Hormonal Communication

The endocrine system provides chemical communication via hormones — molecules secreted by endocrine glands into the blood, which travel to target cells bearing specific receptors. Hormonal responses are typically slower to initiate but longer-lasting than nervous responses. This topic covers the endocrine system, blood glucose regulation, and the detailed structure and function of the kidneys.

Key Definition: A hormone is a chemical messenger produced by an endocrine gland, secreted into the blood, and transported to distant target cells that possess specific complementary receptors for that hormone.

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Nervous vs Endocrine Communication

Feature	Nervous System	Endocrine System
Signal type	Electrical impulses (action potentials)	Chemical (hormones in the blood)
Speed	Very fast (milliseconds)	Slower (seconds to hours)
Duration	Short-lived (milliseconds)	Longer-lasting (hours, days, or even weeks)
Target	Specific (individual muscles/glands via neurones)	Widespread (all cells with the specific receptor)
Pathway	Along neurones	Via the bloodstream
Nature of response	Precise, localised	More general, often affecting multiple organs

In practice, the two systems work together. The hypothalamus links the nervous and endocrine systems, controlling the pituitary gland — often called the "master gland" because it produces hormones that regulate other endocrine glands (e.g., TSH stimulates the thyroid, ACTH stimulates the adrenal cortex, FSH and LH regulate the gonads).

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The Adrenal Glands

Each adrenal gland sits on top of a kidney and has two distinct regions with different embryological origins and functions:

• Adrenal medulla — secretes adrenaline (epinephrine) and noradrenaline (norepinephrine); controlled by the sympathetic nervous system; mediates the rapid "fight or flight" response.
• Adrenal cortex — secretes steroid hormones including cortisol (stress response, anti-inflammatory, promotes gluconeogenesis), aldosterone (regulates Na⁺ reabsorption and K⁺ secretion in the kidneys), and small amounts of sex hormones.

Adrenaline and the Fight-or-Flight Response

Key Definition: A second messenger is a small intracellular signalling molecule (such as cAMP) that is produced in response to a hormone (the first messenger) binding to a cell-surface receptor. Second messengers relay and amplify the hormonal signal inside the cell.

Adrenaline is a first messenger that cannot cross the cell membrane (it is a water-soluble amine). It binds to adrenergic receptors on target cell membranes and activates a second messenger system:

1. Adrenaline binds to a specific receptor on the cell surface membrane.
2. This causes a conformational change that activates a G protein on the intracellular side of the membrane.
3. The activated G protein activates the enzyme adenylyl cyclase.
4. Adenylyl cyclase catalyses the conversion of ATP to cyclic AMP (cAMP) — the second messenger.
5. cAMP activates protein kinase A (PKA), which phosphorylates (and thereby activates) a cascade of enzymes.
6. The enzyme cascade leads to the desired cellular effect — e.g., activation of glycogen phosphorylase, which breaks down glycogen to glucose-1-phosphate (glycogenolysis).

This amplification cascade means that a single molecule of adrenaline can result in the production of millions of glucose molecules — a small extracellular signal produces an enormous intracellular response.

flowchart TD
    Adr["Adrenaline
(first messenger)"] -->|"Binds to receptor
on cell surface"| Gp["G protein activated"]
    Gp --> AC["Adenylyl cyclase activated"]
    AC -->|"ATP → cAMP"| cAMP["cAMP
(second messenger)"]
    cAMP --> PKA["Protein kinase A
activated"]
    PKA --> Cascade["Enzyme cascade
(amplification)"]
    Cascade --> Effect["Glycogen phosphorylase activated
→ Glycogenolysis
→ Glucose released"]

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The Pancreas — Both Endocrine and Exocrine

Key Definition: An endocrine gland secretes hormones directly into the blood (ductless). An exocrine gland secretes its products into a duct that leads to a body surface or cavity.

The pancreas is unusual because it functions as both an endocrine and an exocrine gland:

Function	Cells/Structures	Product	Destination
Exocrine	Acinar cells; pancreatic ducts	Pancreatic juice containing digestive enzymes (trypsinogen, lipase, amylase) and sodium hydrogencarbonate	Duodenum (via the pancreatic duct)
Endocrine	Islets of Langerhans (α cells, β cells, δ cells)	Hormones: glucagon (α cells), insulin (β cells), somatostatin (δ cells)	Blood (directly into capillaries within the islets)

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Blood Glucose Regulation

Blood glucose concentration is normally maintained between 4–8 mmol/L (the set point) by two hormones produced by the islets of Langerhans in the pancreas. This is a classic example of negative feedback with antagonistic hormones (insulin and glucagon work in opposition).

After a Meal (Blood Glucose Rises Above the Set Point)

1. β cells of the islets of Langerhans detect the rise in blood glucose concentration (β cells have glucose transporters and glucokinase, which act as glucose sensors).
2. β cells secrete insulin into the blood.
3. Insulin binds to tyrosine kinase receptors on target cells (liver, muscle, adipose tissue), causing:
   • Increased uptake of glucose via GLUT4 transporters (vesicles containing GLUT4 fuse with the cell membrane, increasing the number of glucose channels).
   • Increased conversion of glucose to glycogen (glycogenesis) in liver and muscle cells.
   • Increased conversion of glucose to fat (lipogenesis) in adipose tissue.
   • Increased rate of cellular respiration of glucose.
   • Increased rate of amino acid uptake and protein synthesis.
4. Blood glucose falls back to the set point → insulin secretion decreases (negative feedback).

