Physiology
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Basic Cellular
You are discussing possible treatments for cerebral oedema.What is the mechanism of action of hypertonic saline:
Answer:
The mechanism of action of hypertonic saline is predominantly through the marked osmotic shift of fluid from the intracellular to the interstitial and intravascular space. When hypertonic saline is administered, extracellular fluid becomes more concentrated relative to intracellular fluid, osmotic potential draws water out of cells and cells lose water and shrink (crenation).Osmosis and Tonicity
Physiology / Basic Cellular / Fluid Spaces
Last Updated: 11th April 2019
Definition
Osmosis is the passive movement of water across a semipermeable membrane from regions of low solute concentration to those of higher solute concentration. Biological membranes are semipermeable in that they usually allow the free movement of water but restrict the movement of solutes.
The creation of osmotic gradients in this way is the primary method of movement of water within the body, and thus the osmotic potential of body fluids is tightly regulated by homeostatic control mechanisms.
Tonicity
Tonicity is a measure of the relative effective osmotic potentials of two solutions separated by a semipermeable membrane. Tonicity is usually used to describe the effective osmotic potential of a solution relative to plasma.
A fluid at the same osmotic potential as plasma is said to be isotonic; one at higher potential is hypertonic and one at lower potential is hypotonic.
Taking on board fluids of differing osmotic potentials has distinct effects on the distribution of water between cells and extracellular fluids.
| Fluid | Water Movement |
|---|---|
| Hypertonic fluid | Extracellular fluid becomes more concentrated relative to intracellular fluid, osmotic potential draws water out of cells, cells lose water and shrink (crenation) |
| Isotonic fluid | Extracellular and intracellular fluid are isotonic, no osmotic potential generated, no net movement of water |
| Hypotonic fluid | Extracellular fluid diluted relative to intracellular fluid, osmotic potential draws water into cells, cells swell and may burst |
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- Biochemistry
- Blood Gases
- Haematology
| Biochemistry | Normal Value |
|---|---|
| Sodium | 135 – 145 mmol/l |
| Potassium | 3.0 – 4.5 mmol/l |
| Urea | 2.5 – 7.5 mmol/l |
| Glucose | 3.5 – 5.0 mmol/l |
| Creatinine | 35 – 135 μmol/l |
| Alanine Aminotransferase (ALT) | 5 – 35 U/l |
| Gamma-glutamyl Transferase (GGT) | < 65 U/l |
| Alkaline Phosphatase (ALP) | 30 – 135 U/l |
| Aspartate Aminotransferase (AST) | < 40 U/l |
| Total Protein | 60 – 80 g/l |
| Albumin | 35 – 50 g/l |
| Globulin | 2.4 – 3.5 g/dl |
| Amylase | < 70 U/l |
| Total Bilirubin | 3 – 17 μmol/l |
| Calcium | 2.1 – 2.5 mmol/l |
| Chloride | 95 – 105 mmol/l |
| Phosphate | 0.8 – 1.4 mmol/l |
| Haematology | Normal Value |
|---|---|
| Haemoglobin | 11.5 – 16.6 g/dl |
| White Blood Cells | 4.0 – 11.0 x 109/l |
| Platelets | 150 – 450 x 109/l |
| MCV | 80 – 96 fl |
| MCHC | 32 – 36 g/dl |
| Neutrophils | 2.0 – 7.5 x 109/l |
| Lymphocytes | 1.5 – 4.0 x 109/l |
| Monocytes | 0.3 – 1.0 x 109/l |
| Eosinophils | 0.1 – 0.5 x 109/l |
| Basophils | < 0.2 x 109/l |
| Reticulocytes | < 2% |
| Haematocrit | 0.35 – 0.49 |
| Red Cell Distribution Width | 11 – 15% |
| Blood Gases | Normal Value |
|---|---|
| pH | 7.35 – 7.45 |
| pO2 | 11 – 14 kPa |
| pCO2 | 4.5 – 6.0 kPa |
| Base Excess | -2 – +2 mmol/l |
| Bicarbonate | 24 – 30 mmol/l |
| Lactate | < 2 mmol/l |
