首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   54347篇
  免费   4499篇
  国内免费   17245篇
  2024年   99篇
  2023年   1972篇
  2022年   2622篇
  2021年   3028篇
  2020年   2755篇
  2019年   2320篇
  2018年   2933篇
  2017年   2783篇
  2016年   3209篇
  2015年   4330篇
  2014年   5134篇
  2013年   4960篇
  2012年   5231篇
  2011年   5025篇
  2010年   4174篇
  2009年   3758篇
  2008年   2868篇
  2007年   3493篇
  2006年   3065篇
  2005年   2271篇
  2004年   1552篇
  2003年   1471篇
  2002年   1194篇
  2001年   1145篇
  2000年   922篇
  1999年   758篇
  1998年   480篇
  1997年   421篇
  1996年   373篇
  1995年   280篇
  1994年   210篇
  1993年   204篇
  1992年   180篇
  1991年   199篇
  1990年   162篇
  1989年   147篇
  1988年   168篇
  1987年   26篇
  1986年   28篇
  1985年   27篇
  1984年   17篇
  1983年   43篇
  1982年   12篇
  1981年   11篇
  1980年   2篇
  1976年   2篇
  1963年   2篇
  1958年   12篇
  1957年   3篇
  1950年   9篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
1.
BackgroundThe utilization of traditional Chinese medicine is a common therapeutic approach for stroke patients in Chinese population, but little is known about the effect of Bu Yang Huan Wu Tang (BYHWT) on post-stroke diabetes.PurposeWe aimed to evaluate the risk of diabetes in stroke patients who used BYHWT.Study designA retrospective cohort study based on a real-world database was conducted.MethodsNewly diagnosed stroke patients receiving inpatient care from 2000 to 2004 were identified using a large-scale insurance database in Taiwan. Propensity score matching was used to select eligible stroke patients who did (n = 9849) and did not (n = 9849) receive BYHWT. These two groups were followed up until the end of 2009 to track incident diabetes. Cox proportional hazard models were used to calculate the adjusted hazard rations (HRs) and 95% confidence intervals (CIs) for post-stroke diabetes associated with BYHWT during the follow-up period.ResultsStroke patients who used BYHWT had a reduced incidence of diabetes (14.1% vs. 19.0%, p < 0.0001) and reduced risk of diabetes (HR 0.77; 95% CI 0.72 to 0.83) compared with the control group. The association between BYHWT and reduced risk of post-stroke diabetes was significant across sexe, age group, and stroke subtype. Additionally, the use of BYHWT was associated with a reduced risk of post-stroke diabetes even after excluding the initial three months of diabetes cases in the sensitivity analysis.ConclusionsStroke patients who received BYHWT therapy had a reduced risk of diabetes, and a positive effect was observed in various subgroups. However, future clinical trials will be necessary to validate the present findings and identify the biochemical mechanism involved.  相似文献   
2.
Haemophilus ducreyi, the chancroid-causing bacterium, produces lipooligosaccharides (HdLOS) that comprise 5–11 partially sialylated monosaccharides. Subcutaneous immunisation of mice with 5 μg of HdLOS purified from H. ducreyi strains 4438 and 7470 induced high levels of anti-HdLOS IgG. The antibody responses displayed T-cell-independent features, and were dependent upon Toll-like receptor 4/MyD88 signalling pathways as demonstrated using knockout mice. The immunogenicity of HdLOS was found to require the intact lipid A moiety. The specificity studies of the anti-HdLOS antibodies, as revealed by absorption studies, antibody detection in ELISA, and immune thin-layer chromatography, indicated that the majority of the anti-LOS antibodies were specific for the inner core of the HdLOS. Antibodies to HdLOS failed to inhibit LOS induction of TNF-α release from human mononuclear cells. The adjuvanticity of HdLOS7470 was assessed in BALB/c mice that were immunised with bovine serum albumin (BSA) with or without the addition of HdLOS. The addition of 5 μg HdLOS resulted in a 10-fold increase in the total anti-BSA IgG antibody level as estimated by ELISA. The highest increase was noted for IgG2b, which contrasted with the predominantly IgG1 subclass response to immunisation with BSA alone, indicating an immunomodulatory activity of the HdLOS.  相似文献   
3.
