全文获取类型
收费全文 | 12196篇 |
免费 | 1041篇 |
国内免费 | 290篇 |
出版年
2024年 | 30篇 |
2023年 | 267篇 |
2022年 | 323篇 |
2021年 | 477篇 |
2020年 | 492篇 |
2019年 | 663篇 |
2018年 | 606篇 |
2017年 | 476篇 |
2016年 | 462篇 |
2015年 | 466篇 |
2014年 | 930篇 |
2013年 | 1260篇 |
2012年 | 658篇 |
2011年 | 766篇 |
2010年 | 588篇 |
2009年 | 507篇 |
2008年 | 586篇 |
2007年 | 541篇 |
2006年 | 438篇 |
2005年 | 363篇 |
2004年 | 288篇 |
2003年 | 206篇 |
2002年 | 164篇 |
2001年 | 103篇 |
2000年 | 95篇 |
1999年 | 112篇 |
1998年 | 116篇 |
1997年 | 95篇 |
1996年 | 75篇 |
1995年 | 73篇 |
1994年 | 72篇 |
1993年 | 65篇 |
1992年 | 68篇 |
1991年 | 55篇 |
1990年 | 44篇 |
1989年 | 45篇 |
1988年 | 36篇 |
1987年 | 35篇 |
1985年 | 139篇 |
1984年 | 127篇 |
1983年 | 81篇 |
1982年 | 91篇 |
1981年 | 78篇 |
1980年 | 70篇 |
1979年 | 62篇 |
1978年 | 50篇 |
1977年 | 42篇 |
1976年 | 28篇 |
1974年 | 36篇 |
1973年 | 25篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
141.
儿童最大有氧活动能力的发展特征 总被引:4,自引:1,他引:3
本文报告了我国463名10-19岁儿童青少年的最大有氧活动能力的发展特征。在青春早期,男女童的最大吸氧量绝对值均随年龄增长而增加,男童由1.75升/分增至3.10升/分,女童由1.44升/分增至2.07升/分,女童增长较少;以后女童即稳定于这一水平,男童仍略有增长。按身高及按最大心率计标的相对值与其有相似的特征。按体重和瘦体重计算的相对值,在男女童都未见随年龄增长的规律。男童VO2max绝对值及各 相似文献
142.
Aysan Etemadi Salar Hemmati Mohammad Shahrivar-Gargari Yasaman Tamaddon Abibiglue Ahad Bavili Maryam Hamzeh-Mivehroud Siavoush Dastmalchi 《化学与生物多样性》2023,20(8):e202300075
Indanone derivatives containing meta/para-substituted aminopropoxy benzyl/benzylidene moieties were designed based on the structures of donepezil and ebselen analogs as the cholinesterase inhibitors. The designed compounds were synthesized and their acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) inhibitory activities were measured. Inhibitory potencies (IC50 values) for the synthesized compounds ranged from 0.12 to 11.92 μM and 0.04 to 24.36 μM against AChE and BChE, respectively. Compound 5 c showed the highest AChE inhibitory potency with IC50 value of 0.12 μM, whereas the highest BChE inhibition was achieved by structure 7 b (IC50=0.04 μM). Structure-activity relationship (SAR) analysis revealed that there is no significant difference between meta and para-substituted derivatives in AChE and BChE inhibition. However, the most potent AChE inhibitor 5 c belongs to meta-substituted compounds, while the most active BChE inhibitor is para-substituted derivative 7 b . The order of enzyme inhibition potency based on the substituted amine group is dimethyl amine>piperidine>morpholine. Compounds containing C=C linkage are more potent AChE inhibitors than the corresponding saturated structures. Molecular docking studies indicated that 5 c interacts with AChE in a very similar way to that observed experimentally for donepezil. The introduced indanone-aminopropoxy benzylidenes could be used in drug-discovery against Alzheimer's disease. 相似文献
143.
摘要 目的:分析Stanford A型主动脉夹层(AD)孙氏手术患者术后血流感染(BSI)的影响因素,并探讨术前血清降钙素原(PCT)、白细胞介素-6(IL-6)、D-二聚体(D-D)对术后发生BSI的预测价值。方法:选取2019年1月~2022年1月贵州医科大学附属医院收治的236例接受孙氏手术的Stanford A型AD患者,根据术后是否BSI分为BSI组和非BSI组。收集患者基础资料和实验室指标,采用多因素Logistic回归分析Stanford A型AD孙氏手术患者术后发生BSI的影响因素,采用受试者工作特征(ROC)曲线分析血清PCT、IL-6、D-D水平对Stanford A型AD孙氏手术患者术后发生BSI的预测价值。结果:BSI组年龄≥60岁、糖尿病史、机械通气、气管切开、人工瓣膜植入比例和术后24 h引流量、血清C反应蛋白、PCT、IL-6、D-D水平高于非BSI组,手术时间、心包纵隔管保留时间长于非BSI组(P<0.05)。多因素Logistic回归分析显示,年龄≥60岁、糖尿病史、机械通气、气管切开、术后24 h引流量上升,血清PCT、IL-6、D-D水平上升为Stanford A型AD孙氏手术患者术后发生BSI的危险因素(P<0.05)。ROC曲线分析显示,血清PCT、IL-6、D-D三项联合预测的Stanford A型AD孙氏手术患者术后发生BSI的曲线下面积大于单独预测。结论:年龄、糖尿病史、机械通气、气管切开、术后24 h引流量、血清PCT、IL-6、D-D水平是Stanford A型AD孙氏手术患者术后发生BSI的影响因素,术前血清PCT、IL-6、D-D水平可作为Stanford A型AD孙氏手术患者术后发生BSI的辅助预测指标。 相似文献
144.
