📚 The Control of Blood Glucose | 血糖的调控
Maintaining a stable blood glucose concentration is critical for the normal functioning of the human body, particularly for the brain, which relies almost exclusively on glucose as a respiratory substrate. In A-Level Biology, the regulation of blood glucose is a key example of negative feedback and hormonal coordination, involving the pancreas, liver, and a network of cellular signalling pathways.
维持稳定的血糖浓度对于人体正常功能至关重要，尤其是对几乎完全依赖葡萄糖作为呼吸底物的大脑而言。在A-Level生物学中，血糖的调节是负反馈和激素协调的一个关键实例，涉及胰腺、肝脏以及一系列细胞信号通路。
1. Importance of Blood Glucose Regulation | 血糖调节的重要性
Glucose serves as the primary fuel for cellular respiration, providing the energy needed for ATP synthesis. Some organs, such as the brain and red blood cells, have an almost absolute requirement for glucose because they cannot effectively utilise fatty acids. A typical fasting blood glucose concentration is approximately 90 mg per 100 cm³ of blood (about 5 mmol dm⁻³).
葡萄糖是细胞呼吸的主要燃料，为ATP合成提供所需能量。有些器官，如大脑和红细胞，对葡萄糖几乎是绝对需求，因为它们无法有效利用脂肪酸。空腹血糖浓度的典型值约为每100毫升血液90毫克（约5 mmol dm⁻³）。
If blood glucose falls too low (hypoglycaemia), cellular respiration is compromised, leading to tiredness, confusion and eventually coma. If it rises too high (hyperglycaemia), water is drawn out of cells by osmosis, causing cellular dehydration and damage to tissues over time. Therefore, precise homeostatic control is essential.
如果血糖过低（低血糖），细胞呼吸会受损，导致疲倦、意识混乱甚至昏迷。如果血糖过高（高血糖），水分会因渗透作用从细胞中流失，导致细胞脱水和组织长期损伤。因此，精确的稳态控制是必不可少的。
2. Sources and Sinks of Blood Glucose | 血中葡萄糖的来源与去除
Glucose enters the blood from three main sources: absorption of digested carbohydrates from the small intestine, breakdown of stored glycogen in the liver (glycogenolysis), and synthesis of new glucose from non-carbohydrate precursors, mainly in the liver (gluconeogenesis).
葡萄糖从三个主要来源进入血液：从小肠吸收消化后的碳水化合物、肝脏中储存糖原的分解（糖原分解），以及主要在肝脏中由非碳水化合物前体合成新葡萄糖（糖异生）。
Glucose is removed from the blood when it is taken up by body cells for respiration, when it is converted to glycogen for storage in the liver and muscles (glycogenesis), and when it is converted into fat. In times of excess, the liver also converts glucose into very-low-density lipoproteins for transport to adipose tissue.
葡萄糖在以下情况从血液中被移除：被身体细胞摄取用于呼吸时、在肝脏和肌肉中转化为糖原储存时（糖原生成），以及转化为脂肪时。当葡萄糖过量时，肝脏还将葡萄糖转化为极低密度脂蛋白，运输至脂肪组织。
3. The Pancreas and the Islets of Langerhans | 胰腺与胰岛
The pancreas functions as both an exocrine gland (secreting digestive enzymes) and an endocrine gland. The endocrine tissue is clustered in groups of cells called the islets of Langerhans. Within each islet, α-cells secrete the hormone glucagon, and β-cells secrete the hormone insulin.
胰腺同时具有外分泌腺（分泌消化酶）和内分泌腺的功能。内分泌组织聚集成团，称为胰岛。在每个胰岛中，α细胞分泌激素胰高血糖素，β细胞分泌激素胰岛素。
The α-cells and β-cells act as the receptor and control centre in the homeostatic pathway. They are sensitive to the concentration of glucose in the blood flowing through the pancreas. A rise in blood glucose stimulates β-cells to release insulin, whereas a fall in blood glucose stimulates α-cells to release glucagon.
α细胞和β细胞在稳态通路中充当感受器和控制中心。它们对流过胰腺的血液中葡萄糖浓度敏感。血糖升高刺激β细胞释放胰岛素，而血糖下降刺激α细胞释放胰高血糖素。
4. Insulin: Secretion and Effects | 胰岛素的分泌与作用
Insulin is a peptide hormone composed of two polypeptide chains linked by disulfide bonds. It is synthesised as proinsulin in the β-cells and packaged into secretory vesicles. When blood glucose concentration rises, glucose enters β-cells via GLUT2 transporters, is phosphorylated and metabolised, leading to an increase in the ATP/ADP ratio. This closes ATP-sensitive K⁺ channels, depolarising the membrane and opening voltage-gated Ca²⁺ channels. The influx of Ca²⁺ triggers exocytosis of insulin-containing vesicles.
