全文获取类型
收费全文 | 1009893篇 |
免费 | 113155篇 |
国内免费 | 661篇 |
出版年
2018年 | 12412篇 |
2017年 | 12088篇 |
2016年 | 13917篇 |
2015年 | 16074篇 |
2014年 | 18231篇 |
2013年 | 25999篇 |
2012年 | 31288篇 |
2011年 | 35276篇 |
2010年 | 24831篇 |
2009年 | 23403篇 |
2008年 | 31481篇 |
2007年 | 33427篇 |
2006年 | 25862篇 |
2005年 | 26098篇 |
2004年 | 25234篇 |
2003年 | 24622篇 |
2002年 | 23843篇 |
2001年 | 41024篇 |
2000年 | 41393篇 |
1999年 | 33313篇 |
1998年 | 12865篇 |
1997年 | 13552篇 |
1996年 | 13003篇 |
1995年 | 12148篇 |
1994年 | 12020篇 |
1993年 | 11917篇 |
1992年 | 28105篇 |
1991年 | 27460篇 |
1990年 | 26922篇 |
1989年 | 26279篇 |
1988年 | 24237篇 |
1987年 | 23673篇 |
1986年 | 21730篇 |
1985年 | 21940篇 |
1984年 | 18350篇 |
1983年 | 15973篇 |
1982年 | 12729篇 |
1981年 | 11363篇 |
1980年 | 10833篇 |
1979年 | 17682篇 |
1978年 | 14126篇 |
1977年 | 13036篇 |
1976年 | 12277篇 |
1975年 | 13371篇 |
1974年 | 14482篇 |
1973年 | 14203篇 |
1972年 | 13117篇 |
1971年 | 12139篇 |
1970年 | 10270篇 |
1969年 | 9800篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
901.
Air transport can move patients safely and rapidly over long distances. However, changes in altitude can have disastrous effects because diminished ambient air pressure may allow gases in closed spaces and tissues to expand rapidly. Even pressurized commercial aircraft do not maintain sea-level pressure: cabin pressures equal to those at yp to 8000 ft may be experienced, diminishing oxygen tension in proportion. Air transport is absolutely contraindicated for patients with untreated pneumothorax, gas gangrene, or air trapped in the cranium and those who have recently undergone abdominal surgery. Special considerations including a planned low-altitude flight are warrented for patients who are anemic, in respiratory or cardiac distress, or immobilized in casts, or who have been engaged in underwater diving immediately before the flight. 相似文献
902.
903.
904.
905.
Uptake of radioactive calcium from guinea pig brain fractions enriched in synaptosomes could be significantly and reproducibly decreased by exposure to high pressure. Calcium efflux from preloaded synaptosomes was unaffected by pressure exposure. It was hypothesized that the development of pressure-induced encephalopathy may be related to an effect of pressure on the central nervous system calcium transport system. 相似文献
906.
W Kornreich H Anderson J Porter W Vale J Rivier 《International journal of peptide and protein research》1985,25(4):414-420
Three new resins have been developed that allow for the solid phase synthesis of C-terminal peptide N-alkylamides using Boc amino acids, usual side chain protecting groups and hydrogen fluoride cleavage and deprotection. These resins were prepared by reacting the appropriate alkylamine (NH2CH3, NH2CH2CH3, NH2CH2CF3) to Merrifield's 1% divinylbenzene cross-linked chloromethylated polystyrene resin. The application of these resins to the synthesis of C-terminal GnRH N-alkylamides illustrates the versatility of this approach. GnRH analogs were tested for their ability to release LH from cultured rat anterior pituitary cells. [DGlu6, Pro9-NHCH2CH3]-GnRH was synthesized for the first time using the solid phase approach and found to be three times more potent than [DGlu6]-GnRH. Other analogs including [DTrp6, Pro9-NHCH2CH3]-GnRH, [DAla6, Pro9-NHCH2CF3]-GnRH and related peptides were found to be equipotent and to have the same properties (HPLC retention times, amino acid analysis and specific rotation) as the corresponding peptides synthesized using less amenable strategies; yields were equivalent or better than those reported earlier. 相似文献
907.
