Potassium helps to decrease blood pressure by flushing excess sodium from the body via the kidneys. This minor miracle is performed by millions of special sodium-potassium pumps found in every cell membrane. These pumps actively push potassium into cells and force sodium out. This process is so efficient that the pumps are able to force three sodium ions out of a cell for every two potassium ions it transports in. As a result, the potassium concentration inside cells is around 30 times greater than in the surrounding, extracellular fluid.
The reason cells do this is to generate a small, negative, electrical charge across the cell membrane which is essential for life.
The electrical changes caused by the flow of potassium in and out of cells allows muscles to contract, your heart to beat and your nerves to conduct messages. These effects all have a profound impact on your blood pressure and circulation, but the main blood pressure lowering effect of potassium occurs within the kidneys.
How potassium lowers blood pressure
Your kidneys filter excess fluid, water-soluble toxins and salts such as sodium and potassium from the blood to produce urine. The sodium-potassium pumps in the membrane of kidney cells capture any passing potassium ions within the filtered fluid and pulls them inside cells in exchange for sodium; for every two potassium ions drawn into a cell, three sodium ions are pushed out. As the filtered fluid passes through the long, convoluted tubules within the kidneys, as much available potassium as possible is reclaimed in this way, and more and more sodium is flushed into the waste to form urine.
The rising sodium concentration within the urine produces a powerful osmotic effect. This attracts water and retains it within the filtered fluid rather than allowing the kidneys to reclaim it. This reduces fluid retention and helps to reduce hypertension and maintain a normal blood pressure.
Your kidneys need potassium
As many as one in two people have inherited kidneys that are less efficient at processing high amounts of sodium. Our ancestors followed a relatively low sodium diet, so this didn’t matter. Now the modern diet is so high in sodium, levels within the body quickly build up.
Excess dietary sodium is associated with fluid retention – quite simply more water must be re-absorbed by the kidneys to prevent body fluids from becoming too concentrated and damaging delicate cell membranes.
Retained fluid increases pressure within the circulation and leads to high blood pressure in many cases.
The balance between blood sodium levels and fluid retention is closely monitored by special salt receptors that regulate the secretion of hormones (eg aldosterone, vasopressin) that tell the kidneys how much water and electrolytes to reabsorb or excrete.
Following a low-sodium diet such as the DASH diet is one of the most effective ways to lower a raised blood pressure as it allows the kidneys to better cope with flushing away excess sodium and water. If you also increase your dietary intake of potassium, you will improve sodium flushing even further, to significantly improve your blood pressure readings and lower your risk of experiencing a stroke. In one study, over 80 per cent of people taking antihypertensive medication were able to halve their drug doses (under medical supervision) just by increasing their dietary intake of potassium.
Foods that are good sources of potassium include seafood, fruit (particularly tomatoes and bananas), vegetables, whole grains and potassium-enriched, low-sodium salts.
Low levels of potassium can develop in people taking certain diuretics (often referred to as water tablets) that flush water from the body without having a potassium-sparing action.
Symptoms that may be due to lack of potassium include poor appetite, fatigue, weakness, low blood glucose, muscle cramps, irregular or rapid heartbeat, constipation, irritability, pins and needles, drowsiness, confusion and poor co-ordination leading to falls. This is a relatively common problem in older people taking diuretics
High potassium levels are rare, as the body usually regulates blood levels well. They can occur in people with kidney problems, however, or in those who take excess potassium supplements. High potassium levels can produce symptoms such as irregular heartbeat, and muscle fatigue.
If you have high blood pressure the best way to increase your intake of potassium is to eat more fruit and vegetables, drink unsweetened juice (eg tomato) and to eat more fish. If you recognise that you may have a low potassium intake, supplements are available, but seek advice before taking them if you are on any prescribed medication.
The upper safe level for long-term use from supplements is suggested as 3700mg. Most supplements contain much lower levels than this, and it is relatively safe to take at the typical supplemental dose of 350mg.
Do not take potassium supplements if you are on a type of medication called an ACE inhibitor, or if you have kidney disease, except under medical advice and supervision.
If your blood pressure is borderline or raised, self-monitoring is key to maintaining good control.
Click here for advice on choosing a blood pressure monitor to use at home.
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