Definition and research guide
This page combines a MeSH definition, publication statistics and selected research abstracts. It is a starting point for reading the literature, not a medical textbook or a systematic evidence review.
Content available: 3 complete abstracts (717 words), 9 references. Full article text is not stored or reproduced here; follow the PMID, DOI or PMC links to read the original sources and their access terms.
Definition
The fluid of the body that is outside of cells. It is the external environment for the cells.
Definition: NLM Medical Subject Headings (MeSH) scope note (Public domain (U.S. National Library of Medicine); attribution requested). View the MeSH record.
Also known as: Extracellular Fluids; Fluid, Extracellular; Fluids, Extracellular; Interstitial Fluid; Fluid, Interstitial; Fluids, Interstitial; Interstitial Fluids; Intercellular Fluid.
Where it fits in the MeSH classification
- Broader topics
- Extracellular Space; Body Fluids
- Narrower topics
- Aqueous Humor; Cerebrospinal Fluid; Extravascular Lung Water; Follicular Fluid; Labyrinthine Fluids; Lymph; Pericardial Fluid; Plasma; Synovial Fluid
Related concepts
- Cells
Research landscape
PubMed indexes 103,395 records tagged with the MeSH heading “Extracellular Fluid” (counts retrieved 2026-10-31). 15,314 of them (14.8%) were published from 2020 onward, and the busiest period so far is through 1999 with 48,903 records.
Reviews account for 8,019 records (7.8%) and meta-analyses for 833. Randomized controlled trials number 2,624 (2.5%) as a share of all records on the topic.
Show the data
| Period | Records |
|---|---|
| through 1999 | 48,903 |
| 2000-2009 | 16,144 |
| 2010-2019 | 23,034 |
| 2020 onward | 15,314 |
Publication-type chart data
| Type | Records | Share |
|---|---|---|
| Reviews | 8,019 | 7.8% |
| Meta-analyses | 833 | 0.8% |
| Randomized controlled trials | 2,624 | 2.5% |
Method: PubMed query "Extracellular Fluid"[MeSH Terms] with publication-type and date filters, retrieved 2026-10-31.
Reading the evidence in anatomy and physiology
Anatomy describes how body structures are built and physiology describes how they normally work, so most findings here are descriptive or mechanistic rather than evidence about treatment. Much of what is known comes from dissection, imaging, tissue studies and experiments in animals, and measurements in healthy volunteers can differ from those in people with disease or at different ages. Normal ranges vary between individuals and populations. Check whether a result was measured in people or in another species, the age and health of the participants, and the method used.
Evidence from open-access studies
These open-access studies come from PubMed’s best-match results for this heading (papers whose main topic it is), ranked here by citation count, with one recent addition. Each abstract is reproduced in full under its recorded open licence so that you can read the authors’ own words. They illustrate the literature; they are not a systematic summary of it. Citation counts measure attention, not reliability or clinical benefit.
| Study / authors | Year | Source label / design | Europe PMC citations | Licence / source |
|---|---|---|---|---|
| Sleep Alters the Velocity of Physiological Brain Pulsations in Humans | 2026 | Research article Design not recorded in structured metadata | 9 | cc by PMID 41632045 · DOI · PMC13045454 |
| Photoacoustic computed tomography monitors cerebrospinal fluid dynamics and glymphatic function | 2026 | Research article Design not recorded in structured metadata | 6 | cc by PMID 41680169 · DOI · PMC13009472 |
| Water exchange across the blood-CSF barrier: A systematic review | 2026 | Systematic review Design not recorded in structured metadata | 5 | cc by PMID 41652968 · DOI · PMC12882841 |
Sleep Alters the Velocity of Physiological Brain Pulsations in Humans [1]
Abstract reading summary — source excerpts
- Opening excerpt
- IntroductionThe clearance of brain metabolites increases during sleep, in association with increased spectral power of the three main cerebrospinal fluid (CSF) flow drivers: cardiovascular, respiratory, and vasomotor brain pulsations.
- Closing excerpt
- Sleep altered the 3D velocity dynamics of all neurofluidic brain pulsations in a manner consistent with increased interstitial space and greater fluid exchange, thus supporting the glymphatic model wherein physiological pulsations drive bulk flow during sleep.
- Interpretation limits
- Verbatim excerpts from the authors’ abstract; the closing excerpt may describe a finding, limitation or recommendation. Design, population, effect size and uncertainty have not been independently extracted or appraised. Read the complete abstract and linked paper before interpreting the findings.
