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Extracellular Fluid Composition and Volume
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Learning Objectives
• Know the distribution of bodily fluids and composition of intracellular and extracellular fluid.
• Know how to calculate the volume of the different fluid compartments.
• Know how to calculate the shifts in osmolarity.• Know what causes the 2 types of edema.• Know what mechanisms the body uses to
prevent edema.
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Water Balance
The body must maintain a relatively constant volume and composition of the bodilyfluids under a wide variety of conditions.
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Distribution of Bodily Fluids
• Most of the H2O is intracellular.
• The total body H2O is ~ 60% of the body weight (~ 42 L).
• The % depends on age, gender, and degree of obesity.
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Composition of Intracellular and Extracellular Fluid
Note: The intracellular compartment of Ca2+ is very low compared to the extracellular.Ca2+ is a very important signaling molecule for electrical signals and protein activation.
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Calculating Compartment fluid Volume
This works only if the indicator uniformly diffused and only into the measured fluidcompartment, and if the indicator itself is not metabolized or excreted.
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Calculating Total Body Water
• Use radioactive water (3H2O) or heavy water (2H2O).
• This will mix with the total body water in just a few hours and the dilution method (previous slide) can be used.
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Determine Extracellular and Intracellular fluid Volume
• For extracellular volume, use a substance that does not cross cell membranes, but does pass through capillary pores. - Radioactive insulin is used for this purpose.
• Then, calculate intracellular volume from: Intracellular volume = total body water – extracellular volume
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Plasma and Interstitial Fluid Volume
• For plasma volume, use an indicator that does not diffuse across plasma membranes or capillary pores.
- 125I-albumin is used for this purpose.
• Interstitial fluid volume can then be calculated from: interstitial fluid volume = extracellular fluid volume – plasma volume
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Measure Blood Volume
• Remember, hematocrit can be measured by centrifuging
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Osmotic Pressure and Osmosis
• Chapter 25 reviews some basic concepts of osmosis and osmotic pressure.
• You should already know this material, plus, we used it to cover osmotic forces that determine the fluid flow at capillaries.
• For cells, remember that H2O passively diffuses across plasma membranes, but not most ions and solutes.
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Adding Different Osmotic Solutions to the Extracellular Fluid
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Calculating Shifts in Osmolarity
• What happens when 2L of 3% solution of NaCl is added to the extracellular fluid of a 70-kg person, whose initial plasma osmolarity is 280 mOsm/L?
• Initially, the total body fluid is 42L, the extracellular fluid is 14L at 280 mOsm/L (3,920 mOsm) and the intracellular fluid is 28L at 280 mOsm/L (7,840 mOsm).
• The NaCl added was 2L x 3g/100ml = 60g or 1,026 mol (MW of NaCl = 58.5 g/mol). 1.026 mol NaCl = 2,051 mOsm (1.026 x 2 x 1,000)
• Now, the total body fluid is 44L and the extracelular fluid is 16L with 5,971 mOsm at 373 mOsm/L.
• Then, the intracellular and extracellular concentrations will equilibrate to 313.9 mOsm/L (13,811 mOsm/44L).
• Thus, the extracellular fluid becomes 19.02L (5,971 mOsm/313.9 mOsm/L) and the intracellular fluid becomes 27.98L (7,840 mOsm/313.9 mOsm/L).
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Edema
• Edema is the presence of excess fluid in the body tissues. Broadly, there are 2 types of edema:
- Intracellular edema (less common)
- Extracellular edema
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Intracellular Edema
• Can be caused by depressed metabolism or inadequate nutrition, such as decreased blood flow.
• The membrane pumps become depressed and the cells are unable to pump out the Na+ that leaks in, causing the edema.
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Extracellular Edema
Caused by:Increases in capillary filtration
Increase capillary filtration coefficientIncrease capillary hydrostatic pressureDecrease plasma osmotic pressureBlockage of lymph flow
The net rate of capillary filtration in the entire body is ~ 2ml/min, which is
returned to the circulation by the lymphatic system
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Changes in Capillary Pressures
• You should already be familiar with these factors.• Factors that increase capillary hydrostatic pressure:
- Excessive retention of salt and H2O
- High venous pressure- Decreased arteriolar resistance
• Factors that decrease capillary osmotic pressure: - Loss of plasma proteins (in urine, from deamaged skin,
failure to produce proteins as occurs in cirrhosis or malnutrition).• Factors that increase capillary permeability:
- Immune reactions (histamine release)- Toxins- Burns
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Lymphatic Blockage
• In the body, the mean capillary forces show a net filtration pressure of 0.3 mm Hg, tending to move fluid outward through the capillary pores.
• The net rate of filtration in the entire body is ~ 2 ml/min, which is returned to the circulation by the lymphatic system.
• Edema that occurs by blockage of the lymphatic can be especially severe, because the lymphatic system returns proteins to the blood that pass through the capillaries.
• Thus, in addition to the back-up of fluid, the osmotic pressure of the interstitial fluid increases.
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Factors that Cause Blockage of Lymph Return
• Cancer
• Infection (elephatiasis is caused by a mosquito-born filaria worm)
• Congenital absence or abnormality of the lymph vessels.
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Factors that Prevent Edema
• Low compliance of the interstitium under normal conditions.
• Increased lymph flow.
• Washdown of interstitial fluid protein.
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Interstitium
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Low Compliance of Interstitium
• This is established by the interstitial gel, which is the proteoglycan polymers in the interstitium.
• This prevents fluid from flowing easily.• As a result, the addition of extra fluid causes a large
incease in interstitial hydrostatic pressure (decreasing capillary filtration rate).
• However, too much interstitial fluid creates channels in the gel, increasing the compliance of the interstitium.
• Note – the interstitial gel also helps to prevent excessive fluid from flowing to your feet when standing.
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Low Compliance of Interstitium
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Increased Lymph Flow
• When fluid begins to accumulate in the interstitium, lymph flow can increase 10- to 50-fold.
• The increased lymph flow also helps decrease the [interstitial proteins], called “washdown” (see text).
• “Washdown” occurs because with increased flow, proteins are removed faster than they filter acrosss the capillaries.