摘要 目的:探讨与分析女贞子提取物对帕金森病大鼠神经炎性反应及内质网应激蛋白激酶R样内质网激酶(PERK)/转录活化因子4(ATF4)通路的影响。方法:采用单侧注射6-羟基多巴胺(6-OHDA)毁损法建立帕金森病大鼠模型,将造模成功的大鼠36只随机平分为三组-模型组、左旋多巴组与女贞子组,每组各12只。左旋多巴组与女贞子组分别灌胃0.5 mL的左旋多巴、女贞子提取物,模型组给予无菌蒸馏水0.5 mL,2次/d,连续给药4周,检测大鼠神经炎性反应、内质网应激PERK/ATF4通路相关蛋白表达变化情况。结果:左旋多巴组与女贞子组治疗第2周、第4周的探究性反应次数显著高于模型组(P<0.05),女贞子组与左旋多巴组对比也有显著提高(P<0.05)。左旋多巴组与女贞子组治疗第2周、第4周的脑组织伊文思蓝含量显著低于模型组(P<0.05),女贞子组与左旋多巴组对比也有显著降低(P<0.05)。左旋多巴组与女贞子组治疗第2周、第4周的血清丙二醛、白介素-1β、肿瘤坏死因子-α、一氧化氮含量显著低于模型组(P<0.05),女贞子组与左旋多巴组对比也有显著降低(P<0.05)。左旋多巴组与女贞子组治疗第2周、第4周的黑质-纹状体组织PERK蛋白、ATF4蛋白相对表达水平显著低于模型组(P<0.05),女贞子组与左旋多巴组对比也有显著降低(P<0.05)。结论:女贞子提取物在帕金森病大鼠的应用能改善神经功能,降低脑组织伊文思蓝含量,还可抑制PERK/ATF4通路的激活,降低血清丙二醛、白介素-1β、肿瘤坏死因子-α、一氧化氮含量,从而持续发挥脑保护作用。 相似文献
145.
146.
147.
148.
Measures of nematode community structure and sources of variability among and within agricultural fields 总被引:4,自引:0,他引:4
Deborah A. Neher Steven L. Peck John O. Rawlings C. Lee Campbell 《Plant and Soil》1995,170(1):167-181
Whole nematode communities, extracted from soil samples taken from agricultural fields, were enumerated by taxonomic family and trophic group (i.e., bacterivores, fungivores, omnivores, plant-parasites, and predators) to evaluate nematode community structure as an indicator for monitoring ecological condition of soil. No differences were found in mixing treatments or methods of packing or shipping samples. However, extraction using Cobb's sifting and gravity method, followed by sucrose centrifugation, gave greater recovery of free-living nematodes than elutriation followed by sucrose centrifugation. Population means and variance of the sampled area were similar when sampled using different strategies for collecting soil samples within fieds, including several patterns, directions and repetitions of transects. Components of variation associated with ratios among the five trophic groups of nematodes and selected indices of community structure were quantified as variation among regions, among counties, among agricultural fields (2-ha area), among transects within agricultural fields, and within composite soil samples. The variance component for'within composite soil samples' was relatively large compared to the other components of variance. Variation within composite soil samples was less for maturity indices (based on life-history strategy characteristics), ratio of bacterivores to plant-parasites, sum of bacterivores and fungivores, populations of plant-parasites, and populations of bacterivores than for trophic diversity indices, populations of fungivores, populations of omnivores, populations of predators, or the ratio of fungivores to bacterivores. With a single composite sample per field, the ability to differentiate ecological condition of soils among fields within a region improved if the variance among and within fields exceeded the variance within composite samples. Given the variance components, power curves indicated that detection of a 10% change (with 0.8 power) in the ecological condition of soils within a region between two time periods would require sampling a minimum of 25 and 50 fields with one composite soil sample analyzed per field for the maturity and trophic diversity index, respectively. More than 100 fieldsper region would be required to detect temporal change in populations of individual trophic groups. Biplots of maturity indices, but not of trophic diversity or populations of individual trophic groups, identified clear differences among fields. Thus, maturity indices, which differentiated among sampling sites better and more efficiently than trophic diversity indices or measures based on populations of individual trophic groups, may be appropriate for use in a regional and/or national monitoring program. 相似文献
149.
Vincent Delpizzo 《Bioelectromagnetics》1995,16(2):132-142
A computer program simulating case-control studies is described. It is used to estimate the minimum sample size required and to assess how this is affected by imprecise exposure assessment. In particular, the consequences of neglecting measurements of nonresidential exposure in case-control studies of residentially exposed adults are investigated. According to this model, while the consequent loss of power is not as large as was predicted by algebraic methods, it would be unwise to neglect it when planning a study. © 1995 Wiley-Liss, Inc. 相似文献
150.
Two types of distributions for the frequencies of occurrence of amino acids in each position of hypervariable regions CDR-1 and CDR-2 were obtained for 2,000 immunoglobulins. The results show that some positions fit an inverse power-law distribution, while others fit an exponential-type distribution. As a result of comparison with structural data in the literature it is proposed that sites in which the frequency distribution fits the inverse power law are critical to maintaining canonical shapes of the recognition regions or are involved in modulating these canonical conformations, while those sites where the distribution fits the exponential law are those which should be exclusively involved in the recognition mechanism.
Correspondence to: F. Lara-Ochoa 相似文献