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   4550篇
  免费   261篇
  国内免费   256篇
  2023年   70篇
  2022年   64篇
  2021年   100篇
  2020年   126篇
  2019年   140篇
  2018年   88篇
  2017年   108篇
  2016年   117篇
  2015年   131篇
  2014年   221篇
  2013年   307篇
  2012年   181篇
  2011年   227篇
  2010年   158篇
  2009年   173篇
  2008年   232篇
  2007年   236篇
  2006年   210篇
  2005年   184篇
  2004年   184篇
  2003年   161篇
  2002年   129篇
  2001年   116篇
  2000年   102篇
  1999年   111篇
  1998年   69篇
  1997年   72篇
  1996年   72篇
  1995年   83篇
  1994年   76篇
  1993年   64篇
  1992年   55篇
  1991年   52篇
  1990年   41篇
  1989年   47篇
  1988年   45篇
  1987年   44篇
  1986年   50篇
  1985年   53篇
  1984年   36篇
  1983年   36篇
  1982年   45篇
  1981年   26篇
  1980年   45篇
  1979年   31篇
  1978年   37篇
  1977年   20篇
  1976年   18篇
  1975年   24篇
  1972年   16篇
排序方式: 共有5067条查询结果,搜索用时 187 毫秒
71.
Despite poor sensitivity and specificity, office blood pressure (BP) determinations are still the “gold standard” for diagnosing gestational hypertension. This prospective blind study evaluates the prognostic value of office values as compared with ambulatory monitoring in pregnancy. We analyzed 2175 BP series systematically sampled from 355 non-preeclamptic pregnant women for 48 h every 4 wks from the first hospital visit until delivery. Women were divided for comparative purposes into three groups: “detected” gestational hypertension, defined on the basis of casual clinical BP>140/90 mm Hg after 20 wks of gestation and hyperbaric index (area of BP excess above the upper limit of a time-specified tolerance interval adjusted for the circadian pattern of the reference population) consistently above the threshold for diagnosing hypertension in pregnancy; “undetected” gestational hypertension, women with office BP<140/90 mm Hg but hyperbaric index consistently above the threshold for diagnosis; and normotension, women with both office values and hyperbaric index below the respective thresholds for diagnosis. Small and insignificant differences in the 24 h mean BP between “detected” and “undetected” gestational hypertension is observed in all trimesters, in contrast with highly significant differences between these two groups and normotensive pregnancies. Normotensive women are characterized by highly significant lesser incidence by 60% in preterm delivery, 70% in intrauterine growth retardation, and 50% in delivery by cesarean section (P<0.001) compared with women with “detected” and “undetected” gestational hypertension (P>0.715). In pregnancy, the hyperbaric index is markedly superior to office BP measurements for diagnosis of what should be truly considered gestational hypertension, and for prediction of the outcome of pregnancy.  相似文献   
72.
《植物生态学报》2018,42(12):1131
生态过程模型的发展为研究者在长时间序列和区域尺度的研究提供了便利, 但模型模拟的准确性受到模型自身结构、模型参数估计合理性的影响。敏感性分析能够定量或定性筛选出对模型模拟结果影响较大的敏感参数, 是模型参数校准过程中的重要工具, 也是建模和应用的先决条件。该文以阔叶红松林为研究对象, 采用全局敏感性分析方法——傅里叶幅度灵敏度检验扩展法(EFAST)对Biome-BGC模型的生理生态参数进行了敏感性分析, 分别分析了红松(Pinus koraiensis)和阔叶树的净初级生产力(NPP)、蒸散(ET)对参数变化的敏感性。结果表明: (1)模拟红松NPP的不确定性高于阔叶树, 但二者的模拟ET的不确定性均较小。阔叶树的NPPET对生理生态参数的敏感性总体上都小于红松。(2)无论是红松、阔叶或其他植被类型, 模拟NPP均表现出对叶片碳氮比、细根碳氮比、比叶面积(SLA)和冠层截留系数的敏感性, 这4个参数的高敏感性主要是由模型自身结构所决定的, 与植被类型和研究地区的关系较小。对模拟ET而言, 细根与叶片碳分配比、新茎与新叶碳分配比和SLA均是影响红松和阔叶树ET的敏感参数, 但红松ET主要受参数与参数间的二阶或多阶交互作用的间接影响, 而阔叶树ET则主要是受到敏感参数直接效应的影响。(3)除了上述影响红松和阔叶树碳水通量的共性参数外, 诸如核酮糖-1,5-二磷酸羧化酶中叶氮含量、叶片与细根周转率、所有叶面积与投影叶面积之比等也是对模拟结果有影响的重要参数, 但是其敏感程度随物种不同和研究区不同而不同, 所以这类参数可以根据具体情况进行参数本地化, 对于其他不敏感参数则可以采用模型缺省值。  相似文献   
73.
Rats given presentations of a citric acid solution while recovering from LiCl-induced illness (i.e., a “medicine effect” treatment) subsequently drank more of an aversively conditioned NaCl solution at test, when the NaCl presentation was immediately preceded by citric acid. That is, citric acid passed a summation test of conditioned inhibition. Such an effect was not observed in a group given explicitly unpaired presentations of LiCl and citric acid. It is proposed that enhanced consumption of an aversive taste due to the previous presentation of a “medicine” taste can provide an animal model of human maladaptive behavior in regards to food consumption.  相似文献   
74.
75.
