Blood Glucose Regulation

Spec mapping: AQA 7402 Section 3.6.4 — the control of blood glucose concentration, illustrated through the antagonistic actions of insulin and glucagon and applied to diabetes mellitus (refer to the official AQA specification document for exact wording).

Blood glucose regulation is the second great case study (alongside thermoregulation and osmoregulation) of homeostatic control by negative feedback with antagonistic hormones. It is a topic that rewards integrated thinking: glucose is the universal cellular fuel; the brain is glucose-dependent and almost glucose-exclusive (it cannot use fatty acids except for ketones during prolonged fasting); plasma glucose must be defended against both hypoglycaemia (cognitive failure, coma, death within minutes if untreated) and chronic hyperglycaemia (vascular and neural damage over years). The pancreas senses plasma glucose, the liver acts as the principal effector tissue, and three hormones — insulin, glucagon and adrenaline — coordinate uptake, storage and mobilisation. The discovery of insulin by Banting and Best in the early 1920s (paraphrasing their classical pancreatic-extract experiments — no verbatim quotation) transformed type-1 diabetes from a uniformly fatal disease into a chronic but manageable condition. This lesson develops the cellular and molecular machinery of glucose sensing and the signalling cascades of insulin and glucagon, then turns to the pathology and management of type-1 and type-2 diabetes.

Key Definition: Blood glucose regulation is the homeostatic maintenance of blood glucose within narrow defended limits (~4–6 mmol dm⁻³ fasting; ~5–8 mmol dm⁻³ post-prandial), achieved through the antagonistic actions of insulin and glucagon supplemented by adrenaline and cortisol.

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Why Blood Glucose Must Be Regulated

• The brain depends on glucose. Adult brain consumes ~120 g glucose day⁻¹ — roughly 60% of basal glucose demand. Brain neurones lack stores of glycogen (a few minutes' worth at most) and cannot synthesise glucose; they depend on a steady plasma supply.
• Cellular respiration substrate. Glycolysis requires glucose; the universal energetic currency of cellular metabolism is generated from glucose oxidation (Course 5, lesson 1).
• Chronic hyperglycaemia is toxic. Glucose at sustained high concentrations glycates proteins non-enzymatically (formation of advanced glycation end-products), damaging blood-vessel walls, basement membranes, and nerves — the long-term complications of poorly-controlled diabetes (retinopathy, nephropathy, neuropathy, accelerated atherosclerosis).
• Acute hypoglycaemia is dangerous. Below ~3 mmol dm⁻³, cognitive function deteriorates rapidly (confusion, slurred speech); below ~2 mmol dm⁻³, seizures and unconsciousness; prolonged severe hypoglycaemia causes permanent neuronal injury.
• Osmotic constraint. Plasma glucose exceeding the renal threshold (~10 mmol dm⁻³) spills into urine (glucosuria — synoptic with lesson 1 on PCT transport maxima), pulling water with it (osmotic diuresis), producing polyuria, dehydration and electrolyte loss — the classical presentation of untreated diabetes.

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The Pancreas: Exocrine and Endocrine in One Organ

The pancreas straddles two functional roles:

• Exocrine pancreas (~98% of mass) — acinar cells secrete pancreatic juice (amylase, lipase, trypsinogen, chymotrypsinogen, sodium bicarbonate) into ducts that drain to the duodenum. Not the focus of this lesson.
• Endocrine pancreas (~2% of mass) — clusters of cells called the islets of Langerhans, secreting hormones directly into the blood.

Islets of Langerhans

Each islet contains several specialised cell populations:

Cell type	Hormone	Trigger	Effect on plasma glucose
β (beta) cells	Insulin	Rising plasma glucose	Lowers it
α (alpha) cells	Glucagon	Falling plasma glucose	Raises it
δ (delta) cells	Somatostatin	Multiple	Suppresses both insulin and glucagon (paracrine)
PP cells	Pancreatic polypeptide	Feeding	Modulates GI function

β cells comprise ~60–80% of islet cells and lie predominantly in the islet core. α cells comprise ~15–20% and lie at the periphery. The architectural arrangement allows paracrine communication: insulin and glucagon influence one another through the islet's local blood flow.

