全文获取类型
收费全文 | 6040篇 |
免费 | 352篇 |
国内免费 | 1篇 |
专业分类
6393篇 |
出版年
2023年 | 62篇 |
2022年 | 87篇 |
2021年 | 164篇 |
2020年 | 112篇 |
2019年 | 140篇 |
2018年 | 187篇 |
2017年 | 148篇 |
2016年 | 229篇 |
2015年 | 325篇 |
2014年 | 375篇 |
2013年 | 412篇 |
2012年 | 519篇 |
2011年 | 524篇 |
2010年 | 317篇 |
2009年 | 236篇 |
2008年 | 326篇 |
2007年 | 355篇 |
2006年 | 345篇 |
2005年 | 258篇 |
2004年 | 253篇 |
2003年 | 214篇 |
2002年 | 212篇 |
2001年 | 51篇 |
2000年 | 35篇 |
1999年 | 33篇 |
1998年 | 50篇 |
1997年 | 39篇 |
1996年 | 44篇 |
1995年 | 32篇 |
1994年 | 28篇 |
1993年 | 24篇 |
1992年 | 28篇 |
1991年 | 18篇 |
1990年 | 16篇 |
1989年 | 20篇 |
1988年 | 17篇 |
1987年 | 8篇 |
1986年 | 11篇 |
1985年 | 14篇 |
1984年 | 7篇 |
1983年 | 11篇 |
1982年 | 12篇 |
1981年 | 7篇 |
1979年 | 12篇 |
1978年 | 10篇 |
1977年 | 9篇 |
1976年 | 6篇 |
1975年 | 9篇 |
1974年 | 8篇 |
1973年 | 8篇 |
排序方式: 共有6393条查询结果,搜索用时 11 毫秒
141.
Roberta Gomes Carvalho Ricardo Louren?o-de-Oliveira Ima Aparecida Braga 《Memórias do Instituto Oswaldo Cruz》2014,109(6):787-796
The geographical distribution of Aedes albopictus in Brazil was updated according tothe data recorded across the country over the last eight years. Countrywide houseindexes (HI) for Ae. albopictus in urban and suburban areas were described for thefirst time using a sample of Brazilian municipalities. This mosquito is currentlypresent in at least 59% of the Brazilian municipalities and in 24 of the 27 federalunits (i.e., 26 states and the Federal District). In 34 Brazilian municipalities, theHI values for Ae. albopictus were higher than those recorded for Ae. aegypti,reaching figures as high as HI = 7.72 in the Southeast Region. Remarks regarding thecurrent range of this mosquito species in the Americas are also presented. NineteenAmerican countries are currently infested and few mainland American countries havenot confirmed the occurrence of Ae. albopictus. The large distribution and highfrequency of Ae. albopictus in the Americas may become a critical factor in thespread of arboviruses like chikungunya in the new world. 相似文献
142.
Luciano Pamplona de Góes Cavalcanti Lia Alves Martins Mota Gustavo Porto Lustosa Mayara Carvalho Fortes Davi Alves Martins Mota Ant?nio Afonso Bezerra Lima Ivo Castelo Branco Coelho Maria Paula Gomes Mour?o 《Memórias do Instituto Oswaldo Cruz》2014,109(1):93-98
In 2009, the World Health Organization (WHO) issued a new guideline that stratifies
dengue-affected patients into severe (SD) and non-severe dengue (NSD) (with or
without warning signs). To evaluate the new recommendations, we completed a
retrospective cross-sectional study of the dengue haemorrhagic fever (DHF) cases
reported during an outbreak in 2011 in northeastern Brazil. We investigated 84
suspected DHF patients, including 45 (53.6%) males and 39 (46.4%) females. The ages
of the patients ranged from five-83 years and the median age was 29. According to the
DHF/dengue shock syndrome classification, 53 (63.1%) patients were classified as
having dengue fever and 31 (36.9%) as having DHF. According to the 2009 WHO
classification, 32 (38.1%) patients were grouped as having NSD [4 (4.8%) without
warning signs and 28 (33.3%) with warning signs] and 52 (61.9%) as having SD. A
better performance of the revised classification in the detection of severe clinical
manifestations allows for an improved detection of patients with SD and may reduce
deaths. The revised classification will not only facilitate effective screening and
patient management, but will also enable the collection of standardised surveillance
data for future epidemiological and clinical studies. 相似文献
143.
