Regulation of Body Fluid Compartments

Question 1

What are some examples of insensible water loss?

Answer 1

• evaporation: through the respiratory tract & skin
• sweat

Question 2

What are the different compartments of body fluids and their relative volume?

Answer 2

Total amount of water ~ 60% of body weight
1. Extracellular (about 20%): interstitial (3/4) and plasma (1/4)
2. Intracellular (about 40%)

Question 3

How does the constituents of extracellular and intracellular fluids differ?

Answer 3

• plasma and interstitial fluid is separated by a highly permeable capillary membranes
• so has similar ionic composition
• but the membrane is not permeable to proteins
• much higher proteins compositions in plasma vs interstitial fluid – only leaks a small amount of protein into the interstitium

Question 4

What’s the characteristic of the membrane that separates the extracellular compartment from the intracellular compartment?

Answer 4

Highly selective membrane
- permeable to water
- not permeable to electrolytes
- therefore have approximate equal osmolarity at steady state

Question 5

What’s the main determinant of fluid distribution between the extracellular and intracellular fluid compartments?

Answer 5

osmotic effects of ions
- a very small difference in solute concentration and cause rapid osmosis of water

Question 6

Define isotonic, hypertonic, and hypotonic.

Answer 6

1. Isotonic. No osmotic force develops across the membranes. Ex. 0.9% NaCl and 5% glucose
2. Hypertonic. Higher concentration of osmotic substance than cell –> water flows out of cell –> reducing the intracellular fluid volume, increasing the the concentration
3. Hypotonic. Lower concentration of osmotic substance than cell –> water flows into the cell –> increasing intracellular volume –> may burst

Question 7

What are 2 basic principles of fluid movements?

Answer 7

1. water moves rapidly across cell membranes
2. cell membranes are almost completely impermeable to most solutes

Question 8

How does fluid shift between extracellular and intracellular compartment based on the osmolarity of the fluids given?

Answer 8

Question 9

What are 3 conditions that can cause intracellular swelling?

Answer 9

1. hyponatremia
2. depressed metabolism
   - cells can’t effectively pump out Na+, leading to excessive Na+ in the cells
3. inadequate nutrients

Inflammation can also make the cell membranes more permeable, leading to water flowing in with the ions

Question 10

What are the 2 causes of extracellular edema?

Answer 10

1. abnormal leakage of fluid from plasma to the interstitial space
2. impaired lymphatic drainage

Question 11

What are the 4 factors that can influence capillary filtration rate?

Answer 11

1. fluid pressure
   - increased fluid hydrostatic pressure: CHF, venous obstruction, etc.
2. oncotic pressure
   - decreased oncotic pressure due to liver failure (decreased protein production), excessive protein loss (nephrotic syndrome), burns
3. membrane permeability
   - permeability can be increased with inflammation, allergic reaction, bacterial infections, toxins, or injury to the cell membranes
4. increased interstitial fluid colloid osmotic pressure
   - impaired lymphatic drainage, preventing protein to be returned from the interstitial space to blood

Question 12

What would happen if a lymph node is infected?

Answer 12

can also cause lymphatic blockage leading to edema

Question 13

What are the 3 major safety factors that can prevent edema?

Answer 13

1. low compliance in tissue
   - therefore, if there is an increase in volume in the interstitial space, there is a large increase in interstitial hydrostatic pressure, making it more difficult for fluid to leave the capillaries
2. Adaptable lymphatic system
   - the lymphatics can accommodate 10-50x increase in fluid filtered from capillaries
3. “Wash down” of interstitial fluid protein
   - as interstitial hydrostatic pressure increases, it promotes lymphatic drainage, further decreasing the protein content, preventing further fluid accumulation