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How β cells sense glucose

Glucose sensing by β cells is one of the most elegant examples of metabolic coupling — the conversion of a substrate concentration directly into an electrical and then a secretory event.

flowchart TB
  G1[Plasma glucose rises] --> G2[Glucose enters β cell<br/>via GLUT2]
  G2 --> G3[Glucose phosphorylated<br/>by glucokinase]
  G3 --> G4[Glycolysis + Krebs<br/>+ oxidative phosphorylation]
  G4 --> G5[ATP:ADP ratio rises]
  G5 --> G6[ATP-sensitive K⁺ channels CLOSE]
  G6 --> G7[Membrane depolarises]
  G7 --> G8[Voltage-gated Ca²⁺ channels OPEN]
  G8 --> G9[Ca²⁺ influx]
  G9 --> G10[Insulin vesicle exocytosis]

1. Glucose enters the β cell through the GLUT2 transporter (facilitated diffusion — insulin-independent). GLUT2 has a high K_m, meaning β cell glucose uptake is proportional to plasma glucose across the physiological range — making the β cell an excellent glucose sensor.
2. Glucokinase phosphorylates glucose to glucose-6-phosphate, committing it to metabolism. Glucokinase has a high K_m, so its flux is also concentration-dependent. The combination of GLUT2 + glucokinase is the molecular glucose sensor.
3. Glycolysis, the link reaction, the Krebs cycle and oxidative phosphorylation (all developed in Course 5) generate ATP. The intracellular ATP:ADP ratio rises in proportion to glucose flux.
4. ATP-sensitive K⁺ channels (K_ATP channels) close at high ATP:ADP. Closed K⁺ channels reduce K⁺ efflux, depolarising the cell membrane.
5. Depolarisation opens voltage-gated Ca²⁺ channels; Ca²⁺ flows into the cell down its electrochemical gradient.
6. Intracellular [Ca²⁺] rises; Ca²⁺ triggers fusion of insulin-containing secretory vesicles with the plasma membrane and exocytosis of insulin.

Sulfonylureas — the oldest oral hypoglycaemic drug class for type-2 diabetes — bind to and close the K_ATP channel directly, bypassing the glucose-sensing step and driving insulin release. Knowing the mechanism makes the drug target obvious; this kind of integrated thinking is the hallmark of A* answers.

Exam Tip: The glucose-sensing sequence (GLUT2 → glucokinase → ATP:ADP → K_ATP close → depolarisation → Ca²⁺ in → exocytosis) is examined as a 6-mark question repeatedly. Learn it as a flowchart and write it with explicit cause-and-effect links.

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Insulin: actions on target tissues

Insulin is a peptide hormone (51 amino acids in two chains linked by disulfide bridges). It binds insulin receptors (tyrosine-kinase-class receptors — distinct from the G-protein-coupled receptors that mediate glucagon action) on target cells, initiating phosphorylation cascades that produce diverse effects.

Liver (hepatocytes)

• Glycogenesis: insulin activates glycogen synthase (by triggering dephosphorylation), converting glucose-6-phosphate into glycogen — the insoluble storage polymer of glucose. The hepatic glycogen store reaches ~100 g in a well-fed adult.
• Increased glycolysis: insulin upregulates glucokinase, increasing glucose flux into the cell and downstream oxidation.
• Lipogenesis: excess glucose carbon is diverted to fatty-acid synthesis and packaged as triglyceride (long-term energy storage).
• Suppressed gluconeogenesis: insulin downregulates PEP-carboxykinase and glucose-6-phosphatase, inhibiting conversion of amino acids and glycerol into glucose.

Skeletal muscle and adipose tissue

• Insulin binding triggers translocation of GLUT4 transporters from intracellular vesicles to the plasma membrane (exocytosis-driven insertion).
• Apical [GLUT4] rises, glucose uptake by facilitated diffusion accelerates.
• Muscle uses glucose for immediate ATP or stores it as glycogen (~400 g total muscle glycogen).
• Adipose tissue converts glucose to glycerol and incorporates it into stored triglyceride.

