Fluids&Osmosis

Question 1

Osmotic stability

Answer 1

Appropriate cell volume and body fluid compartment volume/composition

Question 2

Why are ionic gradients across the PM important?

Answer 2

Secondary active transport of nutrients e.g glucose, Em, electrical activity

Question 3

Body fluid compartments for a 70kg male?

Answer 3

Intracellular fluid: 28L (67%)
Extracellular fluid: 14L (33%):
Plasma: 3.5L (8%)
Interstitual fluid: 10.5L (25%)
Transcellular flud: 0.5L (1%)
60% total body weight is H2O

Question 4

Plasma

Answer 4

Contains proteins (trapped inside)
Separated from interstitial fluid by endothelial barrier 
Free movement of water/most solutes into and out of

Question 5

How can cells affect their osmotic equilibrium?

Answer 5

Each compartment is in osmotic equilibrium
Can add or remove aquaporins to change this

However, water can move freely between the compartments so the compartments have the same osmolarity and osmotic stability
No NET movement of H20

Question 6

Intracellular compartment

Answer 6

Fluid contained inside cells
Mostly cytosol (matrix in which organelles are suspended
Cytosol + organelles = cytoplasm
Contains approx 28 litres of fluid
Remains in osmotic equilibrium

Question 7

EC compartment

Answer 7

Composed of three compartments:- Interstitial, intravascular and transcellular

Its extracellular fluid contains one third of total body water.

Interstitual compartment:- Surrounds tissue cells. Filled with interstiual fluid which provides microenviromnet for cells. (Movement of ions, proteins nutrients across cell barrier) Fluid continually being refreshed by blood capillaries and recollected by lymphatic capillaries. average male had 10.5 litres of fluid.

Intravascular compartment:- Blood plasma (3.5L), lmyph

Transcellular compartment:- Consists of those spaces in the body where fluid does not normally collect in larger amounts. E.g: The eye, CNS. (0.5 L)

Question 8

Osmolarity

Answer 8

Total solute concentration/litre of solution
Osmoles/litre
mOsmol/litre (physiological)
1M solution has an osmolarity of 1 Osmol/litre

Question 9

Osmolarity vs Osmolality

Answer 9

As physiological concentrations are very low, osmolarity and osmolality are often used interchangeably

Question 10

Body fluid osmolality

Answer 10

Often maintained between 280-296 mOsmol/kg H20

Body monitors osmolality through changes in ECF osmolality; body water is the major determinant of ECF and osmolality

Question 11

ICF and ECF composition (mM)

Osmolality: 290mOsmol/kg H20

Answer 11

ICF: 
Na+ 15
K+ 150
Cl- 10
Ca2+ 0.0001
Organic ions: 130mmol/L

ECF:
Na+ 145
K+ 5
Cl- 108
Ca2+ 1
organic ions 0

Question 12

How does cell prevent rupture?

Answer 12

Retention of organic ions leads to problems of osmotic balance such as: Creates an osmotic gradient, creates an electrical gradient
Retention of organic ions = concentration of non-permeable solutes e.g Na+
Requires Na+/K+ ATPase pump

Question 13

Isoosmotic

Answer 13

Solution containing equal solute regardless of permeable or non permeable solutes

Question 14

Composition of ICF and ECF

Answer 14

ICF
Na+ 10-15mmol/L
K+ 120-150mmol/L
Cl- 10-30mmol/L
Organic ions 130mmol/L

ECF
Na+ 120-140mmol/L
K+ 3.5-5mmol/L
Cl- 95-120mmol/L
Organic ions: 0

Em: -70mV

Question 15

Cells at REST

Answer 15

Concentrations of cations and anions in any compartment are always equal
All cells create a negative Em by altering the charge distribution between the ECF and ICF, the result of a few charges moving in the intermediate vicinity of the membrane with negligible effect on bulk ion concentration.
Any ion that moves will effect Em

Question 16

Movement of ions: K+

Answer 16

K+ moves out of the cell ALONG its concentration gradient
Negative charge/electrical attraction will start to attract K+ BACK into the cell
BALANCE of electrical and chemical gradients = equilibrium potential of K+!

Question 17

Why is AT important?

Answer 17

Maintains normal ionic concentration; essential intracellular activities
Indirectly determines cellular gradients of other ions/molecules (secondary AT)
Basis of Em and AP’s

Question 18

What is the relationship LINKING chemical and electrical forces?

Answer 18

Membrane potential is necessary to keep ion from diffusing down concentration gradient
No net movement of ion

Question 19

Nernst equation

Answer 19

Applies to only ONE ion at a time and ONLY ions that can cross the PM
Calculates voltage difference across the membrane

Question 20

Eion’s

Answer 20

K+ -90mV
Na+ +61mV
Cl- -53mV
Ca2+ +120mV
The RMP is the sum of all the individual equilibrium potentials as the PM has permeability to MANY ions
All permeable ions contribute to RMP
e.g some Na+ will leak into the cell which is why the Em doesn’t sit at -90mV

Question 21

What is the major determinant of resting membrane potential?

Answer 21

K+ efflux

Assymetrical distribution of ions across the PM and the different permiabilites of these ions

Question 22

EC ion effects

Answer 22

ECF ionic composition regulated within a narrow range
Disturbances in ECF ion levels can occur
Sensitive to ECF: K+ (range 3.5-5.5mmol/L)

Question 23

Hypolakemia

Answer 23

Low level of K+ in Blood serum
Low level of K+ in ECF
K+ leaves cells along conc grad
Less likely to re-enter cells as less steep conc grad
Inside of cell MORE negative
Lowers RMP (hyperpolarisation)
Threshold for AP less likely to be reached

Question 24

Hyperlakemia

Answer 24

1. High level of K+ in blood serum
2. Higher level of K+ in ECF
3. Less steep conc grad
4. Less of a drive for K+ to leave cells
5. Increased equilibrium of K+
6. Easier to reach threshold
7. Depolarisation starts
8. Na+ channels open
9. AP created.