accuracy of pH measurements typically required, in particular, in relation to the ocean's carbonate system. Since it omits consideration of sulfate and fluoride ions, the free scale is significantly different from both the total and seawater scales. Because of the relative unimportance of the fluoride ion, the total and seawater scales differ only very slightly. Living systems {| class="wikitable" |+pH in living systems |- ! Compartment ! pH |- | Gastric acid || 1.5–3.5 |- | Lysosomes || 4.5 |- | Human skin || 4.7 |- | Granules of chromaffin cells || 5.5 |- | Urine || 6.0 |- | Cytosol || 7.2 |- | Blood (natural pH) || 7.34–7.45 |- | Cerebrospinal fluid (CSF) || 7.5 |- | Mitochondrial matrix || 7.5 |- | Pancreas secretions || 8.1 |} The pH of different cellular compartments, body fluids, and organs is usually tightly regulated in a process called acid–base homeostasis. The most common disorder in acid–base homeostasis is acidosis, which means an acid overload in the body, generally defined by pH falling below 7.35. Alkalosis is the opposite condition, with blood pH being excessively high. The pH of blood is usually slightly basic with a value of pH 7.365. This value is often referred to as physiological pH in biology and medicine. Plaque can create a local acidic environment that can result in tooth decay by demineralization. Enzymes and other proteins have an optimum pH range and can become inactivated or denatured outside this range. Calculations of pH The calculation of the pH of a solution containing acids and/or bases is an example of a chemical speciation calculation, that is, a mathematical procedure for calculating the concentrations of all chemical species that are present in the solution. The complexity of the procedure depends on the nature of the solution. For strong acids and bases no calculations are necessary except in extreme situations. The pH of a solution containing a weak acid requires the solution of a quadratic equation. The pH of a solution containing a weak base may require the solution of a cubic equation. The general case requires the solution of a set of non-linear simultaneous equations. A complicating factor is that water itself is a weak acid and a weak base (see amphoterism). It dissociates according to the equilibrium with a dissociation constant, defined as where [H+] stands for the concentration of the aqueous hydronium ion and [OH−]