Electricity – basics
An electric current is formed by the flow of charge carriers through a conducting material, similar to water flowing through a pipe. The base SI unit ampere (A) is defined…
Capacitance, inductance and defibrillators
Capacitors are passive components that store electrical energy. The ability of the capacitor to store charge is its capacitance and is measured in farads (F). The capacitor consists of two…
Electrical safety
The graph above depicts the relationship between current, frequency and biological effect on sensation and muscle contraction. At high frequencies, greater current flow is required to cause muscle contraction and…
Biological signals
A lead wire and voltmeter attached to the electrode allows measurement of the potential relative to a reference point. The reference point is usually a second skin electrode. Signals are…
CO2 analysis
The pH of the bicarbonate solution falls in proportion to the CO2 concentration, which is measured by the adjacent pH electrode. The Severinghaus electrode has a response time of 2–3…
Ionising radiation
Some isotopes, such as tritium, are unstable and undergo radioactive decay. This is a random process that follows an exponential pattern; the half-life of an isotope is the time taken…
CO2 absorption
Carbonic acid is formed by the reaction between carbon dioxide and water. 2. Carbonic acid reacts with sodium hydroxide to form sodium carbonate. This step also produces water and heat….
Electricity – AC and DC
A direct current (DC) is the unidirectional flow of electrons. The current flow may vary and fall towards zero, but as long as the direction of electron flow does not…
Blood gas analysis
The logarithmic scale produces a 10-fold change in hydrogen ion concentration per unit change in pH. Modern blood gas machines rapidly measure several variables including pH, pCO2, PO2, oxyhaemoglobin concentration…
Gas analysis: oxygen
Oxygen molecules diffuse across the membrane from the sample gas into the sensor, and are reduced to hydroxyl ions at the cathode. The hydroxyl ions then migrate to and oxidise…
