Body fluids

Question 1

What is the distribution of body fluids?

Answer 1

• 60% water - total weight of body is 60% water, which of this is
  
  • 40% body weight intracellular water
  • 20% body weight extracellular water

Question 2

What are the typical volumes of body fluids for each compartment in a 70kg male?

Answer 2

• 28L - Intracellular water
• 11L - Interstitium (extracellular fluid)
• 3L - Plasma water ( this is also extracellular fluid)

Question 3

Why is fluid composition inside and outside of the cell different?

Answer 3

Intracellular has lots of potassium, phosphate

Extracellular has more sodium and chloride

This gradient is important for generating electric fields and action potentials

Movement of molecules:
- Depends on pressure
- Ionic pressure
- Osmotic pressure
- Depends on nature of barrier
- Cell membrane - semi-permeable, allows water only
- Capillary wall - more permeable (capillaries one cell thick), allows water, electrolytes but not proteins (too big)

Question 4

Describe the cell membrane barrier

Answer 4

• Lipid bilayer with proteins
• Impermeable to most ions and proteins; has pumps
• Water allowed to move freely
• Osmotic pressure drives movement of water

Question 5

Describe the capillary wall barrier

Answer 5

• Endothelial cells, semi permeable pores
• Only large molecules (e.g. proteins) can exert an osmotic pressure across it
• Hydrostatic pressure drives movement of water

Question 6

Define diffusion

Answer 6

Movement of molecules from high to low concentration due to random motion

Question 7

Define osmosis

Answer 7

Net diffusion of water across selectively permeable membrane from region of high concentration to low concentration

Question 8

What is osmotic pressure?

Answer 8

Pressure exerted on cell membrane due to differences in solute concentrations between cell and its environment

Osmotic pressure will decrease as water moves into area of lower water concentration from an area of higher water concentration

Question 9

What is the osmole?

Answer 9

Unit of measurement to describe the concentration of solute particles

1 osmole = 1 ionic particle, so Na2SO4 = 3 osmoles, due to 2Na+ and 1 SO42- particle being what Na2SO4 is comprised of

Question 10

What is tonicity?

Answer 10

The effect of a solution (and solutes that cannot cross) on osmotic movement of water across cell membrane

Question 11

Describe the tonicity of NaCl, Glucose and Urea

Answer 11

NaCl - 150mOsm , it’s isosmotic and isotonic

Glucose - 300mOsm, it’s isosmotic, NOT isotonic

Urea - 300mOsm, it’s isosmotic, NOT isotonic

Question 12

What does isosmotic mean?

Answer 12

Isosmotic refers to the situation of two solutions having the same osmotic pressure

Question 13

What does isotonic mean?

Answer 13

Isotonic refers to a solution having the same solute concentration as in a cell or a body fluid

Question 14

Describe 3 methods that can be used to measure volumes in the different compartments

Answer 14

• Dilution method
  
  • Concentration = amount/volume
  • Volume = (amount added - amount lost)/concentration
• Directly measured volumes
  
  • Total body water: Radioactive water, distributed ALL COMPARTMENTS
  • EXTRACELLULAR VOLUME: Radioactive Insulin, excluded from cells
  • PLASMA VOLUME : Radioactive albumin, stays in plasma only
• Indirectly measured volumes
  
  • INTRACELLULAR VOLUME- TBW (total body water) - extracellular volume
  • INTERSTITIAL VOLUME - Extracellular volume - plasma volume

Question 15

Describe the osmotic pressures of ICF and ECF

Answer 15

Osmotic pressures of both tend to be balanced

Question 16

Describe the osmotic pressure gradient

Answer 16

The difference in concentration between two solutions on either side of a semipermeable membrane

Question 17

How does osmolality of a solution influence cell size?

Answer 17

Hypertonic solution - too much solute, not enough water, lots of water moves out of cells, cells become shrunken, crenated

Hypotonic- too much solute inside cell, water moves in to balance, cells begin to lyse, and if too much water moves in, cells begin lysing (exploding basically)

Question 18

What does a solution having high osmolarity mean?

Answer 18

It has a high concentration of solutes, therefore, water would move out of cells in an attempt to create equilibrium of osmotic pressures

Question 19

What does a solution having low osmolarity mean?

Answer 19

It has a low concentration of solutes, therefore, water moves into cells from the extracellular compartment in order to maintain the osmotic pressures in both fluid compartments

Question 20

How does fluid in the interstitium cause oedema?

Answer 20

• Increased capillary fluid pressure
• Decreased capillary Osmotic press
• Leaky membrane (Coefficient)
• Blocked lymphatics

Question 21

Define isosmotic

Answer 21

A solution in which the osmotic pressure of the ICF & ECF
compartments are the same, therefore there is no net movement of water.

Question 22

Define isotonic

Answer 22

A solution in which the osmolarity of intracellular and
extracellular fluids are the same, therefore there is no net movement of water.