12- Renal Physiology Body Fluid Compartments Flashcards
What is the main function of the kidney?
Produce urine and regulate the volume and composition of the body fluids within narrow limits.
Describe the amounts for body fluid compartment, starting with total body water.
Total Body Water = Solids and Water (50-70%)
50-70% Water breaks down into = 2/3 ICF and 1/3 ECF
1/3 ECF breaks down into = 80% Interstitial Fluid and 20% Plasma
These compartments are 2/3 volume of body fluid and consist of cytosol within the cell.
ICF
These compartments are 1/3 volume of body fluid and consist of plasma and interstitial fluid.
ECF
What is the 60-40-20 rule?
- 60% of body weight is total body water
- 40% of body weight or 2/3 total body water is ICF
- 20% of body weight or 1/3 total body water is ECF
If there is a 70 kg man, what are the values of the following?
- Total body water
- ECF and ICF
- Interstital Fluid and Plasma
– Total body water = 70 x 0.6 = 42 L
- ECF = 70 x 0.2 = 14 L
- ICF = 70 x 0.4 = 28 L
- Interstital Fluid = 14 x 0.75 = 10.5 L
- Plasma = 14 x 0.25 = 3.5 L
________ is the only fluid that can be acted on directly to control its volume and composition. If the volume and composition of this is regulated, then the volume and composition of the Interstitial Fluid bathing the cells are also regulated.
Plasma
Any control mechanism that operates on plasma in effect regulates the entire ________.
ECF
T/F. ECF is an intermediary between the cells and the external environment. All exchanges of water and other constituents between the ICF and the external world must occur through the ECF. Water added to the body fluids always enters the ECF compartment first, and fluid always leaves the body via the ECF.
True
In most cases, all fluid is normally contained in the intravascular, intracellular, or interstitial spaces. In certain instances, another shift occurs referred to as ________ ________ (part of ECF).
Third Spacing
This occurs when too much fluid shifts from the blood vessels (intravascular) into the “nonfunctional” area of cells (fluid trapped between tissues and organs of the abdomen) – ascites, interstitial area around lungs – pulmonary edema, often as a result of burns.
Third Spacing
***This is problematic, fluid is being lost from the intravascular space.
Fluid compartments are separated by a ________ ________. Utilizing osmosis, water moves from an area of higher concentration of water (more dilute solution, lower solute concentration) to an area of lower concentration of water (more concentrated solution, higher solute concentration).
Semipermeable Membrane
ECF volume must be closely regulated to help maintain BP. Maintaining _______ balance is of primary importance in the long-term regulation of ECF volume.
Salt
ECF osmolarity must be closely regulated to prevent swelling or shrinking of cells. Maintaining _______ balance is of primary importance in regulating ECF osmolarity.
Water
There is presence of cell ________ in the ICF that cannot permeate the enveloping membranes to leave the cells.
Proteins
Unequal distribution of Na+ and K+ and their attendant anions as a result of the action of the membrane-bound ________ present in all cells. This actively transports Na+ out of and K+ into cells.
Na+/K+ Pump
_______ is the primary ECF cation and ________ is the primary ICF cation.
Na+
K+
_______ is a universal solvent and dissolves various compounds that are either an electrolyte or non-electrolyte.
Water
__________ contain covalent bonds that prevent them from dissociating in solution and therefore have no electrical charge (i.e., glucose, lipids, and urea).
Non-electrolytes
__________ dissociate into ions (ionize) in water. Examples are Mg+, Na+, Cl-, and K+.
Electrolytes
Electrolytes have a higher osmotic power than non-electrolytes because each electrolyte molecule dissociates into at least two ions. Electrolytes have a greater ability to cause a _______ _______.
Fluid Shift
This is the measure of the number of osmotically active particles per kilogram of water.
Osmolality
This is the number of osmotically active particles per liter of total solution.
Osmolarity
All body fluid compartments have approximately the same (OSMOLARITY/OSMOLALITY), expressed as the number of osmotically active particles per kilogram of water (290 mOsm).
Osmolality
This is the term for if you urinate >2.5 L/day. Normal is 1-2 L/day. Causes include Diabetes Mellitus, DI, excess caffeine or alcohol, kidney disease, diuretics, sickle cell anemia, excess water intake, etc.
Polyuria (excessive urine production)
This is the term for if you urinate 300-500 mL/day. Normal is 1-2 L/day. Causes include dehydration, blood loss, diarrhea, cardiogenic shock, kidney disease, and enlarged prostate.
Oliguria (output below the minimum volume)
This is the term for if you urinate <50 mL/day. Normal is 1-2 L/day. Causes include kidney failure or obstruction such as kidney stone or tumor.
Anuria (virtual absence of urine)
How can Obligatory Urine Volume be calculated?
Take the minimal amount of solutes that a person must excrete in a day (i.e., active person is 1000 mOsm/day). Divide it by 1200 mOsm/L (maximal volume of urine concentration by kidney).
1000 / 1200 = 0.83 L/Day
***Less than this is Oliguria for this person.
The rate of ________ ________ represents the rate at which solute-free water is excreted by the kidneys.
Free-Water Clearance
If free-water clearance is positive, then excess water is being excreted by the kidneys. This means urine is ________.
Dilute
In the cases when free-water clearance is negative, excess solutes are being removed from the blood by the kidneys and water is being conserved. This means urine is ________ because we’re conserving water.
Concentrated
Whenever _______ osmolarity is greater than ________ osmolarity, free water clearance will be negative, indicating water conservation.
Urine
Plasma
Free-water clearance (CH2O) is calculated as the difference between water excretion (urine flow rate, V) and osmolar clearance. What does this equation look like?
CH2O = V - Cosm = V - [ (Uosm x V) / (Posm) ]
Uosm = Urine Osmolarity V = Urine Flow Rate P = Plasma Osmolarity
How can we measure the volume in fluid compartments?
Take a small amount of dye or other substance contained in a syringe and inject it into a chamber. The substance is allowed to disperse through the chamber until it is in equal concentrations everywhere. A sample of the fluid is removed and the concentration is analyzed.
***This works for virtually any compartment as long as 1) the indicator disperses evenly, 2) the indicator disperses ONLY in that compartment, and 3) the indicator is not metabolized or secreted.
The main solutes in the ECF are…
- Sodium
- Glucose
- Urea
***Using these we can calculate serum osmolality!
What is the equation for serum osmolality?
Osmolality = (Sodium x 2) + (Glucose/18) + (BUN/2.8)
***Should equal 275-295 mOsm/kg!
What is the “eyeball” way to calculate plasma osmolality?
Plasma Osmolality = 2 (Plasma Na+)
This effect occurs when negatively charged proteins on one side of a semipermeable membrane create an osmotic and electrochemical gradient for water to enter the cell. The proteins are impermeable to the membrane.
Gibbs-Donnan Effect