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   11930篇
  免费   1354篇
  国内免费   3篇
  13287篇
  2022年   76篇
  2021年   137篇
  2020年   144篇
  2019年   223篇
  2018年   243篇
  2017年   257篇
  2016年   326篇
  2015年   532篇
  2014年   534篇
  2013年   732篇
  2012年   720篇
  2011年   642篇
  2010年   545篇
  2009年   472篇
  2008年   573篇
  2007年   586篇
  2006年   571篇
  2005年   618篇
  2004年   583篇
  2003年   630篇
  2002年   637篇
  2001年   122篇
  2000年   107篇
  1999年   159篇
  1998年   202篇
  1997年   146篇
  1996年   148篇
  1995年   143篇
  1994年   128篇
  1993年   92篇
  1992年   128篇
  1991年   105篇
  1990年   93篇
  1989年   93篇
  1988年   72篇
  1987年   88篇
  1986年   78篇
  1985年   95篇
  1984年   102篇
  1983年   83篇
  1982年   107篇
  1981年   119篇
  1980年   90篇
  1979年   94篇
  1978年   90篇
  1977年   80篇
  1976年   83篇
  1975年   53篇
  1974年   73篇
  1973年   66篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Many high‐priority and high‐interest species are challenging to study due to the difficulty in accessing animals and/or obtaining sufficient sample sizes. The recent explosion in technology, particularly social media and live webcams available on the Internet, provides new opportunities for behavioral scientists to collect data not just on our own species, as well as new resources for teaching and outreach. We discuss here the possibility of exploiting online media as a new source of behavioral data, which we termed ‘video mining’. This article proposes epidemiological and ethological field techniques to gather and screen online media as a data source on diverse taxa. This novel method provides access to a rich source of untapped knowledge, particularly to study the behavior of understudied species or sporadic behaviors, but also for teaching or monitoring animals in challenging settings.  相似文献   
992.
993.
994.
Resveratrol (RVT) is a stilbene with a protective effect on the cardiovascular system; however, drawbacks including low bioavailability and fast metabolism limit its efficacy. In this work we described new resveratrol derivatives with nitric oxide (NO) release properties, ability to inhibit platelet aggregation and in vivo antithrombotic effect. Compounds (4af) were able to release NO in vitro, at levels ranging from 24.1% to 27.4%. All compounds (2af and 4af) have exhibited platelet aggregation inhibition using as agonists ADP, collagen and arachidonic acid. The most active compound (4f) showed reduced bleeding time compared to acetylsalicylic acid (ASA) and protected up to 80% against in vivo thromboembolic events. These findings suggest that hybrid resveratrol-furoxan (4f) is a novel lead compound able to prevent platelet aggregation and thromboembolic events.  相似文献   
995.
The annual moult creates the highest physiological stress during a penguin's breeding‐cycle and is preceded by a period of hyperphagia at sea. Although crucial to individual survival, foraging strategies before moult have been little investigated in keystone marine consumers in the Southern Ocean. The Macaroni Penguin Eudyptes chrysolophus demonstrates how individuals may adjust their foraging strategies during this period in line with constraints such as potential intraspecific competition between localities, foraging ability between dimorphic sexes and timing at sea between breeding and non‐breeding population components. We recorded pre‐moult behaviour at sea for 22 Macaroni Penguins from Crozet and Kerguelen Islands (southern Indian Ocean) during 2009 and 2011, using light‐based geolocation and stable isotope analysis. Penguins were distributed in population‐specific oceanic areas with similar surface temperatures (3.5 °C) south of the archipelagos, where they foraged at comparable trophic levels based on stable isotopes of their blood. Bayesian ‘broken stick’ modelling with concurrent analysis of seawater temperature records from the animal‐borne devices showed that within each population, females remained 6 days longer than males in the colder waters before heading back towards their colonies. Finally, 17 other non‐breeding individuals that moulted earlier had a higher mean blood δ15N value than did post‐breeding birds, meaning that early moulters probably fed more on fish than did late moulters. Our findings of such adjustments in foraging strategies developed across locality, sex and breeding status help understanding of the species' contrasted pre‐moult biology across its range and its ecology in the non‐breeding period.  相似文献   
996.
