全文获取类型
收费全文 | 463199篇 |
免费 | 47194篇 |
国内免费 | 3609篇 |
出版年
2018年 | 5430篇 |
2017年 | 4813篇 |
2016年 | 6518篇 |
2015年 | 8364篇 |
2014年 | 10086篇 |
2013年 | 13864篇 |
2012年 | 15206篇 |
2011年 | 15408篇 |
2010年 | 10549篇 |
2009年 | 9586篇 |
2008年 | 13061篇 |
2007年 | 13461篇 |
2006年 | 12544篇 |
2005年 | 11686篇 |
2004年 | 11652篇 |
2003年 | 11335篇 |
2002年 | 10873篇 |
2001年 | 18158篇 |
2000年 | 18078篇 |
1999年 | 14586篇 |
1998年 | 5333篇 |
1997年 | 5590篇 |
1996年 | 5296篇 |
1995年 | 5015篇 |
1994年 | 4903篇 |
1993年 | 4831篇 |
1992年 | 12520篇 |
1991年 | 12299篇 |
1990年 | 12262篇 |
1989年 | 12031篇 |
1988年 | 11276篇 |
1987年 | 10660篇 |
1986年 | 9942篇 |
1985年 | 10343篇 |
1984年 | 8532篇 |
1983年 | 7360篇 |
1982年 | 5687篇 |
1981年 | 5081篇 |
1980年 | 4742篇 |
1979年 | 8132篇 |
1978年 | 6352篇 |
1977年 | 5977篇 |
1976年 | 5713篇 |
1975年 | 6171篇 |
1974年 | 6736篇 |
1973年 | 6620篇 |
1972年 | 6166篇 |
1971年 | 5565篇 |
1970年 | 4799篇 |
1969年 | 4803篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
41.
42.
43.
44.
45.
The frequency of cytokine-producing CD4-/CD8- mononuclear cells was assessed in patients of different age groups (29 infants, aged 1-5 years; 30 schoolchildren, aged 6-14 years, 26 adults, aged > 15 years) with acute Plasmodium falciparum malaria, from Gabon. Fifteen patients were followed up before antimalarial treatment (day 0), during parasite clearance (day 3) and after resolution of parasitemia (day 10). By using flow cytometry for intracellular detection of cytokines, a striking expansion of CD4-/CD8- cells producing the type 1 cytokines interleukin (IL)-2-/interferon (IFN)-gamma+, IL-2+/IFN-gamma+ and IL-2+/IFN-gamma- was observed in adults as compared with children. Type 2 cytokine expression (IL-4+/IFN-gamma-, IL-13+/IFN-gamma-) and type 0 cells (IL-4+/IFN-gamma+, IL-13+/IFN-gamma+) were not significantly different between the three age groups. Patients with severe malaria had a significantly increased frequency of type 2 cytokine-producing CD4-/CD8- cells. Drug-induced clearance of parasitemia was characterized by a decrease of IL-2+/IFN-gamma- and type 2 cytokine expressing CD4-/CD8- cells and by a gradual increase of IL-10+/IFN-gamma- expression. The type 1/type 2 dichotomy observed within the CD4-/CD8- cell population is likely to be of significance in the host response against P. falciparum malaria. 相似文献
46.
Binding of the cationic tetra(tributylammoniomethyl)-substituted hydroxoaluminum phthalocyanine (AlPcN4) to bilayer lipid membranes was studied by fluorescence correlation spectroscopy (FCS) and intramembrane field compensation (IFC) methods. With neutral phosphatidylcholine membranes, AlPcN4 appeared to bind more effectively than the negatively charged tetrasulfonated aluminum phthalocyanine (AlPcS4), which was attributed to the enhancement of the coordination interaction of aluminum with the phosphate moiety of phosphatidylcholine by the electric field created by positively charged groups of AlPcN4. The inhibitory effect of fluoride ions on the membrane binding of both AlPcN4 and AlPcS4 supported the essential role of aluminum-phosphate coordination in the interaction of these phthalocyanines with phospholipids. The presence of negative or positive charges on the surface of lipid membranes modulated the binding of AlPcN4 and AlPcS4 in accord with the character (attraction or repulsion) of the electrostatic interaction, thus showing the significant contribution of the latter to the phthalocyanine adsorption on lipid bilayers. The data on the photodynamic activity of AlPcN4 and AlPcS4 as measured by sensitized photoinactivation of gramicidin channels in bilayer lipid membranes correlated well with the binding data obtained by FCS and IFC techniques. The reduced photodynamic activity of AlPcN4 with neutral membranes violating this correlation was attributed to the concentration quenching of singlet excited states as proved by the data on the AlPcN4 fluorescence quenching. 相似文献
47.
