water follows it so that osmolality stays the same inside and outside cells. Two consequences follow. The amount of sodium in the body (total body Na⁺) decides how big the ECF is: add sodium and water is pulled in with it; lose sodium and the ECF shrinks. The serum sodium concentration is a ratio, total body Na⁺ divided by total body water, so it tells you whether there is too much or too little water relative to sodium. The lecture writes it exactly that way: serum Na⁺ = TBNa⁺/TBW.

Separate the two questionsL4 p.5, 8

  • How much water is there relative to solute? (Are cells swollen or shrunken?)

    Measured by
    Serum Na⁺ concentration, plasma osmolality
    Controlled by
    ADH and thirst (water intake and excretion)

  • How big is the ECF? (Is the patient dry or edematous?)

    Measured by
    Bedside examination: JVP, BP, skin turgor, edema
    Controlled by
    Total body Na⁺, through aldosterone, renal perfusion and natriuretic peptides

That is why a low sodium can occur with a shrunken ECF (thiazide user who lost salt), a normal-looking ECF (SIADH) or an expanded ECF with edema (heart failure). The sodium concentration is low in all three because water is high relative to sodium; the volume differs because total sodium differs. Section 4.2 works through each.

Reading the compartments at the bedside

L4 p.6–8, 12, 19

Each compartment change has its own signs. ECF contraction shows in the circulation and the skin; ECF expansion shows as edema, effusions and raised venous pressure. Cell swelling or shrinking shows mainly in the brain, because the skull leaves no room to swell and stretched bridging veins can tear when the brain shrinks.

Four changes, four sets of signsL4 p.6–8, 12, 19

  • ECF contraction (volume depletion)

    Signs
    Dry mucous membranes, reduced skin turgor (tenting), low BP (postural drop), tachycardia, flat neck veins (low JVP), sunken eyes, capillary refill > 2 s, low urine output, stupor
    Typical causes
    Diarrhea, vomiting, hemorrhage, diuretics, Addison disease

  • ECF expansion

    Signs
    Pitting edema, body-cavity effusions, raised JVP, pulmonary crackles, weight gain
    Typical causes
    Excess saline, heart failure, cirrhosis, nephrotic syndrome

  • Cell swelling (hypotonicity)

    Signs
    Headache, nausea, confusion, seizures, coma (cerebral edema)
    Typical causes
    SIADH, water intoxication, too-rapid correction of hypernatremia

  • Cell shrinking (hypertonicity)

    Signs
    Thirst, lethargy, irritability, seizures, coma; tearing of bridging veins (hemorrhage)
    Typical causes
    Diabetes insipidus without water, HHS, hypertonic sodium gain

Two words for an expanded ECF are worth separating. Edema is excess fluid in the interstitium of a tissue; when it is a transudate in loose subcutaneous tissue, a fingertip pressed for a few seconds pushes the fluid aside and leaves a pit (pitting edema), first in dependent sites: ankles when upright, sacrum when bedbound. An effusion is fluid in a body cavity: pleural effusion, pericardial effusion, or ascites in the peritoneum. Both are ECF on the interstitial side that the circulation cannot use, so a patient can be edematous and still have a poorly filled arterial tree (4.5).

Lecture image

Lecture illustration of the signs of a contracted extracellular fluid volume: postural drop in blood pressure, tachycardia, dry mucous membranes, sunken eyes, decreased skin turgor, decreased consciousness and decreased urine output around a tank labelled decreased ECF volume.
Signs of a contracted ECF from the lecture. All of them reflect a smaller circulating or interstitial volume, not cell size.L4 p.6–7

Worked case · Two patients with a serum sodium of 125 mEq/L

Patient A takes a thiazide, has dry mucous membranes, flat neck veins and a postural drop in blood pressure. Patient B has heart failure, pitting edema to the knees and raised neck veins. Both have a serum Na⁺ of 125 mEq/L.

What do they share, and what separates them?

L4 p.9, 11

Knowledge check · 4.1

Answer, read why, and try again whenever you like. Answers are kept in both study modes.

0 of 4 correct
  1. Q1Effective vs ineffective osmolesL4 p.3

    Which substance can raise measured plasma osmolality without moving water out of cells?

  2. Q2Distribution of infused fluidsL4 p.2, 8

    A 70-kg man receives 1 L of 0.9% saline over 30 minutes. Use the compartment sizes in the figure.

    Intracellular fluidExtracellular fluid 1/3 ≈ 14 L

    Cell membrane (solid blue line)

    Water crosses freely; Na⁺ is pumped out, K⁺ in. Effective osmoles decide which way water moves: osmosis.

    Capillary wall (dashed pink line)

    Water and small solutes cross freely; albumin stays in plasma. Hydrostatic vs oncotic pressure: Starling forces.

    70-kg man, total body water ≈ 42 L (60% of body weight). Teaching schematic.

    About how much of the liter remains in his plasma once it has distributed?

  3. Q3Total body Na⁺ sets ECF volumeL4 p.5–7

    A 30-year-old man has had profuse watery diarrhea for two days. His mucous membranes are dry, his neck veins are flat and his blood pressure falls from 112/70 to 92/60 mm Hg on standing. Serum Na⁺ is 140 mEq/L.

    What has he mainly lost?

  4. Q4Low tonicity swells cellsL4 p.10, 19

    A 26-year-old woman collapses after a marathon during which she drank large amounts of water. She is confused, then has a generalized seizure. Serum Na⁺ is 118 mEq/L.

    What is happening to her brain cells?

Chapter startNext4.2 Fluid shifts