全文获取类型
收费全文 | 6289篇 |
免费 | 394篇 |
国内免费 | 2篇 |
出版年
2023年 | 48篇 |
2022年 | 100篇 |
2021年 | 192篇 |
2020年 | 115篇 |
2019年 | 155篇 |
2018年 | 190篇 |
2017年 | 179篇 |
2016年 | 246篇 |
2015年 | 345篇 |
2014年 | 351篇 |
2013年 | 460篇 |
2012年 | 430篇 |
2011年 | 420篇 |
2010年 | 292篇 |
2009年 | 246篇 |
2008年 | 302篇 |
2007年 | 255篇 |
2006年 | 264篇 |
2005年 | 215篇 |
2004年 | 234篇 |
2003年 | 183篇 |
2002年 | 191篇 |
2001年 | 145篇 |
2000年 | 115篇 |
1999年 | 112篇 |
1998年 | 45篇 |
1997年 | 43篇 |
1996年 | 32篇 |
1995年 | 42篇 |
1994年 | 26篇 |
1993年 | 31篇 |
1992年 | 55篇 |
1991年 | 55篇 |
1990年 | 44篇 |
1989年 | 47篇 |
1988年 | 40篇 |
1987年 | 43篇 |
1986年 | 45篇 |
1985年 | 30篇 |
1984年 | 19篇 |
1983年 | 15篇 |
1981年 | 17篇 |
1979年 | 23篇 |
1978年 | 18篇 |
1977年 | 16篇 |
1975年 | 18篇 |
1974年 | 25篇 |
1973年 | 18篇 |
1969年 | 22篇 |
1968年 | 19篇 |
排序方式: 共有6685条查询结果,搜索用时 15 毫秒
101.
To determine the usefulness of R-wave amplitude changes during exercise testing for the diagnosis of coronary artery disease (CAD) and to understand the discrepancies that have been described in the literature regarding their value, we studied two groups of patients by means of electrocardiographic (EKG) treadmill testing and coronary arteriography. Group I was composed of 149 patients who were studied prospectively. The specificity of R-wave changes measured from preexercise to immediately postexercise (SRV(5)) was 81%, but that of R-wave changes measured from preexercise to peak exercise (URV(5)) was 46%. A group of 156 patients (Group II) evaluated retrospectively showed a high specificity for the SRV(5) (84%) and poor specificity for the URV(5) (39%). The sensitivity of the SRV(5) was 38% in Group I and 42% in Group II. Therefore, if measured during the immediate postexercise period and not at peak exercise, changes in R-wave amplitude may be of value in the diagnosis of coronary artery disease by electrocardiographic exercise testing. 相似文献
102.
De La Calzada CS Verdugo AL Perez JT Alcaine CC Fernandez AS 《Cardiovascular diseases》1981,8(1):73-83
A 50-year-old man presented with hypertrophic obstructive cardiomyopathy (HOC) associated with a left ventricular aneurysm and normal coronary arteries. His history revealed no evidence of myocardial infarction or atypical angina. Physical examination disclosed HOC but did not suggest the presence of an aneurysm. Although the patient was treated medically, heart failure ensued, and he died suddenly while working his farm. Subsequent investigation of the patient's family revealed that three of his five children were also affected by cardiomyopathy, which was especially pronounced in the eldest, a 22-year-old man. The possible hemodynamic relationship between HOC and left ventricular aneurysm is discussed, along with probable indications. The role of left ventricular aneurysm is also presented in relation to the natural history of the disease. 相似文献
103.
Zinc deficiency in wetland rice along a toposequence of hydromorphic soils in the Philippines 总被引:1,自引:1,他引:0
Summary Zinc deficiency in rice induced by prolonged water saturation of the soil is widespread in the Philippines. The poor drainage associated with the disorder is mostly due to upwelling of shallow artesian water in footslope areas and to ponding of water from continually flowing artesian wells common in such areas. One toposequence where no Zn deficiency occurred in the highest field and progressively severe Zn deficiency in successive lower fields, was studied in detail. With lower elevation soils showed an increase in organic matter and free carbonate, associated with increased upwelling and ponding of artesian water very high in dissolved Mg and Ca bicarbonate. 相似文献
104.
