Body Fluid Compartments II

Presenter: Chris Baylis, PhD
Copyright Notice: All contents are copyrighted. No unauthorized distribution allowed.

To appreciate the concept of body fluid osmolality and the passive movement of water.
To understand how the permeability of different solutes influences their osmotic action.
To know the different types of volume expansion and contraction that can occur, specifically hypotonic, isotonic, and hypertonic conditions.

Osmolality is defined as the number of free (dissociated) particles in solution.
Units: osm or mosm/kg of H2O.
- For practical implications, 1 kg of water is equivalent to 1 liter.
- Example: For glucose (Molecular Weight, MW = 180), 180 g/l glucose corresponds to 1 mole/l which equals 1 osm/Kg H2O.
- Example: For albumin (MW = 70,000), 70,000 g albumin/l corresponds to 1 osm/Kg H2O.
It is emphasized that osmolality is determined by the number of particles, not their size.
The majority of extracellular fluid (ECF) osmolality is determined by electrolytes (ions).
- For sodium chloride (NaCl) at low (physiologic) concentrations, dissociation is nearly complete:
- 1 mmol/l of NaCl dissociates to give 1 Na⁺ and 1 Cl⁻, contributing approximately 2 mosm/Kg H2O.

Although the composition of intracellular fluid (ICF) and ECF differs, the total osmolality remains equal at approximately 290-300 mosm/Kg H2O.
Osmotic equilibrium is made possible due to the freely permeable nature of cell membranes to water.
The movement of water is strictly passive, influenced by:
- Hydrostatic and osmotic gradients across capillaries, between the interstitium and plasma.
- Osmotic gradients across cell walls, between cells and the interstitium.
Water movement occurs from regions of low to high solute concentration.

NaCl: Considered to be “impermeable” with a
reflection coefficient of 1.0 that generates a sustained osmotic effect causing no net movement of water.
Glucose:
- If iso-osmotic glucose (300 mosm/Kg H2O) is infused, glucose enters cells and draws water with it.
- Glucose incorporation into other molecules leads to a decrease in osmolality comparable to infusing an isotonic solution.

Urea:
- Permeable and diffuses into cells down a concentration gradient, taking water with it, resulting in no change in cell osmolality but causing cell swelling.
ECF only contributions by solutes:
- NaCl confined to the ECF determines ECF volume.

Hypotonic Expansion:
- Caused by drinking water, leading to the swelling of both ECF and ICF with a drop in osmolality.
- Increase in total body water (TBW).
Isotonic Expansion:
- Results from the infusion of 0.9% NaCl (300 mosm/Kg H2O), resulting in ECF expansion without changing osmolality.
- Increase in TBW leading to edema.
Hypertonic Expansion:
- Ingesting NaCl without water leads to an increase in ECF osmolality, with water moving out of cells to maintain osmotic equilibrium.
- ICF volume decreases while ECF volume increases and osmolality rises, resulting in cell shrinkage.

Hypertonic Contraction:
- Occurs during heavy sweating, leading to the loss of excess water without equivalent sodium chloride loss, decreasing both ECF and ICF volumes.
Isotonic Contraction:
- Characterized by loss of salt and water in equivalent amounts, such as in diarrhea or vomiting, resulting in decreased ECF and possible cardiovascular collapse.
Hypotonic Contraction:
- Involves loss of salt without equivalent water loss (e.g., adrenal insufficiency).
- This causes a decrease in ECF and an increase in ICF volume, leading to cell swelling.

Sodium ions are the major cations in plasma, and directional changes in Na⁺ concentration indicate osmotic changes.
Changes in plasma volume (ECF volume) cause alterations in plasma protein and hematocrit levels.
ICF volume responds primarily to changes in ECF osmotic concentration, rather than ECF volume itself.

Condition	Plasma Na⁺	ICF Volume	Hematocrit	Plasma Protein	ECF Volume
Hypertonic Expansion	↑	↓	↓	↓	↑
Isotonic Expansion	↑	↓	↓	↓	↑
Hypotonic Expansion	↑	↑	↓	↓	↑
Hypotonic Contraction	↓	↑	↑	↑	↓
Isotonic Contraction	↓	↑	↑	↑	↓
Hypertonic Contraction	↓	↑	↓	↑	↓

An example scenario of a 45-year-old man weighing 100 kg who has been ill for 24 hours undergoes specific fluid output measurements, as follows:
- Urine volume: 1.0 L/24 hrs
- Urine concentration of NaCl: 200 mmol/L
- Sweat volume: 2.0 L/24 hrs
- Sweat concentration of NaCl: 50 mmol/L
Initial Plasma osmolality: 310 mosm/Kg H2O
No fluid ingestion and no other solute loss during the 24-hour period.

Initial Condition:
- ICF volume (ICFV) = 36 L (60% total body water), ECF volume (ECFV) = 24 L (40% total body water).
- Total water (TBW) = 60 L.
- Total osmotic particles in body = (36 + 24 L) * 310 mosm = 18,600 mosm.
After 24 hours:
- Total volume lost: 3.0 L (1.0 L urine + 2.0 L sweat).
- Osmotic particles lost: 600 mosm (400 from urine + 200 from sweat).
- Total body fluid volume after 24 hours = 60.0 - 3.0 = 57.0 L.
- Total osmotic particles after 24 hours = 18,600 - 600 = 18,000 mosm.
- Final Osmotic Concentration = 18,000 / 57 = 315.8 mosm/L or Kg H2O.
- Number of osmotically active particles in ICF remains the same (36 L x 310 mosm/Kg H2O = 11,160 mosm).
- New ICF volume calculated as follows:
- New ICF volume = osmotic particles in ICF / new osmotic concentration of ICF =
  \frac{36 \times 310}{315.8} = \frac{11,160}{315.8} = 35.3 L.

The complexities involved in this calculation are highlighted as being more intricate than expected exam questions, emphasizing how osmotic gradients drive water movement between compartments and the dilution principle for calculating body fluid volume.

A man lost in the desert without water who sweats profusely will lose more water than salt.
- ECFV changes: ECF volume will decrease due to water loss, and ECF osmolality will increase since he loses more water.
- ICFV changes: As ECF osmolality rises, water exits the cell to maintain osmotic balance, leading to decreased ICF volume and increased ICF osmolality.

Upon finding the man unconscious, hydration is necessary.
Question: Can pure water be infused intravenously? No, because it would cause cell swelling and rupture, releasing potassium (K⁺) into the plasma, which can be fatal due to hyperkalemia leading to depolarization of nerves and muscles.

A 5% glucose solution should be administered:
- Has an osmolality of 300 mosm/l, preventing cell lysis at the infusion site.
- Distributes throughout the ECF; once inside the cells, glucose is utilized and incorporated into larger molecules, thus ceasing osmotic effects — effectively behaving like water.

Differences in ICF and ECF compositions exist, yet their osmolality is identical due to the free movement of water across cell membranes.
Imbalances in ICF and ECF osmolality result in consequential shifts in cell volume.
The net osmotic effect of solutes relies heavily on their permeability characteristics.
Immediate fluid level adjustments across compartments are short-term, while kidney function regulates long-term volume and composition of body fluids.