IONOMETRY

ION: atom/group of atoms with electric charge (positive charge: cations; negative charge: anions). Many normal substances exist in the body as ions (sodium, potassium, calcium, chloride, bicarbonate, substances known as electrolytes). Chemically, electrolytes are substances that become ions in solution and acquire the capacity to conduct electricity (ions enable the flow of electrical signals through the body). 

The balance of the electrolytes in our bodies is essential for normal function of our cells and organs. If electrolyte levels are too low or too high, cell and organ functions will decline, which could lead to life-threatening conditions. Virtually every metabolic process depends on the presence of electrolytes, which create an "electric potential" needed to do "cellular work".  

The body has complex systems for monitoring and maintaining electrolyte concentrations in a normal narrow range. Physiological functions: 

- maintenance of osmotic pressure and water distribution (water homeostasis)

- conduction of neuromuscular impulses

- acid-base maintenance

- enzyme activation

- electron transfer

- rebuild damaged tissue

Common electrolytes that are measured by doctors with blood testing include sodium, potassium, chloride, bicarbonate, calcium, magnesium, phosphate. These have essential functions in the body. For example, a muscle needs calcium, sodium and potassium to contract. When these substances become imbalanced, it can lead to either muscle weakness or excessive contraction. The heart, muscle and nerve cells use electrolytes to carry electrical impulses to other cells.

Sodium, chloride, potassium, calcium and magnesium (the main electrolytes; nutritional elements) are minerals, and when minerals dissolve in water, they separate into positive and negative ions. For example, when sodium chloride (NaCl) is dissolved in water, it separates into positive sodium ions and negative chloride ions.

Anions: negatively charged ions; migrate towards the anode (positive electrode).

Cations: positively charged ions; migrate towards the cathode (negative electrode).

Major electrolytes (Na+, K+, Cl-, HCO3-) exist as free ions in solution, capable of carrying an electrical charge. Often classified as: major intracellular anion/cation, major extracellular anion/cation. 

Much of the metabolic energy produced by the body is used to establish high intracellular concentrations of potassium (K+) and low concentrations of sodium (Na+), the reverse of the relative concentrations of these ions in the extracellular fluids. The extrusion of sodium requires its movement against a gradient of concentration (higher outside than inside) and electrical potential (inside about 70 mV more negative than outside); work is therefore needed to overcome this electrochemical gradient. 

Functions of the main electrolytes (ions):

- sodium ions: regulate osmotic pressure and the body's water content, transmit nerve signals, contract muscles etc;

- potassium ions: transmit nerve signals and contract muscles including the heart etc;  

- magnesium ions: contract muscles, form bones and teeth, activate enzymes etc;

- calcium ions: transmit nerve signals, contract muscles, form bones and teeth, clot blood;

- chloride ions: regulate osmotic pressure and the body's water content, enable the secretion of stomach acid etc. 

Values differ slightly between serum fluid and plasma fluid; difference is most significant with potassium (serum K+ > plasma K+).

An ion-selective analyser comprises of ion-selective electrode (ISE) and is an analytical technique used to determine the activity of ions in aqueous solution by measuring the electrical potential. It has application in the field of pharmaceuticals and biotechnology. There are four types of ISE: glass electrode, crystalline electrodes, ion-exchange electrodes and enzyme electrodes.

Sodium and potassium and the main and most basic ions analysed. Therefore, more emphasis will be placed on them.  

SODIUM:

Sodium is the major positive ion (cation) in fluid outside of cells. Many processes in the body, especially in the brain, nervous system and muscles require electrical signals for communication. The movement of sodium is critical in the generation of these electrical signals. 

Regulation of osmolality: 

- aldosterone: stimulates sodium (and H2O) retention by kidney (at expense of K+).

Sodium (Na+):

- major cation in ECF (plasma);

- responsible for almost 1/2 the osmotic strength of plasma.

Sodium (Na+) Regulation:

- kidneys are the primary regulators of body sodium and water;

- sodium is freely filtered by glomerulus, 70-80% actively reabsorbed by PCT;

- in times of deficit all sodium is reabsorbed by kidney. 

POTASSIUM:

Potassium is the major positive ion (cation) found inside of cells. Among the many functions of potassium in the body are regulation of the heartbeat and the function of the muscles. 

Potassium levels often change with sodium levels. When sodium levels go up, potassium levels go down, and when sodium levels go down, potassium levels go up. Potassium levels are also affected by a hormone called aldosterone, which is made by the adrenal glands.   

Potassium levels can be affected by how the kidneys are working, the blood pH, the amount of potassium you eat, the hormone levels in your body, severe vomiting, and taking certain medicines, such as diuretics and potassium supplements. Certain cancer treatments that destroy cancer cells can also make potassium levels high. 

Potassium (K+):

- major intracellular cation;

- neuromuscular excitability, heart contractions, maintain ICF volume, maintain H+ concentrations;

- 23x higher in RBC compared to plasma; higher in tissue cells.

Potassium (K+) Regulation:

- Na+-K+-ATP-ase pump continually transports K+ into cells against concentration gradient;

- high intracellular "stores" aid to maintain near-normal extracellular K+ levels in times of deficit;

 - kidney response to conserve K+ is not as immediate and thorough as its response to conserve Na+;

- aldosterone: when sodium is retained, K+ (or H+) is excreted into urine.

CHLORIDE:

Chloride is the major anion (negatively charged ion) found in the fluid outside of cells and in the blood. Chloride plays a role in helping the body maintain a normal balance of fluids. 

Chloride (Cl-):

- major anion in ECF;

- most often Cl- ions shift with Na+ (passive association) and HCO3- to maintain electrical neutrality, osmolality and blood volume. 

Chloride (Cl-) Regulation:

- filtered by glomerulus, passively reabsorbed by the proximal tubules and actively reabsorbed by the ascending Loop of Henle;

- chloride shift: maintains anion-cation balance during buffering process in response to cellular metabolism.

BICARBONATE:

The bicarbonate ion acts as a buffer to maintain the normal levels of acidity (pH) in blood and other fluids in the body. Bicarbonate levels are measured to monitor the acidity of the blood and body fluids.  

Bicarbonate (HCO3-):

- 2nd most abundant anion in ECF;

- maintenance of acid-base balance as the major buffer ion in the carbonic acid/bicarbonate buffer system; major component (>95%) of total CO2 (tCO2).

Bicarbonate (HCO3-) Regulation:

- filtered by the glomerulus and reabsorbed in the proximal and distal tubules;

- kidney has the capacity to reabsorb all or none of the filtered HCO3- as needed to maintain acid/base balance in body.