The structure of atoms of some elements as shown (Fig. 3.9) indicate each element has a distinctive number of protons and electrons.
The atom of all elements (103 known) is composed of the smallest and elementary particles. Neutrons, protons and electrons are the elementary particles. The neutron and proton occur in the centre of an atom where they form an atomic nucleus. The electron orbits the atomic nucleus at some distance from its centre where it is virtually weightless and carries negative electrical charge.
Each neutron and proton (nucleons) has one unit of atomic weight or mass and is about 1800 times heavier than the electron. A proton’s electrical charge is positive, whereas a neutron is electrically neutral. Hence, the numbers of positive and negative charges are exactly balanced and so an atom is electrically neutral.
The chemical properties of the elements however depend upon the number and configuration of electrons. The number of such planetary electrons in the neutral atom of any element is equal to the number of protons in the nucleus of that atom.
An element is characterised by a fixed and definite atomic number of protons in the nuclei of its atom balanced by an equal number of planetary electrons. With the exception of the nucleus of the ordinary hydrogen all atomic nuclei contain one or more neutrons in addition to protons.
The numbers of neutrons in the nuclei of the atoms of an element are not fixed, but vary within certain limits. The atoms of most elements contain nuclei with different numbers of neutrons and consequently of different mass numbers. Each such atomic variety with a specific mass of an element constitutes isotopes.
Therefore, isotopes are atoms of the same elements whose nuclei contain different numbers of neutrons but the same number of electrons and protons. Isotopic atoms (nuclides) can be detected and distinguished physically but not chemically.
They belong to the first class and remain as mixtures in all natural sources and can be isolated by careful fractionation. The isotopes of physiological importance are those of carbon, hydrogen, nitrogen, oxygen, sulphur, chlorine, etc. For instance,—carbon having the familiar atomic wt. 12 has isotopes with atomic wt. 11, 12, 13 and 14 (11C, 12C, 13C 14C or written as C11, C12, C13, C14).
Hydrogen has the usual atomic wt. 1, but isotopic heavy hydrogen has an atomic wt. 2 (2H). Oxygen with atomic wt. 16, has isotopes with atomic wt. 17 and 18 (17O, 18O). Similarly sulphur (atomic wt. 32) has isotopes with wt. 33 and 34 (33S, 34S) and chlorine with the usual atomic wt. of 35 may have an isotopic variety with 37 (37Cl).
Of special interest are the hydrogen isotopes (radio hydrogen) (2H and Hb) and 2H which have been given special names—deuterium and tritium respectively. The deuterium is symbolised by D. D2O (heavy water) is present to some extent in all natural samples of water (tap water contains 1 part in 9,000 parts) and traces of D2O (deuterium oxide) and THO (radioactive tritium oxide) are present throughout the body fluids.
By determining D2O and THO spaces, the total body fluid would be obtained. Electrolysis of water preferentially dissociates H2O but leaves D2O untouched. Consequently after electrolysis of water fairly concentrated solution of D2O can be obtained and can be subsequently isolated by fractionation. Although in chemical properties it is identical with H2O, D2O has different mass, recognised by mass spectrometer.
Some isotopes are stable, but some are unstable due to their relative number of protons and neutrons in the nuclei. These are radioactive isotopes which break down constantly into more stable atoms. Elements like uranium and radium are radioactive, they emit α-, β- and γ-rays. Although majority of the naturally occurring elements are not radioactive, radioactive isotopes of all of them can be prepared artificially by bombardment with a cyclotron or by uranium pile.
The emitting a-radiation from the disintegrating atom is not easily detected. β-radiation and γ-radiation are usually measured with Geiger-Müller counter. In some special cases either Windowless or Scintillation counter is used. The stability of a radioactive isotope is measured by its half-life.
The half-life of a radioactive isotope is the time taken by the radiation to come down to half the original strength. The half-life of radioactive elements varies from minutes to years. Those applied for physiological studies must obviously have sufficiently long life to enable an elaborate study of their life-history in the body.
Half-line of some radioactive isotopes:
42K, 12.4 h.; 24Na, 15 h.; 131I, 8 d.; 59Fe, 45 d.; 32P, 14.3 d., 35S, 87.1 d.; 45Ca, 152 d.; 60Co, 5.3 y.; 14C, 5700 y.; 3H, 12.5 y.; etc. (h denotes hours, d, days and y, years)
Physiological Application of Isotopes:
The isotopes, either natural or radioactive, have identical chemical properties with the parent element and so they are treated similarly in the living system as their normal homologue. A very low concentration of them can however be detected by physical method and so they can be used to ‘label’ molecules and are used extensively in different physiological and clinical studies in recent times.
The latest advancement in the study of metabolic processes is the application of natural and radioactive isotopes. Compounds labelled with such isotopes are administered. The changes undergone in such ‘labelled’ compounds, their migration from place to place and finally their excretion from the body, are traced and observed with the help of suitable instruments and of other physico-chemical properties. Hence, such elements are referred to as tracer elements.
Some physiological studies based on application of radioactive isotopes are listed below:
i. By Dilution of Radioactivity Added:
(a) Measurement of red cell volume with 51Cr labelled red blood corpuscles.
(b) Determination of plasma volume with 131I labelled serum albumin,
(c) Determination of sodium space with either 22Na or 24Na.
(d) Estimation of total body water with iodo-antipyrine labelled with 131I.
ii. Membrane-Transfer Measurement:
(a) Absorption studies of iron (59Fe), vitamin B12 (60Co), fatty acids (14C oleic acid), etc.
(b) Transport of Na and K across the intestinal wall, and
(c) Exchange of ions in the kidney tubules.
iii. Studies Based on Distribution of a Particular Isotope:
(a) Uptake of 131I by thyroid gland in thyroid function test,
(b) Uptake of 32P by malignant tissue in its tracing in the body, and
(c) Distribution of different drugs.
iv. Metabolic Studies:
This is one of the widest applications of radioactive isotope. Studies on synthesis, degradation and isolation of intermediate products in almost all metabolic pathways in normal physiological conditions and its variations in different diseases have been attempted.
They include carbohydrate, protein and nucleic acids, fatty acids and steroids and mineral metabolism. Synthesis and degradation of various hormones have also been studied.
v. As a Radiation Source in Medicine:
(a) 131I in the treatment of hyperthyroidism and thyroid cancer,
(b) 32P in the treatment of leukaemia and polycythemia vera,
The capacity of radioactive isotope to blacken photographic emulsion is termed the technique of autoradiography. A mixture of substances is first of all separated by paper chromatography and then the paper is kept in contact with X-ray film or similar photographic emulsion.
The grade of black hue gives the amount of radioactivity of the particular fraction. If applied on histological section this technique can detect the presence of radioactivity inside the different cellular compartments (nuclei, mitochondria, etc.).