Fluid Movement - Physiology

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Water in the Body

Water is a very important substance in the body making up approximately 70% of it. Over 50% of that water is found in cells referred to as the intracellular fluid. The remainder makes up the base of the fluid surrounding the cell which is referred to as the extracellular fluid or ECF.

Extracellular Fluid (ECF)

The extracellular fluid is subdivided into three parts:

  • The plasma of the blood
  • Tissue fluid (interstitial fluid)
  • Lymph fluid

Water Movement

Water is able to move between the various compartments easily. The movement is determined by the following:

  • Differences in hydrostatic pressure
  • Differences in osmotic pressure

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.


What is Osmolarity

It is a term which is commonly used to represent the osmotic properties of a solution. It is the total number of osmotically active molecules of solute per litre of solvent and its unit is osmol/l. For example a solution with 1 milliosmol of glucose has an osmolarity of 1 milliosmol/l because glucose is an osmotically active molecule. However a solution with 1 milliosmol of NaCl has an osmolarity of 2milliosmol/l because in solution NaCl dissociates and each ion is osmotically active.

The Effects of Osmolarity

The osmolarity of a solution is inversely proportional to the water concentration. Water tends to flow from an area of lower osmolarity to an area of higher osmolarity. Until the osmolarity of both is equal. If the osmolarity of the ECF is changed say after a meal high in salt then because the ECF now has a higher osmolarity than the interstitial fluid water moves from the interstitium into the ECF until a new equilibrium is reached. Water in turn flows out of the intracellular fluid into the interstitium as this now has a higher osmolarity. Thus cell volume changes.

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:

Concentration = Number of dissolved particles ÷ volume

Therefore if concentration is to remain constant volume must increase to compensate for an increase in the number of dissolved particles

Ions and Osmolarity

The Osmolarity of the various compartments is determined by different ions. Most of the bodies K+ 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+ and the anions which follow it. As osmolarity is kept fairly constant the Na+ levels in the ECF determine its volume.

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.

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