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Chapter 21

Body FluidsBody Fluids

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The Importance of WaterThe Importance of WaterWater is used as a solvent, a transport medium,

and participates in metabolic reactions• 50-70% of a persons body weight is water

Body fluid is maintained in the following ways:• Thirst mechanism maintains volume• Kidney activity regulates volume and

composition• Hormones regulate fluid volume and

electrolytes• Buffers, respiration, and kidney function

regulate pH

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Fluid CompartmentsFluid Compartments• Intracellular fluid (ICF)

• Fluid contained in cells• Two-thirds to three-fourths of all

body fluids• Extracellular fluid (ECF)

• Interstitial fluid – fluid between the cells n tissues

• Blood plasma – fluid portion of blood• Lymph – fluid drained by the

lymphatic system• Fluid in special compartments - CSF,

synovila capsule, between serous membranes

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Main fluid compartments showing relative percentage by weight of body fluid. Fluid percentages vary but total about 60% of body weight. Fluids are constantly exchanged among compartments, and each day fluids are lost and replaced.  

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Water BalanceWater Balance• Water gain from

• Beverages• Food• Cellular

respiration• Water loss from

• Kidneys – in urine• Skin - Evaporation• Lungs – water

vapor• Intestinal tract

within feces• Water loss must be

replaced or it will disrupt cell processes

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Sense of Thirst

Control center for thirst (Box 21-1)• Located in hypothalamus• Regulates total fluid volume

• A decrease in fluid volume or increase in concentration in fluids stimulates thirst

• Excessive thirst (polydipsia) may be caused by excessive urination as seen with diabetes

• May not be triggered quickly enough during exercise and/or in the heat which would caused rapid dehydration

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Electrolytes and Their FunctionsElectrolytes and Their Functions

Conduct electrical current in solution• Positive ions (cations)

• Sodium in the most abundant cation in ECF• Required for nerve conduction and maintaining

acid-base balance• Potassium mostly found in ICF

• Also needed for nerve conduction as well as in metabolism of carbs and proteins

• Calcium is stroed predominately in bone• Needed for muscle contractions, nerve

conduction, and blood clotting• Negative ions (anions)

• Phosphate is essential for carb metabolism, bone formation, and acid-base balance

• Chloride helps regulate fluid balance and pH and is used for digetsion in the stomach• Most abundant anion in the ECF

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Electrolyte Balance

• Must be kept in proper concentrations in the ECF and ICF (Read Box 21-2)

• Kidneys do majority of balancing• Hormones involved

• Aldosterone promotes sodium retention• Antidiuretic hormone (ADH) stimulates

water retention when sodium concentrations increase

• Parathyroid hormone increases blood calcuim levels by releasing it from bone

• Calcitonin hormone lowers calcium levels by depositing it in bone

• Know what organs secrete these hormones and what happens when out of balance

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Acid–Base BalanceAcid–Base Balance• pH scale measures hydrogen ion (H+)

concentration (↑ H+ lowers pH, ↓ H+ raises pH)

• Body fluids have normal pH of 7.35 to 7.45• Three-tenths of a point shift in either

direction (↓ 7.0 or ↑ 7.7) is fatal

Three important Buffer systems accept or release H+ as needed to control pH

• Bicarbonate produced by RBCs and Kidney tubules

• Phosphate through out the body• Proteins mostly within cells

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Acid–Base BalanceAcid–Base BalanceRespiration provides short-term regulation• Recall that carbonic acid breaks down to CO2

and H2O which allows CO2 to be expelled at the lungs• ↑ CO2 lowers pH, ↓ CO2 raises pH

Kidney function provides long-term regulation• Reabsorbs or releases hydrogen ions at

the kidneys to increase or decrease pH

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Abnormal pH (Table 21-1)

• Acidosis• pH less than 7.35• Depressed nervous system leading to

confusion, coma and ultimately death• Results from respiratory obstruction

which prevents release of CO2 or kidney failure to eliminate H+

• When carbs are unavailable for energy, fat and protein are used which elevate ketone bodies (acids)• Condition called Ketoacidosis

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Abnormal pH (Table 21-1)

• Alkalosis– pH greater than 7.45– Excites the nervous system to

produce a tingling sensation, muscle twitches, or even paralysis

– Caused by• Decreased CO2 from hyperventilation• Ingestion of too much antacid • Prolonged vomiting and loss of stomach

acids

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Disorders of Body FluidsDisorders of Body Fluids• Edema

• Accumulation of excess fluids in interstitial space (between cells)

• Caused by• Decreased fluid return to the heart, as

in congestive heart failure, leads to a backup of fluid in the lungs (pulmonary edema)

• Protein deficiency decreases plasma proteins reducing blood osmotic pressure and fluid accumulates in tissues

• Kidney failure cannot eliminate water through the production of urine

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Disorders of Body FluidsDisorders of Body Fluids• Water Intoxification

• Disrupts Nervous activity and may lead to coma• dilutes both ICF and ECF

• Caused by too much ADH production or excessive water intake by mouth or IV

• Dehydration• Reduction of body fluids which increases

electrolyte concentration of fluids• Caused by vomiting, diarhea, burn wounds,

excessive perspiration, or inadequate intake.

• Results in nervous and cardiac breakdown• Fluids need to be administered by mouth

or IV ASAP

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Fluid TherapyFluid TherapyFluids administered intravenously (Read Box

21-3)• Normal (.9%) saline (isotonic)• 5% dextrose in .45% saline (hypertonic,

but becomes hypotonic after glucose is metabolized))

• 5% dextrose in water (hypotonic)• Ringer lactate solution increased plasma

volume levels with the addition of buffers• Serum albumin 25% (5X normal) draws

fluids from interstitial spaces into blood• Nutritional solutions of glucose, protein,

and lipids given when oral intake is not possible for extended periods.