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|linkpage =Urinary System - Anatomy & Physiology
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|linktext =URINARY SYSTEM
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==Water in the Body==
 
==Water in the Body==
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These forces play different roles depending on the structure being crossed.  Both influence passage across capillary walls however only osmotic pressure influences that movement across cell membranes.  The reason hydrostatic pressure has little influence over the transport across cell membranes is that the hydrostatic pressure of the intracellular and interstitial fluid is relatively constant.
 
These forces play different roles depending on the structure being crossed.  Both influence passage across capillary walls however only osmotic pressure influences that movement across cell membranes.  The reason hydrostatic pressure has little influence over the transport across cell membranes is that the hydrostatic pressure of the intracellular and interstitial fluid is relatively constant.
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==[[Useful definitions - Renal Anatomy & Physiology| Osmolarity]]==
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==Osmolarity==
    
===What is Osmolarity===
 
===What is Osmolarity===
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The changes in cell size are quickly corrected by transport processes.  If osmolarity is constant then the volume of each compartment of the body is determined by how many particles of solute are dissolved there.  The relationship is expressed by the following formula:
 
The changes in cell size are quickly corrected by transport processes.  If osmolarity is constant then the volume of each compartment of the body is determined by how many particles of solute are dissolved there.  The relationship is expressed by the following formula:
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  '''Concentration = Number of dissolved particles &#247; volume'''
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'''Concentration = Number of dissolved particles &#247; volume'''
    
Therefore if concentration is to remain constant volume must increase to compensate for an increase in the number of dissolved particles
 
Therefore if concentration is to remain constant volume must increase to compensate for an increase in the number of dissolved particles
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==Ions and Osmolarity==
 
==Ions and Osmolarity==
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The Osmolarity of the various compartments is determined by different ions.  Most of the bodies K<sup+</sup> resides intracellulary and it is this and other organic anions which determine the intracellular osmolarity.  On the other hand the osmolarity of the ECF is over 90% determined by Na<sup+</sup> and the anions which follow it.  As osmolarity is kept fairly constant the Na<sup+</sup> levels in the ECF determine its volume.
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The Osmolarity of the various compartments is determined by different ions.  Most of the bodies K<sup>+</sup> resides intracellularly and it is this and other organic anions which determine the intracellular osmolarity.  On the other hand the osmolarity of the ECF is over 90% determined by Na<sup>+</sup> and the anions which follow it.  As osmolarity is kept fairly constant the Na<sup>+</sup> levels in the ECF determine its volume.
    
==Water Replacement==
 
==Water Replacement==
    
When water is ingested it actually takes a rather long time (30-60mins) before the equilibrium is reset between the ECF and ICF.  This is mainly because it takes time to distribute the water between the organs of the body after it is absorbed.
 
When water is ingested it actually takes a rather long time (30-60mins) before the equilibrium is reset between the ECF and ICF.  This is mainly because it takes time to distribute the water between the organs of the body after it is absorbed.
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[[Category:Water Balance and Homeostasis]]
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