The primary function of blood is to maintain a constant environment for the other living tissues of the body. Blood transports nutrients, gases, and wastes to and from the cells of the body. Nutrients from food, digested in the stomach and small intestine, pass into the bloodstream through the lining cells of the small intestine. Blood then carries these nutrients to all body cells. Oxygen enters the body through the air sacs of the lungs. Red blood cells then transport the oxygen to cells throughout the body. Blood also helps remove the waste products released by cells. It carries gaseous waste (such as carbon dioxide) to the lungs to be exhaled. It carries chemical waste, such as urea, to the kidneys to be excreted in the urine.
血液的主要功能是為身體的其他活性組織維持恆定的環境。血液負責進出身體細胞的營養物質、氣體和廢物的運輸。來自食物的營養物質在胃和小腸消化後，通過小腸腸壁細胞進入血流。然後，血液攜帶這些營養物質進入全身各處的細胞。氧氣通過肺泡進入身體。然後，紅細胞將氧氣運輸至全身各處的細胞。血液還幫助清除細胞產生的廢物。血液將氣體廢物（例如二氧化碳）運輸至肺部，呼出體外；將化學廢物（例如尿素）運輸至腎臟，隨尿液排出。
Blood transports chemical messengers called hormones from their sites of secretion in glands, such as the thyroid or pituitary, to distant sites where they regulate growth, reproduction, and energy production.
血液將稱為激素的化學信使從其腺體分泌部位（例如甲狀腺或垂體）運輸至遠端部位，使激素在遠端部位發揮調節生長、生殖和能量生產的作用。
Finally, blood contains proteins, white blood cells and antibodies that fight infection, and platelets (thrombocytes) and other proteins that help the blood to clot.
最後，血液中含有蛋白質、具有抗感染能力的白細胞和抗體，以及有助於凝血的血小板（凝血細胞）和其他蛋白質。
Blood is composed of cells, or formed elements, suspended in a clear, straw-colored liquid called plasma. The cells normally constitute 45% of the blood volume and include erythrocytes (red blood cells), leukocytes (white blood cells), and platelets or thrombocytes (clotting cells). The remaining 55% of blood is plasma, a solution of water, proteins, sugar, salts, hormones, lipids, and vitamins.
血細胞或有形成分是血液的組成成分，懸浮於清澈的淡黃色的液體成分——血漿中。正常情況下，血細胞佔血液總量的45%，包括紅細胞、白細胞和血小板（凝血細胞）。剩餘55%的血液為血漿，血漿是由水、蛋白質、糖、鹽、激素、脂質和維生素構成的溶液。
重難點單詞及短語彙總：
nutrient 營養物質，營養素，營養物
pituitary 垂體
urea 尿素
plasma 血漿
erythrocyte 紅細胞
leukocyte 白細胞
platelet 血小板
thrombocyte 凝血細胞，血小板，血栓細胞（臺灣名）
solution 溶液
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