全文获取类型
收费全文 | 684篇 |
免费 | 90篇 |
专业分类
774篇 |
出版年
2022年 | 6篇 |
2021年 | 12篇 |
2020年 | 6篇 |
2019年 | 7篇 |
2018年 | 9篇 |
2017年 | 11篇 |
2016年 | 12篇 |
2015年 | 26篇 |
2014年 | 27篇 |
2013年 | 34篇 |
2012年 | 40篇 |
2011年 | 24篇 |
2010年 | 20篇 |
2009年 | 26篇 |
2008年 | 32篇 |
2007年 | 33篇 |
2006年 | 32篇 |
2005年 | 24篇 |
2004年 | 15篇 |
2003年 | 27篇 |
2002年 | 21篇 |
2001年 | 26篇 |
2000年 | 34篇 |
1999年 | 12篇 |
1998年 | 15篇 |
1997年 | 10篇 |
1996年 | 13篇 |
1995年 | 12篇 |
1994年 | 8篇 |
1993年 | 5篇 |
1992年 | 16篇 |
1991年 | 15篇 |
1990年 | 9篇 |
1989年 | 13篇 |
1988年 | 6篇 |
1987年 | 12篇 |
1986年 | 10篇 |
1985年 | 9篇 |
1984年 | 8篇 |
1983年 | 8篇 |
1982年 | 9篇 |
1981年 | 7篇 |
1979年 | 6篇 |
1978年 | 7篇 |
1977年 | 6篇 |
1969年 | 5篇 |
1968年 | 5篇 |
1967年 | 4篇 |
1966年 | 5篇 |
1965年 | 4篇 |
排序方式: 共有774条查询结果,搜索用时 10 毫秒
51.
52.
High lung inflation pressures compress alveolar septal capillaries, impede red cell transit, and interfere with oxygenation. However, recently introduced acellular hemoglobin solutions may enter compressed lung capillaries more easily than red blood cells. To test this hypothesis, we perfused isolated rat lungs with fluorescently labeled diaspirin cross-linked hemoglobin (DCLHb; 10%) and/ or autologous red cells (hematocrit, 20). Septal capillaries were compressed by setting lung inflation pressure above vascular pressures (zone 1). Examination by confocal microscopy showed that DCLHb was distributed throughout alveolar septa. Furthermore, this distribution was not affected by adding red blood cells to the perfusate. We estimated the maximum acellular hemoglobin mass within septa to be equivalent to that of 15 red blood cells. By comparison, we found an average of 2.7 +/- 4.6 red cells per septum in zone 1. These values increased to 30.4 +/- 25.8 and 50.4 +/- 22.1 cells per septum in zones 2 and 3, respectively. We conclude that perfusion in zone 1 with a 10% acellular hemoglobin solution may increase the hemoglobin concentration per septum up to fivefold compared with red cell perfusion. 相似文献
53.
The alpha-like globin gene cluster in rabbits contains embryonic zeta-
globin genes, an adult alpha-globin gene, and theta-globin genes of
undetermined function. The basic arrangement of genes, deduced from
analysis of cloned DNA fragments, is 5'-zeta 0-zeta 1-alpha 1-theta 1- zeta
2-zeta 3-theta 2-3'. However, the pattern of restriction fragments
containing zeta- and theta-globin genes varies among individual rabbits.
Analysis of BamHI fragments of genomic DNA from 24 New Zealand white
rabbits revealed eight different patterns of fragments containing
zeta-globin genes. The large BamHI fragments containing genes zeta 0 and
zeta 1 are polymorphic in length, whereas a 1.9-kb fragment containing the
zeta 2 gene and the 3.5-kb fragment containing the zeta 3 gene do not vary
in size. In contrast to this constancy in the size of the restriction
fragments, the copy number of the zeta 2 and zeta 3 genes does vary among
different rabbits. No length polymorphism was detected in the BamHI
fragments containing the theta-globin genes, but again the copy number
varies for restriction fragments containing the theta 2 gene. The alpha 1-
and theta 1-globin genes are located in a nonpolymorphic 7.2-kb BamHI
fragment. The combined data from hybridization with both zeta and theta
probes shows that the BamHI cleavage pattern does not vary within the
region 5'-alpha 1-theta 1- zeta 2-zeta 3-theta 2-3', but the pattern
genomic blot-hybridization patterns for the progeny of parental rabbits
with different zeta-globin gene patterns shows that the polymorphic
patterns are inherited in a Mendelian fashion. Two different haplotypes
have been mapped based on the genomic blot-hybridization data. The
variation in the alpha-like globin gene cluster in the rabbit population
results both from differences in the copy number of the duplication block
containing the zeta-zeta-theta gene set and from the presence or absence of
polymorphic BamHI sites.
