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   1072篇
  免费   300篇
  国内免费   42篇
  1414篇
  2024年   13篇
  2023年   35篇
  2022年   66篇
  2021年   86篇
  2020年   63篇
  2019年   90篇
  2018年   55篇
  2017年   79篇
  2016年   86篇
  2015年   95篇
  2014年   108篇
  2013年   99篇
  2012年   64篇
  2011年   48篇
  2010年   24篇
  2009年   38篇
  2008年   28篇
  2007年   47篇
  2006年   31篇
  2005年   41篇
  2004年   27篇
  2003年   22篇
  2002年   23篇
  2001年   22篇
  2000年   12篇
  1999年   12篇
  1998年   10篇
  1997年   5篇
  1996年   7篇
  1995年   4篇
  1994年   4篇
  1993年   5篇
  1992年   6篇
  1991年   4篇
  1990年   4篇
  1989年   2篇
  1988年   2篇
  1986年   3篇
  1985年   4篇
  1984年   4篇
  1983年   6篇
  1982年   9篇
  1981年   7篇
  1980年   2篇
  1979年   6篇
  1977年   4篇
  1976年   1篇
  1975年   1篇
排序方式: 共有1414条查询结果,搜索用时 15 毫秒
121.
Post‐dispersal seed predation is a key process determining the variability in seed survival in forests, where most seeds are handled by rodents. Seed predation is thought to affect seedling regeneration, colonization ability and spatial distribution of plants. Basic seed traits are the essential factors affecting rodent foraging preferences and thus seed survival and seedling recruitment. Many studies have discussed several seed traits and their effects upon seed predation by rodents. However, the results of those previous studies are usually equivocal, likely because few seed traits and/or plant species tend to be incorporated into these studies. In order to elucidate the relationships between seed predation and seed traits, we surveyed the predation of 48 600 seeds in a natural pine forest, belonging to 30 species, for three consecutive years. The results demonstrated that: (i) seed size and seed coat hardness did not significantly affect seed predation; (ii) total phenolics had a negative effect upon seed predation; (iii) positive effects of nitrogen content upon seed predation were found. From our study, it seems that the better strategy to prevent heavy predation is for plants to produce seeds with higher total phenolics content rather than physical defenses (i.e. hard seed coat) or larger seeds. Additionally, rodent foraging preference may depend more on Nitrogen content than other nutrient content of seeds.  相似文献   
122.
Animals secrete glucocorticoids to deal with daily stressors. Studies have found that supplemental melatonin decreases glucocorticoid metabolite levels in stressed animals. We determined the effect of light interference (LI) and supplemental melatonin on (1) body mass, (2) food intake and (3) glucocorticoid metabolite levels of the striped mouse (Rhabdomys pumilio). Experiment was split into three phases: 8 L: 16 D; 8 L: 16 D with a 15 min light interruption every 4 h; and 8 L: 16 D with a 15 min light interruption every 4 h and melatonin (0.2 μg/ml) added to the water. Body mass was significantly different between phases with lowest body mass (89.17 ± 6.56 g) occurring during standard 8 L: 16 D. LI and melatonin significantly increased body mass. LI increased and melatonin decreased glucocorticoid metabolite levels. LI significantly increased and melatonin significantly decreased assimilation efficiencies possibly due to changes in energetic demands.  相似文献   
123.
BackgroundBariatric surgery is widely performed to improve obesity-related disorders, but can lead to nutrient deficiencies. In this study we examined serum trace element concentrations before and after bariatric surgery.MethodsWe obtained serum trace element concentrations by inductively coupled plasma-mass spectrometry (ICP-MS) method in 437 patients (82% women, median preoperative body-mass index 46.7 kg/m2 [interquartile range 42–51]) undergoing either gastric banding (22.7%), sleeve gastrectomy (20.1%), or gastric bypass (57.3%) procedures. Trace element data were available for patients preoperatively (n = 44); and 3 (n = 208), 6 (n = 174), 12 (n = 122), 18 (n = 39), 24 (n = 44) and 36 months (n = 14) post-operatively. All patients were recommended to take a multivitamin-mineral supplement after surgery.ResultsCopper deficiency was found in 2% of patients before surgery; and after surgery deficiency rates ranged from 0 to 5% with no significant change in median concentrations during follow-up (p = 0.68). Selenium deficiency was reported in 2% of patients before surgery; and after surgery deficiency rates ranged from 11 to 15% with a near-significant change in median concentrations (p = 0.056). Zinc deficiency was reported in 7% before surgery; and after surgery deficiency rates ranged from 7 to 15% with no significant change in median concentrations (p = 0.39).ConclusionsIn bariatric surgery patients recommended to take multivitamin-mineral supplements, serum copper, zinc and selenium concentrations were mostly stable during the first years after bariatric surgery. There was a possible tendency for selenium concentrations to decline during the early postoperative period.  相似文献   
124.
A total of 1,708 small mammals (1,617 rodents and 91 soricomorphs), including Apodemus agrarius (n = 1,400), Microtus fortis (167), Crocidura lasiura (91), Mus musculus (32), Myodes (= Eothenomys) regulus (9), Micromys minutus (6), and Tscherskia (= Cricetulus) triton (3), were live-trapped at US/Republic of Korea (ROK) military training sites near the demilitarized zone (DMZ) of Paju, Pocheon, and Yeoncheon, Gyeonggi Province from December 2004 to December 2009. Small mammals were examined for their intestinal nematodes by necropsy. A total of 1,617 rodents (100%) and 91 (100%) soricomorphs were infected with at least 1 nematode species, including Nippostrongylus brasiliensis, Heligmosomoides polygyrus, Syphacia obvelata, Heterakis spumosa, Protospirura muris, Capillaria spp., Trichuris muris, Rictularia affinis, and an unidentified species. N. brasiliensis was the most common species infecting small mammals (1,060; 62.1%) followed by H. polygyrus (617; 36.1%), S. obvelata (370; 21.7%), H. spumosa (314; 18.4%), P. muris (123; 7.2%), and Capillaria spp. (59; 3.5%). Low infection rates (0.1-0.8%) were observed for T. muris, R. affinis, and an unidentified species. The number of recovered worms was highest for N. brasiliensis (21,623 worms; mean 20.4 worms/infected specimen) followed by S. obvelata (9,235; 25.0 worms), H. polygyrus (4,122; 6.7 worms), and H. spumosa (1,160; 3.7 worms). A. agrarius demonstrated the highest prevalence for N. brasiliensis (70.9%), followed by M. minutus (50.0%), T. triton (33.3%), M. fortis (28.1%), M. musculus (15.6%), C. lasiura (13.2%), and M. regulus (0%). This is the first report of nematode infections in small mammals captured near the DMZ in ROK.  相似文献   
125.

