Body Fluid Compartments Flashcards
1
Q
Extracellular Fluid
A
- includes the blood plasma and interstitial fluid.
- 1/3 total body water
- Major cation: Na+
- Major anions: Cl- and HCO3-
- Plasma 1/4th of ECF
- 1/12 TBW
- Major plasma proteins are albumins and globins
- Major cation: Na+
- Major anion: Cl-, HCO3-, plasma protein
- Interstitial Fluid 3/4 of ECF
- 1/4 TBW
- same as plasma w/ little protein (ultrafiltrate of plasma)
- Major cation: Na+
- Major anion: Cl-, HCO3-
- 60-40-20 rule
- TBW is 60% body weight
- ICF is 40%
- ECF is 20%
2
Q
Intracellular Fluid
A
- sum of the fluid contents of all cells of the body.
- 2/3 of total body water
- Major cations: K+ & Mg2+
- Major anions: protein and organic phosphates
- intracellular water constitutes 30-40% of body weight
3
Q
Transcellular Fluid
A
- specialized fraction of extracellular fluid
- includes cerebrospinal, intraocular, pleural, peritoneal, and synovial fluids plus the digestive secretions.
- Each of the several discontinuous fractions is separated from blood plasma by the capillary endothelium & also a continuous layer of epithelial cells.
4
Q
Measurement of Total body water
A
- Two isotopes of water, deuterium oxide (D2O) and tritiated water (HTO) may be used.
- D2O is quantified by specific gravity techniques
- HTO quantified by measurement of the radioactivity of tritium.
- The drug antipyrine may also be employed.
- measured by chemical means.
- Several hours are allowed for mixing before taking samples.
5
Q
Measurement of Plasma volume
A
- measured as volume of distribution of a substance confined w/in the vascular bed.
- Evans-blue dye and radioiodinated human serum albumin (RISA) are commonly used.
- Evans-blue dye binds with plasma albumin
- RISA is administered precombined with albumin.
- In either instance, the volume is that of plasma albumin
- The concentration of Evans blue is determined colorimetrically
- The concentration of RISA concentration is determined by scintillation counting.
- Also estimated by the distribution of RBCS tagged w/ radioactive phosphorus (32P) or chromium (51Cr).
- From the hematocrit, the blood volume can be calculated.
6
Q
Measurement of Extracellular fluid volume
A
- Less precisely than total body water or plasma volume measure
- No ideal substance is known.
- Some substances used are inulin, sucrose, and radioisotopes of sulfate and mannitol.
- Inulin and sucrose do not diffuse into far places of the extracellular compartments
- tend to underestimate the extracellular volume
- sodium, chloride, bromide and thiocyanate tend to penetrate cells to some extent and overestimate
7
Q
Calculation of intracellular and interstitial water
A
- Intracellular water is calculated as the difference between total body water and extracellular water.
- Intracellular = total body water - extracellular water
8
Q
Extracellular vs. Intracellular Solute Distribution
A
- Predominance of sodium, chloride and bicarbonate in the ECF
- Predominance of potassium, phosphates, and magnesium in ICF
- Substances are transported through the cell membrane by two major processes, diffusion and active transport.
9
Q
Simple diffusion
A
- Random molecular motion requiring an electrochemical gradient for net movement to occur
- net diffusion is always “downhill”)
- Lipid solubility is a major determinant of the diffusibility of any substance
- only very small non-polar substances pass through the membrane by simple diffusion.
- involves no specific interaction between the moving molecule and the proteins of the membrane.
10
Q
Simple facilitated diffusion
A
- produce net movement of a substance only down its electrochemical gradient (thus the term “diffusion”).
- Transport depends on interaction w/ specific membrane proteins that “facilitate” its movement.
- This is an important mechanism for accelerating the movement of non-lipid-soluble molecules
- membrane proteins involved are frequently termed “carriers.”
- exhibits the characteristics of specificity, saturability, and competition. (Not seen in simple)
11
Q
Primary active transport
A
- Molecule interacts w/ membrane carriers and may exhibit specificity, saturability, and competition.
- Hallmark is net uphill transport
- Energy for “active” transport comes directly from splitting ATP or another source of chemical energy.
- “primary” denotes that chemical energy is the direct source of energy for the process
- some cases, ATPase not only splits the ATP but is a component of the actual carrier mechanism.
- Active transport of Na+ is an example of primary active transport.
- Na+ moves across the luminal (apical) membrane into the cell mainly by simple facilitated diffusion.
- Then actively transported across the basolateral membrane into the interstitial fluid.
- This “pump” is primary active involving Na/K-dependent ATPase found only in the basolateral membrane.
- Na+ & water, moves into peritubular capillaries by bulk flow
- final step in the reabsorption of all substances.
12
Q
Coupled facilitated diffusion of two or more substances (Secondary Active Transport)
A
- 2+ substances interact simultaneously w/ the same specific membrane carrier and are transported across the membrane by “facilitated diffusion”.
- This co-transport exhibits specificity, saturability, and competition, just like simple facilitated diffusion.
- Net movement of one of the substances can occur uphill (against its electrochemical gradient).
- The energy liberated by downhill diffusion of one co-transported substance drives the other substance uphill against its electrochemical gradient.
- Sodium is frequently the substance moving downhill in coupled facilitated diffusion systems
- co-transported substance being simultaneously moved uphill undergoes secondary active transport.
- Glucose moves across the luminal membrane by coupled facilitated diffusion w. Na+
- The energy derived from the simultaneous downhill movement of Na+.
- After entering the cell the glucose crosses the basolateral membrane by simple facilitated diffusion
- Overall glucose reabsorption depends upon the primary active Na+ pump in the basolateral membrane.
- This pump maintains the electrochemical gradient for net Na+ diffusion across the luminal membrane,
- Provides the energy for the simultaneous uphill movement of glucose.
- Amino acids, phosphate, and a variety of organic substances undergo secondary active reabsorption by being co-transported with Na+ in this same manner.
13
Q
Osmosis
A
- net movement of water caused by a concentration difference is called osmosis.
- Requires a semipermeable membrane that allows water but not solutes to cross the membrane
- Osmotic pressure is the amount of pressure required to stop osmosis completely.
- The osmotic pressure of nondiffusible particles in a solution is determined by the number of particles per unit volume and not the mass of the particle.
- One osmol is the number of particles in one-gram molecular weight of undissociated solute.
- Two osmols are in each gram of a substance that completely dissociates into two ions, (e.g., NaCl.)
- Osmolality and osmolarity: Two terms that are used almost interchangeably
- The osmolar concentration expressed as osmols per kilogram is referred to as osmolality
- expressed as osmols per liter it is referred to as osmolarity.
14
Q
Addition of water (hypo-osmotic overhydration) effects in ICF and ECF
A
- oral ingestion of a large volume of water or SIADH
- Osmolarity of ECF decreases b/c excess water dilutes
- ECF volume increases b/c water retention
- Water shifts into cells
- ICF osmolarity decreases
- ICF volume increases
- Plasma protein concentration decrease b/c increase ECF volume
- Hematocrit unchanged b/c water enters RBC offsetting diluting effects of increased ECF volume
15
Q
Addition of sodium chloride (hyperosmotic overhydration) effects on ICF and ECF
A
- Hypertonic solution of NaCl IV infusion (or ingestion of sea water)
- Osmolarity of ECF increases b/c osmoles (NaCl) added
- Water shifts from ICF to ECF
- Osmolarity of ICF increases until equal to ECF
- Water shifts so ECF volume increases and ICF decreases
- Plasma protein concentration and hematocrit decrease
- b/c increase in ECF volume
