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   953篇
  免费   66篇
  1019篇
  2022年   9篇
  2021年   10篇
  2020年   4篇
  2019年   9篇
  2018年   14篇
  2017年   15篇
  2016年   38篇
  2015年   41篇
  2014年   50篇
  2013年   73篇
  2012年   66篇
  2011年   52篇
  2010年   44篇
  2009年   32篇
  2008年   40篇
  2007年   61篇
  2006年   63篇
  2005年   52篇
  2004年   50篇
  2003年   57篇
  2002年   50篇
  2001年   7篇
  2000年   8篇
  1999年   6篇
  1998年   12篇
  1997年   4篇
  1996年   10篇
  1995年   13篇
  1994年   5篇
  1993年   5篇
  1992年   9篇
  1991年   6篇
  1990年   6篇
  1989年   6篇
  1988年   3篇
  1987年   5篇
  1986年   3篇
  1985年   5篇
  1984年   6篇
  1983年   3篇
  1982年   3篇
  1981年   5篇
  1980年   3篇
  1979年   9篇
  1978年   3篇
  1977年   6篇
  1976年   5篇
  1974年   4篇
  1969年   3篇
  1967年   4篇
排序方式: 共有1019条查询结果,搜索用时 15 毫秒
71.
72.
The interstitial fluid of the human myocardium was monitored in 13 patients undergoing aortic valve and/or bypass surgery before, during, and after hypothermic potassium cardioplegia. The regulation of glucose and lactate was studied after sampling with microdialysis. The following questions were addressed. 1). Is the rate of transcapillary diffusion the limiting step for myocardial uptake of glucose before or after cardioplegia? 2). Does cold potassium cardioplegia induce a critical deprivation of glucose and/or accumulation of lactate in the myocardium? Before cardioplegia, interstitial glucose was approximately 50% of the plasma level (P < 0.001). Interstitial glucose decreased significantly immediately after induction of cardioplegia and remained low (1.25 +/- 0.25 mM) throughout cardioplegia. It was restored to precardioplegic levels 1 h after release of the aortic clamp. Interstitial glucose then decreased again at 25 and 35 h postoperatively to the levels observed during cardioplegia. Interstitial lactate decreased immediately after induction of cardioplegia but returned to basal level during the clamping period. At 25 and 35 h, interstitial lactate was significantly lower than before and during cardioplegia. Glucose transport over the capillary endothelium is considered rate limiting for its uptake in the working heart but not during cold potassium cardioplegia despite the glucose deprivation following perfusion of glucose-free cardioplegic solution. Lactate accumulated during cardioplegia but never reached exceedingly high interstitial levels. We conclude that microdialysis provides information that may be relevant for myocardial protection during open-heart surgery.  相似文献   
73.
The therapeutic efficacy of protegrin peptide IB-367 was investigated in three rat models of septic shock: (i) rats injected intraperitoneally with 1mg Escherichia coli 0111:B4 lipopolysaccharide, (ii) rats given an intraperitoneal injection of 2 X 10(10) CFU of E. coli ATCC 25922, and (iii) rats in which intra-abdominal sepsis was induced via cecal ligation and puncture. All animals were randomized to receive parenterally isotonic sodium chloride solution, 1mg/kg of IB-367, 60mg/kg piperacillin and 1mg/kg of IB-367 plus 60mg/kg piperacillin. The peptide demonstrated lower level of antimicrobial activity than piperacillin, nevertheless it exhibited the dual properties of antimicrobial and antiendotoxin agent. Finally IB-367 and piperacillin association showed to be the most effective therapeutic approach.  相似文献   
74.
75.
A sural nerve dissected from a recently dead patient displayed an unusual X-ray diffraction pattern, suggesting that in situ and at the time of the patient's death the myelin sheaths were in a swollen state. Diffraction patterns of the swollen type were also recorded from: (1) a sural nerve from the corpse of a neurologically healthy person after soaking the nerve with Ringer solution at pH 5.5; (2) sciatic nerves dissected from rat cadavers at increasing time after death. In all the cases the swollen patterns reversed to the native type upon superfusion with Ringer solution at pH 7.3. The postmortem effect is to decrease the pH of the fluids surrounding the nerves in the cadavers. Our experiments show that the early postmortem processes have the effect of acidifying PNS nerves and that as a consequence of acidification the myelin sheaths swell.  相似文献   
76.
Bacillus pumilus PS213 was found to be able to release acetate from acetylated xylan. The enzyme catalyzing this reaction has been purified to homogeneity and characterized. The enzyme was secreted, and its production was induced by corncob powder and xylan. Its molecular mass, as determined by gel filtration, is 190 kDa, while sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed a single band of 40 kDa. The isoelectric point was found to be 4.8, and the enzyme activity was optimal at 55°C and pH 8.0. The activity was inhibited by most of the metal ions, while no enhancement was observed. The Michaelis constant (Km) and Vmax for α-naphthyl acetate were 1.54 mM and 360 μmol min−1 mg of protein−1, respectively.  相似文献   
77.
78.
Species evolution depends on numerous and distinct forces, including demography and natural selection. For example, local adaptation and population structure affect the evolutionary history of species living along environmental clines. This is particularly relevant in plants, which are often characterized by limited dispersal ability and the need to respond to abiotic and biotic stress factors specific to the local environment. Here we study the demographic history and the possible existence of local adaptation in two related species of Brassicaceae, Cardamine impatiens and Cardamine resedifolia, which occupy separate habitats along the elevation gradient. Previous genome-wide analyses revealed the occurrence of distinct selective pressures in the two species, with genes involved in cold response evolving particularly fast in C. resedifolia. In this study we surveyed patterns of molecular evolution and genetic variability in a set of 19 genes, including neutral and candidate genes involved in cold response, across 10 populations each of C. resedifolia and C. impatiens from the Italian Alps (Trentino). We inferred the population structure and demographic history of the two species, and tested the occurrence of signatures of local adaptation in these genes. The results indicate that, despite a slightly higher population differentiation in C. resedifolia than in C. impatiens, both species are only weakly structured and that populations sampled at high altitude experience less gene flow than low-altitude ones. None of the genes showed signatures of positive selection, suggesting that they do not seem to play relevant roles in the current evolutionary processes of adaptation to alpine environments of these species.  相似文献   
79.

