Human Body Composition Flashcards
2 major fluid compartments of the body
Intracellular fluid (ICF) Extracellular fluid (ECF)
2 subcompartments of the ECF
Intravascular
Interstitial (extravascular)
Percent of body weight occupied by water in an average man
60%
Percent body weight occupied by ICF
40%
Percent body weight occupied by ECF
20%
Percent body weight occupied by water in old age
50%
Percentage of ECF water in the interstitial compartment
75 - 80%
Percentage of ECF water in the vascular compartment
20 - 25%
Name of fluid in the intravascular compartment
Plasma
Via which compartment does water enter and leave the body?
Intravascular compartment
What determines the volume of water that ends up in a given compartment?
Osmotic pressure (the number of osmotically active particles within) Intravascular hydrostatic pressure
Define an osmole
The number of osmotically active particles in a solution
Define an osmotically active particle
A dissolved molecule (solute) that cannot penetrate through a water-permeable membrae
Semi-permeable membrane separating the ECF and ICF
Cell membrane
4 naturally occurring molecules that can freely penetrate cell membranes
Oxygen
Carbon dioxide
Water
Urea
Solute composition of the ICF
Sodium and chloride = very low
Potassium, magnesium and phosphate = very high
Protein = very high (approx. 200 g/L)
Solute composition of the ECF
Sodium = high (140 mmol/L) Chloride = high (105 mmol/L) Potassium = very low (4 mmol/L) Protein = very low (relative to ICF)
Semi-permeable membrane separating the interstitial and intravascular compartments of the ECF
Capillary wall
2 components of the bloodstream
Fluid (plasma)
Cells (blood cells)
What do the capillaries retain in their lumen (despite being porous)
Blood cells
High molecular weight proteins
One of the main reasons why capillaries retain high molecular weight proteins
To create an osmotic pressure to counter-balance the hydrostatic pressure that would otherwise force plasma water out of them into the interstitial compartment and deplete the intravascular compartment
Approximate hydrostatic pressure inside the capillaries relative to the arterial pressure
Considerably less than half (but far from zero)
Pressure in the interstitial compartment
Approximately zero
Define colloid oncotic pressure
Inward directed osmotic pressure (in the capillaries)
WHat exerts the colloid oncotic pressure?
The concentration difference of high molecular weight proteins across the capillary wall
Main high molecular weight protein in the capillary responsible for the colloid oncotic pressure
Albumin
Define the hematocrit
Content of blood occupied by red blood cells (40% normally)
Percentage of blood occupied by plasma
60%
Most acutely critical body compartment
Intravascular compartment
Purpose of the interstitial compartment in terms of water balance
Buffer to minimize volume fluctuations in the intravascular compartment (since it is a zero pressure system)
Speed of equalization between the ECF sub-compartments’ osmolarity and why
Rapid (minutes) because the ECF’s main osmoles (sodium and chloride) permeate easily through the capillary wall
Speed of equalization between the ECF and ICF osmolarity and why
Long (hours) because it occurs as a result of water shifts alone
Standard osmolarity of all fluid compartments at equilibrium
300 mOsm/L
Normal serum sodium concentration
140 mmol/L
Importance of serum sodium concentration measurement
Indication of osmolarity in all of the fluid compartments
What fluid does the intracellular compartment contain
Water inside the body’s cells (mostly skeletal muscle cells)
Anatomic correspondence of the intracellular compartment
Body cell mass (BCM)
Reason why the volume of the ICF compartment is large
1) A lot of cells in the body
2) These cells exert a strong osmotic pressure
Why do the cells of the body exert a strong osmotic pressure?
High concentrations of potassium, magnesium, phosphate and soluble proteins
Define a hypotonic solution
Solution that has a relatively low osmolarity
Define a hypertonic solution
Solution that has a relatively high osmolarity
Effect of putting a cell in a hypotonic environment
Unequal osmotic pressures will drive water into the cell –> swell to a larger volume
Effect of putting a cell in a hypertonic environment
Unequal osmotic pressures will drive water out of the cell –> shrink in size
Define simple dehydration
A medical condition accompanying hypertonicity of the body’s fluid compartments
Define relative dehydration
A disproportionately large amount of sodium in relation to the volume of water in the ECF (hyperosmolarity)
Define hypernatremia
Abnormally high serum sodium concentration
Most frequent cause of simple dehydration
Failure or inability of the patient to drink enough water to compensate for body losses of water
Key symptom pointing to relative dehydration
Thirst