首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   295篇
  免费   17篇
  312篇
  2022年   4篇
  2021年   5篇
  2020年   5篇
  2019年   12篇
  2018年   3篇
  2017年   6篇
  2016年   9篇
  2015年   9篇
  2014年   21篇
  2013年   13篇
  2012年   20篇
  2011年   9篇
  2010年   16篇
  2009年   7篇
  2008年   14篇
  2007年   15篇
  2006年   18篇
  2005年   5篇
  2004年   7篇
  2003年   13篇
  2002年   5篇
  1998年   6篇
  1996年   4篇
  1995年   3篇
  1992年   6篇
  1991年   2篇
  1990年   7篇
  1989年   2篇
  1988年   3篇
  1987年   4篇
  1986年   4篇
  1985年   3篇
  1984年   4篇
  1983年   5篇
  1980年   4篇
  1979年   7篇
  1978年   1篇
  1977年   4篇
  1974年   4篇
  1971年   1篇
  1970年   1篇
  1969年   3篇
  1968年   4篇
  1966年   1篇
  1943年   2篇
  1940年   2篇
  1939年   1篇
  1929年   1篇
  1904年   1篇
  1902年   1篇
排序方式: 共有312条查询结果,搜索用时 0 毫秒
41.
42.
43.
Molecular phylogenetic analyses of 18S rDNA (SSU) gene sequences confirm the placement of Crusticorallina gen. nov. in Corallinoideae, the first nongeniculate genus in an otherwise geniculate subfamily. Crusticorallina is distinguished from all other coralline genera by the following suite of morpho‐anatomical characters: (i) sunken, uniporate gametangial and bi/tetrasporangial conceptacles, (ii) cells linked by cell fusions, not secondary pit connections, (iii) an epithallus of 1 or 2 cell layers, (iv) a hypothallus that occupies 50% or more of the total thallus thickness, (v) elongate meristematic cells, and (vi) trichocytes absent. Four species are recognized based on rbcL, psbA and COI‐5P sequences, C. painei sp. nov., the generitype, C. adhaerens sp. nov., C. nootkana sp. nov. and C. muricata comb. nov., previously known as Pseudolithophyllum muricatum. Type material of Lithophyllum muricatum, basionym of C. muricata, in TRH comprises at least two taxa, and therefore we accept the previously designated lectotype specimen in UC that we sequenced to confirm its identity. Crusticorallina species are very difficult to distinguish using morpho‐anatomical and/or habitat characters, although at specific sites, some species may be distinguished by a combination of morpho‐anatomy, habitat and biogeography. The Northeast Pacific now boasts six coralline endemic genera, far more than any other region of the world.  相似文献   
44.
A number of cardiac myopathies (e.g. familial hypertrophic cardiomyopathy and dilated cardiomyopathy) are linked to mutations in cardiac muscle myosin filament proteins, including myosin and myosin binding protein C (MyBP-C). To understand the myopathies it is necessary to know the normal 3D structure of these filaments. We have carried out 3D single particle analysis of electron micrograph images of negatively stained isolated myosin filaments from rabbit cardiac muscle. Single filament images were aligned and divided into segments about 2x430A long, each of which was treated as an independent 'particle'. The resulting 40A resolution 3D reconstruction showed both axial and azimuthal (no radial) myosin head perturbations within the 430A repeat, with successive crown rotations of approximately 60 degrees , 60 degrees and 0 degrees , rather than the regular 40 degrees for an unperturbed helix. However, it is shown that the projecting density peaks appear to start at low radius from origins closer to those expected for an unperturbed helical filament, and that the azimuthal perturbation especially increases with radius. The head arrangements in rabbit cardiac myosin filaments are very similar to those in fish skeletal muscle myosin filaments, suggesting a possible general structural theme for myosin filaments in all vertebrate striated muscles (skeletal and cardiac).  相似文献   
45.
Summary Dactylicapnos ventii (Khánh) Lidén is described and illustrated. Misidentification when this species was first introduced into cultivation has led to confusion with D. lichiangensis (Fedde) Hand.‐Mazz.  相似文献   
46.
Objectives To assess the evidence for the clinical effectiveness of ultrasound guided central venous cannulation.Data sources 15 electronic bibliographic databases, covering biomedical, science, social science, health economics, and grey literature.Design Systematic review and meta-analysis of randomised controlled trials.Populations Patients scheduled for central venous access.Intervention reviewed Guidance using real time two dimensional ultrasonography or Doppler needles and probes compared with the anatomical landmark method of cannulation.Data extraction Risk of failed catheter placement (primary outcome), risk of complications from placement, risk of failure on first attempt at placement, number of attempts to successful catheterisation, and time (seconds) to successful catheterisation.Data synthesis 18 trials (1646 participants) were identified. Compared with the landmark method, real time two dimensional ultrasound guidance for cannulating the internal jugular vein in adults was associated with a significantly lower failure rate both overall (relative risk 0.14, 95% confidence interval 0.06 to 0.33) and on the first attempt (0.59, 0.39 to 0.88). Limited evidence favoured two dimensional ultrasound guidance for subclavian vein and femoral vein procedures in adults (0.14, 0.04 to 0.57 and 0.29, 0.07 to 1.21, respectively). Three studies in infants confirmed a higher success rate with two dimensional ultrasonography for internal jugular procedures (0.15, 0.03 to 0.64). Doppler guided cannulation of the internal jugular vein in adults was more successful than the landmark method (0.39, 0.17 to 0.92), but the landmark method was more successful for subclavian vein procedures (1.48, 1.03 to 2.14). No significant difference was found between these techniques for cannulation of the internal jugular vein in infants. An indirect comparison of relative risks suggested that two dimensional ultrasonography would be more successful than Doppler guidance for subclavian vein procedures in adults (0.09, 0.02 to 0.38).Conclusions Evidence supports the use of two dimensional ultrasonography for central venous cannulation.  相似文献   
47.
Janus kinase (JAK) 2 is activated by ANG II in vitro and in vivo, and chronic blockade of JAK2 by the JAK2 inhibitor AG-490 has been shown recently to attenuate ANG II hypertension in mice. In this study, AG-490 was infused intravenously in chronically instrumented rats to determine if the blunted hypertension was linked to attenuation of the renal actions of ANG II. In male Sprague-Dawley rats, after a control period, ANG II at 10 ng·kg(-1)·min(-1) was infused intravenously with or without AG-490 at 10 ng·kg(-1)·min(-1) iv for 11 days. ANG II infusion (18 h/day) increased mean arterial pressure from 91 ± 3 to 168 ± 7 mmHg by day 11. That response was attenuated significantly in the ANG II + AG-490 group, with mean arterial pressure increasing only from 92 ± 5 to 127 ± 3 mmHg. ANG II infusion markedly decreased urinary sodium excretion, caused a rapid and sustained decrease in glomerular filtration rate to ~60% of control, and increased renal JAK2 phosphorylation; all these responses were blocked by AG-490. However, chronic AG-490 treatment had no effect on the ability of a separate group of normal rats to maintain normal blood pressure when they were switched rapidly to a low-sodium diet, whereas blood pressure fell dramatically in losartan-treated rats on a low-sodium diet. These data suggest that activation of the JAK/STAT pathway is critical for the development of ANG II-induced hypertension by mediating its effects on renal sodium excretory capability, but the physiological control of blood pressure by ANG II with a low-salt diet does not require JAK2 activation.  相似文献   
48.