Wen-Yeh Hsieh Tang-Ching Kuan Kun-Shan Cheng Yan-Chiou Liao Mu-Yuan Chen Pei-Heng Lin Yuan-Chang Hsu Chen-Yi Huang Wei-Hua Hsu Sheng-Yao Yu Chih-Sheng Lin 《International journal of biological sciences》2012,8(8):1197-1205
Objective: Pleural effusion is common problem, but the rapid and reliable diagnosis for specific pathogenic effusions are lacking. This study aimed to identify the diagnosis based on clinical variables to differentiate pleural tuberculous exudates from other pleural effusions. We also investigated the role of renin-angiotensin system (RAS) and matrix metalloproteinase (MMPs) in the pathogenesis of pleural exudates.Experimental design: The major components in RAS and extracellular matrix metabolism, including angiotensin converting enzyme (ACE), ACE2, MMP-2 and MMP-9 activities, were measured and compared in the patients with transudative (n = 45) and exudative (n = 80) effusions. The exudative effusions were come from the patients with tuberculosis (n = 20), pneumonia (n = 32), and adenocarcinoma (n = 28).Results: Increased ACE and equivalent ACE2 activities, resulting in a significantly increased ACE/ACE2 ratio in exudates, were detected compared to these values in transudates. MMP-9 activity in exudates was significantly higher than that in transudates. The significant correlation between ACE and ACE2 activity that was found in transudates was not found in exudates. Advanced analyses showed significantly increased ACE and MMP-9 activities, and decreased ACE2 activity in tuberculous pleural effusions compared with those in pneumonia and adenocarcinoma effusions. The results indicate that increased ACE and MMP-9 activities found in the exudates were mainly contributed from a higher level of both enzyme activities in the tuberculous pleural effusions.Conclusion: Interplay between ACE and ACE2, essential functions in the RAS, and abnormal regulation of MMP-9 probably play a pivotal role in the development of exudative effusions. Moreover, the ACE/ACE2 ratio combined with MMP-9 activity in pleural fluid may be potential biomarkers for diagnosing tuberculous pleurisy. 相似文献
48.
49.
Self-consistent steady-state axisymmetric configurations of a plasma envelope with a uniform anisotropic conductivity around a rotating magnetized spherical body are considered. A set of electrodynamic and magnetohydrodynamic equations is analyzed under the assumption that the mass velocity of a moving weakly ionized plasma has only the azimuthal component. The equations describing the profile of the angular frequency of the rotating plasma envelope, the magnetic field, the conduction currents, and the plasma density distribution are solved in the limit of a strong anisotropy of the conductivity of a weakly ionized gas. The applicability of the results obtained to a qualitative interpretation of the phenomena occurring in the plasmaspheres of magnetized planets is discussed. 相似文献
50.
Single-chamber microbial fuel cell (SMFC)-I consisted of 4 separator-electrode assemblies (SEAs) with two types of cation exchange membrane (CEM: Nafion and CMI 7000) and an anion exchange membrane (AEM: AMI 7001). SMFC-II consisted of 4 SEAs with Nafion and three types of nonwoven fabric. SMFC-I and -II were inoculated with anaerobic digested and activated sludge, respectively, and operated under fed-batch mode. In SMFC I, AEM-SEA showed a maximum power density (PDmax). Nafion-SEA showed a PDmax in SMFC II, which was similar to that of Nafion–SEA of SMFC I. Although different bacteria were developed in SMFC-I (Deltaproteobacteria and Firmicutes) and SMFC-II (Gammaproteobacteria, Betaproteobacteria and Bacteroidetes), the inoculum type little affects electricity generation. Variations of pH and oxygen in biofilm have influenced microbial community structure and electricity generation according to the electrode and separator material. Although the electricity generation of non-woven fabric-SEA was less than that of Nafion-SEA, the use of non-woven fabrics is expected to reduce the construction and operating costs of MFCs. 相似文献