Fluid compartments L9 Flashcards
What percentage of the body is water in a typical adult?
About 60%.
What is the 60-40-20 rule in fluid compartments?
Total body water = 60% of body weight.
Intracellular fluid (ICF) = 40% of body weight.
Extracellular fluid (ECF) = 20% of body weight.
How does fat content affect body water percentage?
Higher fat content reduces body water percentage because fat does not store water.
Why do females typically have a lower body water percentage than males?
Females have more fat tissue, which stores less water than muscle tissue.
Name the two major fluid compartments in the body.
Intracellular Fluid (ICF).
Extracellular Fluid (ECF).
What are the subdivisions of extracellular fluid?
Plasma (~3 L).
Interstitial fluid (~10 L).
Transcellular fluid (~1 L).
What is transcellular fluid? Give examples.
Specialised fluid separated by membranes, including cerebrospinal fluid (CSF), synovial fluid, peritoneal fluid, and aqueous humour.
What proportion of total body water is in the ICF?
Approximately two-thirds, or 40% of body weight.
hat are the major cations and anions in intracellular fluid (ICF)?
Cations: Potassium (K⁺) and Magnesium (Mg²⁺).
Anions: Proteins and organic phosphates (e.g., ATP).
What are the major cations and anions in extracellular fluid (ECF)?
Cations: Sodium (Na⁺).
Anions: Chloride (Cl⁻) and bicarbonate (HCO₃⁻).
How do sodium-potassium ATPase pumps maintain ion gradients?
They pump Na⁺ out of the cell and K⁺ into the cell, maintaining high intracellular K⁺ and high extracellular Na⁺.
What is the main difference between plasma and interstitial fluid?
Plasma contains large proteins like albumin, while interstitial fluid has fewer proteins.
What is the Gibbs-Donnan effect?
Plasma proteins attract cations (like Na⁺) and repel anions (like Cl⁻), creating slight differences in ion concentrations between plasma and interstitial fluid.
What percentage of blood is plasma?
Plasma makes up about 55% of blood volume.
What is the dilution method used for?
Measuring the volumes of different fluid compartments using specific markers.
What marker is used to measure plasma volume?
Radiolabeled albumin, which stays confined to plasma.
What markers are used to measure extracellular fluid volume?
Mannitol and inulin, which diffuse into the interstitial fluid but not the intracellular fluid.
How is intracellular fluid volume calculated?
By subtracting extracellular fluid volume from total body water.
Write the formula for calculating volume using the dilution method.
Volume = Amount Injected ÷ Concentration Measured.
What can cause reduced plasma albumin levels?
Liver failure, protein malnutrition, or renal disease.
What is a common consequence of low plasma albumin?
Edema, due to reduced oncotic pressure.
Why is maintaining ion gradients essential?
For cellular functions such as neuronal action potentials, muscle contractions, and osmoregulation.
What is osmosis?
The movement of water across a semipermeable membrane from an area of lower solute concentration to higher solute concentration.
What is osmotic pressure?
The pressure required to prevent the movement of water across a semipermeable membrane due to osmosis.
What is oncotic pressure?
The osmotic pressure exerted by plasma proteins, particularly albumin, to retain water in the vascular space.
What happens if a solution is isosmotic but not isotonic?
Water movement may still occur; for example, urea can cross the membrane, causing intracellular osmolarity to rise and water to enter the cell, potentially leading to swelling.
What is the dilution method used for measuring total body water?
Heavy water isotopes (e.g., D₂O) are used because they freely diffuse across all compartments.
What is pulmonary edema?
Fluid accumulation in the lungs, often caused by left-sided heart failure or low plasma oncotic pressure.
What is osmolality?
Osmolality is the number of osmoles of solute per kilogram of solvent, measured in Osm/kg.
What is osmolarity?
Osmolarity is the number of osmoles of solute per liter of solution, measured in Osm/L.
What is the key difference between osmolality and osmolarity?
Osmolality measures solutes per mass of solvent (kg).
Osmolarity measures solutes per volume of solution (L).
Why is osmolality preferred in physiological contexts?
Osmolality is less affected by temperature and pressure changes because it is based on solvent mass, making it more stable in biological systems.
What is the normal range of plasma osmolality in humans?
Approximately 275-295 mOsm/kg.
What is the difference between an isosmotic solution and an isotonic solution?
Isosmotic: Two solutions have the same osmolality.
Isotonic: A solution does not cause net water movement across a semipermeable membrane when applied to cells.
Can a solution be isosmotic but not isotonic? Give an example.
Yes.
Example: Urea is isosmotic with intracellular fluid but not isotonic because urea freely diffuses into cells, increasing intracellular osmolality and causing water influx.
How do you calculate osmolarity for a solute that dissociates in water?
Osmolarity = Molar concentration × Number of particles per molecule.
For example:
NaCl: 1 mol/L dissociates into Na⁺ and Cl⁻, so osmolarity = 2 Osm/L.
CaCl₂: 1 mol/L dissociates into Ca²⁺ and 2 Cl⁻, so osmolarity = 3 Osm/L.
What is the relationship between osmotic pressure and osmolality?
Osmotic pressure is directly proportional to osmolality and is calculated using the van’t Hoff equation:
Osmotic Pressure = nCRT, where:
nC is the osmolality.
R is the gas constant.
T is the temperature in Kelvin.
What happens to cells in a hypertonic solution?
Water leaves the cells, causing them to shrink (crenation).
What happens to cells in a hypotonic solution?
Water enters the cells, causing them to swell and potentially burst (lysis).
What is the role of osmotic pressure in capillary exchange?
It opposes hydrostatic pressure to prevent excessive water loss from capillaries.
How does osmolality affect fluid movement across the blood-brain barrier?
The blood-brain barrier tightly regulates osmolality to prevent swelling or shrinking of brain cells, which could disrupt neurological function.
What does a high plasma osmolality indicate?
Dehydration or hypernatremia (excessive sodium).
What does a low plasma osmolality indicate?
Overhydration or hyponatremia (low sodium).
What is the osmolality of an isotonic saline solution?
~285 mOsm/kg, matching plasma osmolality.
