📚 The Control of Blood Glucose | 血糖的控制
Blood glucose concentration must be kept within a narrow range because glucose is the main respiratory substrate for brain cells, red blood cells and many other tissues. This article explains how the hormones insulin and glucagon, released from the pancreatic islets, work together in a negative feedback system to control blood glucose. It also covers the roles of the liver, GLUT4 transporters, adrenaline, and the causes and consequences of diabetes mellitus.
血糖浓度必须维持在较窄范围内，因为葡萄糖是脑细胞、红细胞和许多其他组织的主要呼吸底物。本文解释胰腺胰岛释放的胰岛素和胰高血糖素如何通过负反馈系统共同调节血糖。文章还涵盖肝脏、GLUT4 转运蛋白、肾上腺素的作用，以及糖尿病的病因和后果。
1. Why Blood Glucose Must Be Controlled | 为什么必须控制血糖
Blood glucose concentration is the amount of glucose dissolved in the blood. In a healthy person it is normally maintained at about 5 mmol dm⁻³, which is roughly 90 mg per 100 cm³. Glucose is the main respiratory substrate for brain cells, red blood cells and many other tissues, because these cells rely heavily on glucose to generate ATP by respiration.
血糖浓度是指溶解在血液中的葡萄糖含量。健康人血糖通常维持在约 5 mmol dm⁻³，大约相当于每 100 cm³ 血液 90 mg。葡萄糖是脑细胞、红细胞和许多其他组织的主要呼吸底物，因为这些细胞高度依赖葡萄糖进行呼吸作用产生 ATP。
If blood glucose falls too low, a condition called hypoglycaemia, cells cannot generate enough ATP. The brain is especially sensitive, so hypoglycaemia can cause confusion, loss of consciousness, coma or even death. If blood glucose rises too high, a condition called hyperglycaemia, the water potential of blood decreases. Water then leaves cells by osmosis, causing dehydration and tissue damage.
如果血糖过低，即低血糖，细胞无法产生足够的 ATP。大脑对低血糖尤其敏感，因此低血糖可引起意识模糊、失去知觉、昏迷甚至死亡。如果血糖过高，即高血糖，血液水势下降，水分通过渗透作用离开细胞，导致脱水并损伤组织。
Control is therefore essential because the brain cannot store large amounts of glucose and cannot quickly switch to alternative fuels. Homeostatic regulation keeps blood glucose within a narrow range despite changes in diet, fasting, stress and exercise.
因此，控制血糖至关重要，因为大脑不能储存大量葡萄糖，也不能迅速改用替代燃料。尽管饮食、禁食、压力和运动发生变化，稳态调节仍将血糖维持在较窄范围内。
2. The Pancreas and Islets of Langerhans | 胰腺与胰岛
The pancreas is both an exocrine and an endocrine gland. Its exocrine role is to secrete digestive enzymes and alkaline fluid into the duodenum. Its endocrine role is carried out by clusters of hormone-secreting cells called the islets of Langerhans, which are scattered throughout the pancreas.
胰腺既是外分泌腺，也是内分泌腺。它的外分泌功能是向十二指肠分泌消化酶和碱性液体。它的内分泌功能由散布在胰腺中的激素分泌细胞团——胰岛完成。
The islets contain two main types of cells involved in blood glucose control. Alpha cells (α-cells) secrete the hormone glucagon, and beta cells (β-cells) secrete the hormone insulin. α-cells are usually located around the periphery of the islet, while β-cells are more central. Both cell types monitor blood glucose concentration and respond in opposite directions to restore normal levels.
胰岛含有参与血糖控制的两种主要细胞。α 细胞分泌胰高血糖素，β 细胞分泌胰岛素。α 细胞通常位于胰岛外周，β 细胞更靠近中央。两种细胞都能监测血糖浓度，并通过相反方向的反应恢复正常水平。
3. Insulin and β-Cells | 胰岛素与 β 细胞
When blood glucose concentration rises above the set point, for example after a carbohydrate-rich meal, β-cells detect the increase and release insulin into the bloodstream. Insulin is a peptide hormone made from proinsulin. It lowers blood glucose by increasing the uptake and use of glucose by target cells, especially muscle, liver and adipose tissue.
