Radioactive Isotopes Reveal Physiology

Vicki L. arrived early at the nuclear medicine department of the health center. As she sat in an isolated cubicle, a doctor in full sterile dress approached with a small metal canister marked with numerous warnings. The doctor carefully unscrewed the top, inserted a straw, and watched as the young woman sipped the fluid within. It tasted like stale water but was actually a solution containing a radioactive isotope, iodine-131.

Vicki's thyroid gland had been removed three months earlier, and this test was to determine whether any active thyroid tissue remained. The thyroid is the only part of the body to metabolize iodine, so if Vicki's body retained any of the radioactive drink, it would mean that some of her cancerous thyroid gland remained. By using a radioactive isotope, her physicians could detect iodine uptake using a scanning device called a scintillation counter (fig. 2A). Figure 2B illustrates iodine-131 uptake in a complete thyroid gland.

The next day, Vicki returned for the scan, which showed that a small amount of thyroid tissue was indeed left and was functioning. This meant another treatment would be necessary. Vicki would drink more of the radioactive iodine, enough to destroy the remaining tissue. This time she drank the solution while in an isolation room, which was lined with paper to keep her from contaminating the floor, walls, and furniture. The same physician administered the radioactive iodine. Vicki's physician had this job because his own thyroid had been removed many years earlier, and therefore, the radiation couldn't harm him.

Thyroid Radioactive Iodine Scan Images

Figure 2A

Physicians use scintillation counters such as this to detect radioactive isotopes.

Figure 2A

Physicians use scintillation counters such as this to detect radioactive isotopes.

Damaged Cells Radioactive Isotope

Figure

Under certain conditions, hydrogen molecules can combine with oxygen molecules to form water molecules.

After two days in isolation, Vicki went home with a list of odd instructions. She was to stay away from her children and pets, wash her clothing separately, use disposable utensils and plates, and flush the toilet three times each time she used it. These precautions would minimize her contaminating her family—mom was radioactive!

Iodine-131 is a medically useful radioactive isotope because it has a

Photos Medical Radioactive Isotopes

Figure 2B

short half-life, a measurement of the time it takes for half of an amount of an isotope to decay to a nonradioactive form. The half-life of iodine-131 is 8.1 days. With the amount of radiation in Vicki's body dissipating by half every 8.1 days, after three months there would be hardly any left. Doctors hoped that the remaining unhealthy thyroid cells would leave her body along with the radioactive iodine.

Larynx

Larynx

Thyroid gland

Trachea-

Thyroid gland

Trachea-

Figure 2B

(a) A scan of the thyroid gland twenty-four hours after the patient receives radioactive iodine. Note how closely the scan in (a) resembles the shape of the thyroid gland as depicted in (b).

Isotopes of other elements have different half-lives. The half-life of iron-59 is 45.1 days; that of phos-phorus-32 is 14.3 days; that of cobalt-60 is 5.26 years; and that of radium-226 is 1,620 years.

A form of thallium-201 with a half-life of 73.5 hours is commonly used to detect disorders in the blood vessels supplying the heart muscle or to locate regions of damaged heart tissue after a heart attack. Gallium-67, with a half-life of 78 hours, is used to detect and monitor the progress of certain cancers and inflammatory illnesses. These medical procedures inject the isotope into the blood and follow its path using detectors that record images on paper or film.

Radioactive isotopes are also used to assess kidney function, estimate the concentrations of hormones in body fluids, measure blood volume, and study changes in bone density. Cobalt-60 is a radioactive isotope used to treat some cancers. The cobalt emits radiation that damages cancer cells more readily than it does healthy cells. ■

inner shells can hold for elements of atomic number 18 and under is as follows:

First shell (closest to the nucleus) 2 electrons

Second shell 8 electrons

Third shell 8 electrons

More complex atoms may have as many as eighteen electrons in the third shell.