胰岛素是由两个多肽链通过二硫键连接而成的肽类激素。它在β细胞中以前胰岛素原的形式合成，并被包装成分泌囊泡。当血糖浓度升高时，葡萄糖通过GLUT2转运蛋白进入β细胞，被磷酸化并代谢，导致ATP/ADP比值升高。这关闭了ATP敏感性K⁺通道，使膜去极化并打开电压门控Ca²⁺通道。Ca²⁺内流触发含胰岛素的囊泡胞吐。
Once in the bloodstream, insulin travels to target tissues, primarily the liver, muscle and adipose tissue. Its effects lower blood glucose by increasing the rate of glucose uptake (especially by muscle and adipose cells), stimulating glycogenesis in the liver and muscles, and inhibiting gluconeogenesis. It also promotes the conversion of glucose into fatty acids and stimulates protein synthesis.
胰岛素进入血液后，运送到靶组织，主要是肝脏、肌肉和脂肪组织。其作用是降低血糖，通过增加葡萄糖的摄取速率（尤其是肌肉和脂肪细胞），刺激肝脏和肌肉中的糖原生成，并抑制糖异生。它还促进葡萄糖转化为脂肪酸，并刺激蛋白质合成。
5. Mechanism of Insulin Action | 胰岛素的作用机制
Insulin exerts its effects by binding to a specific tyrosine kinase receptor on the cell surface membrane. Binding induces receptor autophosphorylation, initiating an intracellular phosphorylation cascade that results in the translocation of glucose transporter vesicles (containing GLUT4) to the plasma membrane, thus increasing the number of glucose transporters and enhancing glucose uptake.
胰岛素通过结合细胞表面膜上的特异性酪氨酸激酶受体发挥其作用。结合引起受体的自身磷酸化，启动细胞内磷酸化级联反应，导致含GLUT4的葡萄糖转运囊泡移位至细胞膜，从而增加葡萄糖转运蛋白的数量并增强葡萄糖摄取。
In the liver, insulin activates enzymes such as glycogen synthase (promoting glycogenesis) and inhibits enzymes such as fructose-1,6-bisphosphatase (reducing gluconeogenesis). Overall, the signal transduction pathway ensures a rapid and amplified response to the hormone.
在肝脏中，胰岛素激活糖原合酶等酶（促进糖原生成），并抑制果糖-1,6-二磷酸酶等酶（减少糖异生）。总的来说，这一信号转导通路确保对激素产生快速且放大的反应。
6. Glucagon: Secretion and Effects | 胰高血糖素的分泌与作用
Glucagon is a single-chain polypeptide of 29 amino acids, secreted by α-cells when blood glucose is low. The stimulus for secretion is a decrease in the intracellular ATP/ADP ratio in α-cells, which opens ATP-sensitive K⁺ channels, keeping the membrane hyperpolarised; paradoxically, the α-cell can still secrete glucagon under low glucose conditions through altered channel activity and Ca²⁺ dynamics. Details are not fully required at A-Level, but the key point is that low blood glucose triggers glucagon release.
胰高血糖素是一条由29个氨基酸组成的单链多肽，由α细胞在血糖低时分泌。分泌的刺激是α细胞内ATP/ADP比值下降，打开ATP敏感性K⁺通道，使膜保持超极化；矛盾的是，在低葡萄糖条件下α细胞仍可通过改变的通道活动和Ca²⁺动态分泌胰高血糖素。A-Level不要求详细机制，只需知道低血糖触发胰高血糖素释放。
The target organ for glucagon is mainly the liver (and, to a lesser extent, adipose tissue). Glucagon raises blood glucose by activating glycogenolysis (the breakdown of glycogen to glucose) and promoting gluconeogenesis. It also inhibits glycogenesis and stimulates lipolysis in adipose tissue, providing fatty acids as alternative fuels, thus sparing glucose for the brain.
胰高血糖素的主要靶器官是肝脏（其次为脂肪组织）。胰高血糖素通过激活糖原分解（糖原分解为葡萄糖）和促进糖异生来升高血糖。它还抑制糖原生成，并刺激脂肪组织的脂肪分解，提供脂肪酸作为替代燃料，从而为大脑节省葡萄糖。
7. The Second Messenger Model for Glucagon | 胰高血糖素的第二信使模型
Glucagon cannot enter its target cells because it is a water-soluble peptide hormone. Instead, it binds to a specific receptor on the plasma membrane of liver cells, triggering a cascade mediated by a second messenger, cyclic AMP (cAMP).