当血糖浓度升高到调定点以上时，例如摄入富含碳水化合物的食物后，β 细胞检测到升高并释放胰岛素进入血液。胰岛素是由胰岛素原加工而成的肽类激素。它通过增加靶细胞（尤其是肌肉、肝脏和脂肪组织）对葡萄糖的摄取和利用来降低血糖。
Insulin also promotes the conversion of glucose into storage forms. In the liver and muscles, glucose is converted to glycogen by glycogenesis; in adipose tissue, glucose is used to synthesise fat. These processes reduce the amount of free glucose in the blood plasma.
胰岛素还促进葡萄糖转化为储存形式。在肝脏和肌肉中，葡萄糖通过糖原生成转化为糖原；在脂肪组织中，葡萄糖用于合成脂肪。这些过程减少了血浆中游离葡萄糖的量。
4. Glucagon and α-Cells | 胰高血糖素与 α 细胞
When blood glucose concentration falls below the set point, for example during fasting or after exercise, α-cells release glucagon. Glucagon is also a peptide hormone, but it raises blood glucose by stimulating the liver to break down glycogen and synthesise new glucose.
当血糖浓度降至调定点以下时，例如禁食或运动后，α 细胞释放胰高血糖素。胰高血糖素也是一种肽类激素，但它通过刺激肝脏分解糖原和合成新的葡萄糖来升高血糖。
The main target of glucagon is the liver. Glucagon activates glycogenolysis, the breakdown of stored glycogen to glucose, and gluconeogenesis, the synthesis of glucose from non-carbohydrate sources such as amino acids, lactate and glycerol. Both processes increase the release of glucose from hepatocytes into the blood.
胰高血糖素的主要靶器官是肝脏。它激活糖原分解，即储存的糖原分解为葡萄糖，以及糖异生，即从氨基酸、乳酸和甘油等非碳水化合物来源合成葡萄糖。这两个过程都增加了肝细胞向血液释放葡萄糖。
5. Negative Feedback Loop | 负反馈调节环
Insulin and glucagon work together in a negative feedback system. A rise in blood glucose triggers insulin release, which lowers blood glucose. As the level returns to normal, β-cells reduce insulin secretion. A fall in blood glucose triggers glucagon release, which raises blood glucose. As it returns to normal, α-cells reduce glucagon secretion.
胰岛素和胰高血糖素在负反馈系统中协同工作。血糖升高触发胰岛素释放，从而降低血糖；当水平恢复正常时，β 细胞减少胰岛素分泌。血糖下降触发胰高血糖素释放，从而升高血糖；当恢复正常时，α 细胞减少胰高血糖素分泌。
Negative feedback therefore prevents overcorrection. The two hormones act antagonistically, meaning they have opposite effects, and their balance keeps blood glucose within a narrow homeostatic range. This control is continuous and automatic.
因此，负反馈可防止过度纠正。两种激素以拮抗方式作用，即效应相反，它们的平衡将血糖维持在较窄的稳态范围内。这种控制是连续且自动的。
6. Action of Insulin on Target Cells | 胰岛素对靶细胞的作用
Insulin binds to specific receptor proteins on the plasma membrane of target cells such as muscle, liver and adipose tissue. This binding triggers a signalling cascade inside the cell, which causes vesicles containing glucose transporter proteins, especially GLUT4, to fuse with the plasma membrane.
胰岛素与肌肉、肝脏和脂肪组织等靶细胞质膜上的特异性受体蛋白结合。这种结合触发细胞内的信号级联反应，使含有葡萄糖转运蛋白（尤其是 GLUT4）的囊泡与质膜融合。
More GLUT4 in the membrane increases the permeability of the cell to glucose, so more glucose enters by facilitated diffusion. Inside the cell, insulin activates enzymes for glycogenesis and glycolysis. This lowers the intracellular glucose concentration, which helps maintain a concentration gradient for further glucose uptake.
