MadSci Network: Anatomy |
Peter! This may seem like a simple question deserving a simple answer. No such luck! As you may know, Na+ (sodium ion) is the most abundant of all ions in extracellular fluid (this is fluid around cells). The kidneys have the prime responsibility for the regulation of all ions, including sodium. The skin does excrete through sweat some sodium, but is a minor amount compared to the kidneys. When the level of sodium gets too high, ADH (antidiuretic hormone) is released from the posterior pituitary gland. It is made in the hypothalamus of the brain & stored in the post. pituitary. ADH decreases the amount of urine produced which in turm increases the amount of blood plasma. The adrenal medulla is then signaled to secrete less aldosterone and this increaces Na+ excretion. The opposite occurs when Na+ is too low. If a person has a diet too high in salt (Na+) the reaction above takes place. Due to the increase in blood volume, cardiac output increases and blood pressure increases. The body has several homeostatic mechanisms to counteract this increase such as vaso dilation, decreasing the heart rate, but these may not be sufficient to lower the BP when a persistant high level of sodium is present. Na+ causes a tremendous increase in the osmotic pressure of the extracellular fluid thus just by this effect, it can change blood volume and blood viscosity dramatically. Many people say that they are OK due to the fact that they don't add salt to their foods. Look at a T.V. dinner, potato chips, soda, etc. for their salt content. They are all very high. So, not only does a low salt diet mean no added salt, it also means to watch the prepared foods as well. I hope this helps. It is very complex so you may want to consult a Human Anatomy and Physiology text for further details. Jim Bridger
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