全文获取类型
收费全文 | 11851篇 |
免费 | 24篇 |
国内免费 | 42篇 |
专业分类
系统科学 | 35篇 |
丛书文集 | 80篇 |
教育与普及 | 30篇 |
理论与方法论 | 42篇 |
现状及发展 | 5124篇 |
研究方法 | 565篇 |
综合类 | 5885篇 |
自然研究 | 156篇 |
出版年
2013年 | 96篇 |
2012年 | 191篇 |
2011年 | 330篇 |
2010年 | 85篇 |
2009年 | 59篇 |
2008年 | 190篇 |
2007年 | 222篇 |
2006年 | 203篇 |
2005年 | 224篇 |
2004年 | 208篇 |
2003年 | 207篇 |
2002年 | 198篇 |
2001年 | 379篇 |
2000年 | 364篇 |
1999年 | 263篇 |
1992年 | 232篇 |
1991年 | 180篇 |
1990年 | 204篇 |
1989年 | 190篇 |
1988年 | 201篇 |
1987年 | 205篇 |
1986年 | 160篇 |
1985年 | 248篇 |
1984年 | 170篇 |
1983年 | 149篇 |
1982年 | 166篇 |
1981年 | 135篇 |
1980年 | 170篇 |
1979年 | 385篇 |
1978年 | 291篇 |
1977年 | 287篇 |
1976年 | 249篇 |
1975年 | 287篇 |
1974年 | 309篇 |
1973年 | 308篇 |
1972年 | 344篇 |
1971年 | 344篇 |
1970年 | 428篇 |
1969年 | 361篇 |
1968年 | 381篇 |
1967年 | 351篇 |
1966年 | 319篇 |
1965年 | 203篇 |
1959年 | 107篇 |
1958年 | 204篇 |
1957年 | 137篇 |
1956年 | 122篇 |
1955年 | 105篇 |
1954年 | 81篇 |
1948年 | 83篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
O. B. Sheynin 《Archive for History of Exact Sciences》1989,39(4):337-377
Communicated by
H. Freudenthal 相似文献
992.
Oxide to metal conversion is one of the most energy-intensive steps in the value chain for metals production. Solid oxide membrane (SOM) electrolysis process provides a general route for directly reducing various metal oxides to their respective metals, alloys, or intermetallics. Because of its lower energy use and ability to use inert anode resulting in zero carbon emission, SOM electrolysis process emerges as a promising technology that can replace the state-of-the-art metals production processes. In this paper, a careful study of the SOM electrolysis process using equivalent DC circuit modeling is performed and correlated to the experimental results. A discussion on relative importance of each resistive element in the circuit and on possible ways of lowering the rate-limiting resistive elements provides a generic guideline for designing optimum SOM electrolysis cells. 相似文献
993.
Endothelium-derived nitric oxide and vascular physiology and pathology 总被引:13,自引:0,他引:13
J.-F. Arnal A.-T. Dinh-Xuan M. Pueyo B. Darblade J. Rami 《Cellular and molecular life sciences : CMLS》1999,55(8-9):1078-1087
In 1980, Furchgott and Zawadzki demonstrated that the relaxation of vascular smooth muscle cells in response to acetylcholine
is dependent on the anatomical integrity of the endothelium. Endothelium-derived relaxing factor was identified 7 years later
as the free radical gas nitric oxide (NO). In endothelium, the amino acid L-arginine is converted to L-citrulline and NO by
one of the three NO synthases, the endothelial isoform (eNOS). Shear stress and cell proliferation appear to be, quantitatively,
the two major regulatory factors of eNOS gene expression. However, eNOS seems to be mainly regulated by modulation of its
activity. Stimulation of specific receptors to various agonists (e.g., bradykinin, serotonin, adenosine, ADP/ATP, histamine,
thrombin) increases eNOS enzymatic activity at least in part through an increase in intracellular free Ca2+. However, the mechanical stimulus shear stress appears again to be the major stimulus of eNOS activity, although the precise
mechanisms activating the enzyme remain to be elucidated. Phosphorylation and subcellular translocation (from plasmalemmal
caveolae to the cytoskeleton or cytosol) are probably involved in these regulations. Although eNOS plays a major vasodilatory
role in the control of vasomotion, it has not so far been demonstrated that a defect in endothelial NO production could be
responsible for high blood pressure in humans. In contrast, a defect in endothelium-dependent vasodilation is known to be
promoted by several risk factors (e.g., smoking, diabetes, hypercholesterolemia) and is also the consequence of atheroma (fatty
streak infiltration of the neointima). Several mechanisms probably contribute to this decrease in NO bioavailability. Finally,
a defect in NO generation contributes to the pathophysiology of pulmonary hypertension. Elucidation of the mechanisms of eNOS
enzyme activity and NO bioavailability will contribute to our understanding the physiology of vasomotion and the pathophysiology
of endothelial dysfunction, and could provide insights for new therapies, particularly in hypertension and atherosclerosis. 相似文献
994.