Prostaglandin E2 (PGE2) is an endogenous lipid molecule involved in normal brain development. Cyclooxygenase‐2 (COX2) is the main regulator of PGE2 synthesis. Emerging clinical and molecular research provides compelling evidence that abnormal COX2/PGE2 signaling is associated with autism spectrum disorder (ASD). We previously found that COX2 knockout mice had dysregulated expression of many ASD genes belonging to important biological pathways for neurodevelopment. The present study is the first to show the connection between irregular COX2/PGE2 signaling and autism‐related behaviors in male and female COX2‐deficient knockin, (COX)‐2?, mice at young (4‐6 weeks) or adult (8‐11 weeks) ages. Autism‐related behaviors were prominent in male (COX)‐2? mice for most behavioral tests. In the open field test, (COX)‐2? mice traveled more than controls and adult male (COX)‐2? mice spent less time in the center indicating elevated hyperactive and anxiety‐linked behaviors. (COX)‐2? mice also buried more marbles, with males burying more than females, suggesting increased anxiety and repetitive behaviors. Young male (COX)‐2? mice fell more frequently in the inverted screen test revealing motor deficits. The three‐chamber sociability test found that adult female (COX)‐2? mice spent less time in the novel mouse chamber indicative of social abnormalities. In addition, male (COX)‐2? mice showed altered expression of several autism‐linked genes: Wnt2, Glo1, Grm5 and Mmp9. Overall, our findings offer new insight into the involvement of disrupted COX2/PGE2 signaling in ASD pathology with age‐related differences and greater impact on males. We propose that (COX)‐2? mice might serve as a novel model system to study specific types of autism.  相似文献   
76.
张晓宇  图力古尔  李玉 《菌物学报》2019,38(7):1099-1110
对采自海南省白沙县鹦哥岭阔叶树腐木上的标本,根据形态学特征和多基因分析,鉴定为脆木耳Auricularia fibrillifera,经分离纯化获得纯菌株作为实验材料,为了充分利用和开发木耳属资源,首次对该种的生物学特性和驯化栽培进行了研究。研究不同碳源、氮源、无机盐和生长因子在固体培养条件下对脆木耳菌丝生长的影响,对以上4个因素进行单因素试验,从中选出3个最优的水平进行正交试验。结果表明:脆木耳菌丝最适生长温度为33℃,最佳碳源为麦芽糖,最佳氮源为牛肉粉,最佳无机盐添加量为1.5% PO43--1% Mg2+,最佳生长因子为玉米汁。驯化栽培过程中,栽培配方为:木屑78%,麸皮20%,石膏粉1%,石灰粉1%,发菌温度为22℃可使脆木耳菌丝在40d左右满袋,且生长旺盛,可培育出子实体。  相似文献   
77.
目的:探讨OSAHS风险与静脉麻醉手术患者术后发生认知功能障碍的关系。方法:采用No SAS评分对55例静脉麻醉手术患者进行OSAHS风险评估,并将其分为对照组23例(NoSAS 8分)和OSAHS组32例(No SAS≥8分),以蒙特利尔认知评估量表(MoCA)对两组患者在术前和术后第一天进行认知功能评估,计算每位患者手术前后Mo CA评分的差值△Mo CA(术前MoCA-术后MoCA),比较两组患者手术前后的MoCA评分及△MoCA。结果:OSAHS组术前MoCA评分(25.83±1.80)明显低于对照组术前MoCA评分(28.05±1.31)(P0.05)。OSAHS组术后MoCA评分(25.13±1.64)较术前无明显变化(P0.05),对照组术后Mo CA评分(26.73±1.17)明显低于术前(P0.05)。OSAHS组△MoCA(0.39±1.03)明显低于对照组(1.32±1.08),主要表现为视空间与执行功能[(0.09±0.29) vs.(0.30±0.32)]、注意力[(0.09±0.60) vs.(0.47±0.70)]和延时回忆力[(0.17±0.39) vs.(0.47±0.51)]两方面(P0.05)。结论:OSAHS高风险患者静脉麻醉术后认知功能障碍的程度较OSAHS低风险人群显著降低。  相似文献   
78.
79.
The dynamic light scattering methods are widely used in biomedical diagnostics involving evaluation of blood flow. However, there exist some difficulties in quantitative interpretation of backscattered light signals from the viewpoint of diagnostic information. This study considers the application of the high‐speed videocapillaroscopy (VCS) method that provides the direct measurement of the red blood cells (RBCs) velocity into a capillary. The VCS signal presents true oscillation nature of backscattered light caused by moving RBCs. Thus, the VCS signal can be assigned as a reference one with respect to more complicated signals like in laser Doppler flowmetry (LDF). An essential correlation between blood flow velocity oscillations in a separate human capillary and the integral perfusion estimate obtained by the LDF method has been found. The observation of blood flow by the VCS method during upper arm occlusion has shown emergence of the reverse blood flow effect in capillaries that corresponds to the biological zero signal in the LDF. The reverse blood flow effect has to be taken into account in interpretation of LDF signals.   相似文献   
80.
为了解决黑果枸杞中花青素稳定性问题,本文采用Box-Behnken设计对黑果枸杞提取物泡腾片配方进行优化,并对其进行质量评价。采用酸碱混合制粒压片法,通过单因素实验,筛选出片剂所需的辅料:崩解剂、填充剂、润滑剂以及甜味剂。采用响应面试验,结合感官评价进行处方优化,从而确定最优配方:柠檬酸32%、黑果枸杞提取物25%、碳酸氢钠24%、乳糖15%、甜蜜素3%、聚乙二醇6000 1%;对最优配方进行质量评价,各项指标均符合规定,其中花青素含量为8.06 mg/g。该泡腾片表面光滑,泡腾效果好,具有黑果枸杞香气,为实际生产提供理论依据。  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