胰高血糖素不能进入其靶细胞，因为它是水溶性肽类激素。相反，它与肝细胞质膜上的特异性受体结合，触发了由第二信使环磷酸腺苷（cAMP）介导的级联反应。
The binding of glucagon to its receptor activates a G-protein, which in turn activates the enzyme adenylyl cyclase. This enzyme catalyses the conversion of ATP to cAMP. cAMP then activates protein kinase A, which phosphorylates and activates a cascade of enzymes, ultimately activating glycogen phosphorylase to break down glycogen and inactivating glycogen synthase.
胰高血糖素与其受体结合后激活G蛋白，G蛋白又激活腺苷酸环化酶。该酶催化ATP转化为cAMP。cAMP随后激活蛋白激酶A，蛋白激酶A通过磷酸化激活一系列酶的级联反应，最终激活糖原磷酸化酶以分解糖原，并使糖原合酶失活。
Glucagon → receptor → G-protein → adenylyl cyclase → cAMP → protein kinase A → phosphorylase kinase → glycogen phosphorylase (active)
胰高血糖素 → 受体 → G蛋白 → 腺苷酸环化酶 → cAMP → 蛋白激酶A → 磷酸化酶激酶 → 糖原磷酸化酶（活化）
This cascade amplifies the original signal enormously: one hormone–receptor complex can generate many cAMP molecules, and each protein kinase can activate many enzyme molecules. This explains how very low concentrations of hormone can produce a large cellular effect.
这一级联反应极大地放大了原始信号：一个激素-受体复合物可产生许多cAMP分子，每个蛋白激酶又可以激活许多酶分子。这解释了为何极低浓度的激素能产生巨大的细胞效应。
8. Negative Feedback Control Loop | 负反馈控制环路
The maintenance of blood glucose concentration is a classical negative feedback system. A rise in blood glucose above the set point is detected by the β-cells, which secrete more insulin. Insulin lowers blood glucose by promoting uptake and storage. As blood glucose falls back to normal, the stimulus for insulin secretion decreases, reducing insulin output. Conversely, a fall in blood glucose is detected by α-cells, which secrete glucagon. Glucagon raises blood glucose, restoring the normal level, and its secretion is then reduced.
血糖浓度的维持是一个经典的负反馈系统。血糖升高超过设定点时被β细胞检测到，β细胞分泌更多胰岛素。胰岛素通过促进摄取和储存来降低血糖。当血糖回落到正常水平时，胰岛素分泌的刺激减弱，胰岛素输出减少。相反，血糖下降由α细胞检测，α细胞分泌胰高血糖素。胰高血糖素升高血糖，恢复正常水平，随后其分泌减少。
This antagonistic pair of hormones ensures that blood glucose oscillates gently around the set point. The system also includes the liver as a central effector, responding to the prevailing insulin-to-glucagon ratio to switch between glucose uptake and release.
这对拮抗激素确保了血糖在设定点附近轻微波动。这套系统还包括肝脏这一核心效应器，根据当前的胰岛素与胰高血糖素比值，在葡萄糖摄取与释放之间切换。
9. Glycogenesis, Glycogenolysis and Gluconeogenesis | 糖原生成、糖原分解与糖异生
Glycogenesis is the conversion of glucose into glycogen. It occurs when blood glucose is high, mainly in the liver and muscles, stimulated by insulin. Glucose molecules are added to a growing glycogen chain under the catalysis of glycogen synthase. Glycogen acts as a compact, osmotically inactive storage form of glucose.
糖原生成是葡萄糖转化为糖原的过程。当血糖高时发生，主要在肝脏和肌肉中，由胰岛素刺激。在糖原合酶的催化下，葡萄糖分子添加到增长的糖原链上。糖原是一种紧凑的、渗透压不活跃的葡萄糖储存形式。
Glycogenolysis is the breakdown of glycogen back to glucose-1-phosphate and then to glucose (in the liver) or glucose-6-phosphate (in muscle, which cannot release free glucose into blood). It is stimulated by glucagon and adrenaline and is catalysed by glycogen phosphorylase.
糖原分解是将糖原分解回1-磷酸葡萄糖，然后在肝脏中转化为葡萄糖，或肌肉中转化为6-磷酸葡萄糖（肌肉不能将游离葡萄糖释放入血）。该过程由胰高血糖素和肾上腺素刺激，并由糖原磷酸化酶催化。
Gluconeogenesis is the synthesis of glucose from non-carbohydrate sources such as amino acids, lactate and glycerol. It occurs mainly in the liver during fasting or prolonged exercise, stimulated by glucagon. Key enzymes include pyruvate carboxylase and phosphoenolpyruvate carboxykinase, which bypass the irreversible steps of glycolysis.
