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   4951篇
  免费   563篇
  2021年   71篇
  2020年   46篇
  2019年   48篇
  2018年   61篇
  2017年   66篇
  2016年   112篇
  2015年   171篇
  2014年   176篇
  2013年   267篇
  2012年   277篇
  2011年   318篇
  2010年   200篇
  2009年   169篇
  2008年   239篇
  2007年   229篇
  2006年   209篇
  2005年   195篇
  2004年   195篇
  2003年   183篇
  2002年   199篇
  2001年   115篇
  2000年   103篇
  1999年   114篇
  1998年   59篇
  1997年   49篇
  1996年   62篇
  1995年   54篇
  1994年   57篇
  1993年   61篇
  1992年   65篇
  1991年   53篇
  1990年   70篇
  1989年   87篇
  1988年   56篇
  1987年   41篇
  1986年   46篇
  1985年   49篇
  1984年   50篇
  1983年   40篇
  1982年   50篇
  1981年   40篇
  1980年   37篇
  1979年   49篇
  1978年   33篇
  1977年   35篇
  1976年   49篇
  1975年   33篇
  1974年   52篇
  1973年   42篇
  1971年   32篇
排序方式: 共有5514条查询结果,搜索用时 890 毫秒
1.
In a study of 2000 unselected pregnant women the development of a mature placental appearance (grade 3) on ultrasonography by 34-36 weeks'' gestation, observed in 15% of cases, was associated with maternal smoking, low parity, low maternal age, and being white. These women had an increased risk of problems during labour and their babies had an increased risk of low birth weight, poor condition at birth, and perinatal death. The women were randomly allocated to two groups: in one group the result of the placental grading was reported to the clinician responsible for care; in the second the result was noted but not reported. There was a significant decrease in the risk of perinatal death in the group where the grading was known. This reduction was responsible for a difference in the principal outcome index, a heterogeneous group of measures of mortality and morbidity, but this difference was not significant. This study alone does not justify routine late scanning, and further, larger trials are required. Nevertheless, the results do provide a basis for the reporting of placental grading when ultrasound examination is performed during the third trimester.  相似文献   
2.
 This paper deals with the use of cladistic methods and cladograms in phylogeny reconstruction in plant groups containing numerous taxa. How accurate are the cladograms as to details? Accuracy tests at the level of details require an independently known phylogeny, which excludes most plant groups, but such tests can be carried out in domesticated and experimental plant groups which have documented pedigrees. Four such tests are known and are presented here: a new case in Gilia and three previously published cases in Avena, Hordeum, and Helianthus. The four cases include domesticated and experimental plants, use of morphological and molecular evidence, and presence of dichotomous as well as reticulate phylogenies. The cladograms of the four plant groups all differ in significant details from the known pedigrees. These results are discussed in relation to problems of interpretation of cladograms. Received March 21, 2000 Accepted August 16, 2001  相似文献   
3.
4.
5.
Cardiolipin (CL) is responsible for modulation of activities of various enzymes involved in oxidative phosphorylation. Although energy production decreases in heart failure (HF), regulation of cardiolipin during HF development is unknown. Enzymes involved in cardiac cardiolipin synthesis and remodeling were studied in spontaneously hypertensive HF (SHHF) rats, explanted hearts from human HF patients, and nonfailing Sprague Dawley (SD) rats. The biosynthetic enzymes cytidinediphosphatediacylglycerol synthetase (CDS), phosphatidylglycerolphosphate synthase (PGPS) and cardiolipin synthase (CLS) were investigated. Mitochondrial CDS activity and CDS-1 mRNA increased in HF whereas CDS-2 mRNA in SHHF and humans, not in SD rats, decreased. PGPS activity, but not mRNA, increased in SHHF. CLS activity and mRNA decreased in SHHF, but mRNA was not significantly altered in humans. Cardiolipin remodeling enzymes, monolysocardiolipin acyltransferase (MLCL AT) and tafazzin, showed variable changes during HF. MLCL AT activity increased in SHHF. Tafazzin mRNA decreased in SHHF and human HF, but not in SD rats. The gene expression of acyl-CoA: lysocardiolipin acyltransferase-1, an endoplasmic reticulum MLCL AT, remained unaltered in SHHF rats. The results provide mechanisms whereby both cardiolipin biosynthesis and remodeling are altered during HF. Increases in CDS-1, PGPS, and MLCL AT suggest compensatory mechanisms during the development of HF. Human and SD data imply that similar trends may occur in human HF, but not during nonpathological aging, consistent with previous cardiolipin studies.  相似文献   
6.
7.
8.
9.
10.
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