سدیم
سدیم فراوانترین الکترولیت موجود در مایع خارج سلولی (ECF) می باشد ، دامنه ی غلظت آن بین 135 تا 145 میلی اکی والان / لیتر است. در نتیجه ، سدیم اولین عامل تعیین کننده اسمولالیته ECF به شمار می آید. به موازات کاهش میزان سدیم ، اسمولالیته نیز دچار تغییر خواهد شد. حقیقت آن است که سدیم نمی تواند به راحتی از غشای سلول عبور نماید ، همین مسئله به علاوه فراوانی یا غلظت بالای آن ، دلیلی است که می تواند نقش اصلی سدیم را در کنترل توزیع آب در سراسر بدن توجیه نماید .
میزان سدیم ، توسط ADH ، تشنگی و سیستم رنین - آنژیوتانسین - آلدوسترون تنظیم می شود. همچنین ، سدیم تنظیم کننده ی اصلی حجم ECF به شمار می آید . جذب یا دفع سدیم در برقراری وضعیت های الکتروشیمیایی ضروری جهت انقباضات عضلانی و انتقال جریانات عصبی نیز، فعال است.
عدم تعادل سدیم اغلب حین انجام درمان های بالینی و در اثر عللی ساده یا پیچیده به وجود می آید و کمبود و فزونی سدیم از جمله شایع ترین اختلالات مربوط به عدم تعادل سدیم می باشند.
فزونی سدیم ( هایپرناترمی - Hypernatremia )
کمبود سدیم (هایپوناترمی - Hyponatremia )
A look at sodium
Sodium is one of the most important elements in the body. It accounts for 90% of extracellular fluid cations (positively charged ions) and is the most abundant solute in extracellular fluid. Almost all sodium in the body is found in this fluid.
Why it's important
The body needs sodium to maintain proper extracellular fluid osmolality (concentration). Sodium attracts fluid and helps preserve the extracellular fluid volume and fluid distribution in the body. It also helps transmit impulses in nerve and muscle fibers and combines with chloride and bicarbonate to regulate acid-base balance. Because the electrolyte compositions of serum and interstitial fluid are essentially equal, sodium concentration in extracellular fluid is measured in serum levels. The normal range for serum sodium level is 135 to 145 mEq/L. As a comparison, the amount of sodium inside a cell is 10 mEq/L.
How the body regulates sodium
What a person eats and how the intestines absorb it determine a body's sodium level. Sodium requirements vary according to the individual's size and age. The minimum daily requirement is 0.5 to 2.7 g; however, the salty American diet provides at least 6 g/day. Even so, sodium levels stay fairly constant because the more sodium a person takes in, the more sodium the kidneys excrete.
Dietary sources of sodium
Major dietary sources of sodium include:
• canned soups and vegetables
• cheese
• ketchup
• processed meats
• table salt
• salty snack foods
• seafood.
Sodium is also excreted through the GI tract and through the skin in sweat. When you think sodium, you should automatically think water as well the two are that closely related in the body. The normal range of serum sodium levels reflects this close relationship. If sodium intake suddenly increases, extracellular fluid concentration also rises, and vice versa.
Not too much!
The body makes adjustments when the sodium level rises. Increased serum sodium levels cause the individual to feel thirsty and the posterior pituitary gland to release antidiuretic hormone (ADH). (For more information about ADH, see chapter 1, Balancing fluids.) ADH causes the kidneys to retain water, which dilutes the blood and normalizes serum osmolality.
When sodium levels decrease and serum osmolality decreases, thirst and ADH secretion are suppressed, and the kidneys excrete more water to restore normal osmolality. (See Regulating sodium and water.)
Aldosterone also regulates extracellular sodium balance via a feedback loop. The adrenal cortex secretes aldosterone, which stimulates the renal tubules to conserve water and sodium when the body's sodium level is low, thus helping to normalize extracellular fluid sodium levels.
The power of the pump
Normally, extracellular sodium levels are very high compared with intracellular sodium levels. The body contains an active transport mechanism, called the sodium-potassium pump, that helps maintain normal sodium levels. This is how the pump works:
In diffusion (a form of passive transport), a substance moves from an area of higher concentration to one of lower concentration. Sodium ions, normally most abundant outside the cells, tend to diffuse inward, and potassium (k) ions, normally most abundant inside the cells, tend to diffuse outward. To combat this ionic diffusion and maintain normal sodium and potassium levels, the sodium-potassium pump is constantly at work in every cell.
However, moving sodium out of the cell and potassium back in can't happen without some help. Each ion links with a carrier because it can't get through the cell wall alone. This movement requires energy (a form of active transport), which comes from adenosine triphosphate (ATP) made up of phosphorus, another electrolyte magnesium, and an enzyme. These substances help move sodium out of the cell and force potassium back into the cell.
Regulating sodium and water
This flowchart shows two of the body's compensatory mechanisms for restoring sodium and water balance.
The sodium-potassium pump allows the body to carry out its essential functions and helps prevent cellular swelling caused by too many ions inside the cell attracting excessive amounts of water. The pump also creates an electrical charge in the cell from the movement of ions, permitting transmission of neuromuscular impulses.
Sodium-potassium pump
This illustration shows how the sodium-potassium pump carries ions when their concentrations change.
- Normal placement
Normally, more sodium (Na) ions exist outside cells than inside. More potassium (K) ions exist inside cells than outside.
- Increased permeability
Certain stimuli increase the membrane's permeability. When this occurs, sodium ions diffuse inward; potassium ions diffuse outward.
- Energy source
The cell links each ion with a carrier molecule that helps the ion return through the cell wall. Energy for the ion's return trip comes from adenosine triphosphate (ATP), magnesium (Mg), and an enzyme commonly found in cells.
فزونی سدیم ( هایپرناترمی - Hypernatremia )
منبع : برونر و سودارث ( ویرایش یازدهم 2008 ) درد ، الکترولیت ، شوک ، سرطان ، مراقبت پایان عمر
مترجمان : دکتر ژیلا عابد سعیدی و نیره ابراهیمی ، دکتر زهره پارسا یکتا ، منصوره فراهانی ، زهرا تذکری