Background

Colorectal cancer is the third most common cancer worldwide. Diet has been hypothesized as involved in colorectal cancer etiology, but few studies on the influence of total dietary antioxidant intake on colorectal cancer risk have been performed.

Methods

We investigated the association between colorectal cancer risk and the total antioxidant capacity (TAC) of the diet, and also of intake of selected antioxidants, in 45,194 persons enrolled in 5 centers (Florence, Naples, Ragusa, Turin and Varese) of the European Prospective Investigation into Cancer and Nutrition (EPIC) Italy study. TAC was estimated by the Trolox equivalent antioxidant capacity (TEAC) assay. Hazard ratios (HRs) for developing colorectal cancer, and colon and rectal cancers separately, adjusted for confounders, were estimated for tertiles of TAC by Cox modeling, stratifying by center.

Results

Four hundred thirty-six colorectal cancers were diagnosed over a mean follow-up of 11.28 years. No significant association between dietary TAC and colorectal cancer incidence was found. However for the highest category of TAC compared to the lowest, risk of developing colon cancer was lower (HR: 0.63; 95% CI: 0.44–0.89, P trend: 0.008). By contrast, increasing TAC intake was associated with significantly increasing risks of rectal cancer (2nd tertile HR: 2.09; 95%CI: 1.19–3.66; 3rd tertile 2.48 95%CI: 1.32–4.66; P trend 0.007). Intakes of vitamin C, vitamin E, and ß-carotene were not significantly associated with colorectal cancer risk.

Conclusions

Further prospective studies are needed to confirm the contrasting effects of high total antioxidant intake on risk of colon and rectal cancers.  相似文献   
80.
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