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Back to 106 Lecture Schedule FLUID AND ELECTROLYTE BALANCE IN THE BODY Body fluids consist of intracellular and extracellular fluid. The intracellular fluid makes up about 2/3 of the body water. The extracellular fluid makes up the remainder and consists of: 1. Interstitial fluid - fluid between the cells. 2. Plasma and lymph - the intra-vascular fluid. 3. Cerebrospinal fluid - found in ventricles of brain and surrounding the brain and spinal cord. 4. G.I. tract fluids - gastric, pancreatic and intestinal juices. 5. Synovial fluid - found inside synovial joint capsules. 6. Eye and ear fluids - aqueous and vitreous humours, perilymph and endolymph. 7. Pleural, pericardial and peritoneal fluids. 8. Glomerular filtrate Water moves from one compartment to another due to osmosis. Osmosis is chiefly influenced by dissolved solutes in fluids. Most of these solutes are electrolytes, i.e., compounds that dissociate into positive and negative ions. Electrolytes may also be essential minerals. Some of them help to maintain the acid-base balance of the body fluids. Fluid Balance - When each compartment of the body contains the appropriate concentration of water and electrolytes, the body is said to be in fluid balance. Concentration - The concentration of an electrolyte depends upon the number of particles it dissociates into. For example, if we dissolve equal numbers of NaCl and CaC12 molecules in water, the concentration of CaC12 will be greater since CaC12 dissociates into three particles and NaC1 dissociates into two particles. Sources of Body Fluids 1. Preformed water represents about 2,300 ml/day of our daily intake. This is the water found in our food and drink. 2. Metabolic water is produced through the catabolic breakdown of nutrients occurring during cellular respiration, This amounts to about 200 ml/day. 3. Combining preformed and metabolic water gives us a total daily intake of 2,500 ml Fluid Balance Normally, there is a balance achieved between our total daily intake and output of water. 1. Our total fluid intake is modified by the induction of the sensation of thirst. This is produced by a reaction of cells in the hypothalamus to the increased osmotic pressure of the blood passing through this region. Another stimulus of thirst would be the degree of dryness of the oral mucosa. 2. Our total output of fluid is regulated primarily by antidiuretic hormone and aldosterone. Both hormones cause the kidney to reabsorb more water from the urine for the body. An abnormally high output of water can be produced by a number of conditions: 1. Blood pressure increases can lead to increased glomerular filtration. 2. Hyperventilation leads to water loss by evaporation. 3. Vomiting and diarrhea. 4. Burns lead to excessive water loss through the skin. One of the primary functions of the kidney is to regulate extracellular fluid pressure. There are two components of extracellular fluid pressure: 1. Hydrostatic (blood) pressure - Blood pressure depends on: a. The
volume of the extracellular fluids 2. Osmotic pressure - depends on the a.
levels of non-diffusing proteins in plasma and interstitial fluids.
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