Renal: Body Fluid Regulation

Question 1

distribution of body fluids; intracellular, extracellular, plasma

Answer 1

ICF = 40% of body weight

ECF = 20% of body weight

plasma = 4-5% of body weight, 20% of ECF

Question 2

sources of water loss in ECF

Answer 2

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)

Question 3

sources of water loss in the gut

Answer 3

saliva and fecal water

Question 4

additions of water balance in the gut

Answer 4

feed water, drinking

Question 5

additions to water balance of ICF

Answer 5

metabolic water; water from metabolism inside the cells

Question 6

water requirements; what is the primary determinant, additional factors

Answer 6

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

Question 7

three pathways for water movement and their details

Answer 7

*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.)

Question 8

how does water move across cell membranes (3)

Answer 8

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.

Question 9

osmolarity vs tonicity

what stops fluid from moving across membrane

Answer 9

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

Question 10

what determines the volume of ECF and ICF, what are components of each, intracellular regulation

what happens overall for water balance

Answer 10

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

Question 11

what are starling forces and details of each, what do they determine

Answer 11

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

Question 12

what do changes in capillary, interstitium or intracellular factors lead to

Answer 12

fluid distribution

Question 13

difference in cations and anions in normal physiology

Answer 13

• 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.)

Question 14

what can fluid loss or lack of intake inlfuence

Answer 14

• 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

Question 15

what happens when ions are lost from the body

Answer 15

• 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

Question 16

what do changes to plasma protein levels and capillary pressure lead to

Answer 16

• 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

Question 17

what is lack of sufficient body fluid called and causes

Answer 17

Lack of sufficient body fluid =
DEHYDRATION

Can be due to excessive loss (through sweat, urine, feces) OR Insufficient intake

Question 18

clinical assessment of hydration (initial, clinical, laboratory, ongoing monitoring)

Answer 18

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.).

Question 19

how is dehydration clinically expressed and what does type of dehydration vary with

Answer 19

• 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)

Question 20

what is the level of hydration for the following clinical signs: not detectable

Answer 20

<5%

Question 21

what is the level of hydration for the following clinical signs: subtle loss of skin elasticity

Answer 21

5-6%

Question 22

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

Answer 22

6-8%

Question 23

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)

Answer 23

10-12%

Question 24

what is the level of hydration for the following clinical signs: death imminent

Answer 24

12-15%

Question 25

balanced vs unbalanced fluids, crystalloids vs colloids

Answer 25

• 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.)

Question 26

fluid movement after IV fluid administration; isotonic solutions

Answer 26

• 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

Question 27

fluid movement after IV fluid administration; hypotonic solution

Answer 27

• Hypotonic Solutions (0.45% NaCl)
• Decrease in ECF osmolality.
• Fluid moves to ICF due to osmotic effects volume of both ECF and ICF increased.

Question 28

fluid movement after IV fluid administration; hypertonic solution

Answer 28

• Hypertonic Solutions (7.5% NaCl)
• Large increase in ECF osmolality.
• High osmotic driving force to move fluid from ICF to ECF

Question 29

what is the major cation in ICF vs ECF

Answer 29

ICF = potassium

ECF = sodium