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   125篇
  免费   22篇
  国内免费   52篇
  2022年   1篇
  2021年   8篇
  2020年   4篇
  2019年   2篇
  2018年   3篇
  2017年   2篇
  2016年   4篇
  2015年   10篇
  2014年   4篇
  2013年   13篇
  2012年   8篇
  2011年   4篇
  2010年   11篇
  2009年   8篇
  2008年   4篇
  2007年   8篇
  2006年   12篇
  2005年   12篇
  2004年   9篇
  2003年   8篇
  2002年   12篇
  2001年   8篇
  2000年   5篇
  1999年   3篇
  1998年   2篇
  1997年   3篇
  1992年   2篇
  1991年   2篇
  1990年   1篇
  1989年   1篇
  1988年   2篇
  1987年   1篇
  1986年   4篇
  1983年   1篇
  1980年   3篇
  1978年   1篇
  1975年   1篇
  1974年   1篇
  1973年   1篇
  1970年   1篇
  1968年   3篇
  1967年   1篇
  1966年   1篇
  1965年   1篇
  1959年   1篇
  1949年   1篇
  1947年   1篇
排序方式: 共有199条查询结果,搜索用时 15 毫秒
1.
Graduates of four rural and four urban family practice programs were interviewed to determine the nature of their practices and the factors that had influenced their practice location decisions. All programs gave residents substantial experience providing continuity of care for underserved populations. Of the 158 physicians surveyed, 58 (46%) were working in areas designated as underserved. The percentage of physicians in underserved areas was higher than that reported in other studies and was much higher than would be expected if practice sites were selected on the basis of population distribution alone. Notable differences in personal and practice characteristics were found between the physicians who chose to work in underserved areas and those who did not and between those who established practices in rural and in urban underserved areas.  相似文献   
2.
Summary Cadmium and zinc uptake parameters were determined for intact corn (Zea mays L.) seedlings grown for 15 and 22 in nutrient solutions containing levels of Cd and Zn that were similar to those found in soil solutions. Uptake of both elements was assumed to follow Michaelis-Menten kinetics. Calculations were based on the concentrations of free ionic Cd (Cd2+) and Zn (Zn2+) rather than the total solution concentration. Rates of Zn uptake were measured by determining depletion of Zn for periods of up to 30 h from solutions containing initial concentrations of 1.5 and 10μmol Zn 1−1. Depletion curves suggested that Zn uptake characteristics were similar at both levels of Zn in solution. The Imax for Zn uptake decreased from 550 to 400 pmol m−2 root surface s−1 between 16 and 22 d of growth while Km decreased from 2.2 to 1.5 μmol Zn2+ 1−1. Cadmium uptake parameters were measured by controlling Cd2+ activities in nutrient solution betwen 6.3 to 164 nmol l−1 by continuous circulation of nutrient solution through a mixed-resin system. Imax for Cd uptake was 400 pmol m−2 root surface s−1 at 15 and 22 d of growth. The magnitude of Km increased from 30 to 100 nmol Cd2+ 1−1 during this time period. The Km value suggests that corn is efficient for Cd uptake. The results of these uptake studies are consistent with the observed uptake of Zn and Cd by corn seedlings in soils.  相似文献   
3.
The quantum yield of tryptophan fluorescence of guinea-pig α lactalbumin is more than twice as large as found for the bovine, goat, or human proteins. This difference is due to the absence of Trp 60 from the guinea-pig amino acid sequence. It is proposed that the presence of this residue results in marked quenching of a second tryptophan residue for the other three proteins. Reference to the “lysozyme analogy model” suggests that the quenched residue is most likely Trp 104, which lies within 7Å of Trp 60, the latter lying within 6Å of two disulfide bridges. It is suggested that transfer of the excited state energy takes place from Trp 104 to 60 with subsequent quenching by the two vicinal disulfide bridges. These observations provide support for the validity of the “lysozyme analogy model”.  相似文献   
4.
5.
Lack of estrogen receptor (ER) and presence of vimentin (VIM) associate with poor prognosis in human breast cancer. We have explored the relationships between ER, VIM, and invasiveness in human breast cancer cell lines. In the matrigel outgrowth assay, ER+/VIM- (MCF-7, T47D, ZR-75-1), and ER-/VIM- (MDA-MB-468, SK-Br-3) cell lines were uninvasive, while ER-/VIM+ (BT549, MDA-MB-231, MDA-MB-435, MDA-MB-436, Hs578T) lines formed invasive, penetrating colonies. Similarly, ER-/VIM+ cell lines were significantly more invasive than either the ER+/VIM- or ER-/VIM- cell lines in the Boyden chamber chemoinvasion assay. Invasive activity in nude mice was only seen with ER-/VIM+ cell lines MDA-MB-231, MDA-MB-435 and MDA-MB-436. Hs578T cells (ER-/VIM+) showed hematogenous dissemination to the lungs in one of five mice, but lacked local invasion. The ER-/VIM+ MCF-7ADR subline was significantly more active than the MCF-7 cells in vitro, but resembled the wild-type MCF-7 parent in in vivo activity. Data from these cell lines suggest that human breast cancer progression results first in the loss of ER, and subsequently in VIM acquisition, the latter being associated with increased metastatic potential through enhanced invasiveness. The MCF-7ADR data provide evidence that this transition can occur in human breast cancer cells. Vimentin expression may provide useful insights into mechanisms of invasion and/or breast cancer cell progression.  相似文献   
6.
