全文获取类型
收费全文 | 496077篇 |
免费 | 59145篇 |
国内免费 | 3753篇 |
专业分类
558975篇 |
出版年
2021年 | 5377篇 |
2019年 | 4538篇 |
2018年 | 5839篇 |
2017年 | 5166篇 |
2016年 | 7617篇 |
2015年 | 10672篇 |
2014年 | 12313篇 |
2013年 | 16296篇 |
2012年 | 18764篇 |
2011年 | 18630篇 |
2010年 | 12446篇 |
2009年 | 11398篇 |
2008年 | 15652篇 |
2007年 | 15854篇 |
2006年 | 14298篇 |
2005年 | 13708篇 |
2004年 | 13388篇 |
2003年 | 12766篇 |
2002年 | 11978篇 |
2001年 | 22501篇 |
2000年 | 22446篇 |
1999年 | 17906篇 |
1998年 | 6583篇 |
1997年 | 6777篇 |
1996年 | 6588篇 |
1995年 | 5993篇 |
1994年 | 6034篇 |
1993年 | 5784篇 |
1992年 | 13898篇 |
1991年 | 13187篇 |
1990年 | 12825篇 |
1989年 | 12671篇 |
1988年 | 11284篇 |
1987年 | 10931篇 |
1986年 | 9990篇 |
1985年 | 9755篇 |
1984年 | 8277篇 |
1983年 | 7154篇 |
1982年 | 5593篇 |
1981年 | 5077篇 |
1980年 | 4692篇 |
1979年 | 7736篇 |
1978年 | 5971篇 |
1977年 | 5462篇 |
1976年 | 5111篇 |
1975年 | 5398篇 |
1974年 | 5839篇 |
1973年 | 5712篇 |
1972年 | 5140篇 |
1971年 | 4757篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
721.
722.
Up regulation of the transforming growth factor-beta 1 (TGF-β1) axis has been recognized as a pathogenic event for progression of glomerulosclerosis in diabetic nephropathy. We demonstrate that glomeruli isolated from diabetic rats accumulate up to sixfold more extracellular adenosine than normal rats. Both decreased nucleoside uptake activity by the equilibrative nucleoside transporter 1 and increased AMP hydrolysis contribute to raise extracellular adenosine. Ex vivo assays indicate that activation of the low affinity adenosine A2B receptor subtype (A2BAR) mediates TGF-β1 release from glomeruli of diabetic rats, a pathogenic event that could support progression of glomerulopathy when the bioavailability of adenosine is increased. 相似文献
723.
724.
C Chatelet J Gaillard Y Pétillot M Louwagie J Meyer 《Biochemical and biophysical research communications》1999,261(3):885-889
Overexpression in Escherichia coli of the fdx4 gene from Aquifex aeolicus has allowed isolation and characterization of the first hyperthermophilic [2Fe-2S](Scys)(4) protein, a homodimer of M = 2 x 12.4 kDa with one [2Fe-2S] cluster per subunit. This protein is undamaged by heating to 100 degrees C for at least three hours. The primary structure, in particular the characteristic distribution of the four cysteine ligands of the metal site, and the spectroscopic properties of the A. aeolicus protein relate it to well characterized [2Fe-2S] proteins from Clostridium pasteurianum and Azotobacter vinelandii. These proteins are also homologous to subunits or domains of hydrogenases and NADH-ubiquinone oxidoreductase (Complex I) of respiratory chains. The A. aeolicus [2Fe-2S] protein is thus representative of a presumably novel protein fold involved in a variety of functions in very diverse cellular backgrounds. 相似文献
725.
726.
