Week 2: Osmosis/Tonicity

Question 1

Osmosis

Answer 1

Movement of water down its concentration gradient due to its thermal motion

Question 2

Water Concentration

Answer 2

Water concentration is determined by the number of solute particles in solution (not on their size)

High solute concentration = low water concentration
Low solute concentration = high water concentration

Question 3

Osmotic Pressure

Answer 3

Osmotic pressure is the pressure applied to a solution to stop osmosis (proportional to the number of solute particles)

It is also measure of the “tendency” of a solution to take in water by osmosis (see this again in Starling Forces)

Basically just the pressure of the water flowing into solution due to osmosis

Question 4

Osmotically Active Particles

Answer 4

The osmol is the unit used to describe the number of solute particles in solution that cause osmosis (osmotically active particles)
Units of concentration:
Osmolality* = # of Osmol/Kg of water (used more often)
Osmolarity* = # of Osmol/L of solution
*for this course, we will consider these to be the same.

Ex. 1.5M CaCl2 –> 1.5M Ca++ and 3M Cl2

Question 5

Tonicity

Answer 5

Tonicity is the ability of a solution to cause osmosis across a biological cell membrane
The osmolality of cellular fluid is approximately 290-300 mOsmoles/kg water (use 300)
Remember – homeostasis!! You want to maintain this number (300) at relatively constant values

Consider 3 solutions: hypotonic (lower), isotonic (same) and hypertonic (higher) solution

WATER ALWAYS MOVES TO THE HIGHER SOLUTE CONCENTRATION

Question 6

Cystic Fibrosis

Answer 6

CF is a genetic disorder that affects (among other organs) the lungs
It’s caused by an abnormal protein called the CF transmembrane conductance regulator (CFTR)
Part of a special chloride transporter that regulates the components of mucus lining the lungs (and sweat/digestive fluids)

Question 7

Periciliary Liquid

Answer 7

Allows cilia to move (between cilia and mucus)

Question 8

What would happen to the periciliary liquid if the Cl- transporter did not function?

Answer 8

Cl- builds up inside the cell
Negative charge will build up inside the cell
We will get more sodium moving in
Cellular fluid will become more concentrated so that H2O moves into the cell rather than out
Periciliary liquid is diminishing
Mucous cannot be removed, airways become narrow (infections)

Question 9

Darrow Yannet Diagram

Answer 9

Helps determine the net movement of water across the cell membrane
A way to look at relative changes in the volume and concentration of the ICF and ECF compartments…

Volume on x
Osmolality on y

ECF < ICF

Question 10

Water Load

Answer 10

Example: hyponatremia

Volume of ECF increases and osmolality decreases (initial)
Water moves into ICF by osmosis
Its concentration decreases
Volume of both compartments increases and osmolality decreases (compared to normal) (Final)

Question 11

Water Loss

Answer 11

Example: sweating (hypotonic of body fluids)

ECF volume decreases and osmolality increases (initial)
Water moves out of ICF into ECF by osmosis
Solutes stay behind in ICF so the Osmolality increases
Volume of ECF and ICF decreases (compared to normal) (Final)
Osmolality of both compartments increases (compared to normal) (Final)

Question 12

Water Loss

Answer 12

Example: sweating (hypotonic of body fluids)

ECF volume decreases and osmolality increases (initial)
Water moves out of ICF into ECF by osmosis
Solutes stay behind in ICF so the Osmolality increases
Volume of ECF and ICF decreases (compared to normal) (Final)
Osmolality of both compartments increases (compared to normal) (Final)

Question 13

Solute Load

Answer 13

Example: drinking too much salt

Osmolality of ECF increases (and vol increase with sea water) (Initial)
Water moves out of ICF into ECF by osmosis
Osmolality of both compartments increases compared to normal (Final)
Volume of ECF increases and decreases in ICF compared to normal (Final)

Question 14

Solute Loss

Answer 14

Example: Giving blood, kidney disease (dialysis) - if you donate 500mL and drink 500mL water, you will gain the volume but you will lose the solute