Note: liver glucose uptake is GLUT2-mediated and is not insulin-dependent; muscle and adipose uptake is GLUT4-mediated and is insulin-dependent. The brain uses GLUT1 and GLUT3 — also insulin-independent. This means that even in untreated type-1 diabetes, the brain continues to take up glucose — but it cannot make use of the dangerous hyperglycaemia, and is exposed to glycation damage.

Net effect

Plasma glucose falls back towards the set point of ~5 mmol dm⁻³.

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Glucagon: actions on target tissues

Glucagon is a peptide hormone (29 amino acids, single chain) acting primarily on the liver via a G-protein-coupled receptor and the cAMP–PKA second-messenger cascade — developed in detail in lesson 4 of this course.

Mechanism

1. Glucagon binds the glucagon receptor (a GPCR) on the hepatocyte plasma membrane.
2. The bound receptor activates the G_s α-subunit, which dissociates with GTP.
3. G_sα activates adenylyl cyclase, which converts ATP to cyclic AMP (cAMP) — the second messenger.
4. cAMP allosterically activates protein kinase A (PKA).
5. PKA phosphorylates target enzymes — activating some (glycogen phosphorylase, hormone-sensitive lipase) and inactivating others (glycogen synthase, pyruvate kinase).

Effects on the liver

• Glycogenolysis: glucagon (via PKA) activates glycogen phosphorylase, which sequentially cleaves glucose units from glycogen. Free glucose-6-phosphate is dephosphorylated by glucose-6-phosphatase and exported via GLUT2 into the blood.
• Gluconeogenesis: glucagon upregulates the gluconeogenic enzymes, allowing conversion of amino acids (from protein), glycerol (from triglyceride hydrolysis), and lactate (from anaerobic muscle metabolism) into new glucose.
• Inhibition of glycogen synthesis: glycogen synthase is phosphorylated and inactivated.
• Lipolysis (in adipose tissue): hormone-sensitive lipase mobilises fatty acids; the liver oxidises some to acetyl-CoA and, under sustained glucagon dominance, produces ketone bodies that can supply the brain during prolonged fasting.

Net effect

Plasma glucose rises back towards the set point.

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Negative Feedback in Plasma Glucose Control

flowchart LR
  H[Plasma glucose HIGH] --> B[β cells secrete insulin]
  B --> U[Glucose uptake by muscle / adipose<br/>+ glycogenesis in liver]
  U --> N1[Plasma glucose falls towards set point]
  L[Plasma glucose LOW] --> A[α cells secrete glucagon]
  A --> R[Glycogenolysis + gluconeogenesis in liver]
  R --> N2[Plasma glucose rises towards set point]

The two hormones operate as an antagonistic pair. Both are continuously secreted at low basal rates; the ratio of insulin to glucagon, rather than absolute levels, sets net liver behaviour. Postprandially, the I/G ratio rises and the liver stores glucose; in fasting, the ratio falls and the liver releases glucose. This is a fundamental architectural feature shared by many homeostatic systems — paired antagonistic hormones whose ratio sets effector behaviour.

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Adrenaline and Cortisol — Stress-Driven Glucose Mobilisation

Two further hormones contribute under stress:

• Adrenaline — secreted by the adrenal medulla under sympathetic activation. Acts via the same cAMP/PKA cascade as glucagon, mobilising hepatic glycogen and skeletal-muscle glycogen for rapid glucose delivery during the fight-or-flight response. Adrenaline also stimulates lipolysis and inhibits insulin secretion. It is the third counter-regulatory hormone alongside glucagon.
• Cortisol — secreted by the adrenal cortex; rises during sustained stress. Promotes gluconeogenesis, protein catabolism, and insulin resistance. Chronic cortisol excess (Cushing's syndrome) produces hyperglycaemia and steroid-induced diabetes.

The system's redundancy reflects evolutionary priority: hypoglycaemia is dangerous and is defended by multiple parallel hormonal pathways; hyperglycaemia is opposed by insulin alone. This is why insulin failure produces uncompensated hyperglycaemia (diabetes mellitus) but partial glucagon failure is well-tolerated.

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Diabetes Mellitus

Diabetes mellitus is the condition of chronic hyperglycaemia. Two principal forms differ in pathology and management.