Paul J. Feldblum Sónia Enosse Karine Dubé Paulo Arnaldo Chadreque Muluana Reginaldo Banze Aristides Nhanala Joana Cunaca Pai-Lien Chen Merlin L. Robb Ricardo Thompson 《PloS one》2014,9(5)
Background
Reliable HIV incidence estimates for Mozambique are limited. We conducted a prospective HIV incidence study as part of a clinical research site development initiative in Chókwè district, Gaza Province, southern Mozambique.Methods
Between June 2010 and October 2012, we recruited women at sites where women at higher risk of HIV infection would likely be found. We enrolled and tested 1,429 sexually active women in the screening phase and 479 uninfected women in the prospective phase. Participants were scheduled for 12+ months follow-up, when they underwent face-to-face interviews, HIV counseling and testing, and pregnancy testing. We observed a total of 373.1 woman-years (WY) of follow-up, with mean (median) of 9.4 (9.7) women-months per participant.Results
The prevalence of HIV was 29.4% (95% confidence interval [CI]: 27.0–31.8%). In multivariable logistic regression analysis, factors that remained significantly associated with prevalent HIV were: older age (OR: 0.6; 95% CI: 0.4–0.7), lower educational level (OR: 0.4; 95% CI: 0.3–0.7), and using hormonal contraception (OR: 0.6; 95% CI: 0.4–0.7) or condoms (OR: 0.5; 95% CI: 0.3–0.7). We observed an HIV incidence rate of 4.6 per 100 WY (95% CI: 2.7, 7.3). The HIV incidence was 4.8 per 100 WY (95% CI: 2.5, 8.3) in women aged 18–24 years, 4.5 per 100 WY (95% CI: 1.2, 11.4) in women aged 25–29 years and 3.2 per 100 WY (95% CI: 0.1, 18.0) in the 30–35 years stratum. None of the demographic factors or time-varying behavioral factors examined was significantly associated with incident HIV infection in bivariable analysis at p≤0.10.Conclusions
We found a high HIV incidence among sexually active young women in Chókwè, Mozambique. HIV prevention programs should be strengthened in the area, with more comprehensive reproductive health services, regular HIV testing, condom promotion, and messaging about multiple sexual partners. 相似文献144.
Clark Tapia Ricardo Cecilia Alfonso Corrado María C. Mandujano Francisco Molina-Freaner 《Evolutionary ecology》2006,20(2):131-142
Stenocereus eruca is a prostrated, self-incompatible cactus endemic to the Sonoran Desert that regenerates primarily through clonal propagation.
Clonal growth is expected to affect mate availability by influencing the number and spatial distribution of mating types.
In this paper we examine the role of clonal growth on female fecundity through a series of pollination experiments in a population
of S. eruca. We set up a pollen supplementation experiment using five distance treatments with pollen collected at 1, 10, 100, 1000 and
25000 m from receptor flowers during the years 2001 and 2002 and evaluated genetic sifmilarities between pairs of receptor-donor
ramets through RAPD markers. Our data on fruit set, number of seeds/fruit, germination and overall fecundity revealed that
S. eruca show a significant reduction in female fecundity when pollination occurs between ramets located at short distances (1 and
10 m), while genetic data showed high levels of similarity at those distances. The reduction in female fecundity is apparently
a consequence of geitonogamy and inbreeding depression. Our data suggest that clonal growth and geitonogamy are likely to
be partially responsible for the low levels of sexual reproduction and seedling recruitment observed in populations of S. eruca.
Co-ordinating editor: H. Kudoh 相似文献
145.
Outward sodium and potassium cotransport in human red cells 总被引:7,自引:0,他引:7
Ricardo Garay Norma Adragna Mitzy Canessa Daniel Tosteson 《The Journal of membrane biology》1981,62(3):169-174
Summary This paper reports some kinetic properties of Na–K cotransport in human red cells. All fluxes were measured in the presence of 10–4 M ouabain. We measured Na and K efflux from cells loaded by the PCMBS method to contain different concentrations of these ions into a medium that contained neither Na nor K (MgCl2-sucrose substitution) in the absence and presence of furosemide. Furosemide inhibited 30–60% of the total efflux depending on the internal ion concentration and the individual subject. We took the furosemide-sensitive fluxes to be a measure of Na–K cotransport. The ratio of Na to K cotransport was 1 over the entire range of internal Na and K concentrations studied. When Na was substituted for K as the only internal cation, cotransport was maximally activated when the Na and K concentrations were between 20 and 90 mmol/liter cells. The concentration of internal Na required to produce half-maximal cotransport was about 13±4 mmol/liter cells (n=4), while the comparable concentration of K was somewhat lower. The activation curve was definitely sigmoid in character, suggesting that at least two Na ions are involved in the transport process. The maximum of Na–K cotransport was about 0.5±0.15 mmol/liter cells × hr (n=5); it had a flat maximum in the medium at about pH 7.0, decreasing in both the acid and alkaline sides. furosemide-resistant effluxes were found to be linear functions of internal Na and K concentrations and to yield rate coefficients of 0.019±0.002 hr–1 and 0.014±0.002 hr–1 (n=7), respectively. These values are of the same order of magnitude expected of ions moving across phospholipid bilayers.Charge de Recherches CNRS. 相似文献
146.
