全文获取类型
收费全文 | 3350篇 |
免费 | 328篇 |
国内免费 | 216篇 |
出版年
2024年 | 6篇 |
2023年 | 57篇 |
2022年 | 48篇 |
2021年 | 81篇 |
2020年 | 124篇 |
2019年 | 141篇 |
2018年 | 135篇 |
2017年 | 126篇 |
2016年 | 168篇 |
2015年 | 134篇 |
2014年 | 168篇 |
2013年 | 231篇 |
2012年 | 127篇 |
2011年 | 140篇 |
2010年 | 116篇 |
2009年 | 155篇 |
2008年 | 173篇 |
2007年 | 183篇 |
2006年 | 137篇 |
2005年 | 139篇 |
2004年 | 113篇 |
2003年 | 106篇 |
2002年 | 103篇 |
2001年 | 98篇 |
2000年 | 72篇 |
1999年 | 71篇 |
1998年 | 88篇 |
1997年 | 54篇 |
1996年 | 66篇 |
1995年 | 61篇 |
1994年 | 43篇 |
1993年 | 42篇 |
1992年 | 58篇 |
1991年 | 38篇 |
1990年 | 53篇 |
1989年 | 37篇 |
1988年 | 29篇 |
1987年 | 20篇 |
1986年 | 24篇 |
1985年 | 21篇 |
1984年 | 32篇 |
1983年 | 9篇 |
1982年 | 13篇 |
1981年 | 14篇 |
1980年 | 10篇 |
1979年 | 11篇 |
1978年 | 6篇 |
1977年 | 2篇 |
1976年 | 7篇 |
1973年 | 2篇 |
排序方式: 共有3894条查询结果,搜索用时 218 毫秒
991.
Abstract: Acute hepatic failure is associated with many biochemical abnormalities in plasma and brain. Changes that correlate well with the degree of behavioral impairment may be important factors in the development of encephalopathy. We measured the concentrations of intermediary metabolites, ammonia, and amino acids in brain and plasma and the rate of whole-brain glucose utilization in rats with an acutely devascularized liver. In all rats an estimate of the grade of encephalopathy (reflected by behavioral impairment) was made. Rats underwent portacaval shunting and hepatic artery ligation (or sham operation) and were kept normoglycemic and normothermic thereafter. We sampled blood and whole brain (by near-instantaneous freeze-blowing) 2, 4, or 6 h later. There were no alterations in levels of high-energy phosphate metabolites in the brain or in metabolites associated with the glycolytic pathway and Krebs cycle, except lactate and pyruvate. Brain glucose use was decreased similarly at all times after surgery. Levels of ammonia and many amino acids were increased in brain and plasma; brain aspartate, glutamate, and arginine levels were decreased. The increases in content of plasma ammonia and brain glutamine, proline, alanine, and aromatic amino acids and the decreases in brain aspartate and glutamate were most strongly correlated with behavioral impairment. 相似文献
992.
对拉萨市(海拔3658m)16名世居藏族和20名已习服的移居汉族健康青年男性的最大氧摄取、最大运动负荷及其影响因素进行了对此研究。结果发现在最大努力作功时,藏族的最大氧摄取量、最大运动负荷量、潮气量、肺通气量以及氧脉搏等均大于移居汉族。说明西藏高原世居藏族具有更佳的氧转运功能,对高原低氧已获得了良好的生理适应。 相似文献
993.
平菇对不同碳氮比麦草培养料主要组分的降解研究 总被引:4,自引:0,他引:4
研究平菇对不同碳氮比麦草培养料主要组分的降解结果表明,麦草栽培平菇后,C/N比值普遍下降,下降幅度随C/N比增加而加大,下降趋势与基物失重、粗纤维降解、呼吸消耗相一致。菇产量以C/N比50—60:1最高,但菇产量的高低与基物失重、粗纤维减少、呼吸损耗多少没有关系,而与培养料的C/N比值却有明显的曲线关系。 相似文献
994.
