W9 L3 - Osmolarity Flashcards
1
Q
Osmosis definition
A
- the process by which fluids move in and out of cells
- Requires semipermeable membrane and aim is to maintain equilibrium
2
Q
Osmotic pressure definition
A
- the pressure required to stop the movement of water across the membrane
- Determined by the total number of particles in the solution
- The greater the number of non-diffusible particles, the higher the osmotic pressure
3
Q
Osmolarity definition
A
- the concentration of solutes per litre of solvent (milliosmoles per litre (mOsm/L))
4
Q
Osmolality meaning
A
the concentration of solutes per kilogram of solvent (milliosmoles per kilogram mOsm/Kg))
5
Q
Definition of an osmole
A
- the number of moles that a solute contributes to the osmotic pressure of a solution
- 1 gram molecular weight of undissociated solute
- BUT where a solute dissociates in water (cations and anions), each will contribute to the number of osmoles
- A solution that has 1 osmole of solute dissolved in 1 Kg water is said to have an osmolality of 1osmole/Kg
- A solution that has 1 osmole of solute dissolved in 1 litre water is said to have an osmolarity of 1osmole/L
- 180g of glucose (equivalent to 1g molecular weight of glucose) is equal to 1 osmole of glucose because glucose stays as one molecule and does not dissociate
- 58.5g of NaCl (equivalent to 1g molecular weight of NaCl) is equal to 2 osmoles because NaCl dissociates into Na+ and Cl-
Na+ and Cl- are both osmotically active
6
Q
How do we measure osmolality?
A
- Osmolality in urine can be measured by an osmometer
Determines kidney’s ability to concentrate urine
“Normal” urine osmolality for a healthy adult is 200-800mOsm/Kg
Chloride, sodium, urea and potassium contribute to urine osmolality
Glucose can contribute when it is abundant in urine (not a normal situation) - Osmolarity can be calculated from osmolality
Serum osmolarity is normally 280-300mOsm/L
7
Q
Interpreting urine osmolality
A
- In humans osmolarity and osmolality are essentially identical
Because clinically we are dealing with very low concentrations of solutes - There are no normal values for urine sodium and osmolality
- Results must be seen in the clinical context, which defines the expected values
- Deviations from these expected values can then be used to identify problems
- Understanding of the physiological principles of renal sodium handling and volume homoeostasis is critical as the kidneys sense plasma volume/perfusion, but not plasma sodium concentration
8
Q
why is urine osmolarity important?
A
- Can indicate dehydration or overhydration
- Can indicate kidney problems
- When fluid balance goes wrong, homeostatic processes kick in
To increase plasma osmolality (when plasma osmolality is low), Body stops releasing antidiuretic hormone (ADH) which results in increase water in urine
To decrease plasma osmolality (when plasma osmolality is high), Body will release ADH which promotes water reabsorption
9
Q
How the body signals for a decrease in osmolality
A
- ADH (antidiuretic hormone)
- The kidney controls water secretion mainly through the action of ADH
- It is secreted by the hypothalamus and stored in the posterior pituitary gland
- Secretion into the circulation is stimulated by osmoreceptors and baroreceptors
- It binds to AVPR2 (Arginine vasopressin receptor-2, or V2 vasopressin receptor-2) in the kidney tubule
- AVPR2 is coupled to Gs, which increases cAMP production which then opens aquaporin-2 channels that allows the reabsorption of water
10
Q
Tonicity
A
- The relative concentration of a solution on the outside of the cell compared to inside the cell
ie the interaction of two solutions across a semipermeable membrane - It relates to the concentration of particles that cannot diffuse through a semipermeable membrane
- It has no units
11
Q
Tonicity: three types of solutions
A
- Hypertonic
- Hypotonic
- Isotonic
12
Q
is urea permeable to cell membranes
A
YES
13
Q
Hypertonic solutions
A
- Tonicity of solution outside of cell is higher than inside the cell
- A cell in a hypertonic solution will lose water and shrink
14
Q
hypotonic solutions
A
- Tonicity of solution outside of cell is lower than inside the cell
- A cell in a hypotonic solution will gain water and swell
15
Q
Isotonic solutions
A
- Tonicity of solution outside of cell is equal to inside the cell
- A cell in a hypertonic solution will not change or be damaged
16
Q
Some applications or osmosis/osmolality in pharmaceutics
A
Osmosis influences the design of medicines:
Oral rehydration therapy
Ophthalmologic solutions
Parental solutions
17
Q
Oral rehydration therapy (ORT)
A
- Food nutrients are taken up by Villus cells which also pumps Na+ from extracellular space and return it circulation. Imbalance of Na+ results in osmotic flow.
- Diarrhoea causing microorganism disturb mechanism: fluid lost in watery stool.
- Diarrhoea causing microorganism do not interfere with Na+ + glucose carrier (co transport only happens when both are present)
- ORT contains glucose and electrolyte: this activates co-transport, re-establish the imbalance of Na+ and leads to transport of water.
18
Q
Ophthalmologic solutions
A
- Solutions added to the eye need to be isotonic and sterile
- Isotonic with blood.
- Osmosis is a colligative property (depends on number of molecules)
- NaCl this corresponds to 154mM or 0.9 %w/v (Saline)
19
Q
Parental solutions: example mAbs solution
A
- Need to be isotonic isosmotic
- Can be done using salts (NaCl or ArgCl) or sugars (sucrose, dextrose, trehalose)
- Difference if IV or SC.
- IV: low concentrations of mAb (1-10 mg/mL), slow (3 hours infusion), large volume reach about 300 Osmol/kg
- SC: high concentrations of mAb (>100 mg/mL), small volume (0.5 mL), can reach high osmolalities (600 Osmol/kg): limit, as cause irritation, discomfort.
- High level of glucose can lead to renal failure especially over 65 yrs old.