相似文献
54.
Smaller lungs in women affect exercise hyperpnea 总被引:2,自引:0,他引:2
McClaran Steven R.; Harms Craig A.; Pegelow David F.; Dempsey Jerome A. 《Journal of applied physiology》1998,84(6):1872-1881
We subjected 29 healthy young women (age: 27 ± 1 yr) with a wide range of fitness levels [maximal oxygenuptake (O2 max): 57 ± 6 ml · kg1 · min1;35-70ml · kg1 · min1]to a progressive treadmill running test. Our subjects had significantly smaller lung volumes and lower maximal expiratory flow rates, irrespective of fitness level, compared with predicted values for age-and height-matched men. The higher maximal workload in highly fit(O2 max > 57 ml · kg1 · min1,n = 14) vs. less-fit(O2 max < 56 ml · kg1 · min1,n = 15) women caused a higher maximalventilation (E) with increased tidal volume (VT)and breathing frequency (fb) atcomparable maximal VT/vitalcapacity (VC). More expiratory flow limitation (EFL; 22 ± 4% ofVT) was also observed duringheavy exercise in highly fit vs. less-fit women, causing higherend-expiratory and end-inspiratory lung volumes and greater usage oftheir maximum available ventilatory reserves.HeO2 (79% He-21%O2) vs. room air exercise trialswere compared (with screens added to equalize external apparatusresistance). HeO2 increasedmaximal expiratory flow rates (20-38%) throughout the range ofVC, which significantly reduced EFL during heavy exercise. When EFL wasreduced with HeO2, VT,fb, andE (+16 ± 2 l/min) weresignificantly increased during maximal exercise. However, in theabsence of EFL (during room air exercise),HeO2 had no effect onE. We conclude that smaller lungvolumes and maximal flow rates for women in general, and especiallyhighly fit women, caused increased prevalence of EFL during heavyexercise, a relative hyperinflation, an increased reliance onfb, and a greater encroachment onthe ventilatory "reserve." Consequently,VT andE are mechanically constrained duringmaximal exercise in many fit women because the demand for highexpiratory flow rates encroaches on the airways' maximum flow-volumeenvelope. 相似文献
55.
Strategies to assess and optimize stability of endogenous amines during cerebrospinal fluid sampling
Marek J. Noga Ronald Zielman Robin M. van Dongen Sabine Bos Amy Harms Gisela M. Terwindt Arn M. J. M. van den Maagdenberg Thomas Hankemeier Michel D. Ferrari 《Metabolomics : Official journal of the Metabolomic Society》2018,14(4):44
Introduction
Metabolic profiling of cerebrospinal fluid (CSF) is a promising technique for studying brain diseases. Measurements should reflect the in vivo situation, so ex vivo metabolism should be avoided.Objective
To investigate the effects of temperature (room temperature vs. 4 °C), centrifugation and ethanol, as anti-enzymatic additive during CSF sampling on concentrations of glutamic acid, glutamine and other endogenous amines.Methods
CSF samples from 21 individuals were processed using five different protocols. Isotopically-labeled alanine, isoleucine, glutamine, glutamic acid and dopamine were added prior to sampling to trace any degradation. Metabolomics analysis of endogenous amines, isotopically-labeled compounds and degradation products was performed with a validated LC–MS method.Results
Thirty-six endogenous amines were quantified. There were no statistically significant differences between sampling protocols for 31 out of 36 amines. For GABA there was primarily an effect of temperature (higher concentrations at room temperature than at 4 °C) and a small effect of ethanol (lower concentrations if added) due to possible degradation. O-phosphoethanolamine concentrations were also lower when ethanol was added. Degradation of isotopically-labeled compounds (e.g. glutamine to glutamic acid) was minor with no differences between protocols.Conclusion
Most amines can be considered stable during sampling, provided that samples are cooled immediately to 4 °C, centrifuged, and stored at ??80 °C within 2 h. The effect of ethanol addition for more unstable metabolites needs further investigation. This was the first time that labeled compounds were used to monitor ex vivo metabolism during sampling. This is a useful strategy to study the stability of other metabolites of interest.56.