Background

The elderly undergo cardiac surgery more and more frequently, often present multiple comorbidities, assume chronic therapies, and present a unique physiology. Aim of our study was to analyze the experience of a referral cardiac surgery center with all types of cardiac surgery interventions performed in patients ≥80 years old over a six years’ period.

Methods

A retrospective observational study performed in a university hospital. 260 patients were included in the study (3.5% of the patients undergoing cardiac surgery in the study period).

Results

Mean age was 82 ± 1.8 years. Eighty-five percent of patients underwent elective surgery, 15% unplanned surgery and 4.2% redo surgery. Intervention for aortic valve pathology and coronary artery bypass grafting were performed in 51% and 46% of the patients, respectively. Interventions involving the mitral valve were the 26% of the total, those on the tricuspid valve were 13% and those on the ascending aortic arch the 9.6%. Postoperative low output syndrome was identified in 44 patients (17%). Mortality was 3.9% and most of the patients (91%) were discharged from hospital in good clinical conditions. Hospital mortality was lower in planned vs unplanned surgery: 3.8% vs 14% respectively. Chronic obstructive pulmonary disease (OR 9.106, CI 2.275 – 36.450) was the unique independent predictor of mortality.

Conclusions

Clinicians should be aware that cardiac surgery can be safely performed at all ages, that risk stratification is mandatory and that hemodynamic treatment to avoid complications is expected.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2253-15-15) contains supplementary material, which is available to authorized users.  相似文献   
126.
In Mizoram (Northeast India), rodent outbreaks are known to occur periodically with the onset of bamboo flowering causing a tremendous destruction to food grains and as per the folk belief, often resulting in famine. In an exploratory survey of rodent pests in bamboo growing atreas for their helminth parasite spectrum, metacestodes of tapeworms were frequently encountered infecting the liver lobes and body cavity of the host. The morphological criteria were found to be closely consistent with the metacestode of Taenia species. In molecular characterization of the parasite, the ribosomal DNA (ITS1, ITS2) and mitochondrial COI were amplified and sequenced. Based upon both morphological data and molecular analysis using bioinformatic tools, the metacestode is identified as confirmed to be representing Cysticercus fasciolaris. The adult form of which (Taenia taeniaeformis) commonly occurs in felid and canid mammalian hosts.  相似文献   
127.
目的:探讨糖原磷酸化酶BB对心脏直视手术后心肌损伤的判定价值.方法:108例行心脏瓣膜置换手术患者,分别于围术期多个时点取桡动脉血,测定血清糖原磷酸化酶BB、心肌肌钙蛋白I(cTnI)、肌酸磷酸激酶同工酶(CKMB)的水平,并检测心肌细胞凋亡程度.结果:血清糖原磷酸化酶BB于体外循环主动脉开放后最早达峰值,其峰值与cT...  相似文献   
128.