Chemotaxis of rat peritoneal cells, of which the eosinophil was the predominant migratory cell type, toward incubates of Trichinella spiralis was studied using a modified Boyden chamber. Excysted muscle larvae, preadults, and adults were incubated in a buffered medium for 20 hr at 37 C. Worms were incubated alone or with serum or spleen cells, or both, from immune and nonimmune rats. Incubates of worm stages alone possessed no chemotactic activity as compared with incubation medium as a negative control and zymosan-activated serum as a positive control. Both normal and immune sera tested alone stimulated cell migration to the same degree. Incubates of spleen cells from either normal or immunized hosts did not show chemotactic activity. Chemotaxis caused by normal and immune sera were not altered by incubation with homologous spleen cells. Addition of larva, preadults, and adult worms to sera, however, enhanced chemotactic activity over sera alone. Chemotaxis caused by larvae plus immune sera was significantly greater than that stimulated by larvae plus normal sera. This difference decreased when preadults were substituted for larvae and was not observed when adult worms were used. Reversal of the chemical gradients showed that active cell migration caused by various incubates was due to Chemotaxis. 相似文献
105.
Ammonia overloading was investigated during glucose and fructose metabolism in isolated hepatocytes under a variety of metabolic conditions. In all assay conditions, the glycolytic flux and oxygen uptake was not modified by 10 mM ammonia. In hepatocytes isolated from rats fed as libitum, the presence of ammonia caused a decrease in the production of lactate (pyruvate); this effect was not observed in anaerobic incubations, in hepatocytes isolated from starved animals, or in fetal hepatocytes. In spite of an overproduction of urea, ammonia detoxification also takes place by the synthesis of alanine, glutamate and aspartate. Addition of 1 mM aminooxyacetate, an inhibitor of aminotransferases, to the incubation medium prevents the formation of these amino acids, and also prevents the decrease of lactate in hepatocytes isolated from fed animals. 相似文献
106.
Adenylate cyclase (ATP pyrophosphate-lyase (cyclizing), EC 4.6.1.1) activity in Blastocladiella emersonii is associated with particulate subcellar fractions. Solubilization after treatment with detergent suggests its localization in a membrane fraction of the zoospore homogenate. The enzyme specifically requires Mn2+ for activity and is not stimulated by NaF. The kinetic characteristics of substrate utilization by B. emersonii adenylate cyclase were investigated with various concentrations of ATP and Mn2+, and in the presence of inhibitors. Plots of enzyme activity versus the actual concentration of the MnATP2- complex give sigmoid curves. An excess of Mn2+ activates the enzyme at low concentrations of substrate and leads to a modification of the enzyme kinetics. The nucleotides 5'-AMP and GTP were shown to be competitive inhibitors of the enzyme. In addition, kinetic data, obtained under conditions in which an inhibitor (ATP) is added in constant proportion to the variable substrate (MnATP2-) concentration, produced reciprocal plots that were linear and intersecting to the right of the ordinate, and secondary replots that were hyperbolic. These kinetic patterns support a model in which: MnATP2- is the substrate; free Mn2+ is an activator at low substrate concentrations, but an inhibitor at high substrate concentrations; and free ATP is not an efficient inhibiyor (Ki greater than 1.10(-4) M). 相似文献
107.
108.
Effects of DAPI on human leukocytes in vitro. 总被引:1,自引:0,他引:1
DAPI (4'-6-diamidino-2-phenylindole), a fluorochrome specific for AT-rich DNA, was supplied for 24 h at various concentrations to human leukocytes in culture. This treatment caused the appearance on the chromosomes of specific areas lacking spiralization. In particular, the centromeric regions of chromosomes 1,9, and 16, a short region on the long arm of chromosomes 1 and 2, and the distal heterochromatic part of the long arm of the Y chromosome were despiralized. The despiralization pattern of DAPI is compared with those previously obtained with Hoechst 33258 and Distamycin A. 相似文献
109.
Doctor Raul Dodsworth Machado Wanderley de Souza Gerson Cotta-Pereira Gustavo de Oliveira Castro 《Cell and tissue research》1976,174(3):355-366
Summary The general ultrastructure of the electrocyte, the basic unit of the electric organs of Electrophorus electricus, is analyzed. Presented herein are detailed observations of the syncytial surface, its fibrillar coat, invaginations of the plasma membrane and synaptic terminals. Using Thiéry's method glycogen granules were identified in the syncytial cytoplasm and inside the synaptic terminals, their size and structure being compatible with the muscular origin of the electric organs, to which the filamentous meshwork found in the cytoplasm may be related. Among the perinuclear-organelles, are dense bodies with crystalline patterns. The mitochondrial matrix contains dense granules, their size and structure varying according to the organ to which they belong and to the fixation method used.This work has been supported by Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Conselho de Ensino para Graduados da UFRJ and Banco Nacional de Desenvolvimento Econômico, FUNTEC-241 相似文献
110.
M Goossens M C Garel J Auvinet O Basset P Ferreira Gomes J Rosa N Arous 《FEBS letters》1975,58(1):149-154