Radiation hybrid map of the mouse genome. 总被引:13,自引:0,他引:13
W J Van Etten R G Steen H Nguyen A B Castle D K Slonim B Ge C Nusbaum G D Schuler E S Lander T J Hudson 《Nature genetics》1999,22(4):384-387
Radiation hybrid (RH) maps are a useful tool for genome analysis, providing a direct method for localizing genes and anchoring physical maps and genomic sequence along chromosomes. The construction of a comprehensive RH map for the human genome has resulted in gene maps reflecting the location of more than 30,000 human genes. Here we report the first comprehensive RH map of the mouse genome. The map contains 2,486 loci screened against an RH panel of 93 cell lines. Most loci (93%) are simple sequence length polymorphisms (SSLPs) taken from the mouse genetic map, thereby providing direct integration between these two key maps. We performed RH mapping by a new and efficient approach in which we replaced traditional gel- or hybridization-based assays by a homogeneous 5'-nuclease assays involving a single common probe for all genetic markers. The map provides essentially complete connectivity and coverage across the genome, and good resolution for ordering loci, with 1 centiRay (cR) corresponding to an average of approximately 100 kb. The RH map, together with an accompanying World-Wide Web server, makes it possible for any investigator to rapidly localize sequences in the mouse genome. Together with the previously constructed genetic map and a YAC-based physical map reported in a companion paper, the fundamental maps required for mouse genomics are now available. 相似文献
995.
996.
997.
B. Thébaud J.-F. Arnal J. C. Mercier A.-T. Dinh-Xuan 《Cellular and molecular life sciences : CMLS》1999,55(8-9):1103-1112
Inhaled nitric oxide (NO) is used to treat various cardiopulmonary disorders associated with pulmonary hypertension. The
rationale is based on the fact that NO, given by inhalation, only dilates those pulmonary vessels that perfuse well-ventilated
lung units. As a result, pulmonary gas exchange is improved while pulmonary vascular resistance is reduced and pulmonary blood
flow is increased. Inhaled NO has been succesfully applied to treat persistent pulmonary hypertension of the newborn, reducing
the need for extracorporeal life support. Although pulmonary hypertension and altered vasoreactivity contribute to profound
hypoxaemia in adult and paediatric acute respiratory distress syndrome (ARDS), the benefit of inhaled NO still remains to
be established in patients with ARDS. ARDS is a complex response of the lung to direct or indirect insults, leading to pulmonary
vasoconstriction and various inflammatory responses. Recent randomized trials suggest that inhaled NO only causes a transient
improvement in oxygenation. Whether this effect is important in the long-term management of ARDS remains to be established.
NO, measured in the exhaled breath, is an elegant and non-invasive means to monitor inflammation of the upper and lower respiratory
tract. In the normal upper airways, the bulk of exhaled NO originates from the paranasal sinuses. Exhaled NO is increased
in nasal allergy and decreased in cystic fibrosis, nasal polyposis and chronic sinusitis. That NO production is increased
in asthmatic airways is also well established. However, several questions still need to be addressed, in particular evaluation
of the sensitivity and specificity of the measurement techniques, and assessment of the bronchodilator action of endogenous
NO. 相似文献
998.
用柱层析和TLC分离小麦PGIP和植保素 总被引:1,自引:0,他引:1
植物抗病能力大小的测定需要比较准确可靠且相对简便的办法。采用接种病菌于植物上观察染菌情况虽然直观,但受病菌本身的生理状态及外界环境影响很大,且不便于定量的表述。因此,选择合适的生理生化指标作为植物抗性大小的衡量标准,对于植物病理的研究具有重要的意义。选择了小麦体系的两种抗性物质- 小麦植保素和 P G I P作为小麦的抗性指标来分离分析 相似文献
999.
Nuclear transport mechanisms 总被引:7,自引:0,他引:7
The term nuclear transport, refers to the movement of a large variety of macromolecules both into and out of the nucleus. Transport must be extremely selective, yet also very efficient. A single type of channel, the nuclear pore complex, mediates all movement across the nuclear envelope. Selectivity is achieved through the use of families of soluble factors that target substrates for import and export and deliver them to their appropriate intracellular destinations. We now have a fairly detailed understanding of the basic mechanisms of protein import into the nucleus. Many of these same principles can be applied to protein export and perhaps RNA export. This review will summarize the current status of what is known about various transport pathways and highlight the questions that remain to be answered. 相似文献
1000.
B S Mankoo N S Collins P Ashby E Grigorieva L H Pevny A Candia C V Wright P W Rigby V Pachnis 《Nature》1999,400(6739):69-73
The skeletal muscles of the limbs develop from myogenic progenitors that originate in the paraxial mesoderm and migrate into the limb-bud mesenchyme. Among the genes known to be important for muscle development in mammalian embryos are those encoding the basic helix-loop-helix (bHLH) myogenic regulatory factors (MRFs; MyoD, Myf5, myogenin and MRF4) and Pax3, a paired-type homeobox gene that is critical for the development of limb musculature. Mox1 and Mox2 are closely related homeobox genes that are expressed in overlapping patterns in the paraxial mesoderm and its derivatives. Here we show that mice homozygous for a null mutation of Mox2 have a developmental defect of the limb musculature, characterized by an overall reduction in muscle mass and elimination of specific muscles. Mox2 is not needed for the migration of myogenic precursors into the limb bud, but it is essential for normal appendicular muscle formation and for the normal regulation of myogenic genes, as demonstrated by the downregulation of Pax3 and Myf5 but not MyoD in Mox2-deficient limb buds. Our findings show that the MOX2 homeoprotein is an important regulator of vertebrate limb myogenesis. 相似文献