Simplified diagrams such as those in figure 2.3 are used to show electron configuration in atoms. Notice that the single electron of a hydrogen atom is located in the first shell, the two electrons of a helium atom fill its first shell, and the three electrons of a lithium atom occur with two in the first shell and one in the second shell.

Radioactive Isotopes

Figure 2.3

Hydrogen (H) Helium (He) Lithium (Li)

Figure 2.3

The single electron of a hydrogen atom is located in its first shell. The two electrons of a helium atom fill its first shell. The three electrons of a lithium atom occur with two in the first shell and one in the second shell.

Atoms such as helium, whose outermost electron shells are filled, have stable structures and are chemically inactive or inert (they cannot form chemical bonds). Atoms with incompletely filled outer shells, such as those of hydrogen or lithium, tend to gain, lose, or share electrons in ways that empty or fill their outer shells. In this way they achieve stable structures.

Atoms that gain or lose electrons become electrically charged and are called ions (i'onz). An atom of sodium, for example, has eleven electrons: two in the first shell, eight in the second shell, and one in the third shell. This atom tends to lose the electron from its outer shell, which leaves the second (now the outermost) shell filled and the new form stable (fig. 2.4a). In the process, sodium is left with eleven protons (11+) in its nucleus and only ten electrons (10-). As a result, the atom develops a net electrical charge of 1+ and is called a sodium ion, symbolized Na+.

A chlorine atom has seventeen electrons, with two in the first shell, eight in the second shell, and seven in the third shell. An atom of this type tends to accept a single electron, thus filling its outer shell and achieving stability. In the process, the chlorine atom is left with seventeen protons (17+) in its nucleus and eighteen electrons (18-). As a result, the atom develops a net electrical charge of 1- and is called a chloride ion, symbolized Cl-.

Because oppositely charged ions attract, sodium and chorine atoms that have formed ions may react together to form a type of chemical bound called an ionic bond (electrovalent bond). Sodium ions (Na+) and chloride ions (Cl-) uniting in this manner form the compound sodium chloride (NaCl), or table salt (fig. 2.4b).

Similarly, a hydrogen atom may lose its single electron and become a hydrogen ion (H+). Such an ion can bond with a chloride ion (Cl-) to form hydrogen chloride (HCl, hydrochloric acid).

Atoms may also bond by sharing electrons rather than by gaining or losing them. A hydrogen atom, for example, has one electron in its first shell but requires two electrons to achieve a stable structure. It may fill this shell by combining with another hydrogen atom in such a way that the two atoms share a pair of electrons. As figure 2.5 shows, the two electrons then encircle the nuclei of both atoms, and each atom becomes stable. In this case, the chemical bond between the atoms is called a co-valent bond. One pair of electrons shared is a single co-valent bond; two pairs of electrons shared is a double covalent bond.

At one extreme is an ionic bond, in which atoms gain or lose electrons. At the other extreme is a covalent bond in which the electrons are shared equally. In between lies the covalent bond in which electrons are not shared equally. Such a bond results in a polar molecule that has equal numbers of protons and electrons, but one atom has more that its share of electrons, becoming

Sodium Atom And Its Particles

Sodium atom (Na) (a) Separate atoms

Chlorine atom (Cl)

Sodium atom (Na) (a) Separate atoms

Chlorine atom (Cl)

Chlorine Atom Chlorine Radioactive Isotope

Chloride ion (CP) _I

Chloride ion (CP) _I

Sodium chloride

(b) Bonded ions

Figure 2.4

(a) If a sodium atom loses an electron to a chlorine atom, the sodium atom becomes a sodium ion, and the chlorine atom becomes a chloride ion. (b) These oppositely charged particles attract electrically and join by an ionic bond.

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Responses

  • Jamie
    Which radioactive isotope is used in measuring of blood?
    5 years ago
  • IRENE
    How are isotopes used to detect disorders in blood vessels?
    5 years ago
  • Harry Ahlberg
    Can a radioactive isotope of sodium combine with chlorine to form table salt?
    4 years ago

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