全文获取类型
收费全文 | 4749篇 |
免费 | 434篇 |
国内免费 | 431篇 |
专业分类
5614篇 |
出版年
2024年 | 27篇 |
2023年 | 122篇 |
2022年 | 150篇 |
2021年 | 193篇 |
2020年 | 267篇 |
2019年 | 249篇 |
2018年 | 248篇 |
2017年 | 259篇 |
2016年 | 291篇 |
2015年 | 283篇 |
2014年 | 289篇 |
2013年 | 326篇 |
2012年 | 173篇 |
2011年 | 169篇 |
2010年 | 169篇 |
2009年 | 199篇 |
2008年 | 188篇 |
2007年 | 245篇 |
2006年 | 219篇 |
2005年 | 185篇 |
2004年 | 169篇 |
2003年 | 143篇 |
2002年 | 149篇 |
2001年 | 121篇 |
2000年 | 99篇 |
1999年 | 99篇 |
1998年 | 103篇 |
1997年 | 62篇 |
1996年 | 58篇 |
1995年 | 71篇 |
1994年 | 40篇 |
1993年 | 38篇 |
1992年 | 26篇 |
1991年 | 26篇 |
1990年 | 15篇 |
1989年 | 11篇 |
1988年 | 18篇 |
1987年 | 13篇 |
1986年 | 11篇 |
1985年 | 13篇 |
1984年 | 14篇 |
1983年 | 4篇 |
1982年 | 17篇 |
1981年 | 14篇 |
1980年 | 2篇 |
1979年 | 7篇 |
1978年 | 9篇 |
1976年 | 2篇 |
1975年 | 2篇 |
1973年 | 2篇 |
排序方式: 共有5614条查询结果,搜索用时 0 毫秒
51.
F. W. Asselbergs M. P. van den Berg S. J. Bakker J. E. Signorovitch H. L. Hillege W. H. van Gilst D. J. van Veldhuisen 《Netherlands heart journal》2008,16(3):73-78
Background B-type natriuretic peptide (BNP) is secreted from cardiomyocytes and may reflect haemodynamic abnormalities predisposing to atrial fibrillation (AF). We aimed to investigate whether N-terminal pro BNP (NT-proBNP) is associated with newly detected AF in subjects obtained from the general population. Methods From the PREVEND programme (n=8592), we selected all subjects with an available baseline and four-year electrocardiogram and NTproBNP levels at baseline. We excluded subjects with AF at baseline and subjects with a serum creatinine >2.0 mg/dl. Results In total, 6494 subjects were eligible for the prospective analysis (aged 49±12 years, 49.7% men). At four years, AF was detected in 41 (0.6%) subjects. Median NT-proBNP levels at baseline in subjects with newly detected AF after four years was 62.2 (22.6 to 208.5) pg/ml as compared with 35.7 (15.9 to 68.7) pg/ml in those with sinus rhythm (p=0.001). Each 1 standard deviation increment in natural log transformed NT-proBNP was associated with a 54% (5% to 126%, p=0.028) increase in risk for AF after adjustment for other risk factors predisposing to AF. NT-proBNP levels above the sex-specific 80th percentile (97 pg/ml in women and 60 pg/ml in men) were associated with a multivariate odds ratio of 2.65 (1.22 to 5.76, p=0.01) for the occurrence of AF. Conclusion Plasma levels of NT-proBNP predict newly detected AF in subjects obtained from the general population independent of cardiovascular risk factors predisposing to AF. (Neth Heart J 2008;16:73-8.) 相似文献
52.
Marco Schmidt Holger Kreft Adjima Thiombiano Georg Zizka 《Diversity & distributions》2005,11(6):509-516
A map of plant species diversity in Burkina Faso is presented based on field observations and specimen data from the Ouagadougou University Herbarium (OUA) and the Herbarium Senckenbergianum (FR). A map of collecting intensity and field observations illustrates centres of botanical research activities in Burkina Faso. To overcome problems associated with biased sampling intensity, distributions of species have been modelled and extrapolated to maps of vascular plant diversity, life forms and diversity of four selected families (Poaceae, Cyperaceae, Dioscoreaceae and Rubiaceae). The area of most intensive collection and observation is around Gorom‐Gorom and Fada N’Gourma. Modelled diversity generally increases towards the south, as does the proportion of phanerophytes, lianas and hemicryptophytes, while the opposite trend is observed for therophytes. Poaceae diversity is highly correlated with total vascular plant diversity, making the family especially suitable as an indicator for overall plant diversity. Cyperaceae are rather evenly distributed throughout the country, Dioscoreaceae are restricted to the Sudanian Zone. Rubiaceae have their highest diversities in the very south. Our approach can be transferred to areas with a similar database, certainly to other areas within West Africa. Future research should focus on distribution data for rare species, enabling our approach to evaluate the West African system of protected areas. 相似文献
53.
54.
Ho Cheng Koc Jing Xiao Weiwei Liu Yong Li Guokai Chen 《International journal of biological sciences》2022,18(12):4768
The pandemic of COVID-19 is the biggest public health crisis in 21st Century. Besides the acute symptoms after infection, patients and society are also being challenged by the long-term health complications associated with COVID-19, commonly known as long COVID. While health professionals work hard to find proper treatments, large amount of knowledge has been accumulated in recent years. In order to deal with long COVID efficiently, it is important for people to keep up with current progresses and take proactive actions on long COVID. For this purpose, this review will first introduce the general background of long COVID, and then discuss its risk factors, diagnostic indicators and management strategies. This review will serve as a useful resource for people to understand and prepare for long COVID that will be with us in the foreseeable future. 相似文献
55.