Trees can adjust xylem anatomical structure related with potential hydraulic functions to cope with climate variability. We therefore need a better understanding of how climate variability constrains wood anatomy and tree radial growth. Pinus tabuliformis dominates natural forests and plantations over the western Qinling Mountains, which is one of the ecologically vulnerable areas in China. Here, we investigated the response of P. tabuliformis tree-ring anatomical structure to climate variability by applying wood anatomy analysis, and evaluated the influences of anatomical traits on potential hydraulic functions and the climate significance of intra-annual density fluctuations (IADFs). We found that with the increasing temperature from spring to summer, the negative effect of temperature on the formation and enlargement of earlywood and transition-wood tracheids was gradually enhanced. However, spring precipitation not only had a direct and positive influence on the formation of earlywood, but also had a delaying impact on the transition-wood cell enlargement. Besides, the smaller earlywood tracheid size of P. tabuliformis could be a substantially characteristic reflecting spring drought. The contribution of lumen diameter on conduit wall reinforcement was dominated in earlywood, while the contribution of cell wall thickness was greater than that of lumen diameter in latewood. The different contributions of anatomical traits on conduit wall reinforcement would further affect the response of potential hydraulic function to climate. IADFs of P. tabuliformis could be a potential indicator to reflect the abnormal summer precipitation events in the western Qinling Mountains. IADFs with strong and weak intensity indicated years with high and low rates of change in mid-summer precipitation, respectively. Future warmer and drier climate in the western Qinling Mountains will likely result in the production of smaller tracheids to ensure hydraulic safety, which means the stronger drought resistant of P. tabuliformis in the future. In this study, we linked the xylem anatomy and potential hydraulics functions with intra-seasonal climate variability in the context of climate warming and drying, and proposed some xylem anatomical indices reflecting potential drought events.  相似文献   
4.
5.
6.
《Developmental cell》2022,57(8):995-1008.e5
  1. Download : Download high-res image (144KB)
  2. Download : Download full-size image
  相似文献   
7.
Na+/H+ antiporters comprise a super-family (CPA) of membrane proteins that are found in all kingdoms of life and are essential in cellular homeostasis of pH, Na+ and volume. Their activity is strictly dependent on pH, a property that underpins their role in pH homeostasis. While several human homologues have long been drug targets, NhaA of Escherichia coli has become the paradigm for this class of secondary active transporters as NhaA crystal structure provided insight into the architecture of this molecular machine. However, the mechanism of the strict pH dependence of NhaA is missing. Here, as a follow up of a recent evolutionary analysis that identified a ‘CPA motif’, we rationally designed three E. coli NhaA mutants: D133S, I134T, and the double mutant D133S-I134T. Exploring growth phenotype, transport activity and Li+-binding of the mutants, we revealed that Asp133 does not participate directly in proton binding, nor does it directly dictate the pH-dependent transport of NhaA. Strikingly, the variant I134T lost some of the pH control, and the D133S-Il134T double mutant retained Li+ binding in a pH independent fashion. Concurrent to loss of pH control, these mutants bound Li+ more strongly than the WT. Both positions are in close vicinity to the ion-binding site of the antiporter, attributing the results to electrostatic interaction between these residues and Asp164 of the ion-binding site. This is consistent with pH sensing resulting from direct coupling between cation binding and deprotonation in Asp164, which applies also to other CPA antiporters that are involved in human diseases.  相似文献   
8.
《Cell reports》2020,30(1):98-111.e5
  1. Download : Download high-res image (124KB)
  2. Download : Download full-size image
  相似文献   
9.
The functioning of the vertebrate eye depends on its absolute size, which is presumably adapted to specific needs. Eye size variation in lidless and spectacled colubrid snakes was investigated, including 839 specimens belonging to 49 genera, 66 species and subspecies. Variations of adult eye diameters (EDs) in both absolute and relative terms between species were correlated with parameters reflecting behavioral ecology. In absolute terms, eye of arboreal species was larger than in terrestrial and semiaquatic species. For diurnal species, EDs of terrestrial species do not differ from semiaquatic species; for nocturnal species the ED of terrestrial species is larger than fossorial species but not different from semiaquatic species. In relative terms, ED did not differ significantly by habitat for diurnal species. Although the ED of terrestrial species is larger than fossorial species there were no differences for nocturnal species between semiaquatic and fossorial snakes. In contrast to other vertebrates studied to date, colubrid EDs in absolute and relative terms are larger in diurnal than in nocturnal species. These observations suggest that among colubrid snakes, eye size variation reflects adaptation to specific habitats, foraging strategies and daily activities, independently of phylogeny. J. Morphol. 2012. © 2012 Wiley Periodicals, Inc.  相似文献   
10.
Food availability is important to the dynamics of animal social organizations or populations. However, the role of winter food availability in animal population dynamics is still controversial. We carried out an experimental study to test Lack’s hypothesis that reduced food in winter limits survival and spring numbers of breeding individuals of social groups, using the Mongolian gerbil (Meriones unguiculatus) as model species. We established 24 gerbil social groups in 24, 10 × 10 m, pens in September 2008. We provided wheat seeds as supplemental food in 12 enclosures from September 2008 to March 2009; the other 12 enclosures, not provided with supplemental food, served as controls. We live-trapped gerbils at a 2-week interval from September to April. Supplemental food during winter increased biweekly survival by 10% relative to that in control groups. Only four control social groups survived to the end of our study whereas all 12 food-supplemented social groups survived through our study period. Supplemental food also increased cumulative numbers of recruits and group sizes of gerbils. We conclude that winter food availability limits winter survival and spring social groups or population sizes of Mongolian gerbils.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号