Florian Degen 《Journal of Industrial Ecology》2023,27(3):964-976
The market for electric vehicles is growing rapidly, and there is a large demand for lithium-ion batteries (LIB). Studies have predicted a growth of 600% in LIB demand by 2030. However, the production of LIBs is energy intensive, thus contradicting the goal set by Europe to reduce greenhouse gas (GHG) emissions and become GHG emission free by 2040. Therefore, in this study, it was analyzed how the energy consumption and corresponding GHG emissions from LIB cell production may develop until 2030. Economic, technological, and political measures were considered and applied to market forecasts and to a model of a state-of-the art LIB cell factory. Notably, different scenarios with trend assumptions and above/below-trend assumptions were considered. It could be deduced that, if no measures are taken and if the status quo is extrapolated to the future, by 2030, ∼5.86 Mt CO2-eq will be emitted due to energy consumption from European LIB cell production. However, by applying a combination of economic, technological, and political measures, energy consumption and GHG emissions could be decreased by 46% and 56% by 2030, respectively. Furthermore, it was found that political measures, such as improving the electricity mix, are important but less dominant than improving the production technology and infrastructure. In this study, it could be deduced that, by 2030, through industrialization and application of novel production technologies, the energy consumption and GHG emissions from LIB cell production in Europe can be reduced by 24%. 相似文献
995.
Analía G. Karadayian Bárbara Paez Juanita Bustamante Silvia Lores-Arnaiz Analía Czerniczyniec 《Journal of biochemical and molecular toxicology》2023,37(1):e23232
Atrazine (2-chloro-4-ethylamino-6-isopropylamino-s-triazine) has been described as a potential toxic for dopaminergic metabolism both in vivo and in vitro. Its main metabolite diamino-chloro triazine (DACT) has been shown to achieve higher levels in brain tissue than atrazine. The aim of this study was to evaluate the in vitro effects of atrazine and DACT on striatal mitochondrial function, active oxygen species generation, and nitric oxide (NO) content. Incubation of mitochondria with atrazine (10 µM) was not able to modify oxygen consumption. However, a 50% increase in malate-glutamate state 4 respiratory rates was observed after DACT treatment (100 µM) without changes in respiratory state 3. Atrazine was able to inhibit complex I–III activity by 30% and DACT induced a tendency to decrease by 17% in the striatum. Regarding reactive oxygen species (ROS), DACT increased H2O2 production by 43%. Also, superoxide anion levels were higher (14%) after atrazine exposure than in control mitochondria. Incubation of striatal mitochondria with atrazine and DACT induced membrane depolarization by 15% and 19%, respectively. Also, atrazine increased NO content by 10% but no significant changes were observed after exposure of mitochondria to DACT. Glutathione peroxidase activity was inhibited (56%) by DACT and atrazine inhibited superoxide dismutase activity by 60%. Also, cardiolipin oxidation (15%) was observed after atrazine treatment. Summing up, the obtained results suggest that in vitro atrazine and DACT induce ROS production affecting striatal mitochondrial function. The atrazine effects would be attributed to a direct effect on the mitochondrial respiratory chain and superoxide dismutase activity while DACT appears to disturb glutathione-related enzyme system. 相似文献
996.
We investigated the role of seed packaging (division of total seed volume among individual seeds) and fruit color in determining feeding preferences of American Robins (Turdus migratorius). Experiments were conducted using artificial fruits with either 8 small plastic beads or a single large one with equivalent volume. Other fruit characters were held constant. As predicted, large seeds were voided rapidly by regurgitation, resulting in higher pulp consumption rates for large-seeded fruits than for small-seeded ones, whose seeds were passed through the gut. Most birds apparently used this difference in profitability as a choice criterion: four of seven preferred large-seeded fruits. That three individuals did not do so suggests that birds may differ in their ability to perceive minor differences in fruit profitability, or to use them as choice criteria. Pulp color was also important: blue fruits were preferred by all seven birds. This preference was surprising, since Robins commonly feed on red fruits in the field. 相似文献
997.
