Requirement, Occurrence

The daily requirement of vitamin A (Table 6.3) is provided to an extent of 75% by retinol intake (as fatty acid esters, primarily retinyl palmitate), while the remaining 25% is through P-carotene and other provitaminactive carotenoids. Due to the limited extent of carotenoid cleavage, at least 6 g of P-carotene are required to yield 1 g retinol. Vitamin A absorption and its storage in the fiver occur essentially in the form of fatty acid esters. Its content in fiver is 250pg/g fresh tissue, i.e. a total of about 240-540 mg is stored. The fiver supplies the blood with free retinol, which then binds to proteins in blood. The plasma concentration of retinol averages 1.78 pmol/1 in women and 2.04 lamol/l in men. 

A hypervitaminosis is known, but the symptoms disappear if the intake of retinal is decreased. Vitamin A occurs only in animal tissue above all in fish fiver oil, in fivers of mammals, in milk fat and in egg yolk. Plants are devoid of vitamin A but do contain carotenoids which yield vitamin A 

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Safety-critical computer software components Those computer software components (processes, functions, values or computer program state) whose errors (inadvertent or unauthorized occurrence, failure to occur when required, occurrence out of sequence, occurrence in combination with other functions, or erroneous value) can result in a potential hazard, or loss of predictability or control of a system (MIL-STD-882). 

Category 4. Operating conditions highly hypothetical after which plant restart is not required. Occurrence frequency 10-7/year < f < lO-4/year. The lower value (lO-7/year) must be coherent with the figure of 10-6/year (chosen as a probabilistic target for unacceptable consequences) and a very low number N of families of evrats in the fourth category (N < 10). 

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