Introduction

Statins may theoretically reduce postoperative atrial fibrillation (AF) in patients after cardiac valvular surgery due to preservation of endothelial function and anti-ischaemic, anti-inflammatory and anti-remodelling effects.

Methods

Two hundred seventy-two patients who underwent cardiac workup and subsequently cardiac valvular surgery without AF and concomitant coronary artery bypass grafting (CABG) at our hospital were selected. Preoperative drug use and postoperative AF were recorded. AF was defined as any episode of AF longer than 10 s. In addition, results from echocardiography and blood samples were retrieved.

Results

Baseline characteristics were as follows: mean age was 65 ± 11 years, 142 (52%) patients were male, 189 (70%) had undergone aortic valve surgery and the mean left ventricular ejection fraction was 57 ± 12%. Statins were used by 79 patients (29%). Statin users, more often, had a prior percutaneous coronary intervention (25% vs 9%, p < 0.001) or CABG (24% vs 4%, p < 0.001), diabetes mellitus (22% vs 5%, p < 0.001) and more often used β-blockers (51% vs 24%, p < 0.001). Patients in the non-statin group more often had surgery on more than one valve (10% vs 3%, p = 0.043) and had a higher cholesterol level (222 ± 48 vs 190 ± 43 mg/dl, p < 0.001). Postoperative AF occurred in 54% (43/79) of the patients with and in 55% (106/193) of the patients without statins (p = 0.941). There was also no difference in the timing of onset of AF or duration of hospital stay.

Conclusion

In this observational study, statin use was not associated with a reduced incidence of AF in patients after cardiac valvular surgery.  相似文献   
129.
毛天巍  陈郁希  李强  李学超  阚毅 《生物磁学》2011,(18):3446-3448,3484
目的:探讨胃转流术后远端肠管黏膜的适应性变化及生长因子的表达情况。方法:将8周龄的Wistar大鼠随机分为对照组、假手术组和胃转流术组,术后8周取吻合口远端肠管行常规病理切片检查,测量肠黏膜厚度和绒毛高度,采用免疫荧光法检测肠粘膜中表皮生长因子(EGF)和胰岛素样生长因子-1(IGF—1)的表达。结果:胃转流术组黏膜厚度(672±39与500±31um,P〈0.01)和绒毛高度(445±19与342±15um,P〈0.01)均显著高于假手术组(P〈0.01),而对照组和假手术组间均无显著差异。与假手术组比较,胃转流术组肠粘膜中EGF和IGF-1的表达显著升高(P〈0.01),而对照组和假手术组间无显著差异。结论:胃转流术后远端肠管粘膜发生适应性增生,同时伴随着EGF和IGF—1表达水平的升高。  相似文献   
130.
李孝智  郭峰  汪清 《生物磁学》2011,(8):1536-1538
目的:探讨后腹腔镜保留肾单位手术(LNSS)术式对术肾肾功能的影响。方法:2009年1月-2010年12月行经后腹膜途径腹腔镜保留肾单位手术例26。男16例,女10例。随访时间〉1年的有18例。年龄40-70岁。肾细胞癌23例,肾嫌色细胞癌2例,肾平滑肌瘤1例。肿瘤直径2.6-3.2cm,平均2.9cm。所有患者术中阻断肾动脉,剪刀切除肿瘤。术中肾动脉阻断时间20-30min,平均26.4分钟。分别于术前、术后2周、术后3月、术后1年测定双肾显像、血清肌酐、血清胱抑素。结果:18例患者术肾术前GFR及占总GFR的比例分别为53.2±7.8 ml/min和50.7±3.6%,术后两周GFR及占总GFR的比例分别为31.2±8.9 ml/min和35.8±5.8%,术后3月GFR及占总GFR的比例分别为34.7±8.6 ml/min和38.4±5.3%,术后1年GFR及占总GFR的比例分别为41.1±9.7 ml/min和43.2±6.2%。血清肌酐及血清胱抑素术前术后对比因为受对侧肾功能的影响不能反映对分肾功能的影响。结论:LNSS术式对于直径小于4cm的肾早期肿瘤是切实可行的。  相似文献   
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