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The Body fluid Compartments:

Extracellular and Intracellular Fluids; Interstitial Fluid and Edema

Maria Clarideth N. De Castro-Guanzon,MD

 

 

 


FLUID INTAKE AND OUTPUT

 

Daily Intake of water

 

2 major sources:

1.       Food in the form of liquid and water: 2000ml/day

2.       Synthesized from the oxidation of carbohydrates: 200ml/day

 

Daily intake and Output of Water (ml/day)

 

 

Normal

Prolonged Exercise

Intake

 

 

  Fluids ingested

2100

?

  From metabolism

200

200

     Total intake

2300

?

Output

 

 

  Insensible – Skin

350

350

  Insensible – Lungs

350

650

  Sweat

100

5000

  Feces

100

100

  Urine

1400

500

     Total output

2300

6600

 

 

BODY FLUID COMPARTMENTS

 

1.       Extracellular fluid – 20% body weight (14L)

a.       Plasma- ¼ of ECF

b.       Interstitial fluid- ¾ of ECF

3.       Intracellular Fluid- 40% of total body wt.

Transcellular Fluid- includes synovial, peritoneal, pericardial, cerebrospinal, intraocular spaces (1-2 liters).

 

BLOOD VOLUME

 

-          Contains both ECF (fluid in the plasma) and ICF (fluid in the RBC).

-          Considered to be a separate fluid compartment because it has a chamber of its own (circulatory system).

-          Average bld. Vol. = 5L (7% of body wt.).

-          60% is plasma, 40% is RBC.

 

 

HEMATOCRIT (packed red cell volume)

 

-          Is the fraction of blood composed of rbc as determined by centrifuging in the hct. tube

-          Male - 0 .40, Female - 0 .36 (below these = anemia)

-          hct. = 0.10 – not compatible with life

-          hct. 0.65% - polycythemia

 

CONSTITUENTS OF ECF AND ICF

 

-          Ionic composition of plasma and interstitial fluids are almost similar.

-          Difference lies on the concentration of plasma proteins “Donnan Effect“

-          Concentration of of (+) charged ions (cations) is slightly greater in the plasma because the plasma proteins, being negatevely charged tend to attract cations ( Na+, K+).

-          Conversely, (-) charged ions (anions) tend to have slightly higher concentration in the interstitial fluid bec. The (-) charges of the plasma    proteins repel the (-) charged anions.

 

Major Cations and Anions

ECF

ICF

Na++

K+

Ca++

Mg++

Cl-

PO4 and organic ions

HCO3

Proteins

 

MEASUREMENT OF BODY FLUID VOLUME

 

Total body water                        3H20, 2H20 antipyrine

Extracellular Fluid           22Na, 125I-iothalamate, thiosulfate, inulin

Intracellular fluid            (calculated as Total body water-ECF)

Plasma volume              125-albumin, Evans blue dye (T-1824)

Blood volume                 51 Cr-labeled red blood cells; or calculated as Blood volume= Plasma volume / (1 -hematocrit)

Interstitial fluid               Calculated as ECF vol. - Plasma volume

 

 

BASIC PRINCIPLES OF OSMOSIS AND OSMOTIC PRESSURE

 

OSMOSIS - net diffusion of H20 across a selectively permeable membrane from a region of high H20 concentration to that with a lower H20 concentration.

 

Rate of osmosis - rate of diffusion of water.

 

MOLES AND OSMOLES

 

OSMOLES

-          refers to the total number of particles in a solution

-          Unit for expressing osmotic activity of solutes in the body fluid

-          Also known as mOsm/L

 

1 osmole (osm) = 1 mole of solute particles

ex.        1 mole of glucose/liter= 1 osm/L

1 mole NaCl/liter = 1 osm/L

1 mole NaCl à Na+ + Cl- = 2 osm/L

 

OSMOLARITY - osmoles per liter solution

OSMOLALITY - osmoles per kg water

 

OSMOTIC PRESSURE

-          The amount of pressure required to prevent osmosis

-          Indirect measurement of water and solute concentration high osmotic pressure

= high solute conc.

= low H20 conc.

