2. Body Fluids & Osmosis 2024
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67 Terms
What is the main component of the human body by weight?
Water.
What percentage of body weight is water?
50–70%, depending on leanness.
Why does fat content affect body water percentage?
Fat contains little water.
What are the two main compartments of total body water (TBW)?
Intracellular fluid (ICF) and extracellular fluid (ECF).
What fraction of TBW is intracellular fluid (ICF)?
Two-thirds.
What fraction of TBW is extracellular fluid (ECF)?
One-third.
What are the two main components of ECF?
Plasma volume and interstitial fluid.
What percentage of ECF is interstitial fluid?
80%.
What percentage of ECF is plasma volume?
20%.
What does blood volume (BV) include?
Plasma volume and blood cell volume.
What is hematocrit?
The fraction of blood volume occupied by red blood cells.
What is the normal range for hematocrit?
35–50%, higher in men than women.
What is transcellular fluid?
A small component of ECF found in organs like the GI tract and cerebrospinal fluid.
Is transcellular fluid included in standard compartment calculations?
No, it is usually ignored.
What is the “60-40-20 Rule” for estimating fluid volumes in a 70 kg person?
TBW = 60% BW, ICF = 40% BW, ECF = 20% BW.
How is ECF further subdivided using the 60-40-20 rule?
Interstitial = 80% of ECF, Plasma = 20% of ECF.
What is the osmolality of most body fluid compartments?
About 285 mOsm/kg.
Why is osmolality similar across compartments?
High membrane water permeability allows rapid equilibration.
What is osmosis?
The movement of water down its concentration gradient.
How is water concentration related to osmolality?
Inversely; high osmolality = low water concentration.
In which direction does osmosis occur in terms of osmolality?
From low to high osmolality.
What other force influences water movement across membranes?
Hydrostatic pressure gradient.
What happens when water moves from pure water to a solute solution?
Water moves to the side with solute due to lower water concentration.
What is osmotic pressure (π)?
The pressure needed to stop net osmotic water movement.
What equation defines osmotic pressure?
π = RTCS.
What does CS represent in the van’t Hoff equation?
Solute concentration.
What is RT at 37°C in mmHg·L/mol?
19,300 mmHg·L/mol.
What osmotic pressure results from a 1 mM solute difference?
19.3 mmHg.
How does increasing osmolality affect osmotic pressure?
It increases it.
What is the osmotic pressure of pure water?
Zero.
In which direction does osmosis occur in terms of osmotic pressure?
From low to high osmotic pressure.
What type of membrane is assumed in basic osmosis models?
A semi-permeable membrane (permeable only to water).
What happens if the membrane is also permeable to solute?
Effective osmotic pressure is lower than predicted.
What is the reflection coefficient (σS)?
The fraction of solute molecules reflected by the membrane.
What is σS for an impermeable solute?
1.
What is σS for a very permeable solute?
Close to 0.
What equation accounts for solute permeability in osmotic pressure?
πeff = σS RTCS.
What does a low σS mean for osmotic effectiveness?
Low effective osmotic pressure.
What is the physiological relevance of the reflection coefficient?
It modifies how effectively a solute draws water.
What equation describes net water (volume) flow across membranes?
JV = LP (Δp – σPΔπ).
What does JV represent?
Volume flow of solution (e.g., mL/min).
What does LP represent?
Membrane hydraulic permeability.
What does Δp represent?
Hydrostatic pressure difference across the membrane.
What does Δπ represent?
Osmotic (oncotic) pressure difference across the membrane.
Why is there a negative sign before the osmotic term in the equation?
Osmotic pressure pulls water in, while hydrostatic pressure pushes water out.
When is osmotic equilibrium reached?
When net flow JV = 0.
What condition defines osmotic equilibrium from the equation?
Δp = σPΔπ.
Where is this equation (Starling’s Law) physiologically applied?
In filtration across capillary endothelia.
Why don’t small solutes create an osmotic pressure gradient across capillaries?
They equilibrate between plasma and interstitial fluid due to high permeability.
What is the approximate osmotic pressure created by total solutes at 285 mOsm?
~5,400 mmHg.
Why do plasma proteins generate oncotic pressure despite low molarity?
Their large size and plasma restriction create an osmotic gradient.
What is the typical plasma protein concentration?
~70 g/L.
What is the main plasma protein contributing to oncotic pressure?
Albumin.
What is the albumin concentration in plasma?
~50 g/L.
What is the molar concentration of albumin?
~0.73 mM.
What is the total plasma oncotic pressure?
~25 mmHg.
What does σP represent in capillary Starling forces?
Reflection coefficient for proteins in a specific capillary bed.
Where is σP lowest?
In discontinuous capillaries (e.g., liver).
Where is σP highest (σP = 1)?
In tight continuous capillaries (e.g., brain).
What determines the effective oncotic pressure across capillaries?
The product of σP and Δπ.
Why are extracellular and intracellular osmolalities equal?
Because water moves rapidly between compartments to equalize osmolality.
What allows rapid water movement across most cell membranes?
High permeability to water due to aquaporins.
What are aquaporins (AQPs)?
Integral membrane proteins that act as selective water channels.
What is the structure of an aquaporin?
A tetramer of four identical subunits.
How selective are aquaporins for water?
Highly selective; they primarily allow water passage only.
Where are aquaporins especially abundant?
In epithelial tissues involved in fluid transport.
Why is osmotic equilibration across cells so rapid?
Aquaporins enable fast water movement in response to small osmolality changes.