GLOSSARY OF TERMS

Mass and Water Distribution

The figure below provides a graphical representation of the relationship of mass and water distribution.

Dehydration

Refers to loss of total body water (TBW), with TBW consisting of intra-cellular water of fluid (ICF) and extra-cellular water or fluid (ECF).

A decrease in TBW can be due to a loss of ICF, ECF or both. Dehydration results from prolonged sweating without adequate fluid (water and electrolyte) replacement, excess kidney water and electrolyte losses (diuresis) that results from disease or use of diuretics, and excessive bleeding (trauma, ulcers).

Euhydration

The state of normal hydration

Extra-cellular Fluid (ECF) Compartment

All of the fluid that is outside of the cells, including interstitial fluid (fluid surrounding cells within tissues and organs), lymph and blood plasma (the non-cellular fluid of the blood).

Comprises ~1/3 of Total Body Water, or ~20% of total body mass.

Interstitial fluid and plasma provides a medium for gas and ion exchange with cells, transfer of nutrients and excretion of metabolic end products.

Plasma volume (PV) and interstitial fluid account for 5% and 15%, respectively of Total Body Mass.

Fat Mass (FM)

All extractable lipids from adipose and other tissues in the body.

FM = Total Body Weight – Lean Body Mass (LBM)

Impedance

Impedance, expressed in Ohms, is the measure of the degree to which an electric circuit resists electric-current flow when a voltage is impressed across its terminals.

Impedance is the measurement of the resistance to the flow of current in a body.

Water and its electrolytes are a good conductor of current. Hence, the less the fluid volume, the higher the impedance value. Impedance would therefore be greater in a state of dehydration than normal hydration.

Interstitial Fluid

Fluid surrounding cells within tissues and organs.

It is a sub-compartment of the ECF.

Comprises ~80% of ECF volume or ~15% of total body mass.

Intra-cellular Fluid (ICFV)

The fluid within ALL cells of the body.

Comprises ~2/3 of Total Body Water, or ~40% of total body mass.

ICFV is estimated as the difference between TBW and ECFV.

ICF=TBW-ECF

Intravascular Fluid or Plasma Volume

It is a sub-compartment of the ECF.

Comprises ~20% of ECF volume, or ~5% of total body mass.

Lean Body Mass (LBM)

Consists of all water, muscle (protein), bone, minerals and other non-fat tissue.

~75% of LBM consists of water.

Plasma Volume

Also known as intravascular fluid, the fluid non-cellular portion of the vascular (blood) compartment.

It is a sub-compartment of the ECF.

Comprises ~20% of ECF volume or ~5% of total body mass.

Rehydration

Refers to re-gain of water.

Sodium (Na+)

Sodium (Na+) is the major osmotically active cation in the ECF compartment. Therefore changes in total body Na+ content are paralleled by corresponding changes in ECF volume.

When total Na+ content is low, ECFV volume will also be low (extra-cellular dehydration).

When total Na content is high, volume overload may develop, but the kidneys rapidly excrete the extra Na+ and water.

Total Body Water (TBW)

The total amount of fluid in the body of the horse whether in cells, outside cells, in blood etc.

Since most of the body water is contained in the Lean Body Mass (75% of LBM = TBW), the body water percentage will increase with a loss of fat weight and a gain in lean tissue.

• Total body mass is ~60 % water by mass in a euhydrated horse
• Water is located either inside and outside the cells
  • 2/3 is located inside cells (ICFV)
  • ~1/3 is located outside cells (ECFV) of which:
    • 1/5 ECFV is located inside blood vessels (intra-vascular)
    • 4/5 ECFV is located in extra-vascular (interstitial) space.

Distribution of water between the ICF and ECF compartments is determined by electrolyte distribution and the concentrations of osmotically and oncotically active molecules in the ICF and ECF compartments

Distribution of water within the ECFV between the intra-vascular and interstitial spaces is determined by proteins (oncotic effects).

Total Body Fat (Fat)

All bodies contain fat. The fat may be deposited in large tissue masses called adipose, which is primarily found under the skin and in the abdominal region.
Fat is also deposited within many other types of cells, including skeletal muscle.
Fat is a storage form of metabolic fuel.

Volume Depletion

See dehydration (note: horses only very rarely vomit).

Reproduced with the permission from Dr M Lindinger