SOLUTE & WATER TRANSPORT

LEARNING OBJECTIVES

• Explain transcellular transport • Explain osmosis • Explain osmolarity and tonicity • Explain how effective solutes regulate fluid compartments

TRANSCELLULAR TRANSPORT

Na+

glucose

glucose

3 Na+

2 K+ ATP

blood lumen

CONCENTRATION OF WATER

High [water] Low [water]

Na+

Na+ Na+

Na+

Add solute

Na+

Na+

Na+

Na+

Equal size compartments when membrane is permeable to solute

Na+ Na+

Na+ Na+

Na+

Na+ Na+

Na+

Na+

Na+

Na+

Na+

Osmosis causes change in compartment size when membrane is impermeable to solute

OSMOSIS

Na+ Na+

OSMOSIS IS MOVEMENT OF WATER

Occurs by diffusion ONLY! Uses aquaporin channels = facilitated diffusion Has highest concentration when pure water

IMPORTANT TERMS Molarity = # mole/vol (where 1 mole = 6 x 1023 molecules). Osmolarity = (# molecule/vol) x (# particles/molecule). Osmolality = (# molecule/Kg water) x (# particles/molecule). In body, osmolarity ~ osmolality = 300 mOsM. Consider ALL molecules to calculate osmolarity!

TONICITY & CELL VOLUME

Tonicity = # non-penetrating molec/volume

300 mOsM

>300 mOsM

< 300 mOsM

Always comparative!

CHANGES IN BODY FLUID COMPARTMENTS

Condition

Total Body Water

ECF volume

ICF volume

Effect on ECF osmolarity

IV isotonic saline (NaCl)

increase =

Diarrhea (isotonic loss)

decrease =

Excessive NaCl intake

= increase

Excessive sweating (hypotonic loss)

decrease increase

GENERAL CONCEPTS • Transcellular movement is by secondary

active transport.

• The two fluid compartments of the body, ICF and ECF, are in osmotic balance.

• Water moves by simple diffusion in a

process called osmosis. • Cellular volume is critically dependent on

the steady state movement of solutes and water across the plasma membrane in exchange with the ECF. Cells swell in hypo-tonic conditions and shrink in hyper-tonic conditions.

QUESTIONS? 1. Urea NaCl Glucose Solution X. 100mM 50mM 20mM Solution Y. 100mM 100mM 50mM A. What is the tonicity of X relative to a cell of 300mOsM? B. Which solution has the most water? 2. If a cell is 100mOsM, then it will swell in which of the following solutions? A. 100 mOsM B. 50 mOsM C. 200 mOsM 3. Jane drank 1 bowl of isotonic tomato soup. How would the water from the soup distribute between the ICF and ECF compartments? A. All remains in the ECF B. 2/3 in ECF and 1/3 in ICF C. 1/3 in ECF and 2/3 in ICF D. 1/2 in ECF and 1/2 in ICF