Dehydration and blood pressure: what actually happens inside your body

Many people often think that dehydration simply causes thirst or fatigue, but few people know that this condition can directly affect blood pressure and indirectly be related to cardiovascular risks, including high cholesterol.

Cholesterol is a type of fat (lipid) found in cell membranes and blood, playing an important role in the production of hormones, vitamin D and bile acids that help digest fat. Our bodies produce most of their own cholesterol, the rest comes from food.

High cholesterol occurs when the amount of cholesterol in the blood exceeds the necessary level, especially when LDL (Low-Density Lipoprotein, often called “bad cholesterol”) is high. This condition can cause plaque to form in the artery walls, causing atherosclerosis, narrowing the lumen and increasing the risk of heart attack or stroke.

Chronic dehydration, by increasing blood pressure and damaging blood vessel walls, can contribute to the rapid formation of cholesterol plaques. This explains why staying hydrated not only helps maintain stable blood pressure but also helps prevent long-term cardiovascular complications.

How dehydration can increase blood pressure

When you are dehydrated, the amount of blood in your body decreases, but the blood becomes thicker and more concentrated. This causes the heart to pump harder to push blood throughout the body. At the same time, the body will activate a defense mechanism by releasing a number of hormones, typically vasopressin and angiotensin.

1. Vasopressin – water retention, vascular constriction

Vasopressin, also known as antidiuretic hormone (ADH), is secreted from the pituitary gland after the brain receives a signal that the concentration of salt in the blood is increasing or the blood volume is decreasing. This hormone causes the kidneys to reabsorb more water, helping to retain precious fluids in the body. However, vasopressin also has the effect of constricting blood vessels, increasing peripheral vascular resistance, thereby increasing blood pressure.

2. Angiotensin – salt retention, water retention, increased blood pressure

Angiotensin is part of the renin–angiotensin–aldosterone system (RAAS), a key mechanism that regulates blood pressure and fluid balance. When dehydrated, the kidneys secrete the enzyme renin, which converts angiotensinogen (a protein produced by the liver) into angiotensin I. Then, the enzyme ACE (angiotensin-converting enzyme) will convert angiotensin I into angiotensin II (the most active form). Angiotensin II is a strong vasoconstrictor, and also stimulates the adrenal glands to secrete aldosterone, helping the kidneys retain sodium and water. The combination of water retention and vasoconstriction will cause a significant increase in blood pressure, especially when dehydration occurs for a long time or frequently.

When both vasopressin and angiotensin work together in a state of dehydration, blood pressure can increase significantly above normal levels. These changes are helpful in the short term to maintain blood pressure and protect vital organs. However, if dehydration persists, this mechanism becomes a burden on the cardiovascular system, increasing the risk of high blood pressure and other complications.

Signs of dehydration and the connection to blood pressure

The body sends clear signals when its water reserves are insufficient. You may feel constantly thirsty, have a dry mouth, feel tired or sluggish. Some people also experience dizziness, lightheadedness, muscle cramps, difficulty concentrating or irritability. When dehydration is severe, urine often turns dark yellow or amber, along with the risk of electrolyte imbalance. In severe cases, dehydration can cause urinary tract infections, kidney stones, and even hypovolemic shock, a life-threatening condition.

Hydration status is closely related to blood pressure because about 60% of blood volume is plasma, and nearly 90% of plasma is water. When the amount of water in the body decreases, blood volume also decreases, forcing the body to activate physiological mechanisms to maintain blood pressure, such as vasoconstriction or increased heart rate. If this condition persists, vasoconstriction and persistently high pressure can become a risk factor for vascular damage, leading to chronic hypertension.

Tips to keep your body hydrated

Here are some helpful tips:

• Drink enough water: Everyone has different water needs, depending on their level of activity, the weather and their health status. People who exercise or work outdoors in hot weather will need more water than people who work in cool environments. A simple rule is to drink when you feel thirsty and not stay thirsty for too long.
• Limit caffeinated drinks: Caffeine has a mild diuretic effect, causing the body to excrete water more quickly through urine. If you consume too much coffee, strong tea or energy drinks, you may inadvertently reduce your body’s water reserves. This does not mean you have to eliminate it completely, but you should adjust it appropriately and replenish fluids along the way.
• Replenish water when sweating, sick or in hot weather: When the body loses water through sweat, fever or diarrhea, the amount of fluid lost increases rapidly. In these cases, you need to increase your water intake or supplement electrolyte solutions to avoid imbalance.
• Eat water-rich foods: Besides drinking water, you can take advantage of natural water sources from food. Watermelon, cucumber, spinach, strawberries… not only contain abundant water but also provide vitamins, minerals and antioxidants, helping the body to be healthy overall.