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Fluids, Electrolytes, & Acid-Base Study Guide

Spring Semester 2005

Learning Objectives, Topics & Keywords



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Readings: Martini, Chapter 27

1. Understand the distribution and balance of fluids within the body

total body water = about 60% of total body weight (for lean males)

distributed between two "compartments" (separated by cell membranes)

extracellular fluid (ECF)= 1/3

plasma

interstitial fluid (between cells)

lymph

intracellular fluid (ICF) = 2/3

inside the cells

ECF osmotic pressure = ICF osmotic pressure (almost always)

ins & outs (fluid balance is when ins = outs, and in the right compartments)

water gain

drinking liquids

controlled by thirst

drying of oral mucosa

ECF osmotic pressure is sensed by osmoreceptor neurons in hypothalamus

also, angiotensin II - acts on hypothalamus to stimulate thirst

food has moisture

metabolism leads to "water of respiration" (carbohydrate + O2 --> CO2 + H2O)

water loss

urine

ADH controls water loss (controls urine osmolarity and volume)

secretion, by posterior pituitary, regulated by osmoreceptors in hypothalamus

aldosterone

limits sodium ion loss in kidney

sweat

feces

ventilation

lost because inhaled air is saturated with water and then exhaled

2. Know the main electrolytes and where they are distributed

sodium, calcium, chloride, bicarbonate ions mainly in ECF

(almost) all cells have sodium-potassium pumps (Na-K ATPase)

total body sodium sets extracellular volume

potassium, proteins mainly in ICF

 

3. Understand the control of acid-base balance

normal blood pH is 7.4 (7.35 — 7.45) (recall: pH 7.0 is neutral)

acidosis

pH below normal

alkalosis

pH above normal

buffer systems

resist changes in pH by absorbing or releasing hydrogen ions

volatile buffer: the carbonic acid - bicarbonate system (most important)

can accept or release hydrogen ion

carbon dioxide is volatile!

pCO2 controlled by alveolar ventilation

non-volatile buffers

phosphate ions in blood

proteins in blood

kidney must secrete hydrogen ion to really get it out of the body

 

4. Understand pathologic acid-base imbalances and their compensation

respiratory causes

respiratory acidosis (e.g., COPD --> reduced alveolar ventilation --> increased pCO2)

respiratory alkalosis (e.g., mania or fever --> increased alveolar ventilation --> decreased pCO2)

renal compensation

acidosis --> increased secretion of hydrogen ion into DCT

alkalosis --> decreased recovery of bicarbonate ion in PCT

metabolic causes

metabolic acidosis

(e.g., diabetic ketoacidosis; ischemia --> glycolysis --> lactic acid production;

diarrhea --> loss of HCO3-)

metabolic alkalosis

(e.g. vomiting --> loss of gastric juice --> loss of hydrogen ions)

respiratory and renal compensation

respiratory --> adjust alveolar ventilation to change pCO2

renal --> PCT varies absorption of HCO- ; DCT varies secretion of H+


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Durham Technical Community College
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Last updated 27 April 2005