Body Fluid compartments

I. Water compartments of the body

• A. Intracellular

• B. Extracellular

  • i. Interstitial

  • ii. Plasma

  • iii. Transcellular

II. Compare/contrast water compartments

• A. Size

• B. Composition

• C. Osmolality

III. How do we have different composition/ movement of solutes

• A. Different permeability

• B. Types of transport across the membrane for solutes -- Protein transporters

• C. Review of Simple diffusion of solutes

IV. Movement of water

• A. Osmosis – movement across cell membranes due to unequal particles

• B. Hydrostatic pressure – movement across capillaries

V. Examples of when water vs. solute moves

• Watch how diuretics and laxatives exploit these principles

VI. Water content in the body (overall quantities)

• Water makes up about 45-75% of body weight

• Example calculation for a 70 kg man with approximately 0.60 fraction of water:

  • $70\ \text{kg} \times 0.60 = 42 \ \text{kg} \approx 42 \ \text{L}$

• Note on units: $1\ \text{kg} = 2.2\ \text{lb}$

• Water is divided into two main compartments: intracellular water and extracellular water

VII. Body fluid compartments and percentages

• Intracellular Fluid (ICF): $30-40\%\ \text{Body Wt}$

• Extracellular Fluid (ECF): total outside cells; subdivided into:

  • Interstitial fluid: $16\%$

  • Plasma fluid: $4-5\%$

  • Transcellular fluid: $1-3\%$

• Example/transcellular note: Transcellular fluid includes water in chambers lined by epithelial membranes; e.g., lumen of stomach (gastric secretions occupy this space)

VIII. Key properties of compartments

• Different compositions (different amounts of individual particles)

• Differences due to membrane permeability and transport mechanisms

• Different volumes across compartments

• Same osmolality across compartments: $0.3\ \text{Osmolal} = 300\ \text{mOsmolal} \ (actually closer to 280\ \text{mOsmolal})$

IX. Movement of water: mechanisms and drivers

• Movement of water is always passive and unsaturable

• Aquaporins provide water channels: widely distributed Aquaporin-1 (AQP1); collecting duct has Aquaporin-2 (AQP2)

• Osmosis:

  • Defined as movement of water due to a chemical potential energy difference, dependent on water concentration on two sides of a membrane

  • For physiologists, higher solute concentration => lower water concentration

  • Driving force for water movement across cell membranes

• Hydrostatic pressure:

  • Pressure of the fluid on vessel walls or container (energy/mole change)

  • In animal cell membranes, pressure is not a driving force across the membrane itself (membranes are flexible), but it drives movement of plasma water across capillary walls

• Analogy/example: squeezing a bottle increases hydrostatic pressure to push water out

X. Osmotic concepts and terminology

• Osmosis is movement of water toward an area with more solute when the membrane is permeable to water but not to solute

• Osmotic pressure is the hydrostatic pressure required to counterbalance osmosis

• Osmolarity concepts:

  • Isosmotic: same osmolarity as body fluids

  • Hyperosmotic: higher osmolarity than body fluids

  • Hyposmotic: lower osmolarity than body fluids

• Osmotic pressure is attributed to the osmolarity of a solution; osmosis occurs when water moves from a solution with fewer particles to one with more particles (particles can’t cross the membrane)

• Mnemonic from the lecture: “particles suck (in water)”

XI. Diabetes example: hyperosmolar shifts

• Increased blood glucose raises solute concentration in the extracellular fluid (ECF)

• ECF osmolarity rises; water moves from intracellular fluid (ICF) to ECF via osmosis

• Result: cells lose water and shrink; ECF compartments become relatively more solute-rich

• Conceptual mapping: ICF <→ ECF osmotic imbalance leads to water flow toward ECF; the diagram shows extracellular compartments with higher osmolarity (e.g., 304 mOsm) compared to baseline ~300 mOsm

• Clinical implication: dehydration at the cellular level with potential polyuria and volume shifts

XII. Kwashiorkor and edema: protein deficiency effects

• Kwashiorkor: nickname meaning “disease of the displaced child”; occurs with weaning off breast milk and diets high in corn

• Corn lacks tryptophan; low dietary protein leads to hypoalbuminemia

• Low plasma protein reduces oncotic (colloidal) pressure in capillaries

• Net effect: fluid shifts from plasma into the interstitial space, leading to edema and abdominal distention (peritoneal cavity edema)

• Additional clinical features: failure to grow, lethargy, depressed mentality

XIII. Solutions and solutes: fundamentals

• Water is the most abundant liquid and a universal solvent

• A solution comprises a solvent (e.g., water) and solutes (dissolved substances)

• Distinguish solute vs solvent:

  • Solvent: the dissolving medium (water in biological fluids)

  • Solute: dissolved substances (salts, sugars, proteins, gases, etc.)

XIV. Concentration and measurement of solution strength

• Concentration: a measure of how much solute is dissolved in solvent

• Common ways to express concentration:

  • Mass/volume (m/v) or gm% (grams per 100 mL; g/100 mL)

  • Molarity: $M = \text{moles per liter} = \text{mol/L}$

  • Molality: $m = \text{moles per kilogram of solvent} = \text{mol/kg}$

  • Osmolality: $\text{Osmolality} = \text{Osmol per kilogram of solvent} = \text{Osm/kg}$

XV. Practice: calculating gm%

• Question 1: What is the gm% of (5.0 \times 10^{1}) g of NaCl in 100 mL of water?

  • Answer: gm% = (5.0 \times 10^{1}\ \,\text{g} / 100\ \,\text{mL} = 50\% \text{w/v})

• Question 2: What is the gm% of 0.009 g in 100 mL of water?

  • Answer: gm% = (0.009\ \,\text{g} / 100\ \,\text{mL} = 0.009\% \text{w/v})

• Note: Among the concentration units, gm% (g per 100 mL) is the simplest to interpret