Stroke and Hydration

What is a stroke?

A stroke is a condition where the blood in the body fails to reach the brain, due to defects in blood vessels. It is life-threatening, as the lack of oxygen being delivered into the brain immediately causes the brain cells to die. The rapid process can lead to brain injuries, loss of body functions and mortality. Strokes occur more frequently amongst older male, and are often associated with smoking, high fat diet, poor hydration, etc.1 To identify the symptoms of strokes, F.A.S.T. assessment could be done on the patients and urgent treatment is often essential to prevent further damage. 

There are two major types of strokes: an ischaemic stroke, which is caused by artery blockages, and a haemorrhagic stroke, which is caused by rupture of blood vessels in the brain. Whilst the former can be a result of excessive fat deposit in the artery; the latter occurs when the blood pressure reaches an enormous level, creating bulges in the vascular walls and causing them to break.2 For both scenarios, conditions such as obesity and diabetes are the major risk factors.  

A third type of condition, termed transient ischaemic attacks (TIA), describes short episodes of “mini-strokes”. Blood supplies temporarily stop going through the brain, and the patients would experience short-term stroke-like symptoms. Despite only lasting for a short period, from a few minutes to 24 hours, TIA can lead to a major stroke within a few months if untreated. Therefore, a “Mini-stroke” is a significant warning sign for the patients to  visit the doctor and receive urgent treatment.1,2   

Symptoms of a stroke

It is important for stroke patients to receive early diagnosis and urgent treatment. F.A.S.T assessment could be used to identify the symptoms of a stroke:

  • F = Face Drooping
  • A = Arm Weakness
  • S = Speech Difficulty
  • T - Time to call the emergency phone line (911 in the US and 999 in the UK)

Symptoms such as sudden numbness, confusion and dropping can also occur without known causes. Even if they only last for a few minutes, it is important to call the emergency line and assist the patients when visiting the doctors.3

Staying hydrated helps prevent a stroke

As a stroke is induced by problems in the vessels, the severity of the condition heavily depends on blood flow around the body. Therefore, it is crucial for us to maintain a stable blood volume and blood pressure by staying hydrated.4,5

Our blood is mainly composed of plasma (55%), white blood cells and platelets (<1%), and red blood cells (45%). Of which, 90% of plasma is made up of water, allowing the blood cells and nutrients to flow.6,7 

As a mixture of solid and liquid, our blood can dissolve substances such as fat and sugar; whereas the insoluble blood cells and platelets are evenly dispersed in plasma.8,9 In both cases, water is proved to be one of the most important components in blood, as it balances the fluidity and prevents the blood from clotting in the circulation

A higher water intake leads to an increased blood volume and a reduced density, meaning that the amount of substances in every unit of blood would decrease. Drinking water also reduces plasma concentration. These factors would all lower the pressure in the blood vessels, and prevent them from being blocked or bursting.10,11,12  As a result, the risk of having a stroke would be lowered when we are well hydrated. 

Dehydration increases the long-term risk of a stroke

Dehydration is a major risk factor of strokes due to its ability to chronically increase plasma sodium concentration, heart rate and blood pressure.

Long-term dehydration means that not enough water would be available for sugar, fat, salts, etc. to dissolve in the plasma. Insoluble particles would also “pile up” in the vessels, and red blood cells would no longer be able to deliver oxygen and other nutrients effectively due to lower fluidity.13 This leads to an increase in blood density. 

Excessive sodium concentration also activates our sympathetic nervous system, which causes the heart rate to increase. This means that more concentrated blood would be pumped with more force in narrowed vessels. As a result, blood pressure would constantly be high due to the “system overload”.14

Furthermore, dehydration and high sodium consumption trigger the release of an antidiuretic hormone called vasopressin, preventing water loss. However, high vasopressin levels cause further contractions and narrowing of the blood vessels, further inducing high blood pressure.15 

Overall, long-term hypertension, which can be caused by chronic dehydration, leads to decreased blood flow and increased arterial damage, both of which are major risk factors in the occurrence of strokes. 

