Physiology
Basic Cellular
Which of the following ions has a higher concentration in intracellular fluid than in extracellular fluid:
Answer:
- Sodium (Na+) is the principal extracellular cation; approximately 140 mmol/L (93%) is extracellular and 10 mmol/L (7%) intracellular.
- Of the other cations, most Ca2+ in the cell is transported actively either out of the cell or into the endoplasmic reticulum and mitochondria, leaving very low levels of free Ca2+ in the intracellular fluid.
- Intracellularly the main anions are protein and phosphate (PO₄³⁻), whereas extracellularly the main anions are chloride (Cl-) and bicarbonate (HCO3-).
Relative Ionic Distribution in Fluid Spaces
Physiology / Basic Cellular / Fluid Spaces
Last Updated: 26th July 2024
The extracellular and the intracellular fluid compartments differ markedly in terms of the concentrations of the ions that are dissolved in them. It should be noted that, within any one compartment, there must be electrical neutrality, i.e. the total number of positive charges must equal the total number of negative charges.
Intracellular vs Extracellular Fluid
The most important difference is the relative concentrations of the cations:
- Sodium (Na+) is the principal extracellular cation; approximately 140 mmol/L (93%) is extracellular and 10 mmol/L (7%) intracellular.
- Potassium (K+) is the principal intracellular cation; approximately 4 mmol/L is extracellular (3%) and 140 mmol/L intracellular (97%).
Of the other cations, most Ca2+ in the cell is transported actively either out of the cell or into the endoplasmic reticulum and mitochondria, leaving very low levels of free Ca2+ in the intracellular fluid. Mg2+ is a predominantly intracellular ion.
Intracellularly the main anions are protein and phosphate, whereas extracellularly the main anions are chloride (Cl-) and bicarbonate (HCO3-).
Relative Ionic Distribution Between Fluid Spaces. (Image by OpenStax College [CC BY 3.0 , via Wikimedia Commons)
Na/K+ ATPase Pump
Ion channel proteins allow the cell to determine the flow of ions across its own membrane. In most circumstances, relatively few channels are open so that the leakage of ions is low. There is, however, always a steady movement of ions across the membrane, with Na+ and K+ following their concentration gradients into and out of the cell, respectively. Uncorrected, the leak would lead to the equalisation of the compositions of the two compartments, effectively eliminating all bioelectrical signalling. This is prevented by the action of the Na+/K+ ATPase pump which is effectively responsible for maintaining the ionic gradient between the intracellular and extracellular fluid.
Gibbs-Donnan Equilibrium
Negatively charged intracellular proteins (and other large fixed anions e.g. phosphate ions) that cannot cross the plasma membrane of cells, effectively repel Cl- ions, which can diffuse freely across the plasma membrane, forcing them out of the cell. The electrical force driving the Cl- ions out is balanced by the chemical gradient driving them back in, a situation known as the Gibbs-Donnan equilibrium.
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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 |
