Physiology
Basic Cellular
Regarding diffusion, which of the following statements is CORRECT:
Answer:
Passive diffusion refers to movement down a concentration gradient and accounts for movement across small distances e.g. within the cytosol or across membranes. Diffusion of a substance across a membrane is affected by the difference in concentration across the membrane and the membrane area over which diffusion occurs. It is also affected by the permeability (p) of the membrane. The permeability is related to the membrane thickness and composition, and the diffusion coefficient of the substance. The rate of diffusion of a substance within a solution is described by Fick's law, which can be modified to describe the rate of diffusion across a membrane.Diffusion and Permeability
Physiology / Basic Cellular / Vessel Fluid Dynamics
Last Updated: 11th April 2019
Passive diffusion refers to movement down a concentration gradient and accounts for movement across small distances e.g. within the cytosol or across membranes.
Diffusion Within a Solution
The rate of diffusion in a solution is described by Fick's law which states:
Js = -DA (ΔC/Δx)
where,
- Js = Amount of substance transferred per unit time
- ΔC = Difference in concentration
- Δx = Diffusion distance
- A = Surface area over which diffusion occurs
- D = Diffusion coefficient
The diffusion coefficient is a measure of how easy it is for the substance to diffuse - it is related to temperature, solvent viscosity and the size of the molecule.
Diffusion Across a Membrane
Diffusion across a membrane is also affected by the permeability (p) of the membrane. The permeability is related to the membrane thickness and composition, and the diffusion coefficient of the substance.
Therefore Fick's equation for diffusion across a membrane can be rewritten as:
Js = -pAΔC
where,
- Js = Amount of substance transferred per unit time
- ΔC = Difference in concentration across the membrane
- A = Membrane area over which diffusion occurs
- p = Membrane permeability
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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 |