目的: 人动脉血来源是右心系统并在肺脏进行气体交换的静脉血,右心系统的静脉血是否存在波浪式信号目前尚没有证据支持,本研究旨在对比同时间动、静脉血中信号的连续变化特点。方法: 选择心功能正常,需要连续监测动脉血流动力学变化的患者6 例,4男2女,年龄(59.00±16.64) 岁,体质量(71.67±10.37)kg,左心射血分数(LVEF)(61.33±2.16)%。患者签署知情同意书后,选择心功能正常需要监测动、静脉血流动力学变化的患者6 例,连续同时桡动脉、颈内静脉逐搏取血,测定PaO2。选取2个典型呼吸周期,用于分析同时段动、静脉血气的波浪式变化。分别比较患者血氧分压最高和最低值,以验证同时段动、静脉血气是否都存在周期性波浪式信号变化。此外,将患者动脉、静脉血气周期性波浪式信号的变化幅度进行统计学t 检验分析,比较有无差异。结果: 共6例患者,抽取动、静脉血液充满肝素化细长塑化管需要15~16次心跳,即取血需要15~16次心跳,全部覆盖超过2个呼吸周期。所有患者动脉血气中PaO2均呈现明显的波浪式变化(P<0.05),幅度是(9.96±5.18)mmHg,是均值的(8.09±2.43)%。患者静脉血气中PaO2波动幅度并不明显,为(1.63±0.41)mmHg,是均值的(3.91±1.22)%,与动脉血气组相比有明显统计学差异(P<0.05)。结论: 采用同时连续逐搏动、静脉取血血气分析法证实,患者自主呼吸时动脉血气有明显的周期性波浪式变化信号,而静脉血气几乎没有周期性波浪式变化信号(很弱),说明动脉血气波浪式信号主要是由于肺通气过程中吸气和呼气期产生肺泡中氧分压规律性上升和下降,通过离开肺毛细血管与肺泡氧气压力平衡的动脉化血液,经过左心室搏血进入动脉血管系统所致。  相似文献   
7.
目的: 为探讨新生儿自主呼吸产生机制,前文已对新生儿出生后自主呼吸开始前脐带动静脉氧气和二氧化碳差值进行了人群组间分析;而本部分则对相关信息进行个体化分析。方法: 在产前经所有胎儿父母签署知情同意书,新生儿出生后还没有呼吸之前在脐带动脉和脐带静脉分别连续逐搏取血,仅有3例同时采集到Pua和Puv血液样本进行血气分析测定,计算分析脐带静脉和脐带动脉的异同和动态变化。结果: 虽然准备了数十产妇,但仅有3例同时采集到Pua和Puv血液样本,同一时间的PuvO2显著高于PuaO2P均<0.01),平均相差(24.17±7.09) mmHg;而PuvCO2显著低于PuaCO2P均<0.01),平均相差(-7.67±3.70) mmHg。在同一时间的Puv-uaO2显著高于Puv-uaCO2P<0.05)。结论: 新生儿出生后自主呼吸前,全部氧气供应由脐带静脉运输,只要胎盘开始剥离则新生儿的PuaO2随时间(心跳次数)逐渐降低,当PuaO2达到触发呼吸阈值(最低值)诱发第一次吸气开始其自主呼吸。  相似文献   
8.
Surfactant protein-A (SP-A) belongs to a family of collagen-containing C-type lectins called collectins. SP-A is expressed by renal tubule epithelial cells. We investigated the distribution of SP-A in renal cell carcinomas (RCC) using immunohistochemical techniques and western blotting. We used 35 formalin fixed, paraffin embedded (FFPE) RCC tissue samples. We compared results with clinico-pathological parameters of RCC including age, sex, Fuhrman grade, tumor volume, tumor node metastasis (TNM) and clinical stage. SP-A was localized in the glomerulus and renal tubule epithelium in nontumor tissue and strong SP-A immunoreactivity was observed in tumor tissue. SP-A was expressed in the RCC tumor cells (64%) and nontumor cells (34%) in males and RCC tumor cells (90%) and nontumor cells (30%) in females. There was a significant correlation between SP-A immunoreactivity in tumor cells and gender, age, tumor diameter, Fuhrman grade and tumor diameter. Western blot analysis supported the immunohistochemical findings. We present evidence for involvement of SP-A in RCC and suggest that increased SP-A expression in RCC is associated with favorable prognosis.  相似文献   
9.
10.
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