147.
Ricardo Pablo Garay 《生物化学与生物物理学报:生物膜》1982,688(3):786-792
Human erythrocytes are able to incorporate cyclic AMP (cAMP) in amounts larger than those required to saturate cAMP-dependent protein kinase. In contrast to previous observations in avian red blood cells in which cAMP stimulates the Na+/K+ cotransport system, we demonstrate that cAMP inhibits this system in human erythrocytes. The cotransport inhibition is enhanced by addition of phosphodiesterase inhibitor 1-methyl-3-isobutylxanthine to the incubation medium. The cAMP concentration giving half-maximal cotransport inhibition showed a wide variation among different individuals (from 0.1 to 5 mM external cAMP concentration). In contrast to cAMP, cyclic GMP showed little effect on the cotransport system. Ca2+ introduced into the cell interior was an inhibitor of the Na+/K+ cotransport system. These results suggest that in human cells in which endogeneous levels of cAMP and Ca2+ are modulated by hormones, the Na+/K+ cotransport system may be under hormonal regulation. 相似文献
148.
James R. Probert Catherine L. Parr Ricardo M. Holdo T. Michael Anderson Sally Archibald Colin J. Courtney Mustaphi Andrew P. Dobson Jason E. Donaldson Grant C. Hopcraft Gareth P. Hempson Thomas A. Morrison Colin M. Beale 《Global Change Biology》2019,25(10):3406-3423
Fire is a key driver in savannah systems and widely used as a land management tool. Intensifying human land uses are leading to rapid changes in the fire regimes, with consequences for ecosystem functioning and composition. We undertake a novel analysis describing spatial patterns in the fire regime of the Serengeti‐Mara ecosystem, document multidecadal temporal changes and investigate the factors underlying these patterns. We used MODIS active fire and burned area products from 2001 to 2014 to identify individual fires; summarizing four characteristics for each detected fire: size, ignition date, time since last fire and radiative power. Using satellite imagery, we estimated the rate of change in the density of livestock bomas as a proxy for livestock density. We used these metrics to model drivers of variation in the four fire characteristics, as well as total number of fires and total area burned. Fires in the Serengeti‐Mara show high spatial variability—with number of fires and ignition date mirroring mean annual precipitation. The short‐term effect of rainfall decreases fire size and intensity but cumulative rainfall over several years leads to increased standing grass biomass and fuel loads, and, therefore, in larger and hotter fires. Our study reveals dramatic changes over time, with a reduction in total number of fires and total area burned, to the point where some areas now experience virtually no fire. We suggest that increasing livestock numbers are driving this decline, presumably by inhibiting fire spread. These temporal patterns are part of a global decline in total area burned, especially in savannahs, and we caution that ecosystem functioning may have been compromised. Land managers and policy formulators need to factor in rapid fire regime modifications to achieve management objectives and maintain the ecological function of savannah ecosystems. 相似文献
149.
Luis Molina y Vedia Mónica Torruella Ricardo Attar Ernesto Podesta Juan A. Reig Mirtha M. Flawia Héctor N. Torres 《Biochemical and biophysical research communications》1983,113(3):778-783
A monoclonal antibody against soluble adenylate cyclase was obtained. The antibody inhibits cyclase activities from several lower eucaryotic organisms but not activities associated to testicular cytosol or turkey erythrocyte membranes. 相似文献
150.
Because previous work showed that in the newborn brain, but not in the adult brain, glutamate decarboxylase (GAD) is notably susceptible to heat, we have studied the possible involvement of GAD inhibition in febrile convulsions and the related changes in gamma-aminobutyric acid (GABA) content. Rats of different ages were subjected to hyperthermia, and GAD activity was determined in brain homogenates by measuring the release of 14CO2 from labeled glutamate and by measuring the formation of GABA. The latter method gave considerably lower values than the former in the youngest rats, and was considered more reliable. With this method, we found a 37-48% inhibition of GAD activity in rat pups 2-5 days old, which showed febrile seizures at progressively higher body temperatures, whereas in 10- and 15-day-old animals, which did not show convulsions, GAD activity was not affected by hyperthermia. Whole-brain GABA levels, however, did not change at any age. In contrast to GAD, choline acetyltransferase and lactic dehydrogenase activities were not altered by hyperthermia at any of the ages studied. These results suggest that a decreased efficiency of the inhibitory neurotransmission mediated by GABA, consequent to the inhibition of GAD activity, may be a factor related to febrile convulsions. 相似文献