Lecture 13: water, fluid, spaces and compartments

Question 1

In what 2 compartments are body fluids distributed into?

Answer 1

1. intracellular fluid compartment ICF
2. **Extra cellular fluid compartment ** ECF which is subdived into …
   * interstitial fluid (fluid in the spaces between cells)
   * intravascular fluid (fluid within the blood)

Question 2

what % of body weight is water?

Answer 2

60 %

Question 3

what % of body weight is water & what % of it is distributed into the 2 compartments?

Answer 3

body weight is 60 % water
* 40% of body weight water is ICF
* 20% of body weight water is ECF

Question 4

what is the equation for measuring blood volume?

Answer 4

BV (blood volume) = volume of blood (both red cells & plasma)/ 1- the haemtocrit

Question 5

How do the kidneys regulate water excretion?

Answer 5

• water diffuses across the plasma membrane of most cells via water channels called aquaporins
• the type and number of aquaporins can be altered in response to various signals eg ADH
• the late distal tubules and collecting tubules of the kidney express numerous aquaporins that can be increased or decreased depending on the** total amount of water in the body**

Question 6

Compare the number of aquaporins expressed in situations of dehydration vs overhydration

Answer 6

• dehydration - high expression of aquaporins in tubules as this increases water absorption back into blood
• overhydration - low expression of aquaporins as more water needs to be excreted

Question 7

In which body fluid compartments does the fluid exchange occur?

Answer 7

there is continual internal fluid exchange between the
1. plasma (intravascular fluid) and the interstitial fluid
2. the interstitial fluid and the intracellular fluid

Question 8

what is the exchange of water between body fluid compartments facilitated by?

Answer 8

it is facilitated by 2 forces : **hydrostatic pressure and osmotic pressure **

Question 9

Explain osmotic pressure

Answer 9

• the pressure that must be applied to a solution to** prevent the net movement of water into it **
• ie the pressure needed to stop osmosis

Question 10

Explain** hydrostatic pressure**

Answer 10

• hydrostatic pressure is the** pressure that is exerted by a fluid **at equilibrium due to the force of gravity that pushes fluid out of the capillaries into the interstitial fluid

Question 11

Explain oncotic pressure

Answer 11

• type of osmotic pressure generated by impermeable proteins in the solution
• the pressure exerted by large plasma proteins in the plasma fluid to pull water back into the capillaries
• albumin plays a role in this

Question 12

what are the** main drivers **of solute movement between the intracellular and interstitial fluid compartment?

Answer 12

• osmotic pressure
• electrochemical gradient (ie the seperation of charge across a plasma membrane (membrane potential) provides the electrical force that drive positive ions into the cell and negative ions out)

Question 13

what are the main drivers for solute movement between the interstitial fluid and the plasma compartment?

Answer 13

• hydrostatic pressure
• oncotic pressure

Question 14

Explain the exchange between the interstitial fluid and intracellular fluid

Answer 14

• the cell membrane between the IF and the ICF has a low permeability to certain ions eg Na+
• however, water can cross freely across the cell membrane so that ICF and IF are in osmotic equilibrium
• changes in the ionic content of the ICF or IF will cause movements of water between the 2 compartments

Question 15

what would happen if we added more Na+ into the ECF - ie the interstitial fluid eg by ingesting NaCl?

Answer 15

the extra solute in the interstitial fluid would attract water from the intracellular fluid until the osmolarity of the intracellular and interstitial fluids were equal

Question 16

Explain the exchange between intravascular fluid and interstitial fluid compartments

Answer 16

• remember the plasma fluid compartment contains the capillaries - where exchange takes place
• water and electrolyes move continously throug**h capillary walls between the plasma and interstitial fluid in both directions
• At the** arteriolar end** of the capillary, hydrostatic pressure contributes to the movement of H20 and electrolytes as this pressure** favours filtration** - ie pushing out
• At the **venous end **of the capillary, the osmotic pressure is greater than the hydrostatic pressure which causes a movement of fluid back into the capillary (as plasma proteins pull fluid in)

Question 17

what are the units of osmotic pressure?

