Half-life, biological effective

Isotope Organ of reference Radiological half-life Biological half-Ufe Effective half-life... 

Once a radioactive chemical is introduced into the body, little can be done to speed its removal. Some removal will eventually occur as a result of excretion (urine, feces, perspiration, or expired air) and the activity will diminish as a result of radioactive decay. The effective half-life of a radioisotope depends upon both its physical half-life (radioactive decay) and its biological half-life (biological... 

HALF-LIFE, EFFECTIVE Time required for a radioactive nuclide present in an organism to be diminished by 50% as a result of both radioactive decay and biological elimination. Symbol Teff Tcff = tb X L/(tb+L). (See also HALF-LIFE, BIOLOGICAL and HALF-LIFE, PHYSICAL)... 

The therapeutic utility of systemically administered ASON had been limited by their short plasma half life (sometimes even less than 3 min). This is due to their sensitivity to nuclease digestion. When the first-generation ASON were chemically modified, e.g., by replacing the oxygen in the phosphodiester bond with sulfur (phosphorothiorate) they obtained an increased stability in biological fluids while their antisense effect has been maintained. First-generation agents can be delivered via intravitreal injection, parenterally, by topical cream, enema, and inhaled aerosol. These antisense... 

Triiodothyronine (3, 5,3-L-triiodothyronine, T3) is a thyroid hormone. It is producedby outer ring deiodination of thyroxine (T4) in peripheral tissues. The biologic activity of T3 is 3-8 times higher than that of T4. T3 is 99.7% protein-bound and is effective in its free non-protein-bound form. The half-life of triiodothyronine is about 19 h. The daily tur nover of T3 is 75%. Triiodothyronine acts via nuclear receptor binding with subsequent induction of protein synthesis. Effects of thyroid hormones are apparent in almost all organ systems. They include effects on the basal metabolic rate and the metabolisms of proteins, lipids and carbohydrates. 

The biological half-life of a radioisotope is the time required for the body to excrete half of the radioisotope. The effective half-life is the time required for the amount of a radioisotope in the body to be reduced to half its original amount, as a result of both the decay of the radioisotope and its excretion. Sulfur-35 (tu2 = 87.4 d) is used in cancer research. The biological half-life of sulfur-35 in the human body is 90. d. What is the effective half-life of sulfur-35 ... 

Barium-140 (tl/2 = 12.8 d) released in the fire at the Chernobyl nuclear plant has been found in some agricultural products in the region. The biological half-life of barium-140 in the human body is 65 d. What is the effective half-life (see Exercise 17.55) of barium-140 ... 

Effective Half-time - Biological half-time x Radioactive half-life... 

PbB concentrations reflect the absorbed dose of lead. However, the interpretation of PbB data depends on a knowledge of the past history of exposure to lead. This is because in the body, bone constitutes the major lead sink and this results in lead having a long body half-life. Thus, in the absence of intense exposure to lead for a considerable period up to its body half-life, the PbB concentrations reflect recent lead exposures. However, if intermittent exposure to lead is occurring in several distinct environments, the PbB concentration reflects both recent and past exposures to lead. Thus, biological effects for populations with the same PbB concentrations may not be the same since different exposure times scales may be involved. This is the reason why free erythrocyte protoporphyrin (FEP) and erythrocyte zinc protoporphyrin (ZPP) have been used as additional biological markers since their elevation is more related to chronic lead exposure than acute lead exposure (see Section 2.7). 

Retention of cerium deposited in these tissues is generally considered to be very long, with a biological half-time in excess of a thousand days. Therefore, the effective half-time of retention in these internal organs will be approximately equal to the physical half-life. 

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