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Biomedical subjects

Richard F Burton

Publications and source records attributed to Richard F Burton.

7 recordsLinked to original sources
PubMed2004

The mathematical treatment of leaf venation: the variation in secondary vein length along the midrib.

BACKGROUND AND AIMS: In some dicotyledonous leaves and leaflets, the secondary veins run more-or-less straight to the margins and have well-defined lengths. For a given half-lamina of length L, an equation, previously proposed, relates the lengths of these veins, p, to the distances, l, between the leaf tip and their insertions on the midrib: p = B2(x+y)l(x)(L - l)(y)/L(x+y-1), where B, x and y are fitted parameters. Aspects of the formula are re-examined, including its general applicability, significance and usefulness. METHODS: Length measurements were made on leaves of various dicotyledons, notably Ulmus glabra, U. procera, Alnus viridis, A. glutinosa, Corylus avellana and Crataegus monogyna. Equations were fitted by non-linear regression. KEY RESULTS: The equation has now been applied descriptively to 23 species of eight families, but it is sometimes preferable or necessary to replace the measured length, L, with a fourth parameter that may differ significantly from it. Within a given species, values of the indices x and y are positively correlated. Leaves of some U. glabra depart qualitatively from the general pattern. As an example of hypothesis testing, the equation was used to show that the retuse or emarginate leaf tips of A. glutinosa are not due to stunting. CONCLUSIONS; That the equation applies to many species suggests that the underlying processes of leaf growth are quantitatively similar. Although relevant knowledge of these is scant, consideration of mathematical relationships may help their elucidation.

Cotyledon↗
PubMed2004

Defining stress.

Explore the source record for details and available documents.

Adaptation, Psychological↗
PubMed2002

Misinformation, partial knowledge and guessing in true/false tests.

CONTEXT: Examiners disagree on whether or not multiple choice and true/false tests should be negatively marked. Much of the debate has been clouded by neglect of the role of misinformation and by vagueness regarding both the specification of test types and "partial knowledge" in relation to guessing. Moreover, variations in risk-taking in the face of negative marking have too often been treated in absolute terms rather than in relation to the effect of guessing on test unreliability. OBJECTIVES: This paper aims to clarify these points and to compare the ill-effects on test reliability of guessing and of variable risk-taking. METHODS: Three published studies on medical students are examined. These compare responses in true/false tests obtained with both negative marking and number-right scoring. The studies yield data on misinformation and on the extent to which students may fail to benefit from distrusted partial knowledge when there is negative marking. A simple statistical model is used to compare variations in risk-taking with test unreliability due to blind guessing under number-right scoring conditions. CONCLUSIONS: Partial knowledge should be least problematic with independent true/false items. The effect on test reliability of blind guessing under number-right conditions is generally greater than that due to the over-cautiousness of some students when there is negative marking.

Communication↗
PubMed2002

Temperature and acid-base balance in ectothermic vertebrates: the imidazole alphastat hypotheses and beyond.

The 'imidazole alphastat hypothesis' states that intracellular and extracellular pH, partly via buffering by imidazole groups, change with temperature in a way that keeps imidazole and protein ionization constant, thus maintaining cell function and minimizing shifts of base equivalents and total CO(2), while adjustment of P(CO(2)) involves imidazole-based receptors. 'The hypothesis', which is actually several hypotheses, has been variously perceived and judged, but its underlying conceptual framework remains largely valid, and is reformulated using differential equations requiring less information input than their integral equivalents. Their usefulness is illustrated with published data on temperature responses in fish cells and whole tetrapods. Mathematical modelling allows general principles to be explored with less immediate concern for uncertainties in experimental data and other information. In tetrapods, it suggests that warming is followed by a loss of base equivalents from the body, and that this loss is due to metabolic adjustments that are not part of pH homeostasis. Uncertainties include intracellular buffer values, local variations in P(CO(2)) within the body, the possible role of buffering by bone mineral, and the temperature dependence of pK values for CO(2)/HCO(3)(-) and imidazole groups. The equations utilize a single, notional, temperature-dependent pK value for all non-bicarbonate buffers in a given body compartment. This approximates to the 'passive component' of pH adjustment to temperature change as measured by the homogenate technique. Also discussed are the diversity of cell responses within individual animals, relevant aspects of the control of ventilation, metabolism and transmembrane transport, and the basis of optimum pH-temperature relationships.

Acid-Base Equilibrium↗
PubMed2002

Evolutionary determinants of normal arterial plasma pH in ectothermic vertebrates.

Mean values of normal arterial pH in different species of fish, amphibians and reptiles at 15 and 25 degrees C, taken from the literature, are negatively correlated with arterial P(CO(2)) and plasma [Na(+)]. At either temperature, the data accord with the hypothesis that extracellular acid-base homeostasis evolved to maintain an optimal pH at particular cell-surface sites that are similar in all species. These hypothetical sites bear fixed negative charges that attract H(+), but which are partially screened by Na(+); for the surface pH to be constant, the bulk interstitial pH should then vary inversely with [Na(+)], as is the case. At the same time, the bulk interstitial fluid must be more acid than arterial plasma by an amount that increases with decreasing arterial P(CO(2)). With allowance made for additional screening by Ca(2+) and Mg(2+), the relevant cell-surface pH is probably approximately 6.2.

Amphibians↗