997.
The present study was carried out to assess the effects of protease inhibitor (PI) therapy on basal whole body protein metabolism and its response to acute amino acid-glucose infusion in 14 human immunodeficiency virus (HIV)-infected patients. Patients treated with PIs (PI+, 7 patients) or without PIs (PI-, 7 patients) were studied after an overnight fast during a 180-min basal period followed by a 140-min period of amino acid-glucose infusion. Protein metabolism was investigated by a primed constant infusion of l-[1-(13)C]leucine. Dual-energy X-ray absorptiometry for determination of fat-free mass (FFM) and body fat mass measured body composition. In the postabsorptive state, whole body leucine balance was 2.5 times (P < 0.05) less negative in the PI+ than in the PI- group. In HIV-infected patients treated with PIs, the oxidative leucine disposal during an acute amino acid-glucose infusion was lower (0.58 +/- 0.09 vs. 0.81 +/- 0.07 micromol x kg FFM(-1) x min(-1) using plasma [(13)C]leucine enrichment, P = 0.06; or 0.70 +/- 0.10 vs. 0.99 +/- 0.08 micromol x kg FFM(-1) x min(-1) using plasma [(13)C]ketoisocaproic acid enrichment, P = 0.04 in PI+ and PI- groups, respectively) than in patients treated without PIs. Consequently, whole body nonoxidative leucine disposal (an index of protein synthesis) and leucine balance (0.50 +/- 0.10 vs. 0.18 +/- 0.06 micromol x kg FFM x (-1) x min(-1) in PI+ and PI- groups respectively, P < 0.05) were significantly improved during amino acid-glucose infusion in patients treated with PIs. However, whereas the response of whole body protein anabolism to an amino acid-glucose infusion was increased in HIV-infected patients treated with PIs, any improvement in lean body mass was detected.  相似文献   
998.
Author index     
(1) Cyclic AMP stimulated alanine transport in isolated hepatocytes by approx. 30%, in the range 0.2–5 mM alanine. (2) Alanine utilisation was also stimulated by cyclic AMP. The rates of transport and metabolism were comparable, both in the presence and absence of cyclic AMP. (3) At concentrations of alanine above 1 mM, addition of ouabain, or the reduction of the Na+ concentration, could partially inhibit transport without affecting the rate of metabolism. (4) At these alanine concentrations, stimulation of metabolism by cyclic AMP was associated with a decrease in the intracellular to extracellular alanine concentration ratio. (5) At alanine concentrations below 0.5 mM, or at higher concentrations when transport was inhibited by reducing the Na+ concentration, cyclic AMP caused an increase in the alanine concentration ratio. (6) It is concluded that at concentrations of alanine above 1 mM, alanine transport is not rate-limiting for alanine metabolism in hepatocytes from fed rats, and cyclic AMP stimulates alanine metabolism primarily by an effect on an intracellular reaction. At physiological concentrations of alanine, however, alanine transport appears to be rate-limiting in agreement with a previous report.  相似文献   
999.
1000.
In this paper, we propose a Bayesian design framework for a biosimilars clinical program that entails conducting concurrent trials in multiple therapeutic indications to establish equivalent efficacy for a proposed biologic compared to a reference biologic in each indication to support approval of the proposed biologic as a biosimilar. Our method facilitates information borrowing across indications through the use of a multivariate normal correlated parameter prior (CPP), which is constructed from easily interpretable hyperparameters that represent direct statements about the equivalence hypotheses to be tested. The CPP accommodates different endpoints and data types across indications (eg, binary and continuous) and can, therefore, be used in a wide context of models without having to modify the data (eg, rescaling) to provide reasonable information-borrowing properties. We illustrate how one can evaluate the design using Bayesian versions of the type I error rate and power with the objective of determining the sample size required for each indication such that the design has high power to demonstrate equivalent efficacy in each indication, reasonably high power to demonstrate equivalent efficacy simultaneously in all indications (ie, globally), and reasonable type I error control from a Bayesian perspective. We illustrate the method with several examples, including designing biosimilars trials for follicular lymphoma and rheumatoid arthritis using binary and continuous endpoints, respectively.  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