糖异生是从非碳水化合物来源如氨基酸、乳酸和甘油合成葡萄糖的过程。主要在禁食或长时间运动时于肝脏中发生，由胰高血糖素刺激。关键酶包括丙酮酸羧化酶和磷酸烯醇丙酮酸羧激酶，它们绕过了糖酵解的不可逆步骤。
10. The Role of Adrenaline | 肾上腺素的作用
Adrenaline (epinephrine) is a hormone and neurotransmitter released from the adrenal medulla during stress or vigorous exercise. It promotes glycogenolysis in the liver and muscle, rapidly raising blood glucose to prepare the body for ‘fight or flight’. Adrenaline also binds to cell surface receptors and uses cAMP as a second messenger, following a cascade very similar to that of glucagon.
肾上腺素是一种由肾上腺髓质在应激或剧烈运动时释放的激素和神经递质。它促进肝脏和肌肉中的糖原分解，快速升高血糖，使身体为“战或逃”做准备。肾上腺素同样结合细胞表面受体，并以cAMP作为第二信使，遵循与胰高血糖素极为相似的级联反应。
In muscle, adrenaline’s activation of glycogenolysis provides glucose-6-phosphate for glycolysis, enabling rapid ATP production. In the liver, the glucose produced is exported to the blood to supply other tissues. This dual effect complements glucagon’s primarily liver-targeted action.
在肌肉中，肾上腺素激活糖原分解为糖酵解提供6-磷酸葡萄糖，使ATP快速生成。在肝脏中，产生的葡萄糖被输出到血液供应其他组织。这一双重效应补充了胰高血糖素主要作用于肝脏的功能。
11. Diabetes Mellitus: Type 1 and Type 2 | 糖尿病：1型与2型
Diabetes mellitus is a metabolic disorder characterised by chronic hyperglycaemia. Type 1 diabetes is an autoimmune condition in which the body’s immune system destroys the β-cells of the islets, leading to little or no insulin production. It typically develops in childhood or adolescence and requires lifelong insulin injections.
糖尿病是一种以慢性高血糖为特征的代谢紊乱疾病。1型糖尿病是一种自身免疫性疾病，身体的免疫系统破坏了胰岛的β细胞，导致胰岛素产生极少或完全没有。通常在儿童期或青少年期发病，需要终身注射胰岛素。
Type 2 diabetes is more common and usually develops later in life, often associated with lifestyle factors such as obesity and physical inactivity. In type 2, the β-cells may still produce insulin, but target tissues become resistant to its action (insulin resistance), or insulin secretion becomes insufficient to compensate. Management includes diet, exercise, oral hypoglycaemic drugs, and sometimes insulin.
2型糖尿病更为常见，通常在晚年发病，常与肥胖和缺乏运动等生活方式因素相关。在2型糖尿病中，β细胞可能仍能产生胰岛素，但靶组织对其作用产生抵抗（胰岛素抵抗），或者胰岛素分泌不足以代偿。管理手段包括饮食、运动、口服降糖药物，有时需要胰岛素。
Common diagnostic tests include the oral glucose tolerance test, measurement of fasting blood glucose and the HbA1c test (which measures glycated haemoglobin to indicate average blood glucose over the previous 2–3 months). Urine tests for glucose and ketones can also provide evidence of poor control.
常见的诊断检测包括口服葡萄糖耐量试验、空腹血糖测量和HbA1c检测（测量糖化血红蛋白，反映过去2-3个月的平均血糖）。尿液葡萄糖和酮体检测也能提供控制不佳的证据。
12. Consequences of Dysregulation | 血糖失调的后果
Prolonged hyperglycaemia damages blood vessels and nerves. In the kidneys, this can lead to nephropathy; in the retina, it can cause diabetic retinopathy and blindness; and in peripheral nerves, it can lead to neuropathy. High blood glucose also reduces the immune response and hinders wound healing.
长期高血糖会损害血管和神经。在肾脏中，可导致肾病；在视网膜中，可导致糖尿病性视网膜病变和失明；在周围神经中，可导致神经病变。高血糖还会降低免疫反应，阻碍伤口愈合。
In untreated type 1 diabetes, cells switch to fat metabolism, producing excessive ketone bodies that acidify the blood (ketoacidosis), which can be fatal. Hypoglycaemia, often caused by too much insulin or prolonged exercise without food intake, starves the brain of glucose, leading to dizziness, seizures and unconsciousness.
在未经治疗的1型糖尿病中，细胞转向脂肪代谢，产生过量酮体，使血液酸化（酮症酸中毒），可能致命。低血糖常由胰岛素过量或长时间运动而无食物摄入引起，使大脑缺乏葡萄糖，导致头晕、抽搐和意识丧失。
The precise regulation of blood glucose is therefore not just an academic concept but a physiological necessity, maintaining the delicate metabolic balance required for survival.
因此，血糖的精确调控不仅是一个学术概念，更是一种生理必需，维持着生存所需微妙的代谢平衡。
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