Introduction
The main goal of this study was to review the evidence related to the effect of population ageing on various functions of the health system, including stewardship, financing, resource generation, and service provision.

Methods
This study is a systematic review. The related articles were searched in PubMed, ProQuest, Web of Science, and Google Scholar from 2000 to 2022 using the related keywords. Original and review articles that had examined the effects of population aging on at least one of the functions of the health system were included in this study. Quality evaluation of selected articles was done with strengthening the reporting of observational studies in epidemiology (STROBE) checklist. To analyze the extracted data, the World Health Organization’s (WHO) health system performance assessment framework was used, which includes the four functions of stewardship, financing, resource generation, and service provision. All steps of this systematic review were done independently by two authors and disagreements were resolved with the discussion and participation of the third author. From the 1954 identified articles, 203 were removed due to being duplicates and 1751 remained for screening. After reviewing the title and abstract of the remaining articles, 94 articles were screened by full text. Finally, 23 articles met the entry criteria and selected for the review.

Results
There was a upward growth trend in the studies from 2002 to 2021. Among the selected articles, four articles studied with the effects of population aging on stewardship function of the health system. Most of the studied was related to financing function (n=18). Three articles were related to the function of resource development, and nine articles were related to the function of health service provision. A summary of findings is provided in Table 1.



Conclusion
The effects of population aging on all the functions of the health system, especially the function of financing, are very noticeable. The findings of this study can provide appropriate information to health care providers so that, by proper and timely planning and management, they can improve the performance of the health system in the face of the negative consequences of population aging, which can ultimately improve the health of society and help meet expectations and fair financing in the health system.

Ethical Considerations
Compliance with ethical guidelines
This study was approved by the Ethics Committee of Tehran University of Medical Sciences, Tehran, Iran (Code: IR.TUMS.SPH.REC.1401.044).

Funding
This study was extracted from the PhD dissertation of Alireza Hajizadeh, approved by Department of Health Management, Policy and Economics, School of Public Health, Tehran University of Medical Sciences, Tehran, Iran (Code: 9921383004).

Authors' contributions
All authors equally contributed to preparing this article.

Conflicts of interest
The authors declared no conflict of interest.


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