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Fleur sam Ch. 5Everything that enters/leaves cell has to pass through ECF.
Water is only molecule that moves freely between cells and ECF.
Bc of this free movement of H2O, the extracellular and intracellular compartments reach a
state of osmotic equilibrium in which the fluid concentrations are equal on the 2 sides of the
cell membrane (conc= amount of solute per volume of solution).
Conc of ECF and ICF are equal, some solutes are more concentrated in 1 of the 2 body
compartments than in the other, so body is in state of chemical disequilibrium
Na+, Cl-, HCO3- are more concentrated in extracellular fluid.
K+ more concentrated in intracellular fluid.
Ca2+ more concentrated in ECF, but also stored by many cells inside organelles like ER &
mitochondria.
Extracellular fluid is not at equilibrium between its 2 sub compartments:
- Plasma: liquid matrix of blood, found inside circulatory system.
Proteins/large anions are concentrated inside plasma but cannot cross leaky
exchange epithelium of blood vessels so are mostly absent from interstitial fluid.
- interstitial fluid.
Smaller molecules/ions like Na+/Cl- are small enough to pass freely between
endothelial cells and have same conc. in plasma and interstitial fluid.
Conc. differences of chemical disequilibrium are hallmark of living organism, only the
continual input of energy keeps to body in this state.
If solutes leak across cell membrane diving intracellular compartment & extracellular
compartment, energy is needed to return them to the compartment they left bv. Na/K ATP-
ase, K+ that leak out of cell & Na+ that leak into cell are returned to their original
compartment.
When cells die and cannot use energy they obey the second law of thermodynamics and
return to a state of randomness that is marked by loss of chemical disequilibrium.
Body as whole is electrically neutral, but few extra negative ions are found in intracellular
fluid, while their matching positive ions are located in extracellular fluid. This ionic imbalance
results in state of electrical disequilibrium. Changes in this disequilibrium create electrical
signals.
Homeostasis is not the same as equilibrium
The body is in osmotic equilibrium, but in chemical and electrical disequilibrium.
Osmotic equilibrium, chemical/electrical disequilibrium are dynamic steady states.
Goal of homeostasis is to maintain the dynamic steady state of the body’s compartments.
Body fluid compartment:
- intracellular fluid (ICF), 2/3 of total body water volume (contains proteins)
- extracellular fluid (ECF), 1/3 of total body water volume
o interstitial fluid: between circulatory system and cells. 25% of ECF.
, o blood plasma: liquid matrix of body, contains proteins (which are not in interstitial
fluid). 75% of ECF.
5.1 Osmosis and tonicity
Water can move freely in/out of almost every cell in human body by water-filled ion
channels and special water channels created by the protein aquaporin (AQP).
In men approximately 60% of total body weight is water.
Each kg of water has volume of 1L (so total body water=42 L in man of 70 kg).
Adult women have less water per kg of body mass than men bc they have more
adipose tissue. Large fat droplets in adipose tissue occupy most of the cell’s volume,
displacing more aqueous cytoplasm.
Age also influences the body water content. Infants have relatively more water than
adults, and water contents decreases as people grow older than 60.
Variability in body water content is important in prescribing drugs, when less body
waterhigher conc. of drug in plasma.
Water moves freely until body is in state of osmotic equilibrium. The movement of
water across a membrane in response to a solute concentration gradient is called
osmosis. Once conc. are equal, net movement of water stops.
The pressure on the membrane that opposes the osmotic movement of water is
known as the osmotic pressure. This is expressed in atmospheres (atm) or
millimetres of mercury(=kwik) (mm Hg). (pressure of 1 mm Hg= pressure
exerted(/applied) on 1 cm2 area by a 1 mm high column of mercury).
Conc. often expressed in Molarity= number of moles of dissolved solute per liter of
solution (mol/L).
1 mol= 6.02 x 1023 molecules.
Osmolarity is the number of osmotically active particles per liter of solution (not the
number of molecules, bc some molecules dissociate into ions when they dissolve in a
solution, the number of particles in solution is not always same as number of
molecules). Water moves by osmosis in response to the total concentration of all
particles (ions/uncharged molecules/both) in the solution.
Osmolality is expressed in osmoles per liter (osmol/L or OsM) or for very dilute
physiological solutes: miliosmoles/liter (mOsM)
Molarity (mol/L) x particles/molecule (osmol/mol) = osmolarity (osmol/L)
Osmolarity describes only number of particles in solution, says nothing about
composition of particles.
Normal osmolarity in human body ranges from 280-296 miliosmoles per liter (mOsM).
Osmolality is the conc. expressed as osmoles of solute per kg of water.
Bc biological solutions are dilute and little of their weight comes from solute,
osmolarity and osmolality used interchangeably.
, osmolality used in clinical situations bc easy to estimate body water by weighting
people.
Fluid loss in dehydration of people: weight loss=fluid loss. (bc. 1L of water=1kg). can
tell you how much fluid needs to be replaced.
Osmolarity is a property of every solution. If two solutions contain the same number
of solute particles per unit volume, we say that the solutions are isosmotic. If there
are differences, the highest osmolarity solution is hyperosmotic, the lower, fewer
osmoles per unit volume is hypoosmotic.
Osmolarity is colligative property of solutions, meaning it depends strictly on number
of particles per liter of solution.
Osmolarity says nothing about what the particles are/how they behave. To know if
osmosis will take place, properties of membrane and of solutes on each side of it
must be known.
If membrane only permeable to water, not to any solutes: water moves by osmosis
from hypoosmotic hyperosmotic.
But most membranes are selectively permeable and allow some solutes to cross in
addition to water.
To predict movement of water you must know tonicity of solution.
Tonicity is a physiological term to describe a solution and how that solution would
affect cell volume if cell was places in solution and allowed to come to equilibrium:
If a cell gain water/swells, the solution is hypotonic to the cell
If the cell shrinks and loses water, the solution is hypertonic
If the cell in the solution doesn’t change size at equilibrium, the solution is isotonic.
How does tonicity different from osmolarity?
Osmolarity describes the number of solute particles in a solution. It has units (bv
osmoles/L) can be measured by osmometer. Tonicity is comparative(vergelijkend)
and therefore has no units.
Osmolarity can be used to compare two solutions and their relationship is reciprocal
(wedekerig) (bv. Solution A=hyperosmotic to B, so B=hypoosmotic to A). Tonicity
always compares a solution to a cell. tonicity is used to describe only the solution.
Osmolarity alone does not tell you what happens to a cell placed in a solution.
Tonicity will tell what happens to a cell volume at equilibrium when the cell is placed
in the solution.
Osmolarity can’t be used to predict tonicity as tonicity not just depends on the
concentration but also on the nature (=whether solute particles can cross cell
membrane) of the solutes in the solution.
Solute particles that can enter a cell are called penetrating solutes. Particles that
cannot cross the cell membrane are called nonpenetrating solutes.
Tonicity depends on conc. of nonpenetrating solutes only.
NaCl is functionally nonpenetrating solute bc Na+/Cl- do not enter cell (few Na+ ions
may leak but are directly transported back to extracellular fluid by Na/K-ATP-ase).
Cells are filled with other types of nonpenetrating solutes, solutes inside the cell are
unable to leave as long as cell membrane remains intact.
If you know composition/osmolarity of solution, how to know tonicity of solution
without putting cell in solution? Know the relative concentrations of nonpenetrating
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