ECF volume

Question 15

Starling Forces

Answer 15

Starling Forces: Factors that influence the fluid exchange at endothelium – between plasma and interstitial fluid

There are 4 forces acting on this fluid:
2 hydrostatic pressures (fluid pressures)
2 colloid osmotic (oncotic) pressures (osmotic pressure created by proteins in plasma and interstitial fluid)

Question 16

Capillary hydrostatic pressure of the plasma (Pc)

Answer 16

Pressure from the blood (plasma) in the capillary
Varies from 25 mmHg (arteriole end) to 10 mmHg (venous end)
Causes filtration (plasma → interstitium) - pushes plasma fluid into the interstitial space

ONLY PRESSURE THAT VARIES ALONG THE CAPILLARY LENGTH!

Question 17

Interstitial fluid hydrostatic pressure (PIF)

Answer 17

Pressure on the fluid in the interstitial space
Varies from organ to organ (-6 to +6 mmHg)
Can cause filtration (when pressure is neg.) or reabsorption (when pressure is positive)

NEGATIVE in subcutaneous tissue
POSITIVE in encapsulated organs (have hard connective tissue surrounding; e.g. brain, kidneys) **don’t need to know which organs have which type of pressure

Question 18

Interstitial colloid osmotic (oncotic) pressure (πIF)

Answer 18

Pressure caused by osmosis due proteins in the interstitial space (not too many)
Pressure = +5 mmHg 
Causes filtration (water moves out of capillary)

Question 19

Plasma colloid osmotic (oncotic) pressure (πP)

Answer 19

Pressure caused by osmosis due to proteins in the plasma (more than the interstitial space)
Pressure = +28 mmHg (almost 6 times greater) 
Causes reabsorption (pulls water back into the capillary)

Question 20

Kf

Answer 20

Kf = filtration coefficient
Represents the permeability of the capillary and surface area of the endothelium
Varies in different tissue: very high in kidney compared to muscle
But constant in any single capillary bed (under normal conditions)
Assume it equals 1 unless otherwise stated

Question 21

NFM

Answer 21

+ means FILTRATION

• means REABSORPTION

Question 22

Lymphatic system

Answer 22

A series of small channels leading to larger and larger vessels
Found in skin, GI, genitourinary and resp. systems
Part of immune system
Drains the tissue of excess interstitial fluid

Fluid moved along by:

1. Pressure gradient (higher in interstitial fluid, lower in the lymphatic capillaries)
2. Smooth muscle pumps and valves (fluid cannot move back out; ensures that fluid moves in one direction)
3. Contraction of skeletal muscle (skeletal muscle pump) squeezes vessels

Moves to larger vessels
Goes to lymph nodes
Fluid enters back into the vena cava

Question 23

Edema

Answer 23

The swelling of tissue due to ↑ interstitial fluid volume

Fluid shifts are usually isotonic

Question 24

Causes of Edema

Answer 24

1. Decreased plasma proteins seen in severe malnutrition
   This will decrease plasma osmotic pressure (πP) which will cause and increase in filtration into the interstitial compartment, which causes edema
2. Partial blockage of venous return to heart
   Increases capillary hydrostatic pressure of the plasma (Pc) which will cause an increase in filtration from the capillary
3. Blockage of lymphatic system
   This will prevent the uptake of the excess fluid into the lymphatic system that is filtered at the capillary. This will produce edema over a long term
4. Burns damage capillaries and allergic/inflammatory reactions release histamine and cytokines
   Damaged tissue releases chemicals that will increase the permeability of the capillaries. This will increase Kf resulting in an increase in filtration.
   Increases Kf (more leaky)

Question 25

Kwashiorkor Malnutrition

Answer 25

Kwashiorkor malnutrition is seen in severely malnourished children – particularly those with very low protein diets.
Symptoms include a severely bloated belly and swelling in the hands and feet.
It is caused by an imbalance of fluid between the plasma and interstitial space