How does water move in the body?
• The cell membrane is semi-permeable
• Water can move freely
• Water is in equilibrium between cells and
extracellular fluids (osmotic equilibrium)
• Ions and solutes are disequilibrium
• Osmosis water moves along its
concentration gradient across a semipermeable membrane
Distribution of solutes in the body
fluid compartments
plasma
Interstitial fluid
Intracellular fluid
Ions and solutes are in disequilibrium
Ions and solutes are in disequilibrium
• Water can cross the cell membrane
Na+
Na+
K+
K+
proteins
Osmosis
• water moves along its concentration
gradient across a semi-permeable
membrane
• Water moves to dilute a solute
Osmosis
Osmotic pressure is pressure exerted
to counter the movement of water
to dilute something
Osmolarity
• Describes the number of particles in solution
• Know this and the direction of water movement
can be predicted
•
•
•
•
# of particles in 1 liter of solution
Is expressed as osmoles/L, or OsM
If very dilute: milliosmoles/L, or mOsM
Human body, approx 300 mOsM
Osmolarity: number of particles in 1L
• 1 M glucose = 1 OsM glucose
• 1M NaCl = 2 OsM NaCl, because NaCl
disassociates to 2 ions in solution.
Na+ Cl-
Compare the osmolarity of 2 solutions:
• Solution A
• Solution B
• 1 OsM glucose
• 2 OsM glucose
• A is hyposmotic to B
• B is hyperosmotic to A
• (A has fewer particles
than B)
• (B has more particles
than A)
Compare the osmolarity of 2 solutions:
• Solution B
• Solution C
• 2 OsM glucose
• 1 OsM NaCl
• B is hyperosmotic to C
• C is hypotonic to B
• (B has more particles/L
than A)
• (C has fewer particles/L
than B)
Compare the osmolarity of 2 solutions:
• Solution A
• Solution C
• 1 OsM glucose
• 1 OsM NaCl
• A is isosmotic to C
• C is isosmotic to A
Osmosis, the diffusion of water across the
cell membrane, has consequences on cells
• After water leaves a cell, the volume
changes (it can shrink)
Tonicity
• Describes how the cell volume will change
in a solution
P is penetrating solute
N is nonpenetrating solute
Water moved into the cell to dilute the solutes.
• Cell gains volume in a hypotonic solution
• Cell looses volume in a hypertonic solution
• Cell keeps the same volume in an isotonic
solution.
Tonicity indicates how the cell
volume will change in a solution
• In a hypotonic solution, the cell has a
higher concentration of a nonpenetrating
solute than the solution, water moves in.
• In a hypertonic solution, the cell has a
lower concentration of nonpenetrating
solute than the solution, water leaves the
cell
During intavenous injection:
•
•
•
•
0.9% (normal) saline  isotonic
D5--.9% saline (5% dextrose)  isotonic
D5W  hypotonic
0.45% saline  hypotonic
• Vs dehydration  hypotonic
• Vs blood loss  isotonic