Predation influences prey diversity and productivity while it effectuates the flux and reallocation of organic nutrients into biomass at higher trophic levels. However, it is unknown how bacterivorous protists are influenced by the diversity of their bacterial prey. Using 456 microcosms, in which different bacterial mixtures with equal initial cell numbers were exposed to single or multiple predators (Tetrahymena sp., Poterioochromonas sp. and Acanthamoeba sp.), we showed that increasing prey richness enhanced production of single predators. The extent of the response depended, however, on predator identity. Bacterial prey richness had a stabilizing effect on predator performance in that it reduced variability in predator production. Further, prey richness tended to enhance predator evenness in the predation experiment including all three protists predators (multiple predation experiment). However, we also observed a negative relationship between prey richness and predator production in multiple predation experiments. Mathematical analysis of potential ecological mechanisms of positive predator diversity—functioning relationships revealed predator complementarity as a factor responsible for both enhanced predator production and prey reduction. We suggest that the diversity at both trophic levels interactively determines protistan performance and might have implications in microbial ecosystem processes and services. 相似文献
57.
Abstract Opportunistic sightings and strandings of Caperea marginata (n=196) from the vicinity of Australia and New Zealand (1884 to early 2007) were used to relate geographic and temporal patterns to oceanographic and broad-scale climatic variability. Records were not uniformly distributed along the coast and more (69%) were from Australia than New Zealand. Seven coastal whale ‘hotspots’ were identified which accounted for 61% of records with locality data. Half of the hotspot records were from southeast (37) and northwest (20) Tasmania—others each had 9–15 events. Upwelling and/or high zooplankton abundance has been documented near all whale hotspots. Records of C. marginata occurred in all months, with 75% in spring and summer. Inter-annual variability showed broad agreement between increased whale records (usually in spring/summer) and strongly positive ‘Niño 3.4’ during 1980–1995 but not thereafter. Coastal upwelling and productivity increase during climatic phenomena such as El Niño and are likely to be quickly beneficial to plankton-feeding whales such as C. marginata. 相似文献
58.
Variable Responses of Lowland Tropical Forest Nutrient Status to Fertilization and Litter Manipulation 总被引:2,自引:0,他引:2
Emma J. Sayer S. Joseph Wright Edmund V. J. Tanner Joseph B. Yavitt Kyle E. Harms Jennifer S. Powers Michael Kaspari Milton N. Garcia Benjamin L. Turner 《Ecosystems》2012,15(3):387-400
Predicting future impacts of anthropogenic change on tropical forests requires a clear understanding of nutrient constraints
on productivity. We compared experimental fertilization and litter manipulation treatments in an old-growth lowland tropical
forest to distinguish between the effects of inorganic nutrient amendments and changes in nutrient cycling via litterfall.
We measured the changes in soil and litter nutrient pools, litterfall, and fine root biomass in plots fertilized with nitrogen
(N), phosphorus (P), or potassium (K), and in litter addition and litter removal treatments during 7 years. Soil inorganic
N and litter N increased in double-litter plots but not in N-fertilized plots. Conversely, litter P and soil pools of P and
K increased in fertilized plots but not in the double-litter plots. Soil and litter pools of N and K decreased in the no-litter
plots. Changes in litterfall with added nutrients or litter were only marginally significant, but fine root biomass decreased
with both the litter and the K addition. Differences between the two experiments are mostly attributable to the coupled cycling
of carbon and nutrients in litter. Increased nutrient inputs in litter may improve plant uptake of some nutrients compared
to fertilization with similar amounts. The litter layer also appears to play a key role in nutrient retention. We discuss
our findings in the context of possible impacts of anthropogenic change on tropical forests. 相似文献
59.
Dr. Ch. T. Harms Dr. I. Potrykus 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1978,53(2):57-63
Summary A simple effective technique for the fractionation of protoplast populations is described. Protoplasts are separated by low-speed centrifugation in an iso-osmotic, discontinuous density gradient system on the basis of differences in their buoyant densities. At a constant osmolality of 660±20 mOs/kg H2O, the gradients provide a density range from 1.017 to 1.069 g/cm3 at 20 °C which corresponds to the buoyant densities of most protoplast types studied. Characteristics of the KMC/S-density gradient system and factors affecting the fractionation were investigated. Protoplasts were isolated from various tissues and cultivars of tobacco, barley, wheat, rye, oat and maize. Their density-dependent distribution profiles in KMC/S-gradients and their average buoyant densities were determined under standardized conditions. Great differences in the buoyant densities were found between protoplasts of different tissues. Mixed populations of two types of protoplasts, differing in buoyant density by about 15–20 mg/cm3, were separated to give highly purified fractions. Factors affecting the buoyant densities of protoplasts have been investigated. Ploidy level and species differences did not significantly affect the fractionation profiles. However, an age-dependent variation in the average buoyant density of tobacco mesophyll protoplasts was observed. Fractionation of tobacco mesophyll protoplasts and their subsequent regeneration to plants demonstrates the practicability and physiological compatibility of the KMC/S-density gradient system under sterile conditions. The morphogenetic potential of protoplasts was not affected by the separation procedure or the gradient components. 相似文献
60.