When to contact a doctor

It is necessary to contact a doctor immediately after the patient experiences stroke symptoms, even if they last for a short period of time. “Mini-strokes” can develop into a major stroke within a few months and should not be ignored. 


Both ischaemic and haemorrhagic strokes are severe conditions that can impact patients’ lives. They are strongly associated with the health of our blood, and can be related to many other conditions, including cardiovascular and kidney diseases. Hydration is a crucial factor to prevent strokes, as it maintains the fluidity of blood, with a balanced concentration and volume. When we are well hydrated, healthy blood pressure would be ensured, and the risk of strokes would be greatly reduced.


  1. NHS. Stroke [Internet]. NHS. 2019. Available from:
  2. ‌CDC. About Stroke [Internet]. Centers for Disease Control and Prevention. 2019. Available from:
  3. ‌American Stroke Association. Stroke Symptoms [Internet]. 2018. Available from:
  4. ‌Ragav Sharma, Sandeep Sharma. Physiology, Blood Volume [Internet]. StatPearls Publishing; 2018. Available from:
  5. ‌NHS. What is blood pressure? [Internet]. NHS. 2019. Available from:
  6. American Red Cross. Blood Components [Internet]. 2019. Available from:
  7. Composition of the Blood | SEER Training [Internet]. 2020. Available from:
  8. MedlinePlus. Blood [Internet]. National Library of Medicine; 2019. Available from:
  9. ‌1. Colloids [Internet]. Chemistry LibreTexts. 2019. Available from:
  10. Friedman SM, Mclndoe RA, Tanaka M. The relation of blood sodium concentration to blood pressure in the rat. Journal of Hypertension. 1990 Jan;8(1):61–6.
  11. Kenner T. The measurement of blood density and its meaning. Basic Research in Cardiology. 1989 Mar;84(2):111–24.
  12. Endo Y, Torii R, Yamazaki F, Sagawa S, Yamauchi K, Tsutsui Y, et al. Water drinking causes a biphasic change in blood composition in humans. Pflügers Archiv. 2001 Jun;442(3):362–8.
  13. Hannah. The Importance of Water - The Heart Foundation [Internet]. The Heart Foundation. 2019. Available from:
  14. Jm W. The Role of the Sympathetic Nervous System in Hypertension [Internet]. Current opinion in nephrology and hypertension. 1993. Available from:
  15. Levi DI, Wyrosdic JC, Hicks A-I, Andrade MA, Toney GM, Prager-Khoutorsky M, et al. High dietary salt amplifies osmoresponsiveness in vasopressin-releasing neurons. Cell Reports. 2021 Mar;34(11):108866.

Yongyi Dai

Master of Science – MSc Translational Neuroscience, Imperial College London, United Kingdom

Daisy (Yongyi) is a student, currently undertaking a master’s degree in Translational Neuroscience. She aims to study and research effective gene therapies to treat neurodegenerative diseases, such as Alzheimer’s disease.

She has completed individual research projects, including “How does age affect our cooperation?” and “Composing a piece of music to aid children with autism.” She led the Sing-Along Surrey project at Royal Holloway University of London between 2020 and 2021 to connect students with residents in local care homes; and she fundraised for charities including Dementia UK and Children’s Hospice South West. presents all health information in line with our terms and conditions. It is essential to understand that the medical information available on our platform is not intended to substitute the relationship between a patient and their physician or doctor, as well as any medical guidance they offer. Always consult with a healthcare professional before making any decisions based on the information found on our website.
Klarity is a citizen-centric health data management platform that enables citizens to securely access, control and share their own health data. Klarity Health Library aims to provide clear and evidence-based health and wellness related informative articles. 
Klarity / Managed Self Ltd
Alum House
5 Alum Chine Road
Westbourne Bournemouth BH4 8DT
VAT Number: 362 5758 74
Company Number: 10696687

Phone Number:

 +44 20 3239 9818