908.
Differential enhancement of spontaneous transition mutations in the lacI gene of an Ung- strain of Escherichia coli 总被引:1,自引:0,他引:1
In this communication, the contribution of cytosine deamination to spontaneous mutagenesis in the lacI gene of E. coli was examined. In a wild-type strain, 75% of the amber mutations recovered were G:C----A:T transitions and 60% of these were at the 5-methylcytosine spontaneous hotspots Am6, Am15 and Am34. In a strain deficient for uracil-DNA glycosylase (Ung-), 96% of the amber mutations were G:C----A:T transitions while only 15% of these occurred at the hotspot sites. This shift in the mutational distribution demonstrates that cytosine deamination is a potent mutagenic process, which is enhanced in the absence of glycosylase. Moreover, some amber sites were greatly enhanced in the Ung- strain while others were only slightly enhanced. This result suggests that the rate of cytosine deamination at individual sites may be influenced by surrounding base composition. Therefore, we examined the neighboring sequences and found a strong correlation between the fold-increase in mutation and the A/T richness of the surrounding sequence. It is suggested that A/T-rich regions denature more often, forming transient single strands in which cytosine residues would be expected to deaminate more readily. 相似文献
909.
The mechanisms responsible for the difference in neurovirulence to inbred mice between two variants of the Miyama strain of herpes simplex virus type 1 (HSV-1) were studied. After intraperitoneal (i.p.) inoculation, the +GC (LPV) variant reached the spinal cord and the brain, and caused death. Conversely, the -GCr variant lacked the ability to gain access to the central nervous system (CNS) after the same route of infection and failed to kill susceptible mice. The initial virus growth after i.p. inoculation, as indicated by the number of infective centers (ICs) produced by the peritoneal exudate cells (PECs), was compared between these two variants. The virulent +GC (LPV) strain induced much more ICs than the attenuated -GCr variant. When the attenuated variant was preinoculated i.p. 24 hr before the challenge inoculation with the virulent variant by the same route, the production of ICs by the pathogenic variant was highly inhibited, and growth of this variant did not occur in the CNS. Thus, mice were protected from lethal infection by the virulent variant by preinoculation with the attenuated one. Moreover, the ability of mice to resist i.p. infection by HSV-1 was shown to be age-dependent. 相似文献
910.
M N Berry R B Gregory A R Grivell D C Henly C D Nobes J W Phillips P G Wallace 《Biochimica et biophysica acta》1988,936(3):294-306
The lipophilic triphenylmethylphosphonium cation (TPMP+) has been employed to measure delta psi m, the electrical potential across the inner membrane of the mitochondria of intact hepatocytes. The present studies have examined the validity of this technique in hepatocytes exposed to graded concentrations of inhibitors of mitochondrial energy transduction. Under these conditions, TPMP+ uptake allows a reliable measure of delta psi m in intracellular mitochondria, provided that the ratio [TPMP+]i/[TPMP+]e is greater than 50:1 and that at the end of the incubation more than 80% of the hepatocytes exclude Trypan blue. Hepatocytes, staining with Trypan blue, incubated in the presence of Ca2+, do not concentrate TPMP+. The relationships between delta psi m and two other indicators of cellular energy state, delta GPc and Eh, or between delta psi m and J0, were examined in hepatocytes from fasted rats by titration with graded concentrations of inhibitors of mitochondrial energy transduction. Linear relationships were generally observed between delta psi m and delta GPc, Eh or J0 over the delta psi m range of 120-160 mV, except in the presence of carboxyatractyloside or oligomycin, where delta psi m remained constant. Both the magnitude and the direction of the slope of the observed relationships depended upon the nature of the inhibitor. Hepatocytes from fasted rats synthesized glucose from lactate or fructose, and urea from ammonia, at rates which were generally linear functions of the magnitude of delta psi m, except in the presence of oligomycin or carboxyatractyloside. Linear relationships were also observed between delta psi m and the rate of formation of lactate in cells incubated with fructose and in hepatocytes from fed rats. The linear property of these force-flow relationships is taken as evidence for the operation of thermodynamic regulatory mechanisms within hepatocytes. 相似文献