Most techniques currently available to measure blood flow in bone are time consuming and require destruction of the tissue, but laser-Doppler technology offers a less invasive method. This study assessed the utility of laser-Doppler perfusion imaging (LDI) to measure perfusion in cortical bone. Twelve mature New Zealand White rabbits were assigned to one of three groups: normal control, constriction (norepinephrine), or dilatation (nitroprusside). The left and right medial tibiae were consecutively scanned at red (634-nm) and near-infrared (810-nm) wavelengths to examine the repeatability of LDI output. The pharmacological intervention groups were injected with the respective drug, and LDI measurements at 810 nm were obtained concurrently with colored microsphere-determined flow in all of the groups. LDI effectively quantified blood flow in cortical bone and detected physiologically induced changes in perfusion. A significant positive correlation was found between microsphere-determined flow and LDI output (r = 0.6, P < 0.05). Repeatability of consecutive LDI measurements was within 5%. The effectiveness of LDI to measure perfusion in bone suggests this method has potential for investigating the role of blood flow in bone metabolism and remodeling. 相似文献
727.
H. Wiksell V. Ekstrand C. Wadström G. Auer 《Physica medica : PM : an international journal devoted to the applications of physics to medicine and biology : official journal of the Italian Association of Biomedical Physics (AIFB)》2009,25(3):128-132
A large and increasing number of women in the western world will at some point during their life be investigated morphologically for some type of breast lesion. Fine Needle Aspiration (FNA) is one morphological method which is considered to be the fastest, cheapest and the most patient-friendly approach. However, the frequency of conclusive samples using this method varies and is often too low, especially when performed by unexperienced operators. In this study we have developed and tested a new semi-automated instrument (“CytoTest”) designed for FNA which is intended to improve the efficacy of the technique by increasing the percentage of conclusive samples. A total of 443 consecutive aspiration procedures on palpable breast lesions were performed to compare this new “CytoTest” equipment with the standard protocol using the same type of needles. We conclude that by increasing the extent and frequency of the reciprocatory motions used by an experienced sampling operator as well as enhancing the ejection pressure, the cellular yield can be increased almost three folded compared to the standard protocol. For cases with high amounts of non-diagnostic material (such as blood or cystic fluid) which were discarded, up to four times more sample could be obtained. Furthermore, the frequency of sparse samples under 1 mg was halved with use of the “CytoTest”. 相似文献
728.
729.
In Exp. 1A, hybrid mice (N = 10) were provided with food and 25% (v/v) ethanol as the only source of liquid for 72 h, beginning at the detection of the copulatory plug (08:00 h, Day 1). Control mice received food and tap water. Food consumption (P less than 0.001) but not total caloric intake (P greater than 0.05) was less for the alcohol-treated mice than the controls. Ethanol-derived calories averaged 35% of caloric intake during the 72 h of treatment. Alcohol-treated animals showed a dramatic weight loss until Day 5 while controls gained weight (P less than 0.05). Ethanol consumption did not influence pregnancy rate, litter size or litter weight. In Exp. 1B, animals were treated as in Exp. 1A, but were killed at various times between 24:00 h, Day 1, and 08:00 h, Day 4. Trunk blood was used to determine haematocrit and serum to determine alcohol concentration. Haematocrit was greater (P less than 0.05) for all alcohol-treated mice than for controls at all time periods sampled except one. Dehydration was therefore probably responsible for the weight loss seen in Exps 1A and 1B. Average blood alcohol concentrations fluctuated with time of day and day of treatment. Average maximum concentration was 91.4 mg ethanol/100 ml serum. In Exp. 2, hybrid mouse 2-cell embryos were cultured in vitro in 0 or 0.1% ethanol (Exp. 2A) and 0 or 1.0% ethanol (Exp. 2B) for 8 days.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
730.
R L Allen R Hopewell C Prottey 《Comparative biochemistry and physiology. B, Comparative biochemistry》1986,84(4):523-529
Histidine ammonia lyase was purified to homogeneity from guinea-pig liver and epidermis. Both enzymes had similar molecular weights, subunit composition and pH optima. Km values for the two were similar at pH 9.2 but different at pH 7.0. Both enzymes were stimulated by low thiol concentrations and inhibited at higher concentrations, but to different extents. Antibody to the hepatic enzyme showed complete identity against hepatic enzyme but incomplete identity against epidermal enzyme. 相似文献