Exam #4: Water Balance Flashcards
What are the names of the two main body fluid compartments and the percentage of
total body water in each?
Total Body Water (TBW)= all of the water in the body (60% body-weight on average)
- Extracellular fluid= 1/3 (20% of bodyweight)
- Intracellular fluid= 2/3 (40% of bodyweight)
Remember the 60-40-20 rule*
What are the names of the two main subdivisions of the ECF and the percentage of the ECF in each division?
- Interstitial fluid= 75%
- Plasma= 25%
What are the main cations in the ICF? What are the main cations in the ECF?
- Main ICF cations= K+ & Mg++
- Main ECF cations= Na+ & Ca++
What are the main anions in the ICF? What are the main anions in the ECF?
Main ICF anions=
- Proteins
- Organic Phosphate (ATP, ADP, AMP)
- Sulfate ion
- Bicarbonate
Main ECF anion= Cl-
State the main difference between the interstitial fluid and the intravascular portion (plasma) of the extracellular fluid compartment. Why do they differ?
The interstitial fluid contains much less protein & no RBCs
*****Remember the interstitial fluid is filtered plasma; large proteins and cells are not able to pass through pores in capillaries to make it into the ISF
Explain why the ICF (intracellular fluid) is nearly always identical to the ECF (extracellular fluid) in osmotic concentration.
- The volume of a body fluid compartment is dependent on the solute concentration
- Water shifts freely across cell membranes
- If there is a shift in osmolarity of one compartment relative to another, water will shift until a new steady state is established
How does the size and osmolality of the ICF and ECF compartments change with the ingestion of pure water?
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**Remember:
1) Id. if any change is occurring in the ECF
2) Decide if the change will increase, decrease, or not change the osmolarity of the ECF
3) Determine which direction water will flow based on the change in ECF osmolarity - Increase in volume of the ECF WITHOUT SOLUTE
- This will DECREASE the ECF OSMOLARITY
- Water will flow into the ICF
Thus, there will be an increase in ECF & ICF volume & a decrease in total osmolarity
How does the size and osmolality of the ICF & ECF compartments change with the addition of isotonic saline?
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**Remember:
1) Id. if any change is occurring in the ECF
2) Decide if the change will increase, decrease, or not change the osmolarity of the ECF
3) Determine which direction water will flow based on the change in ECF osmolarity - Volume of the ECF will INCREASE
- The change WILL NOT change the osmolarity
- Water will NOT flow to equilibrate
Thus, osmolarity will remain the same, but the ECF volume will INCREASE; ICF volume will remain the same size
How does the size and osmolality of the ICF & ECF compartments change with the addition of hypertonic saline?
-
**Remember:
1) Id. if any change is occurring in the ECF
2) Decide if the change will increase, decrease, or not change the osmolarity of the ECF
3) Determine which direction water will flow based on the change in ECF osmolarity - ECF volume will INCREASE
- Osmolarity of the ECF will also INCREASE
- Water will flow from the ICF into the ECF, further increasing ECF volume
Thus, ICF volume will decrease, ECF volume will increase, and total osmolarity will increase
Explain what is meant by the term “ineffective osmole”.
- The hypothalamic osmoreceptor is the “sensor” for changes in osmotic pressure.
- Increased solute concentration in the ECF drives water out of the ICF of the osmoreceptor, triggering the release of ADH
- For a solute to cause a change in osmotic pressure, it must not pass freely through the cell membrane; IF IT DOES, then it is an INEFFECTIVE OSMOLE
E.g. Na+ does not freely pass the cell membrane= “effective osmole;” urea FREELY passes through the cell membrane; thus, it is an “ineffective osmole”
Explain why elevated plasma urea concentrations do not elicit ADH release.
Urea is an “ineffective osmole,” it freely passes across the cell membrane & thus does not elicit any change in osmotic pressure at the level of the hypothalamic osmoreceptors
How do plasma osmolality and baroreceptor signals control the release of ADH.
Osmolality= shrinkage of hypothalamic osmoreceptors (increase osmolarity of the ECF) causes a release of ADH from the posterior pituitary
Baroreceptors= stretch receptors in areas of low pressure & high pressure detect changes in ECF volume
- These changes have little effect on ADH secretion until blood volume has decrease by 10%, once this threshold has been reached, ADH secretion drastically increases
Explain how plasma osmolality and baroreceptor signals affect thirst.
Osmolality receptors= thirst is triggered by shrinkage of these receptors i.e. an increase in ECF osmolality
Baroreceptors= thirst is also triggered by a decrease in blood volume, but requires a drastic change in volume for a response
How does angiotensin affect thirst?
Angiotensin II acts on the brain to INCREASE thirst
Describe the alterations in blood pressure, sympathetic tone, renin secretion, angiotensin II production, and atrial natriuretic peptide production that occur when excess NaCl ingestion occurs in a salt sensitive individual.
Blood pressure= initially there will be an increase in blood pressure, but the resulting homeostatic mechanisms will bring blood pressure back to normal
Sympathetic tone= DECREASED
- Angiotensin II will be decreased, thus, there will be a decrease in sympathetic tone i.e. systemic vasoconstriction
Renin Secretion=
- Increased ECF NaCl leads to ECF volume expansion–> DECREASED RENIN secretion
Angiotensin II production
- DECREASED RENIN= DECREASED angiotensin I & consequently less angiotensin II
ANP= INCREASED
- plasma volume expansion stretches the atira
- INCREASED ANP is released in response to stretching
