How Does Sodium Affect Blood Pressure?
Duyet Le, and Si Lai
CU Boulder, Fall 2006
We tested the
effects that sodium intake, specifically salt, would have on Blood Pressure
(BP). The volume of extracellular fluid is directly related to the amount of
accumulation of salt in the body. When there is excess salt in the body the
osmolality of the body fluids increases, and this in turn stimulates the thirst
center, making the person drink extra amounts of water to dilute the
extracellular salt to a normal concentration.
Salt increases the extracellular fluid volume. An increase in extracellular fluid volume will increase blood pressure because the mean circulatory filling pressure that the heart exerts on blood volume, subsequently increases venous return of blood to the heart, which increases cardiac output from the heart to the arteries, resulting in an increase in blood pressure (Guyton 260). We hypothesize that salt will increase the extracellular fluid volume of blood.
To test this hypothesis, we recruited three non-smokers, and non-diabetic male volunteers. First, we took a reading of their initial Blood Pressure (BP) prior to any salt consumption. Then, we had them consume 60 milligrams of salt. We waited fifteen minutes until taking their next reading. Since excess salt intake increases extracellular fluid volume, we predicted the final BP reading to be higher than the initial BP reading.
Extracellular fluid volume did increase BP. Our results indicated that BP had a significant difference between before salt ingestion (mean= 88.5 mmHg), and after salt ingestion (mean= 98.8 mmHg). A t-Test was constructed to determine that p < .05, therefore, is a significant difference between BP at rest, and BP after salt was consumed.
Our results remained consistent with our hypothesis. One potential problem with our experimental design involves the many uncontrolled variables. Blood pressure can also be affected by a psychological variable like anxiety and/or emotional distress. Also, after the subjects consumed the salt, they drank a bottle of water. This added volume would increase the workload of the heart and cause resistance to blood flow. Moreover, arteriole vasoconstriction and resistance to blood flow may have combined with volume overload to create a higher BP (Guyton 261). An expansion of this experiment may include the role of the Renin-Angiotensin System in maintaining a normal blood pressure. This system has an impact on BP because it reduces the rate of secretion of renin to a much lower level, this leads sequentially to decreased renal retention of salt and water in the extracellular fluid, which brings BP back to normal (Guyton 261). We could test to see how much salt concentration would be needed for the Renin-Angiotensin System to secrete renin from the kidneys and return the extracellular fluid back to normal. To test this, we could see how long BP drops back to normal after salt consumption. Our experiment was not listed on the class webpage, but there was a similar experiment that tested if chocolate is an aphrodisiac. They tested chocolate consumption and its impact on blood pressure. Their results were inconsistent with their hypothesis, however, they were correct about milk chocolate increasing BP more than dark chocolate. Milk chocolate contains more sodium than dark chocolate. Since the substance with more sodium increased BP, our results are consistent with theirs because our substance, salt, has a high sodium concentration and also increased BP.
Guyton, Arthur. Textbook of Medical Physiology. Vol: 7. West Washington Square
Philadelphia, PA: W. B. Saunders Co., 1986.