Third-instar praying mantids (Tenodera sinensisSaussure: Mantidae) were fed either a sequence of unpalatable milkweed bugs (Oncopeltus fasciatusDallas: Lygaeidae) and palatable fruit flies (Drosophila melanogasterMeigen: Drosophilidae) or a control diet of palatable flies only. Mantids fed a sequence of 3 days of unpalatable bugs and 4 days of palatable flies took, on average, 5 days longer to develop to the fourth instar than the controls, and consequently, their growth rate was less than that of the controls. They ate 10–15 unpalatable bugs in that time, but also, because the stadium was prolonged, they ended up eating as many flies as the faster-growing controls and therefore attained similar biomass. Mantids subjected to episodes of 2 days with unpalatable bugs followed by 4 days of flies had reduced growth rates that were a function of both more time spent in the stadium and less weight gained than the controls. Mantids subjected to episodes of 1 day with unpalatable bugs followed by 4 days of flies did not gain as much weight as the controls but had similar growth rates. Mantids fed unpalatable bugs on Days 1 and 6 and palatable flies on the other days and mantids fed flies for 4 days and then 1 day without food grew at the same rate. In this case, eating unpalatable prey for 2 days of 7 (with 7 days = average stadium duration) slowed weight gain as much as missing food for a day. We conclude that, depending on the sequence, episodes (in this case, 1 or more days) of eating unpalatable prey can reduce the daily rate of consumption sufficiently to have a negative impact on biomass gained, stadium duration, and growth rate. 相似文献
998.
Synopsis Disease-free blue tilapia,Oreochromis aureus, exposed to cadmium (0.5 ppm and 10 ppm) showed very significant and dramatic decrease in food consumptions. Food consumptions
returned to near normal 21 days after fish were returned to cadmium-free water. Along with anorexia there were decreases in
body weights during the period when fish were exposed to cadmium. Fish in cadmium-free water that were pair-fed to cadmium-exposed
fish (0.5 ppm) did not gain weight but there were no decreases in body weights. Cadmium was still in tissues 34 days after
the fish were placed in cadmium-free water and the accumulation was highest in the kidney; this was followed by liver, brain,
gill filaments and muscles. The accumulations of the heavy metal (in kidneys and gills) were significantly higher in fish
exposed to high than low cadmium levels. There were no differences in complement levels (haemolytic acitivity) in cadmium-exposed
and cadmium-free fish. However, cadmium-exposed fish did not produce detectable haemagglutinating antibodies against sheep
red blood cells while cadmium-free fish responded well to the antigen. The anorexia in cadmium-exposed fish contributed to
the depression in antibody production. 相似文献
999.
Summary In sepsis tissue O2 uptake may be abnormally limited because of a depressed O2 consumption/O2 transport relationship. This study has been performed to assess patterns of O2 consumption, CO2 production and O2 transport in septic patients undergoing total parenteral nutrition; more in particular, this study has investigated the interdependence between the patterns of blood O2 uptake and simultaneous CO2 release, and the availability of substrates (amino acids, glucose and fat). It has been shown that the O2 consumption/O2 transport relationship is significantly influenced by the exogenous amino acid load, which tends to increase O2 uptake and O2 consumption at any given O2 transport, thus suggesting a favourable effect of amino acid administration on energy metabolism. The data on CO2 production and CO2 release, in addition to reconfirming the results of previous studies, have shown that the changes in O2 uptake and in CO2 production mediated by substrate doses have a quantifiable impact on blood O2-CO2 exchange interactions. 相似文献
1000.
Abstract: Direct observations of feeding sea otters ( Enhydra lutris ) at 11 sites in southeast Alaska showed infaunal clams to be the primary prey utilized by otters throughout the region. Foraging dive times associated with clam and sea urchin prey were significantly longer than those for more easily captured prey (crabs and mussels). Dive times and surface intervals were also generally correlated with water depth or apparent difficulty in obtaining buried prey. Male otters, which fed more extensively on clams than females, made significantly longer foraging dives than females. Foraging success remained high, even at sites where prey numbers were found to be very low during a related study. The very deeply burrowing geoduck clam ( Panope abrupta ), while common at several otter feeding sites, was rarely captured by otters. These results, combined with those of a companion study on prey numbers, indicate that butter clams ( Saxidomus giganteus ) account for the majority of the sea otter diet in southeast Alaska, and that sea urchins may represent relatively short-term prey in comparison to infaunal bivalves in regions where both prey types co-exist. Furthermore, the importance of butter clams in the sea otter diet and the tendency for this bivalve to retain chronically high levels of paralytic shellfish poisoning toxins in southeast Alaska increases the probability that toxic phytoplankton blooms influence sea otter distribution in this region. 相似文献