Background

The associations between nutritional biomarkers and measures of sleep quantity and quality remain unclear.

Methods

Cross-sectional data from the National Health and Nutrition Examination Surveys (NHANES) 2005–2006 were used. We selected 2,459 adults aged 20–85, with complete data on key variables. Five sleep measures were constructed as primary outcomes: (A) Sleep duration; (B) Sleep disorder; (C) Three factors obtained from factor analysis of 15 items and labeled as “Poor sleep-related daytime dysfunction” (Factor 1), “Sleepiness” (Factor 2) and “Sleep disturbance” (Factor 3). Main exposures were serum concentrations of key nutrients, namely retinol, retinyl esters, carotenoids (α-carotene, β-carotene, β-cryptoxanthin, lutein+zeaxanthin, lycopene), folate, vitamin B-12, total homocysteine (tHcy), vitamin C, 25-hydroxyvitamin D (25(OH)D) and vitamin E. Main analyses consisted of multiple linear, logistic and multinomial logit models.

Results

Among key findings, independent inverse associations were found between serum vitamin B-12 and sleep duration, 25(OH)D and sleepiness (as well as insomnia), and between folate and sleep disturbance. Serum total carotenoids concentration was linked to higher odds of short sleep duration (i.e. 5–6 h per night) compared to normal sleep duration (7–8 h per night).

Conclusions

A few of the selected serum nutritional biomarkers were associated with sleep quantity and quality. Longitudinal studies are needed to ascertain temporality and assess putative causal relationships.  相似文献   
49.
Hydrazine can support a rapid oxygen uptake in illuminated chloroplasts. The oxygen uptake rate is inhibited by 3-(3,4-dichlorophenyl)-1, 1-dimethylurea but is only slightly increased by added methyl viologen, and little H2O2 is produced. The pH optimum for hydrazine-dependent oxygen uptake is much higher than that of the Hill reaction. Addition of Mn (II) increases the rate of oxygen uptake in the light and causes the reaction to continue in the dark, the dark rate being dependent on the duration of the preceding light period. Flash yield experiments show that at least six electrons are transferred from hydrazine per flash compared to one electron per flash when water is the electron donor.  相似文献   
50.
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号