Read complete abstract (370 words)
IntroductionThe clearance of brain metabolites increases during sleep, in association with increased spectral power of the three main cerebrospinal fluid (CSF) flow drivers: cardiovascular, respiratory, and vasomotor brain pulsations. However, little is known about how the increased power of these pulsations affects the velocity and direction of fluid flow in the sleeping brain.ObjectivesTo address this knowledge gap, we mapped the CSF oscillatory flow velocity in relation to the changing physiological pulsations in the brains of 22 healthy volunteers during sleep and waking.MethodsWe used the ultrafast magnetic resonance imaging sequence known as magnetic resonance encephalography (MREG) for tracing the pulsatile movement of water molecules inside the cranium. First, we conducted a phantom validation study with optical flow analysis to confirm that MREG accurately tracks pulsatile water molecule flow in a porous tissue medium. Next, we obtained MREG recordings for mapping the three physiological pulsations without aliasing in the human brain across the awake and sleep states; we thereby quantified the brain-wide 3D velocity →V$\mathop \to \limits_{\mathrm{V}} $ vectors (i.e., the velocity vs and 3D direction v̂${\mathrm{\hat{v}}}$ ) of each pulsation band, using comprehensive dense optical flow analysis during EEG-verified sleep in comparison to the awake state. Finally, we assessed relationships among the spectral power of the physiological pulsations, their 3D velocity →V$\mathop \to \limits_{\mathrm{V}} $ , and slow-delta EEG power, which is known to depict the increased interstitial volume during sleep.ResultsIn our phantom study, dense optical flow analysis reliably detected water flow in tissue driven by external pulsations. In healthy volunteers, sleep increased flow velocities ( →V$\mathop \to \limits_{\mathrm{V}} $ ) of the pulsations by more than 20% in concert with elevations in respiratory pulsations and vasomotor waves, while the velocity of cardiovascular pulsations (vs) declined by the same percentage. There was a significant anticorrelation between cardiac mean spectral power and slow delta EEG mean power, and a significant correlation between vasomotor mean spectral power and slow delta EEG mean power over the whole brain.ConclusionsPhantom studies validated the optic flow analysis of fast MREG recordings. Sleep altered the 3D velocity dynamics of all neurofluidic brain pulsations in a manner consistent with increased interstitial space and greater fluid exchange, thus supporting the glymphatic model wherein physiological pulsations drive bulk flow during sleep.
Abstract reproduced from the open-access article under the recorded cc by licence; © the authors. PMID 41632045 · DOI · PMC13045454
Photoacoustic computed tomography monitors cerebrospinal fluid dynamics and glymphatic function [2]
Abstract reading summary — source excerpts
- Opening excerpt
- Cerebrospinal fluid (CSF) continuously circulates through the brain and surrounding tissues to remove metabolic waste, a process that becomes less efficient with ageing and in neurodegenerative disease.
- Closing excerpt
- These results establish PACT as a non-invasive platform for measuring CSF and glymphatic function in vivo, providing a way to study how brain fluid transport is altered in ageing and neurological disorders.
- Interpretation limits
- Verbatim excerpts from the authors’ abstract; the closing excerpt may describe a finding, limitation or recommendation. Design, population, effect size and uncertainty have not been independently extracted or appraised. Read the complete abstract and linked paper before interpreting the findings.
Read complete abstract (155 words)
Cerebrospinal fluid (CSF) continuously circulates through the brain and surrounding tissues to remove metabolic waste, a process that becomes less efficient with ageing and in neurodegenerative disease. Visualizing this drainage in living animals has been difficult because existing imaging tools either lack depth, require radioactive tracers, or are too slow to capture dynamic flow. Here, we show that whole-body photoacoustic computed tomography (PACT) enables three-dimensional, real-time tracking of CSF transport in mice using indocyanine green. We visualize CSF movement from the spinal canal into the brain, quantify its efflux under different anesthesia conditions, and detect impaired clearance in a mouse model of Alzheimer's disease. Compared with healthy animals, diseased mice retain the tracer in the brain for several days, indicating reduced waste removal. These results establish PACT as a non-invasive platform for measuring CSF and glymphatic function in vivo, providing a way to study how brain fluid transport is altered in ageing and neurological disorders.
Abstract reproduced from the open-access article under the recorded cc by licence; © the authors. PMID 41680169 · DOI · PMC13009472
Water exchange across the blood-CSF barrier: A systematic review [3]
Abstract reading summary — source excerpts
- Opening excerpt
- Interest in cerebrospinal fluid (CSF) physiology and its relevance to neurological disease has increased markedly in recent years.
- Closing excerpt
- These findings challenge the classical concept of unidirectional CSF production and absorption, supporting instead a dynamic equilibrium where distributed, gradient-driven water flux maintains brain water homeostasis.
- Interpretation limits
- Verbatim excerpts from the authors’ abstract; the closing excerpt may describe a finding, limitation or recommendation. Design, population, effect size and uncertainty have not been independently extracted or appraised. Read the complete abstract and linked paper before interpreting the findings.