Answer 17

• osmoles - ie osmolality
• usually expressed as mOsm/Kg

note the osmole is comparable to the mole

Question 18

what is the difference between osmolality and osmolarity?

Answer 18

1. osmolality = the number of solute particles per weight of solvent in** kilograms **
2. osmolarity = the number of solute particles per volume of solution in L

Question 19

what is the tonicity of a solution?

Answer 19

tonicity of a solution relates to its effect on the volume of the solution

recal isotonic, hypertonic and hypotonic solution

Question 20

what is an isomotic solution?

Answer 20

a solution that has the same osmolarity or solute concentration as another solution
* if this solution is seperated by a semi permeable membrane (ie permeable to h20 but not solutes), water will move in equal parts out of each solution and inter the other

Question 21

what is a hypertonic solution?

and give example

Answer 21

a solution that has a higher solute concentration / osmolarity than another solution
* example : the inferior of a red blood cell in comparison to the solute concentration of fresh water

Question 22

what is a** hypotonic solution**?

Answer 22

a solution with a lower osmolarity or solute concentration than another solution

Question 23

what are the 3 dehydration states?

Answer 23

1. isotonic dehydration
2. hypertonic dehydration
3. hypotonic dehydration

Question 24

explain isomotic dehydration

clinical causes, and what the outcome is ie does volume of ECF increase

Answer 24

• causes: blood loss (haemorrhage) & diarrhoea / vomiting (gastointestinal fluids-faeces and vomit are isotonic)
• water does not move between the compartments as osmolality stays the same
• volume of ECF decreases
• there is no change in volume or osmolality of the ICF compartment

Question 25

Describe hyperosmotic dehydration

state the causes, what happens to the ECF, what happens to ICF?

Answer 25

causes: decreased water intake, diabetes insipidus, alocholism etc IE we are loosing excess water over salt
* the volume of the ECF decreases
* the osmolality of the ECF increases
* This causes water to move from the ICF into the ECF
* the volume of the ICF decreases
* the osmolality of both compartments will equalise at a later stage

Question 26

Describe Hypoosmotic dehydration

state causes, what happens to the ECF, what happens to the ICF?

Answer 26

causes: addisons disease (renal loss of NaCl) & sweating from running on very hot day - ie a loss of solute in excess of water
* the volume of the ECF decreases
* the osmolality of the ECF decreases - as solutes have been removed
* water moves from the ECF into the ICF
* the volume of the ICF increases
* osmolalities of the ICF and ECF equalise at later stages

Question 27

What are three different over hydration scenarios?

Answer 27

• iso-osmotic over hydration
• hyperisomotic overhydration
• hypoisomotic overhydration

Question 28

Describe isoosmotic overhydration

what are the causes, explain ECF and then ICF

Answer 28

causes: **oral admin of large volumes of isotonic NaCl solution **
* volume of the ECF increases
* as the administered solution is isotonic, there is no change in the osmolality of the ECF
* both compartments have the same osmolality, so water will not move between the compartments
* volumes of ICF does not change

Question 29

Describe hyperosmotic overhydration

state causes, describe ECF and then ICF

Answer 29

causes: **administration of a hypertonic NaCl solution **
* volume of ECF increases
* because the administered solution is hypertonic, the osmolality of the ECF increases
* this causes water to move via osmosis from the ICF into the ECF
* therefore the volume of the ICF decreases and the osmolaity increases (as water is leaving)
* osmolality equalises at later stage

Question 30

Describe hypoosmotic overhydration

Answer 30

causes: administration of hypotonic Nacl solution OR ingestion of large volumes of h20
* the volume of the ECF increases
* as the administered solution is hypotonic, the osmolality of the ECF decreases
* this causes the movement of water via osmosis from the ECF into the ICF
* the volume of the ICF increases and the osmolality decreases
* the osmolality equalises at later stages