11/25/2014 Medical Physiology Water Transport/Osmosis Steven Grassl

Question 1

What is the Total Body Water (TBW) in liters?

Answer 1

~60% of body weight *1 liter of water weighs 1 Kg

Question 2

How many liters does a standardized patient weighing 70 Kg have?

Answer 2

TBW of 42 liters

Question 3

The ICF is __ of TBW, or __ L for a 70 Kg patient (__ of body weight)

Answer 3

2/3; 28; 40%

Question 4

The ECF is __ TBW, or __ L for a 70 Kg patient (__ of body weight)

Answer 4

1/3; 14; 20%

Question 5

How is the ICF volume maintained within narrow limits?

Answer 5

By solute transport mechanisms driving water into, or out of, the cell by the process of osmosis

Question 6

Why is the ICF volumen important?

Answer 6

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

Question 7

How is the ECF volumen distributed?

Answer 7

In 2 compartments or volumes, which exist inside or outside the vasculature

Question 8

Describe the intravascular fluid compartment of the ECF

Answer 8

• 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

Question 9

Describe the extravascular fluid compartment of the ECF

Answer 9

• 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

Question 10

How is the ECF volume (especially the intravascular volume) regulated?

Answer 10

By balancing the excretion of water by the kidney to match the consumption of water by the mouth

Question 11

Why is maintaining the ECF intravascular volume within normal limits essential?

Answer 11

To maintain blood pressure which drives blood flow to and from the organs

Question 12

What is the mot abundant molecule diffusing across the cell membrane?

Answer 12

Water

Question 13

How does the movement of water molecules across the cell membrane, or water transport occur?

Answer 13

• in either direction across the cell membrane - by both mediated and unmediated pathways

Question 14

Define Aquaporins

Answer 14

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

Question 15

How do Aquaporins mediate membrane transport of water?

Answer 15

By a process of facilitated diffusion *they do not mediate transport of solutes across the membrane

Question 16

What is the unmediated pathway through which water diffuses across cell membranes?

Answer 16

Process of simple diffusion –> no membrane protein interaction

Question 17

What is faster: mediated transport of water by Aquaporin or water transport by simple diffusion?

Answer 17

Mediated transport of water by Aquaporin

Question 18

Where can the trans-membrane water-channel protein, Aquaporin, be found?

Answer 18

• 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

Question 19

Define osmosis

Answer 19

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

Question 20

How does osmosis occur?

Answer 20

Simple diffusion and facilitated diffusion (aquaporins)

Question 21

What is the direction of osmosis?

Answer 21

Either direction across the cell membrane, depending on the direction of the water concentration gradient across the cell membrane

Question 22

Define osmolarity (mOsmoles/L)

Answer 22

The concentration of water is inversely related to the sum total of all the solute concentrations in the compartment

Question 23

If the sum total of all the solute concentrations in a compartment, or osmolarity (mOsmoles/L), is high, the water concentration is __

Answer 23

Low

Question 24

the sum total of all the solute concentrations in a compartment, or osmolarity (mOsmoles/L) is low, the water concentration is __

Answer 24

High

Question 25

What drives net diffusion of water and solute across a semi-permeable membrane?

Answer 25

Water and solute concentration gradient

Question 26

What happens at equilibrium where trans-membrane solute transport is equivalent in both directions?

Answer 26

No solute concentration gradient exists across the membrane

Question 27

What is the rate of net solute diffusion across the membrane dependent upon?

Answer 27

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

Question 28

Define chemical potential

Answer 28

Measure of the free energy of a solution