Read complete abstract (192 words)
Interest in cerebrospinal fluid (CSF) physiology and its relevance to neurological disease has increased markedly in recent years. Classical descriptions portray CSF as a unidirectional flow from the choroid plexus to the dural venous sinuses and rarely distinguish between its solutes and the solvent (water) component, which constitutes ~99% of CSF. We conducted a systematic literature review to evaluate current evidence for water exchange between CSF and blood across the blood-CSF barriers (BCSFB). Eighteen studies met the inclusion criteria: 15 in experimental animals and six with humans, spanning more than 70 years and employing diverse methodologies. This literature review shows that CSF water moves freely and bidirectionally between CSF and blood across multiple BCSFB sites along the craniospinal axis, including the choroid plexus, ependymal surfaces, pial vessels, and perivascular spaces. The net direction of movement varies locally with hydrostatic, osmotic, and molecular gradients that transiently favor either inflow or outflow. Blood-CSF water exchange occurs predominantly by diffusion and is modulated by aquaporins and local vascular forces. These findings challenge the classical concept of unidirectional CSF production and absorption, supporting instead a dynamic equilibrium where distributed, gradient-driven water flux maintains brain water homeostasis.
Abstract reproduced from the open-access article under the recorded cc by licence; © the authors. PMID 41652968 · DOI · PMC12882841
Key reviews and meta-analyses
The most cited review-type papers among PubMed’s best-match results for this heading, listed with bibliographic details only. Follow the links for the full text and access terms.
- Research article The Efficacy of Platelet-Rich Plasma on Tendon and Ligament Healing: A Systematic Review and Meta-analysis With Bias Assessment [4]
- Research article PRP Injections for the Treatment of Knee Osteoarthritis: A Meta-Analysis of Randomized Controlled Trials [5]
- Research article Synovial Fluid Biomarkers for the Diagnosis of Periprosthetic Joint Infection: A Systematic Review and Meta-Analysis [6]
- Research article Comparative effectiveness of platelet-rich plasma injections for treating knee joint cartilage degenerative pathology: a systematic review and meta-analysis [7]
- Research article Cerebrospinal fluid drainage to prevent paraplegia during thoracic and thoracoabdominal aortic aneurysm surgery: a systematic review and meta-analysis [8]
- Research article Platelet-rich plasma versus hyaluronic acid in the treatment of knee osteoarthritis: a meta-analysis [9]
References
- Elabasy A, Helakari H, Väyrynen T, Rajna Z, Huotari N, Raitamaa L, et al.. Sleep Alters the Velocity of Physiological Brain Pulsations in Humans. . 2026. PMID 41632045 · DOI · PMC13045454
- Choi S, Kim J, Jeon H, Lee Y, Lim G, Ju WM, et al.. Photoacoustic computed tomography monitors cerebrospinal fluid dynamics and glymphatic function. . 2026. PMID 41680169 · DOI · PMC13009472
- Hjørnevik T, Eide PK. Water exchange across the blood-CSF barrier: A systematic review. . 2026. PMID 41652968 · DOI · PMC12882841
- Chen X, Jones IA, Park C, Vangsness CT. The Efficacy of Platelet-Rich Plasma on Tendon and Ligament Healing: A Systematic Review and Meta-analysis With Bias Assessment. Am J Sports Med. 2018. PMID 29268037 · DOI · PMC6339617
- Filardo G, Previtali D, Napoli F, Candrian C, Zaffagnini S, Grassi A. PRP Injections for the Treatment of Knee Osteoarthritis: A Meta-Analysis of Randomized Controlled Trials. Cartilage. 2021. PMID 32551947 · DOI · PMC8808870
- Lee YS, Koo KH, Kim HJ, Tian S, Kim TY, Maltenfort MG, et al.. Synovial Fluid Biomarkers for the Diagnosis of Periprosthetic Joint Infection: A Systematic Review and Meta-Analysis. J Bone Joint Surg Am. 2017. PMID 29257013 · DOI
- Chang KV, Hung CY, Aliwarga F, Wang TG, Han DS, Chen WS. Comparative effectiveness of platelet-rich plasma injections for treating knee joint cartilage degenerative pathology: a systematic review and meta-analysis. Arch Phys Med Rehabil. 2014. PMID 24291594 · DOI
- Cinà CS, Abouzahr L, Arena GO, Laganà A, Devereaux PJ, Farrokhyar F. Cerebrospinal fluid drainage to prevent paraplegia during thoracic and thoracoabdominal aortic aneurysm surgery: a systematic review and meta-analysis. J Vasc Surg. 2004. PMID 15218460 · DOI
- Tang JZ, Nie MJ, Zhao JZ, Zhang GC, Zhang Q, Wang B. Platelet-rich plasma versus hyaluronic acid in the treatment of knee osteoarthritis: a meta-analysis. J Orthop Surg Res. 2020. PMID 32912243 · DOI · PMC7488405
About this article
- Sources
- NLM Medical Subject Headings (definition, synonyms); NCBI PubMed (counts, identifiers); Europe PMC (open-access abstracts and citation counts).
- Verification
- Every reference was resolved against PubMed on 2026-10-31; title, year and DOI matched.
- Review state
- automated source verified; no human expert review has been recorded yet.
- Version
- 1, last updated 2026-10-31.
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Evidence strength
EvidenceHelix's computed view of the study designs behind this topic: how common meta-analyses, randomized trials and reviews are, how recent the literature is, and a plain-language reading. It describes the design mix of the literature. It is not a quality appraisal of any study, and it is not medical advice.