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Physiology
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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
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