11/25/2014 Medical Physiology Water Transport/Osmosis Steven Grassl Flashcards
What is the Total Body Water (TBW) in liters?
~60% of body weight *1 liter of water weighs 1 Kg
How many liters does a standardized patient weighing 70 Kg have?
TBW of 42 liters
The ICF is __ of TBW, or __ L for a 70 Kg patient (__ of body weight)
2/3; 28; 40%
The ECF is __ TBW, or __ L for a 70 Kg patient (__ of body weight)
1/3; 14; 20%
How is the ICF volume maintained within narrow limits?
By solute transport mechanisms driving water into, or out of, the cell by the process of osmosis
Why is the ICF volumen important?
Because an optimal concentration of intracellular solutes, organic and inorganic, anionic and cationic, is essential for optimal function of all cells, given the enormous diversity of organ-specific cell functions
How is the ECF volumen distributed?
In 2 compartments or volumes, which exist inside or outside the vasculature
Describe the intravascular fluid compartment of the ECF
- its volume is approximately 7% of body weight or 5 L for a 70 Kg patient - subdivided according to the measurement of hematocrit, where 55% of blood volume is plasma (3 L) and 45% of blood volume is cellular (2 L) *plasma is approximately 25% of the ECF
Describe the extravascular fluid compartment of the ECF
- remaining 75% ECF volume, approximately 11 L - edema is the pathophysiological shift of fluid from the intravascular to extravascular compartment and indicates an increase in TBW measured as an increase in body weight
How is the ECF volume (especially the intravascular volume) regulated?
By balancing the excretion of water by the kidney to match the consumption of water by the mouth
Why is maintaining the ECF intravascular volume within normal limits essential?
To maintain blood pressure which drives blood flow to and from the organs
What is the mot abundant molecule diffusing across the cell membrane?
Water
How does the movement of water molecules across the cell membrane, or water transport occur?
- in either direction across the cell membrane - by both mediated and unmediated pathways
Define Aquaporins
water molecule-specific channels that mediate the regulated transport of water across the cell membrane by insertion or removal of the Aquaporin proteins from the cell membrane
How do Aquaporins mediate membrane transport of water?
By a process of facilitated diffusion *they do not mediate transport of solutes across the membrane
What is the unmediated pathway through which water diffuses across cell membranes?
Process of simple diffusion –> no membrane protein interaction
What is faster: mediated transport of water by Aquaporin or water transport by simple diffusion?
Mediated transport of water by Aquaporin
Where can the trans-membrane water-channel protein, Aquaporin, be found?
- human red blood cells - kidney epithelial cells - trans-epithelial formation of aqueous humor in the eye - CSF formation in the brain - fluid exchange in the lung - formation of bile fluid - interstitial fluid formation in skeletal, cardiac and smooth muscle
Define osmosis
Net movement of water molecules across the cell membrane driven by a difference in water concentration across the membrane *from the intracellular or extracellular compartment down the water concentration gradient from the compartment where water is at higher concentration to the compartment where water is at lower concentration
How does osmosis occur?
Simple diffusion and facilitated diffusion (aquaporins)
What is the direction of osmosis?
Either direction across the cell membrane, depending on the direction of the water concentration gradient across the cell membrane
Define osmolarity (mOsmoles/L)
The concentration of water is inversely related to the sum total of all the solute concentrations in the compartment
If the sum total of all the solute concentrations in a compartment, or osmolarity (mOsmoles/L), is high, the water concentration is __
Low
the sum total of all the solute concentrations in a compartment, or osmolarity (mOsmoles/L) is low, the water concentration is __
High
What drives net diffusion of water and solute across a semi-permeable membrane?
Water and solute concentration gradient
What happens at equilibrium where trans-membrane solute transport is equivalent in both directions?
No solute concentration gradient exists across the membrane
What is the rate of net solute diffusion across the membrane dependent upon?
The magnitude of the solute concentration gradient across the membrane *as the solute concentration gradient across the membrane declines with transfer of solute, the rate of net solute diffusion declines and approaches zero because the rate of uni-directional solute transport becomes equivalent in both directions across the membrane
Define chemical potential
Measure of the free energy of a solution