During Fasting or Exercise (Blood Glucose Falls Below the Set Point)

1. α cells of the islets of Langerhans detect the fall in blood glucose concentration.
2. α cells secrete glucagon into the blood.
3. Glucagon binds to receptors on liver cells (not muscle cells — muscles cannot release glucose into the blood), activating the cAMP second messenger pathway, causing:
   • Glycogenolysis — breakdown of glycogen to glucose (glucose-6-phosphatase in liver cells converts glucose-6-phosphate to glucose, which is released into the blood).
   • Gluconeogenesis — synthesis of glucose from non-carbohydrate sources (amino acids, glycerol, lactate).
4. Blood glucose rises back to the set point → glucagon secretion decreases (negative feedback).

flowchart TD
    High["Blood glucose RISES
(e.g., after a meal)"] --> Beta["β cells detect rise"]
    Beta --> Ins["Insulin secreted"]
    Ins --> InsEff["Glycogenesis in liver & muscle
Glucose uptake via GLUT4
Increased respiration"]
    InsEff --> Normal1["Blood glucose returns
to set point (4–8 mmol/L)"]
    Normal1 -.->|"Negative feedback:
insulin secretion decreases"| Beta

    Low["Blood glucose FALLS
(e.g., during exercise)"] --> Alpha["α cells detect fall"]
    Alpha --> Gluc["Glucagon secreted"]
    Gluc --> GlucEff["Glycogenolysis in liver
Gluconeogenesis"]
    GlucEff --> Normal2["Blood glucose returns
to set point (4–8 mmol/L)"]
    Normal2 -.->|"Negative feedback:
glucagon secretion decreases"| Alpha

Exam Tip: Remember that glucagon acts primarily on the liver, not on muscle. Muscle cells lack glucose-6-phosphatase, so they cannot release glucose into the blood — they use their glycogen stores for their own respiration. This is a commonly tested distinction.

Diabetes Mellitus

Feature	Type 1	Type 2
Cause	Autoimmune destruction of β cells by T lymphocytes	Target cells become resistant to insulin (insulin receptor down-regulation); β cell function may also decline
Onset	Usually childhood/adolescence (but can occur at any age)	Usually later in life; strongly linked to obesity, physical inactivity, and genetic predisposition
Insulin production	Little or none	Insulin is produced (often initially at normal or elevated levels) but cells respond poorly
Treatment	Insulin injections (or insulin pump); careful blood glucose monitoring; matching insulin dose to carbohydrate intake	Lifestyle changes (diet and exercise); oral hypoglycaemic drugs (e.g., metformin); sometimes insulin injections
Prevalence trend	Relatively stable	Rapidly increasing worldwide, particularly in developed countries

Worked Example 1 — Blood Glucose Regulation Scenario

A patient eats a meal containing 80 g of carbohydrate. Blood glucose rises from 5.0 mmol/L to 9.5 mmol/L within 30 minutes.

1. β cells detect the rise above the set point (approximately 5.0 mmol/L).
2. β cells secrete insulin into the hepatic portal vein.
3. Insulin stimulates glycogenesis in liver and muscle cells, and glucose uptake via GLUT4 in muscle and adipose tissue.
4. Over the next 90 minutes, blood glucose returns to 5.0 mmol/L.
5. The fall in blood glucose is detected by β cells, which reduce insulin secretion (negative feedback).
6. If blood glucose continues to fall (e.g., during subsequent exercise), α cells secrete glucagon, stimulating glycogenolysis in the liver.

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The Oestrous Cycle (Menstrual Cycle)

The menstrual cycle is controlled by the interplay of four hormones in a combination of negative and positive feedback:

Hormone	Source	Key Actions
FSH (follicle-stimulating hormone)	Anterior pituitary	Stimulates development of follicles in the ovary; stimulates oestrogen secretion by the follicle
LH (luteinising hormone)	Anterior pituitary	Surge of LH triggers ovulation (day 14); stimulates the empty follicle to develop into the corpus luteum
Oestrogen	Developing follicle (granulosa cells)	Stimulates repair and proliferation of the endometrium (uterus lining); at low levels inhibits FSH/LH (negative feedback); at high levels stimulates a surge of LH (positive feedback)
Progesterone	Corpus luteum	Maintains the thick, vascularised endometrium; inhibits FSH and LH (negative feedback); if no implantation occurs, the corpus luteum degenerates, progesterone falls, and menstruation occurs

A described diagram of hormone levels during the 28-day cycle would show: FSH peaking early (days 1–5), oestrogen rising gradually and peaking just before day 14, a sharp LH surge on day 13–14 triggering ovulation, and progesterone rising from day 14 to about day 21 then declining if pregnancy does not occur. Menstruation occurs from days 1–5 as progesterone and oestrogen are at their lowest.

Exam Tip: The switch from negative to positive feedback for oestrogen is a favourite exam topic. At low concentrations, oestrogen inhibits FSH and LH release (negative feedback). As the dominant follicle matures, oestrogen levels rise above a critical threshold, and the effect switches to positive feedback — oestrogen now stimulates a surge of LH from the anterior pituitary, which triggers ovulation.

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The Kidneys and Osmoregulation

The kidneys regulate water balance (osmoregulation), excrete metabolic waste products (urea, creatinine), and control blood ion concentrations.

Nephron Structure — Detailed

Key Definition: The nephron is the functional unit of the kidney. Each kidney contains approximately 1 million nephrons. Each nephron consists of a Bowman's capsule, proximal convoluted tubule (PCT), loop of Henle, distal convoluted tubule (DCT), and collecting duct.