全文获取类型
收费全文 | 22307篇 |
免费 | 1985篇 |
国内免费 | 2304篇 |
专业分类
26596篇 |
出版年
2024年 | 41篇 |
2023年 | 209篇 |
2022年 | 444篇 |
2021年 | 707篇 |
2020年 | 598篇 |
2019年 | 688篇 |
2018年 | 687篇 |
2017年 | 581篇 |
2016年 | 782篇 |
2015年 | 1260篇 |
2014年 | 1459篇 |
2013年 | 1610篇 |
2012年 | 2042篇 |
2011年 | 1764篇 |
2010年 | 1256篇 |
2009年 | 1156篇 |
2008年 | 1435篇 |
2007年 | 1304篇 |
2006年 | 1229篇 |
2005年 | 1131篇 |
2004年 | 1042篇 |
2003年 | 950篇 |
2002年 | 796篇 |
2001年 | 555篇 |
2000年 | 463篇 |
1999年 | 427篇 |
1998年 | 278篇 |
1997年 | 212篇 |
1996年 | 194篇 |
1995年 | 171篇 |
1994年 | 149篇 |
1993年 | 110篇 |
1992年 | 138篇 |
1991年 | 109篇 |
1990年 | 91篇 |
1989年 | 89篇 |
1988年 | 59篇 |
1987年 | 70篇 |
1986年 | 53篇 |
1985年 | 28篇 |
1984年 | 18篇 |
1983年 | 35篇 |
1982年 | 16篇 |
1981年 | 17篇 |
1979年 | 17篇 |
1978年 | 14篇 |
1977年 | 13篇 |
1975年 | 10篇 |
1972年 | 12篇 |
1970年 | 11篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
化疗性静脉炎小鼠模型的建立 总被引:1,自引:0,他引:1
目的通过静脉注射盖诺(vinorelbine,VNB)为化疗性静脉炎(chemotherapy induced phlebitis,CIP)研究,提供效果稳定的CIP模型。方法49只成年小鼠随机分为6个实验组和1个对照组。实验组小鼠分别从右侧鼠尾静脉注射不同浓度的等体积VNB溶液,对照组则注射等体积生理盐水。注射后第5天对CIP临床表现进行分级评价后处死。制作石蜡切片并进行镜下分级。结果随注射VNB浓度及剂量的增加,小鼠静脉炎发生率也逐渐增高,但浓度剂量过高动物出现中毒死亡。用3.2mg/ml的VNB按38mg/kg注射组CIP发生率达100%,无动物死亡,出现红斑、水肿、条索状改变等典型CIP临床症状和内皮脱落、炎细胞浸润、组织水肿等CIP镜下改变,对照组未出现类似变化。结论本实验通过鼠尾静脉注射VNB成功建立了小鼠CIP模型。 相似文献
92.
93.
农田小麦在土壤湿润条件下的气孔活动 总被引:1,自引:0,他引:1
1984年在北京大屯农业生态系统试验站用MK 3型自动气孔计对土壤湿润、睛朗天气条件下冬小麦气孔传导力的日变化,季节变化、以及叶龄和梯度变化进行了观测。发现叶片传导力(gl)的日变化为双峰曲线,正午gl的降低可能是强光和高的饱和差作用的结果,从而限制了光合潜力的发挥。近轴面传导力(gd)和远轴面传导力(gb)的比(α)不是一个常数,这是因为两面气孔对环境条件的反应不同。gl和冠层传导力(g(?))都以孕穗期为最高。灌浆时期gl和gc有所降低,反映了叶片的过早衰老。这是影响本地区小麦产量的重要因素之一,其原因不仅与土壤水分有关,还与大气条件和病虫害有关。本文还讨论了gl随时龄的变化。观测表明,气孔计可以迅速准确地掌握作物群体的生理动态,有可能用于指导农田灌灌和管理。 相似文献
94.
95.
96.