膜上更多的 GLUT4 增加了细胞对葡萄糖的通透性，因此更多葡萄糖通过易化扩散进入细胞。在细胞内，胰岛素激活糖原生成和糖酵解的酶。这降低了细胞内葡萄糖浓度，有助于维持进一步摄取葡萄糖所需的浓度梯度。
For A-level, you do not need to learn every phosphorylation step in the insulin signalling cascade. The key point is the outcome: increased glucose uptake and increased conversion of glucose to glycogen and fat, so blood glucose falls.
在 A-level 阶段，你不需要记住胰岛素信号级联中的每一个磷酸化步骤。关键点是结果：葡萄糖摄取增加，葡萄糖转化为糖原和脂肪增加，因此血糖下降。
7. Action of Glucagon on Hepatocytes | 胰高血糖素对肝细胞的作用
Glucagon binds to receptors on the plasma membrane of hepatocytes. This activates the enzyme adenylate cyclase, which converts ATP to cyclic AMP (cAMP). cAMP acts as a second messenger inside the liver cell.
胰高血糖素与肝细胞质膜上的受体结合。这激活腺苷酸环化酶，将 ATP 转化为环 AMP（cAMP）。cAMP 在肝细胞内作为第二信使发挥作用。
The rise in cAMP activates protein kinase A, which phosphorylates and activates enzymes such as glycogen phosphorylase. This promotes glycogenolysis, in which stored glycogen is broken down to glucose 1-phosphate and then converted to glucose 6-phosphate. In hepatocytes, glucose 6-phosphatase removes the phosphate group to form free glucose, which leaves the liver and enters the blood.
cAMP 升高激活蛋白激酶 A，蛋白激酶 A 磷酸化并激活糖原磷酸化酶等酶。这促进糖原分解，储存的糖原被分解为 1-磷酸葡萄糖，再转化为 6-磷酸葡萄糖。在肝细胞中，6-磷酸葡萄糖酶去除磷酸基团形成游离葡萄糖，葡萄糖离开肝脏进入血液。
Glucagon also stimulates gluconeogenesis by increasing the transcription of enzymes such as phosphoenolpyruvate carboxykinase (PEPCK). This allows the liver to release glucose into the blood even when glycogen stores are low, for example during prolonged fasting.
胰高血糖素还通过增加磷酸烯醇丙酮酸羧激酶（PEPCK）等酶的转录来刺激糖异生。这使得肝脏在糖原储备较低时（例如长时间禁食）也能向血液释放葡萄糖。
8. Role of the Liver in Glucose Homeostasis | 肝脏在葡萄糖稳态中的作用
The liver is the central organ for blood glucose control because it can store, release and manufacture glucose. After a meal, insulin promotes glycogenesis in hepatocytes, storing glucose as glycogen. Between meals or during fasting, glucagon promotes glycogenolysis and gluconeogenesis, releasing glucose into the hepatic vein and then into the general circulation.
肝脏是血糖控制的中枢器官，因为它可以储存、释放和制造葡萄糖。餐后胰岛素促进肝细胞中的糖原生成，将葡萄糖以糖原形式储存。在两餐之间或禁食期间，胰高血糖素促进糖原分解和糖异生，将葡萄糖释放到肝静脉并进入全身循环。
The liver contains the enzyme glucose 6-phosphatase, which allows it to release free glucose into the blood. Muscle cells lack this enzyme, so although they store glycogen, they cannot release free glucose into the circulation. The liver is therefore the main source of blood glucose during fasting.
肝脏含有 6-磷酸葡萄糖酶，因此能够向血液释放游离葡萄糖。肌细胞缺乏这种酶，所以虽然肌细胞储存糖原，但不能将游离葡萄糖释放到循环中。因此，肝脏是禁食期间血糖的主要来源。
Key liver processes can be summarised as:
肝脏的关键过程可概括为：
glycogenesis: glucose → glycogen
glycogenolysis: glycogen → glucose
gluconeogenesis: non-carbohydrates → glucose
The balance between these pathways determines whether the liver stores or releases glucose, depending on the insulin-to-glucagon ratio.
这些途径之间的平衡决定肝脏是储存还是释放葡萄糖，具体取决于胰岛素与胰高血糖素的比值。
9. Glucose Transporters and GLUT4 | 葡萄糖
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