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基本外科：電解質（醫學英語）
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Dehydration Types: Pathophysiology, Lab Tests and Values
Dehydration occurs when more water and fluids leave the body than enter it. Even low levels of dehydration can cause headaches, lethargy, and constipation. The human body is roughly 75 percent water. Without this water, it cannot survive. Water is found inside cells, within blood vessels, and between cells. A sophisticated water management system keeps our water levels balanced, and our thirst mechanism tells us when we need to increase fluid intake. Although water is constantly lost throughout the day as we breathe, sweat, urinate, and defecate, we can replenish the water in our body by drinking fluids. The body can also move water around to areas where it is needed most if dehydration begins to occur. Most occurrences of dehydration can be easily reversed by increasing fluid intake, but severe cases of dehydration require immediate medical attention.
Types of Dehydration
Dehydration can be categorized into isotonic, hypertonic and hypotonic, depending on how it affects the tonicity of the extracellular fluid. The extracellular fluid includes the blood and the fluid between the cells, interstitial fluid.
Tonicity is the ability of the solution on one side of the cell membrane to attract water from the solution on the other side of the membrane. Normally, the solutions inside and outside the cells have the same ability to attract water, so they are isotonic. The tonicity of the fluid depends on osmotically active solutes. The main osmotically active solute that determines the tonicity of the extracellular fluid is sodium.
What are osmosis and osmolality?
Osmosis is the movement of water through the membrane from a solution with lower tonicity to a solution with higher tonicity. Substances that increase the tonicity of solutions and thus osmosis are called osmotically active substances. Sodium is the main osmotically active substance in the extracellular fluid. In dehydration, the concentration of sodium in the extracellular fluid can change significantly. The measure of the amount of osmotically active substances in the solution is osmolality, which is expressed in milliosmoles of a solute per kilogram of water (mOsm/L). The normal range of the blood plasma osmolality is 285-295 mOsm/kg.
Why is it important to know if dehydration is iso-, hyper- or hypotonic? Because the type of dehydration can suggest its cause and because hypotonic dehydration must be treated with great caution to avoid severe neurological damage.
Isotonic Dehydration
Isotonic dehydration occurs when proportionally the same amount of water and sodium is lost from the body, so the sodium concentration of the extracellular fluid and hence its tonicity do not change. Isotonic dehydration is the most common type of dehydration.
Lab test values in isotonic dehydration
Blood tests: Osmolality: 285-295 mOsm/kg，normal range. Sodium: 130-150 mmol/liter (a slightly wider than normal range, which is 135-145 mmol/L).
Urine tests: 24-hour urine volume: decreased; Urine specific gravity: increased.
Possible causes of isotonic dehydration
Excessive sweating, repeated vomiting, diarrhea, including secretory diarrhea in cholera, severe bleeding.
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參考譯文：
脫水類型：病理生理學、實驗室檢測和數值
脫水發生時，更多的水和液體離開身體，而不是進入身體。即使脫水程度低也會導致頭痛、昏睡和便秘。人體大約75%是水。沒有水，人就無法生存。水存在於細胞內、血管內和細胞間。一個複雜的水管理系統保持我們水的平衡，我們的口渴機制告訴我們何時需要增加液體的攝入。儘管在我們呼吸、出汗、排尿和排便的過程中，一整天都在不斷流失水分，但我們可以通過喝水補充體內的水分。如果開始脫水，身體也可以將水轉移到最需要的地方。大多數脫水的發生可以通過增加液體的攝入而輕易逆轉，但嚴重的脫水情況需要立即就醫。
脫水類型
脫水可分為等滲性、高滲性和低滲性，這取決於它如何影響細胞外液體的張力。細胞外液包括血液和細胞間液、間質液。
張力是細胞膜一側的溶液從膜的另一側的溶液中吸水的能力。通常，細胞內外的溶液具有相同的吸水能力，所以它們是等張的。流體的張力取決於具有滲透活性的溶質。決定細胞外液張力的主要滲透性活性溶質是鈉。
什麼是滲透和滲透壓？
滲透是水通過膜從低張力溶液到高張力溶液的運動。增加溶液張力和滲透作用的物質稱為滲透活性物質。鈉是細胞外液中主要的滲透活性物質。在脫水過程中，細胞外液中鈉的濃度會發生顯著變化。溶液中滲透活性物質量的測量是滲透壓，它以每千克水（mOsm/L）中溶質的毫摩爾表示。血漿滲透壓正常範圍為285-295mOsm/kg。
為什麼說了解脫水是等滲、高滲還是低滲很重要呢？因為脫水的類型可以提示其原因，並且由於低滲性脫水必須非常小心地治療，以避免嚴重的神經損傷。
等滲性脫水
等滲性脫水發生時，按比例等量的水和鈉從體內流失，細胞外液的鈉濃度和其張力沒有改變。等滲性脫水是最常見的脫水類型。
等滲性脫水的實驗室檢測值
血液檢測：滲透壓：285-295 mOsm/kg（正常範圍），鈉：130-150 mmol/L（略寬於正常範圍，135-145 mmol/L）。
尿檢：24小時尿量：減少；尿比重：增加。
等滲性脫水的可能原因：出汗過多，反覆嘔吐，腹瀉，包括霍亂分泌性腹瀉，嚴重出血。
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重點詞彙
dehydration [ˌdihaɪˈdreɪʃən] 脫水
headaches [ˈhɛˌdeɪks] 頭痛
lethargy [ˈleθərdʒi] 昏昏欲睡、無精打採
constipation [ˌkɑːnstɪˈpeɪʃn] 便秘
sophisticated [səˈfɪstɪkeɪtɪd] 複雜的
urinate [ˈjʊrɪneɪt] 小便
defecate [ˈdefəkeɪt] 排便
replenish [rɪˈplenɪʃ] 補充
isotonic [ˌaɪsoʊˈtɑːnɪk] 等滲
hypertonic [ˌhaɪpərˈtɑnɪk] 高滲
hypotonic [ˌhaɪpə'tɒnɪk] 低滲
tonicity [toʊ'nɪsətɪ] 張力
interstitial fluid [ˌɪntərˈstɪʃl] 組織間液
solutes 溶質
sodium [ˈsoʊdiəm] 鈉
osmosis [ɑzˈmoʊsɪs] 滲透作用
osmolality [ˌɒzme'lælətɪ] 滲透壓
milliosmoles 毫摩爾
isotonic dehydration 等滲脫水
proportionally [prə'pɔʃənlɪ ] 按比例地
sweating [ˈswetɪŋ] 出汗
vomiting [ˈvɑːmɪtɪŋ] 嘔吐
diarrhea [ˌdaɪə'riə] 腹瀉
cholera [ˈkɑːlərə] 霍亂
bleeding [ˈbliːdɪŋ] 出血
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