Type 1 Diabetes

• Pathology: autoimmune destruction of pancreatic β cells, mediated by autoreactive T cells and circulating autoantibodies (anti-GAD, anti-IA-2). The β-cell mass declines progressively until insulin secretion fails entirely.
• Onset: usually in childhood, adolescence or young adulthood. Once called "juvenile-onset"; adult-onset forms (LADA) occur but are less common.
• Genetic background: certain HLA class II alleles (HLA-DR3, HLA-DR4) confer susceptibility; environmental triggers (possibly viral, e.g., Coxsackie B; possibly dietary) are thought to initiate autoimmunity in genetically susceptible individuals.
• Clinical features: rapid onset of polyuria, polydipsia, weight loss, fatigue. Ketoacidosis is the acute life-threatening complication if insulin therapy is delayed — uncontrolled lipolysis and ketone-body production produce metabolic acidosis.
• Management: exogenous insulin replacement, by subcutaneous injection or continuous insulin pump. Doses are titrated to carbohydrate intake and to monitored plasma glucose; modern systems combine continuous glucose monitoring with insulin pumps (closed-loop "artificial pancreas" devices).
• Historical context: before insulin (Banting and Best, ~1921 — paraphrasing their pancreatic-extract work), type-1 diabetes was uniformly fatal within months to years. The development of recombinant human insulin transformed prognosis.

Type 2 Diabetes

• Pathology: target tissues (muscle, adipose, liver) become insulin-resistant — insulin signalling is impaired, GLUT4 translocation is reduced, and hepatic glucose output continues despite raised insulin. β cells initially compensate with hyperinsulinaemia; over time, β-cell function declines and insulin secretion falls.
• Onset: typically in middle age and beyond; increasingly common in younger adults and adolescents reflecting rising obesity rates.
• Risk factors: obesity (especially central/visceral), physical inactivity, high-sugar/high-refined-carbohydrate diet, family history (~30–70% heritability), age, ethnicity (higher incidence in South Asian and African-Caribbean populations), gestational diabetes history. Lifestyle factors are modifiable; genetic background is not.
• Clinical features: insidious onset; many cases diagnosed incidentally on screening. Polyuria, polydipsia, fatigue, recurrent infections (candidiasis), visual disturbance possible. Chronic complications (retinopathy, nephropathy, neuropathy, accelerated atherosclerosis) dominate the morbidity.
• Management: first-line lifestyle modification — weight loss, dietary change (reduced refined carbohydrate), increased physical activity. Pharmacological treatment escalates through metformin (improves hepatic insulin sensitivity), sulfonylureas (close K_ATP channels), GLP-1 receptor agonists (enhance insulin secretion), SGLT2 inhibitors (block glucose reabsorption in the PCT — synoptic with lesson 1), and ultimately exogenous insulin if endogenous secretion fails.

Comparison

Feature	Type 1	Type 2
Mechanism	Autoimmune β-cell destruction	Insulin resistance ± reduced secretion
Onset	Childhood / adolescence usually	Adulthood (and increasingly younger)
Insulin production	Minimal or absent	Normal or raised initially; declining over time
Body habitus	Often normal weight or thin	Usually overweight or obese
Ketoacidosis risk	High (acute)	Low (usually only under severe stress)
Treatment	Insulin (essential, lifelong)	Lifestyle; oral agents; eventually insulin
Modifiable by diet/exercise?	No	Yes (substantially)
Prevalence	~5–10% of diabetes cases	~90–95% of diabetes cases

Diagnosis

• Fasting plasma glucose ≥ 7.0 mmol dm⁻³, or
• Random / 2-hour OGTT plasma glucose ≥ 11.1 mmol dm⁻³, or
• HbA1c ≥ 48 mmol mol⁻¹ (a measure of glycated haemoglobin reflecting average glucose over 2–3 months).

Glucosuria detected by urinalysis dipstick — once a primary diagnostic test — is now used as a screening rather than a diagnostic step, because it depends on exceeding the renal threshold (~10 mmol dm⁻³).

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Common Errors and Mark-Loss Patterns

Many candidates lose marks on this topic by: