Renal: Body Fluid Regulation Flashcards
distribution of body fluids; intracellular, extracellular, plasma
ICF = 40% of body weight
ECF = 20% of body weight
plasma = 4-5% of body weight, 20% of ECF
sources of water loss in ECF
evaporation (skin, respiratory)
renal free water loss - ADH (variable, dependent on overall water status)
obligatory renal losses - solute load (cant concentrate urine past a certain amount)
sources of water loss in the gut
saliva and fecal water
additions of water balance in the gut
feed water, drinking
additions to water balance of ICF
metabolic water; water from metabolism inside the cells
water requirements; what is the primary determinant, additional factors
caloric intake is the primary determinant
caloric intake is a log function of body weight, smaller animals need relatively more caloried, therefore need more water
small dog = 60ml/kg per day
large dog = 40ml/kg per day
additional: environment, activity, dietary solutes
three pathways for water movement and their details
*Lipid pathway: Diffusion of water across lipid bilayer.
- High [Water] + Large surface area counteract general low permeability.
*Water channels: Aquaporins
- Found in tissues where there is a high, regulated water flux (kidneys, gut, rbc)
*Pores/Intercellular gaps - specialized capillaries and lymphatics (eg. glomerulus etc.)
how does water move across cell membranes (3)
Osmotic Pressure
= Hydrostatic pressure required to oppose the movement of water through a semi- permeable membrane in response to an osmotic
gradient.
Osmotic Gradient
= The difference in particle concentrations on 2 sides of a membrane .
Osmolarity
= The concentration of solutes in a solution that exert an osmotic force.
osmolarity vs tonicity
what stops fluid from moving across membrane
Osmolarity refers to the concentration of all the solutes that are in the solution – those that can readily cross a membrane and those that cannot. (Example: Urea can readily cross, Na+ cannot).
Tonicity refers only to the concentration of solutes that cannot readily cross membranes, thus influencing water movement.
So fluid will move across a membrane until the tonicities on each side are the same
what determines the volume of ECF and ICF, what are components of each, intracellular regulation
what happens overall for water balance
The volume of ECF and ICF are determined by the number of osmotically active particles in each compartment.
For ECF: Sodium, Anions (Chloride, Bicarbonate) and Glucose
For ICF: Potassium and Anions (Chloride, Bicarbonate)
The total body content of sodium is the major determinant of extracellular fluid (intravascular + extravascular) volume.
Intracellular fluid volume regulation is more passive, as K+ moves out of
cells through leaky “channels” if the level becomes unbalanced relative to
extracellular sodium.
Therefore, when the body actively regulates water balance overall, it
retains or excretes sodium which then favours water retention or loss.
This is done through hormones that act on the brain for water
consumption and kidney for water loss
what are starling forces and details of each, what do they determine
Physical pressure (P) (hydrostatic) that is generated by the heart and circulatory system. This is usually greater inside the vessel versus outside (Pcap ), so it tends to force fluid out. It is counterbalanced a little by physical pressure in the interstitial fluid (Pif).
Oncotic pressure (π) – the osmotic pressure generated by proteins
dissolved in plasma. These don’t tend to leave the vascular system so they typically “pull” fluid back into the vasculature.
Collectively these are called “Starling” forces – they often determine net movement of fluid into and out of capillaries because ion concentrations are usually stable and balanced. The renal system balances the ions in several complex pathway
what do changes in capillary, interstitium or intracellular factors lead to
fluid distribution
difference in cations and anions in normal physiology
- In normal physiology the levels of cations (+ve) and anions (-ve) in the different body compartments don’t change too much (they tend to be electrochemically balanced and maintained by the various cell transporters like Na/K ATPase etc.)
what can fluid loss or lack of intake inlfuence
- Fluid loss (or lack of intake) can influence fluid volumes in extra and intracellular compartments – when the problem is only a loss of water, then the loss to each compartment is equally shared between all compartments
what happens when ions are lost from the body
- When ions (particularly Na+) are lost to the body due to vomiting, diarrhea or malfunctioning kidneys etc. then fluid balance between compartments also becomes perturbed as the body is limited in its ability to shift fluids osmotically
what do changes to plasma protein levels and capillary pressure lead to
- Changes in plasma protein levels and capillary pressure are other forces that lead to changes in distribution of fluids and can lead to fluid accumulation when they are not properly balanced with osmotic effects – edema and effusions result
what is lack of sufficient body fluid called and causes
Lack of sufficient body fluid =
DEHYDRATION
Can be due to excessive loss (through sweat, urine, feces) OR Insufficient intake
clinical assessment of hydration (initial, clinical, laboratory, ongoing monitoring)
Initial assessment:
* Skin tent (persistent = dehydrated),
* Mucous membranes – moist vs. tacky vs. dry
Other signs: Clinical
Not specific for dehydration but often seen
* Tachycardia
* Sunken eyes
* Delayed capillary refill time
* Blood pressure – decreased in severe cases
Laboratory:
* PCV and total plasma protein – increased in dehydration (if high)
Ongoing monitoring:
* Body weight – not useful at time of presentation, but any change over the course of time in hospital likely
reflects a change in hydration. Should be closely monitored.
* Urine output – provide the kidneys are functional, urine production should match fluid intake (orally or by I.V.).
how is dehydration clinically expressed and what does type of dehydration vary with
- Expressed clinically as % loss in body weight
- Type of dehydration (and fluid required) varies with osmolality of fluid that is lost (or not taken in)
what is the level of hydration for the following clinical signs: not detectable
<5%
what is the level of hydration for the following clinical signs: subtle loss of skin elasticity
5-6%
what is the level of hydration for the following clinical signs: delay in return of skin to normal position, slight increase in CRT, dry mucous membranes
6-8%
what is the level of hydration for the following clinical signs: skin stands in place when tented, increase in CRT, eyes sunken, dry mucous membranes, shock (tachycardia, cold extremities, weak pulses)
10-12%
what is the level of hydration for the following clinical signs: death imminent
12-15%
balanced vs unbalanced fluids, crystalloids vs colloids
- Balanced: Ion concentrations are similar to extracellular fluid
- Unbalanced: Ion concentrations different from ECF
- Crystalloids: Solutions containing ions and solutes capable of moving freely between different fluid containing body compartments.
- Colloids: Solutions containing larger molecular weight molecules that remain in plasma – exert an osmotic effect to retain volume of fluids in the vascular space (dextrans, hetastarch etc.)
fluid movement after IV fluid administration; isotonic solutions
- Isotonic Solutions (0.9% NaCl, Plasmalyte A/148, LRS)
- Increase in ECF volume.
- No osmotic driving force to move fluid to ICF so volume of ICF unchanged
fluid movement after IV fluid administration; hypotonic solution
- Hypotonic Solutions (0.45% NaCl)
- Decrease in ECF osmolality.
- Fluid moves to ICF due to osmotic effects volume of both ECF and ICF increased.
fluid movement after IV fluid administration; hypertonic solution
- Hypertonic Solutions (7.5% NaCl)
- Large increase in ECF osmolality.
- High osmotic driving force to move fluid from ICF to ECF
what is the major cation in ICF vs ECF
ICF = potassium
ECF = sodium