EFFECT OF DIFFERENT CONCENTRATION OF IMPERMEANT SOLUTES IN THE ECF ON CELL VOLUME

 

Intracellular fluid = 282 mOsm/L

 

1. Isotonic fluid

-          No shift of fluid

-          No shrinking of cell

ex. 0.9% NaCl soln., 5% glucose solution

2. Hypotonic fluid- swelling of cell

-          equilibrium

ex. NaCl soln. less than O.9%NaCl

3. Hypertonic

-          H20 will move out of the cell

-          Cell will shrink

ex. > 0.9% NaCl soln.

 

VOLUMES AND OSMOLALITIES OF ECF AND ICF IN ABNORMAL STATES

 

Basic principles:

1.       Water moves rapidly across cell membranes.

2.       Cell membranes are almost completely impermeable to many solutes, therefore, the number of solutes in the ECF remains constant unless solutes are added to or lost from the ECF.

 

CLINICAL ABNORMALITIES OF FLUID REGULATION

 

1. Hyponatremia- serum N< 142meq/L

 

a.       Hypo-osmotic dehydration – dec. ECF vol.

-          loss of NaCl

-          ex. Vomiting , diarrhea, diuretic overuse

-          Addison’s disease - dec. aldosterone

b.       hypo-osmotic overhydration

-          sec. addition of water to ECF

-          ex. Excessive secretion of ADH         

 

2. Hypernatremia- serum Na > 142 meq/L

 

a.       loss of H20 from the ECF

-          ex. Central DI - inability to secrete ADH therefore inc. NaCl in ECF, dilute urine

-          Nephrogenic DI- inability of the kidneys to respond to ADH

b.       hyperosmotic overhydration - excessive secretion of aldosterone

           

EDEMA

-          Excess fluid in the body tissues

-          Mainly occurs in ECF

 

Intracellular edema

2 predisposing conditions:

1.       Depression of metabolic systems of the tissue

2.       Lack of adequate nutrition to the cells

 

Extracellular edema

Causes:

1.       Abnormal leakage of fluids from plasma to the interstitial spaces across the capillaries (inc. capillary filtration)

2.       Failure of lymphatics to return fluid from interstitium back into the blood.

 

SUMMARY OF CAUSES OF EXTRACELLULAR EDEMA

            I. Increase capillary pressure

            II. Decrease Plasma Proteins

            III. Increase capillary permeability

            IV. Blockage of Lymph return

 

3 major factors that cause increased capillary filtration of fluid and protein into the interstitium:

1.       increased capillary hydrostatic pressure

2.       decreased plasma colloid oncotic pressure

3.       Increased capillary permeability (causes leakage of proteins and fluids through the pores of the capillaries)

 

EDEMA CAUSED BY HEART FAILURE

-          Inc. venous pressure

-          Inc. cap. Pressure

-          Dec. arterial pressure à activation of RAS

-          Left-sided heart failure à inc. pulmonary vasc. Pressure and pulm. Cap. Pressure à pulmonary edema

 

EDEMA CAUSED BY DEC. KIDNEY EXCRETION OF SALT AND WATER

Main effects:

1.       inc.interstitial fluid vol.

2.       hypertensiom

-          ex. Acute glomerulonephritis

 

EDEMA CAUSED BY DEC. PLASMA PROTEINS

-          ex. Nephrotic syndrome, liver cirrhosis

 

SAFETY FACTORS THAT NORMALLY PREVENT EDEMA

 

1.       Low compliance of interstitium when interstitial fluid is in the negative range (-3mmHg) in the gel form).

2.       The ability of lymph flow to increase 10-50 folds

3.       Washdown of interstitial fluid protein conc.  Which reduces interstitial fluid colloid osmotic pressure as capillary filtration increases.

 

SUMMARY OF SAFETY FACTORS THAT PREVENT EDEMA

 

1.       The safety factor caused by low tissue compliance in the negative pressure range is          about 3mmHg.

2.       The safety factor caused by increased lymph flow is about 7 mmHg.

3.       The safety factor caused by washdown of proteins from the interstitial spaces is about 7mmHg.

           

Total safety factor = 17mmHg