Ralph M. Perhac Jr. 《人类与生态风险评估》1996,2(2):381-392
The public thinks about (i.e., defines) environmental human health risks in different terms than the “experts.”; And because the manner in which we conceive of risk goes a long way in determining how risk management is conceived and carried out, any definition of risk has important normative elements. I argue that environmental policy based on the public's conception of risk fails to adequately protect fundamental human rights to health and liberty, by taking undue account of certain psychological factors that enter into the public's perception of risk. A survey of some of these factors is offered in an attempt to determine their policy relevance. The traditional scientific conception of risk, although not adequate to entirely define risk policy, serves as an important anchor for the protection of these rights by focusing on the probability or number of adverse health effects. 相似文献
56.
Eric Abella Roth 《Human ecology: an interdisciplinary journal》1990,18(4):441-455
Previous analysis of Rendille household herd composition revealed a transition from camel to cattle ownership for sedentary impoverished Rendille pastoralists of northern Kenya. In an attempt to delineate determinants of livestock holdings, logistic regression analysis of 112 household herds from the Rendille settlement of Korr, Marsabit District, Kenya was undertaken. Results indicated that household wealth, measured in present livestock holdings, past drought losses, and livestock sales, formed better predictors of cattle ownership than did household characteristics pertaining to labor supply, wage earners, age-set membership, and birth order of household head. These results are discussed in light of pastoral strategies designed to minimize risk. 相似文献
57.
Biological invaders can have dramatic effects on the environment and the economy. To most effectively manage these invaders,
we should consider entire pathways, because multiple species are dispersed through the same vectors. In this paper, we use
production-constrained gravity models to describe movement of recreational boaters between lakes – potentially the most important
pathway of overland dispersal for many aquatic organisms. These models are advantageous because they require relatively easily
acquired data, hence are relatively easy to build. We compare linear and non-linear gravity models and show that, despite
their simplicity, they are able to capture important characteristics of the recreational boater pathway. To assess our model,
we compared observed data based on creel surveys and mailed surveys of recreation boaters to the model output. Specifically,
we evaluate four metrics of pathway characteristics: boater traffic to individual lakes, distances traveled to reach these
lakes, Great Lakes usage and movement from the Great Lakes to inland waters. These factors will influence the propagule pressure
(hence the probability of establishment of invasive populations) and the rate of spread across a landscape. The Great Lakes
are of particular importance because they are a major entry point of non-indigenous species from other continents, hence will
act as the origin for further spread across states. The non-linear model had the best fit between model output and empirical
observations with r2 =0.80, r2 =0.35, r2 =0.57, and r2 =0.36 for the four metrics, respectively. For the distances traveled to individual lakes, r2 improved from 0.35 to 0.76 after removal of an outlier. Our results suggest that we were able to capture distances traveled
to most but not all lakes. Thus, we demonstrate that production-constrained gravity models will be generally useful for modeling
invasion pathways between non-contiguous locations. 相似文献
58.
Hubert E. Blum 《Journal of cellular and molecular medicine》2010,14(9):2257-2263
Molecular and cell biology have revolutionized not only diagnosis, therapy and prevention of human diseases but also greatly contributed to the understanding of their pathogenesis. Based on modern molecular and biochemical methods it is possible to identify on the one hand point mutations and single nucleotide polymorphisms. On the other hand, using high throughput array technologies, it is possible to analyse thousands of genes or gene products simultaneously, resulting in an individual gene or gene expression profile (signature). These data increasingly allow to define the individual risk for a given disease and to predict the individual prognosis of a disease as well as the efficacy of therapeutic strategies (individualized medicine). In the following sections some of the recent advances of predictive medicine and their clinical relevance will be addressed. 相似文献
59.
Variation between mouse major urinary protein genes isolated from a single inbred line 总被引:13,自引:0,他引:13
We describe ten Charon 4A genomic DNA clones from BALB/c mice which include at least seven different major urinary protein (MUP) genes. We have established the orientation of all seven sequences, and have placed six of them in precise register by means of restriction site maps and Southern blot hybridization with cloned cDNA sequences. Four of the seven genomic sequences (family I sequences) form hybrids with six independent cDNA clones that have a high thermal stability and hybridize more strongly with mRNA from three inbred mouse lines. Hybrids between the remaining three genomic sequences and the cDNA clones have a lower thermal stability and hybridize less strongly with mRNA from the three inbred lines. Homologies between different cloned sequences extend over as much as 15 kb. No clone contains parts of two MUP genes, and no homology has been detected between the 3' flanking region of one MUP gene and the 5' flanking region of another. 相似文献
60.
Prey animals must balance antipredatory behaviour with foraging behaviour. According to the risk allocation hypothesis, prey increase antipredatory behaviour and reduce foraging activity during pulses of high risk, but with continuous risk, other activities must continue and antipredatory behaviour decreases despite the risk. We studied the impact of lynx presence on the vigilance behaviour of wild roe deer under conditions of (i) a pulsed elevated risk by experimentally spreading lynx urine as an olfactory cue, and (ii) continuous risk by comparing an area where lynx was eradicated 160 years ago to an area where lynx has been re‐introduced 30 years ago. Roe deer were extremely vigilant in response to the predator olfactory cue; however, roe deer vigilance did not differ measurably among areas with or without potential lynx presence. Deer were more vigilant before sunset than during the night at both study areas, probably due to long‐term adaptation of roe deer to human hunting during daytime. Vigilance decreased from August to September even though activity of lynx increases in autumn, which may be a result either of increased foraging due to decrease in food quality in autumn, or of changes in social organization of the deer. Our results suggested that the degree of vigilance depends on environmental cues. We found that roe deer respond to lynx urine despite a long absence of lynx in the ecosystem. Our results support the risk allocation hypothesis for responses to pulses of high risk but not for responses to continuous elevated levels of risk. 相似文献