Fluid and Electrolytes

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Fluid and Electrolytes Imbalances

Fluid Compartments:
* Extracellular Fluid (ECF) – This is fluid found outside of the cells and the amount of ECF decreases with age. In the newborn for example, approximately ½ of the body fluid is contained in the ECF. By the time the infant has reached one year old; the ECF has decreased to approximately 1/3 of the total volume. In an average 70 kg adult the ECF is approximately 15 liters of total volume. ECF can further be divided into the following: * Interstitial Fluid – This is the fluid that surrounds the cells and equals approximately 11 to 12 liters of fluid in adults (Lymph fluid is included in interstitial fluid). * Intravascular Fluid – This is the fluid contained within the blood vessels (plasma volume). The average adult blood volume is approximately 5 to 6 liters or which approximately 3 liters is plasma. The remaining 2 or 3 liters of volume consists of red blood cells and erythrocytes (which transport oxygen and important body buffers). * Transcellular Fluid – This is fluid contained within specialized cavities of the body and is also considered to be part of the ECF. Examples of trancellular fluid include: * Cerebral Spinal Fluid

* Pericardial Fluid
* Pleural Fluid
* Synovial Fluid
* Intraocular Fluid
* Digestive Secretions
Intracellular Fluid (ICF) – This is the fluid that is contained within the cell. In adults it is approximately 2/3’s of the body’s fluid (27 liters). The ICF also known as “cellular soup” is comprised mostly of potassium, organic anions, proteins and other small cations and anions. Note: The 60-40-20 rule is that in total 60% of our body weight is water with 40% of that being intracellular with the remaining 20% being extracellular.  

Factors That Affect Movement of Water and Solutes:
Membranes – Each of the fluid compartments are separated by specific permeable membranes that allow the movement of water and some solutes (not plasma proteins for example because they are large molecules). Because permeability is selective; the composition of each compartment (ECF, ICF) maintains its own unique composition. Specific semi-permeable membranes include: * Cell Membranes – these membranes separate intracellular fluid from interstitial fluid, and are composed of lipids and proteins. * Capillary Membranes – these membranes separate intravascular fluid from interstitial fluid. * Epithelial Membranes – these membranes separate interstitial fluid and intravascular fluid from transcellular fluid. The Transport Process:

In addition to the use of membranes (permeable and semi-permeable); the movement of water and solutes are determined by the following processes:  
Diffusion – This is the random movement of particles in all directions from an area of high concentration to low concentration. One example of diffusion is the movement of oxygen from the alveoli of the lungs into the blood stream. Another example of diffusions occurs when cations follow anions and vice versa. Substances may diffuse across the cell wall (which is composed of lipids and proteins) under the following conditions: * The substance is small enough to pass through the protein pores (water, urea). * The substance is lipid soluble (oxygen and carbon dioxide). * The substance is transported by a “carrier substance” (Fore example, because glucose is such a large molecule it must combine on the outside of the cell with a carrier substance to be moved into the cell). Factors that increase diffusion include:

* Increased temperature
* Increased concentration of solutes
* Decreased size or weight of solutes
* Increased surface area available for diffusion
* Decreased distance across which the solute mass must diffuse Active Transport – The need for active transport (energy) is also a requirement for simple diffusion. Active transport also relies on the availability of carrier...
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