根据担子菌丝裂原活化蛋白质激酶激酶激酶(MAPKKK)蛋白的保守序列设计两对简并引物,通过巢式简并PCR方法获得草菇VV-MAPKKK基因中的保守片段,然后通过和草菇基因组信息比对,获得了VV-MAPKKK基因全长序列。VV-MAPKKK基因长度为4434bp,包含4个内含子,编码1405个氨基酸残基,推定的氨基酸序列与新型隐球菌(Cryptococcus neoformans)、巴西芽生菌(Paracoccidioides brasiliensis)和异旋孢腔菌(Cochliobolus heterostrophus)的MAPKKK同源蛋白相似性分别为58%、57%和56%。对VV-MAPKKK蛋白的系统发生学分析的结果表明,VV-MAPKKK与担子菌中的Hog信号传导途径的MAPKKK同源蛋白聚在同一进化支上,这些数据都支持所获得的VV-MAPKKK为Hog-MAPKKK蛋白在草菇中的同源物的推定。 相似文献
97.
Chen Chen John Wang Jeff Kwong JinHee Kim Aaron van Donkelaar Randall V. Martin Perry Hystad Yushan Su Eric Lavigne Megan Kirby-McGregor Jay S. Kaufman Tarik Benmarhnia Hong Chen 《CMAJ》2022,194(20):E693
Background:The tremendous global health burden related to COVID-19 means that identifying determinants of COVID-19 severity is important for prevention and intervention. We aimed to explore long-term exposure to ambient air pollution as a potential contributor to COVID-19 severity, given its known impact on the respiratory system.Methods:We used a cohort of all people with confirmed SARS-CoV-2 infection, aged 20 years and older and not residing in a long-term care facility in Ontario, Canada, during 2020. We evaluated the association between long-term exposure to fine particulate matter (PM2.5), nitrogen dioxide (NO2) and ground-level ozone (O3), and risk of COVID-19-related hospital admission, intensive care unit (ICU) admission and death. We ascertained individuals’ long-term exposures to each air pollutant based on their residence from 2015 to 2019. We used logistic regression and adjusted for confounders and selection bias using various individual and contextual covariates obtained through data linkage.Results:Among the 151 105 people with confirmed SARS-CoV-2 infection in Ontario in 2020, we observed 8630 hospital admissions, 1912 ICU admissions and 2137 deaths related to COVID-19. For each interquartile range increase in exposure to PM2.5 (1.70 μg/m3), we estimated odds ratios of 1.06 (95% confidence interval [CI] 1.01–1.12), 1.09 (95% CI 0.98–1.21) and 1.00 (95% CI 0.90–1.11) for hospital admission, ICU admission and death, respectively. Estimates were smaller for NO2. We also estimated odds ratios of 1.15 (95% CI 1.06–1.23), 1.30 (95% CI 1.12–1.50) and 1.18 (95% CI 1.02–1.36) per interquartile range increase of 5.14 ppb in O3 for hospital admission, ICU admission and death, respectively.Interpretation:Chronic exposure to air pollution may contribute to severe outcomes after SARS-CoV-2 infection, particularly exposure to O3.By November 2021, COVID-19 had caused more than 5 million deaths globally1 and more than 29 400 in Canada.2 The clinical manifestations of SARS-CoV-2 infection range from being asymptomatic to multiple organ failure and death. Identifying risk factors for COVID-19 severity is important to better understand etiological mechanisms and identify populations to prioritize for screening, vaccination and medical treatment. Risk factors for severity of COVID-19 include male sex, older age, pre-existing medical conditions and being from racialized communities.3–5 More recently, ambient air pollution has been implicated as a potential driver of COVID-19 severity.6–10Long-term exposure to ambient air pollution, a major contributor to global disease burden,11 could increase the risk of severe COVID-19 outcomes by several mechanisms. Air pollutants can reduce individuals’ pulmonary immune responses and antimicrobial activities, boosting viral loads.8 Air pollution can also induce chronic inflammation and overexpression of the alveolar angiotensin-converting enzyme 2 (ACE) receptor,7 the key receptor that facilitates SARS-CoV-2 entry into cells.12,13 Exposure to air pollution contributes to chronic conditions, such as cardiovascular disease, that are associated with unfavourable COVID-19 prognosis, possibly owing to persistent immune activation and excessive amplification of cytokine development.10 Thus, greater exposure to long-term air pollution may lead to severe COVID-19 outcomes.Reports exist of positive associations between long-term exposure to particulate matter with diameters equal to or smaller than 2.5 or 10 μm (PM2.5 and PM10), ground-level ozone (O3) and nitrogen dioxide (NO2), and metrics of COVID-19 severity (e.g., mortality and case fatality rate).8–10 However, most studies to date have used ecological and cross-sectional designs, owing to limited access to individual data, which leads to ambiguity in interpreting the results, thus hindering their influence on policy. 6,14 Ecological designs do not allow for disentangling the relative impacts of air pollution on individual susceptibility to infection and disease severity.14 Residual confounding by factors such as population mobility and social interactions is also problematic. Therefore, a cohort study with data on individuals with SARS-CoV-2 is a more appropriate design.6,14 Studies that have used individual data were conducted in specific subpopulations15,16 or populations with few severe cases,17 or had limited data on individual exposure to air pollutants.18 In Canada, 1 ecological study found a positive association between long-term exposure to PM2.5 and COVID-19 incidence,19 but no published study has explored the association between air pollution and COVID-19 severity.We aimed to examine the associations between long-term exposure to 3 common air pollutants (PM2.5, NO2 and O3) and key indicators of COVID-19 severity, including hospital admission, intensive care unit (ICU) admission and death, using a large prospective cohort of people with confirmed SARS-CoV-2 infection in Ontario, Canada, in 2020. The air contaminants PM2.5, NO2 and O3 are regularly monitored by the Canadian government, and are key pollutants that are considered when setting air-quality policies. They originate from varying sources (NO2 is primarily emitted during combustion of fuel, O3 is primarily formed in air by chemical reactions of nitrogen oxides and volatile organic compounds, and PM2.5 can be emitted during combustion or formed by reactions of chemicals like sulphur dioxide and nitrogen oxides in air) and they may affect human health differently.20,21,22 相似文献
98.
亚热带典型常绿阔叶树种叶片叶绿素含量与其高光谱特征间的关系 总被引:4,自引:2,他引:2
叶绿素在植物的生理生态过程中非常重要.利用高光谱数据,揭示光谱反射率上特征光谱与叶绿素含量间的关系将有助于理解叶绿素光谱反射特征的规律.选取了6种亚热带常绿阔叶树种,抽取一定数量的叶片样品,分别测量了叶片的光谱反射率及其叶绿素含量,并建立了光谱反射率曲线上的红边位置附近的光谱反射率和叶绿素总量间的回归模型.结果显示:对所研究的树种而言,红边位置处一窄波长范围内的平均光谱反射率与叶绿素总量间的相关性普遍较高,回归效果显著.从高光谱遥感的角度结合植物的生理生态特点分析了其机理.建立的有关红边位置处的光谱反射率与叶绿素总量间的回归模型普遍具有较高的拟合度,其应用前景较好. 相似文献
99.
100.
谭清苏铁性别相关的RAPD标记研究 总被引:1,自引:0,他引:1
以谭清苏铁(Cycas tanqingii D.Y.Wang)雌雄植株半年生羽叶为材料,用优化的CTAB法分别提取其全基因组DNA,进行RAPD单因子梯度实验和正交实验以优化扩增条件。应用160个RAPD随机引物检测基因组DNA,雌雄植株均扩增出1450多条带,其中引物S0465扩增出与谭清苏铁雌株高度相关的RAPD标记,其大小约为500bp,该标记与雄株没有关联。 相似文献