2.4 The Endocrine System

Endocrine Glands and Hormones

A gland is an organ or cluster of cells that synthesises and secretes specific chemical substances. The human body contains two distinct categories of glands:

• Exocrine glands: Possess ducts (tubes) to carry their secretions onto an internal or external body surface. Examples include salivary glands, sweat glands, and the enzyme-secreting tissue of the pancreas.
• Endocrine glands: These are ductless glands that secrete hormones directly into the blood.

A hormone is a chemical messenger produced by an endocrine gland and transported in the blood to alter the activity of one or more target organs.

Target tissues have specific protein receptors on their cell membranes or inside their cytoplasm with shapes that match the complementary hormone. When a hormone binds to its receptor, it triggers a specific cellular response. Most hormones are proteins (for example, insulin) or steroids made from lipids (for example, testosterone and oestrogen).

Summary of Course Glands

Comparing Endocrine and Nervous Coordination

Vertebrates maintain a constant internal environment and react to external changes using two interacting communication systems: nervous and hormonal. While both link sensory receptors to effectors via a control centre, their pathways and timescales differ substantially:

Some events demand rapid responses, such as blinking when an object nears the eye or withdrawing your hand from a hot surface. Others, such as physical growth, kidney osmoregulation, or managing the menstrual cycle, require the sustained, widespread control that hormones deliver. Both pathways link receptors and effectors through a control centre; they differ in how the message travels.

The Seven Major Glands and Their Secretions

You need to know the location, secretions, and roles of the seven major endocrine glands on your course:

1. The Pituitary Gland

Located at the base of the brain just beneath the hypothalamus. Known as the 'master gland' because several of its secretions stimulate other endocrine glands:

• Thyroid-Stimulating Hormone (TSH): Stimulates the thyroid to secrete thyroxine.
• Follicle-Stimulating Hormone (FSH): Stimulates follicle development and oestrogen production in ovaries; stimulates sperm development in testes.
• Luteinising Hormone (LH): Triggers ovulation and corpus luteum formation in females; stimulates testosterone secretion in males.
• Anti-Diuretic Hormone (ADH): Released into the blood to act on the distal convoluted tubule and collecting duct of the nephron, increasing water reabsorption during osmoregulation.
• Oxytocin: Stimulates uterine wall contractions during labour and milk ejection during breastfeeding. Note: Oxytocin during childbirth acts via positive feedback, where contractions cause more oxytocin release to drive delivery, unlike standard homeostatic loops.
• Prolactin: Stimulates milk production in mammary tissue (you meet this again in Reproduction).

2. The Thyroid Gland

Located in the neck across the trachea, just below the larynx. It combines the amino acid tyrosine with iodine to produce thyroxine.

• Function: Regulates basal metabolic rate (the rate of cellular respiration and energy release in resting cells) and guides growth and development.

3. The Parathyroid Glands

Four small glands embedded in the posterior (rear) surface of the thyroid gland.

• Secretion: Parathyroid Hormone (PTH).
• Function: Elevates blood calcium by stimulating calcium release from bones, increasing calcium absorption from food in the gut, and reducing calcium loss in urine.

4. The Adrenal Glands

Located like caps on the top of each kidney.

• Secretion: Adrenaline.
• Function: Coordinates the emergency 'fight-or-flight' stress reaction. It raises heart rate and breathing rate, elevates blood pressure, promotes glycogen breakdown in the liver to boost blood glucose, and diverts blood to skeletal muscles.

5. The Pancreas

The pancreas lies in the abdomen, behind the stomach, with its head resting in the loop of the duodenum. It is a dual-function gland: exocrine cells secrete digestive enzymes into pancreatic juice, while clusters of endocrine cells called the islets of Langerhans secrete hormones directly into blood capillaries.

• Secretion: Insulin.
• Function: Lowers blood glucose levels by enabling cells to take up glucose and stimulating liver and muscle cells to convert soluble glucose into insoluble glycogen.

6. The Gonads (Testes and Ovaries)

• Testes: Located externally within the scrotum of males. They secrete testosterone, which controls male puberty, secondary sexual characteristics (such as voice deepening and facial hair), and sperm production.
• Ovaries: Located in the pelvic cavity of females. They produce oestrogen (stimulates endometrial repair and female secondary characteristics) and progesterone (maintains the vascular lining of the endometrium for pregnancy and inhibits further ovulation).

Hormonal Manipulations in Sport, Health, and Agriculture

Because hormones trigger widespread changes at very low concentrations, applying synthetic analogues or natural extracts has substantial practical impacts:

1. In Sport

• Anabolic steroids: Synthetic substances structurally derived from testosterone. Athletes take them illegally to boost protein synthesis, increase muscle bulk, and accelerate muscle recovery between training sessions. Hazard: Misuse causes severe health risks, including liver damage, elevated blood pressure, cardiovascular disease, testicular shrinkage, and infertility.
• Erythropoietin (EPO): A hormone that stimulates red blood cell production in bone marrow. Endurance competitors abuse it to improve oxygen transport to working muscles. Hazard: Too many red blood cells make the blood dangerously viscous, sharply increasing the danger of blood clots, strokes, and heart failure.

2. In Healthcare

• Insulin therapy: Genetically modified bacteria produce human insulin via recombinant DNA technology. Daily subcutaneous injections or continuous pump infusions manage Type 1 diabetes, where pancreatic islet cells fail to produce native insulin.
• Hormone Replacement Therapy (HRT): Prescribed to alleviate menopausal symptoms in women by supplementing declining oestrogen and progesterone, helping protect bone density against osteoporosis.
• Hormonal contraception: The combined oral contraceptive pill supplies synthetic oestrogen and progesterone. These maintain constant negative feedback on the pituitary gland, inhibiting FSH and LH release, which prevents follicle development and stops ovulation.

3. In Agriculture

• Livestock reproductive management: Progesterone sponges or intravaginal devices synchronise oestrus ('heat') cycles across groups of sheep and cattle. When removed simultaneously, females enter oestrus together, allowing artificial insemination to take place on a single date and ensuring grouped lambing or calving.
• Growth regulation in meat production: While synthetic growth hormones have been used internationally to accelerate muscle development in cattle, the practice is banned across the European Union to protect food safety and consumer health.
• Plant growth regulators: Synthetic auxins act as selective weedkillers on broad-leaved weeds in grass pastures and serve as rooting powders for vegetative cuttings. Ethene gas is applied commercially in warehouses to ensure uniform, controlled ripening of fruit such as bananas.