Fuel and control: the systems that supply movement and the hormones that govern it

The digestive tract is a single continuous tube from mouth to anus, and food inside it is technically still outside the body — it has not entered until it is absorbed. Two processes run throughout: mechanical digestion, physically breaking food into smaller pieces, and chemical digestion, using enzymes to break the molecular bonds.

• Mouth — teeth break food down mechanically while salivary amylase begins the chemical digestion of starch. Chewing matters more than it sounds: it multiplies the surface area available to every enzyme downstream.
• Oesophagus — carries food to the stomach by peristalsis. It is muscular action, not gravity, which is why you can swallow lying down.
• Stomach — churns food mechanically and secretes hydrochloric acid and the enzyme pepsin, which begins protein digestion. Food leaves as a liquid called chyme, and the rate at which it does so — gastric emptying — is the single most important variable in whether eating before exercise causes trouble.
• Small intestine — the duodenum, jejunum and ileum. Most chemical digestion and almost all absorption happen here. Its lining is folded into millions of villi, each covered in microvilli, giving a surface area of around 250 square metres.
• Large intestine — absorbs water and electrolytes, hosts the gut microbiota, and forms waste for elimination.

Three accessory organs sit outside the tube and feed into it:

• Liver — produces bile, stores glycogen and releases glucose to maintain blood sugar, and processes absorbed nutrients. Every nutrient absorbed from the small intestine passes through it first.
• Gallbladder — stores bile and releases it into the duodenum, where it emulsifies fat into small droplets so enzymes can reach it.
• Pancreas — secretes digestive enzymes for all three macronutrients into the small intestine, and secretes insulin and glucagon into the blood.

Digestion reduces each macronutrient to a form the blood can carry and a cell can use. What happens next is the part that matters for movement.

The trade-off between the last two rows is worth stating now, because Lesson 10 depends on it. Fat carries far more energy per gram but requires oxygen to release it and releases it slowly. Carbohydrate yields less per gram but can be broken down rapidly, and with or without oxygen. That is why a marathon runner is largely fat-fuelled and a sprinter is entirely carbohydrate-fuelled, and why running out of glycogen ends a race even though the runner still carries ample fat.

Micronutrients supply no energy at all, and every one of them matters because of something you have already studied:

• Iron — makes haemoglobin. Lesson 5's entire second half depends on it.
• Calcium — builds and maintains bone, and is required for muscle contraction itself. Lessons 1 and 2.
• Vitamin D — needed to absorb calcium, so a deficiency weakens bone even with adequate calcium intake.
• B group vitamins — required by the reactions that release energy from food. No B vitamins, no usable energy from any macronutrient.
• Sodium and potassium — maintain fluid balance and make nerve conduction and muscle contraction possible. These are what is lost in sweat.
• Antioxidant vitamins such as C and E — limit cell damage, and vitamin C also markedly improves absorption of plant-source iron.

The useful summary for an exam: macronutrients supply the energy and the building materials; micronutrients make it possible to use them. A diet adequate in kilojoules but deficient in iron will still leave an athlete unable to perform, for reasons Lesson 5 explains exactly.

The endocrine system is a set of glands that secrete hormones directly into the bloodstream. A hormone travels everywhere but acts only on cells with a matching receptor, which is how a chemical released in the neck can act specifically on a muscle in the leg.

The body has two control systems, and the comparison is examinable:

The major glands, and the hormones this course needs:

• Hypothalamus and pituitary — the control centre. The hypothalamus links the nervous and endocrine systems, and directs the pituitary, which releases growth hormone and governs several other glands. The pituitary also releases antidiuretic hormone, which conserves water.
• Thyroid — releases thyroxine, which sets the body's overall metabolic rate.
• Adrenal glands, above the kidneys — release adrenaline and noradrenaline for the immediate stress response, cortisol for the sustained one, and aldosterone, which conserves sodium.
• Pancreas — releases insulin and glucagon, which between them hold blood glucose within a narrow range.
• Ovaries and testes — release oestrogen and testosterone, which influence bone density, muscle mass and recovery.

Most of this is run by negative feedback: a change triggers a response that reverses the change and then switches the response off. Blood glucose rises after a meal, insulin is released, glucose moves into cells, blood glucose falls, insulin secretion stops. It is the same logic as a thermostat, and it is why hormone levels oscillate around a set point rather than holding still.

Follow a training session and the endocrine system's role becomes a sequence rather than a list.

• Before and at the start — adrenaline and noradrenaline. Released from the adrenal glands, they raise heart rate and stroke volume, dilate the bronchioles to ease ventilation, dilate the arterioles supplying working muscle while constricting those supplying the digestive organs, and trigger the breakdown of glycogen to glucose. This is the "fight or flight" response, and it explains the racing heart before a game has started — the endocrine system is preparing for movement that has not yet happened, which is something the nervous system alone cannot do.
• During — glucagon and cortisol. As muscles consume blood glucose, glucagon releases more from liver glycogen, holding blood glucose steady. In prolonged exercise cortisol contributes, mobilising fat and, if it continues long enough, protein as fuel.
• During — antidiuretic hormone and aldosterone. As sweating reduces blood volume, these hormones conserve water and sodium at the kidneys. This is the endocrine defence of the plasma volume whose loss Lesson 5 identified as raising heart rate.
• After — insulin and growth hormone. When food is eaten after training, insulin drives glucose into muscle cells to restore glycogen, and amino acids in for repair. Growth hormone, released during exercise and especially during deep sleep, drives tissue growth and repair — which is the physiological reason sleep is a training variable and not a lifestyle preference.

The dot point asks explicitly for factors that impact on the efficiency of the systems, and names stress. Take it first, and separate two things that are usually confused.

Acute stress is useful. The adrenaline response before competition is exactly what should happen: it raises heart rate and cardiac output, releases fuel, and diverts blood to the muscles that need it. It is a preparation for movement, and it works.

Chronic stress is the problem, and it damages both systems in specific ways:

• On digestion. The same diversion of blood away from the digestive organs that is helpful for ten minutes is harmful when sustained. Chronic stress reduces gut blood flow and alters motility, producing nausea, cramping, altered bowel habit and worsened symptoms of conditions such as irritable bowel syndrome. Appetite is disrupted in both directions — suppressed in some people, increased in others, with cortisol also driving preference towards energy-dense food.
• On the endocrine system. Persistently elevated cortisol keeps the body in a mobilising state when it should be rebuilding: it promotes the breakdown of muscle protein, impairs recovery and adaptation from training, suppresses immune function so illness becomes more frequent, and disrupts sleep — which reduces growth hormone release, compounding the impaired recovery.
• The cycle. Poor sleep raises stress, raises cortisol, impairs recovery, worsens performance, and raises stress further. This is worth drawing as a loop, because it explains why athletes and students under pressure often deteriorate faster than the workload alone accounts for.

The other factors affecting efficiency:

Low energy availability deserves emphasis, because it is where this lesson's physiology meets a genuine health issue affecting the age group this course is written for. When intake does not cover the cost of training, the body reduces spending on functions it can defer — reproductive hormones, bone maintenance, immune function, growth. The consequences are severe and include stress fractures and long-term reductions in bone density, and the early signs are easy to mistake for dedication. It is the clearest illustration that the endocrine system does not fail loudly: it downgrades quietly, and the symptoms appear far from the cause.

Record everything you eat and drink for one day, along with the timing of any training or sport. This is an audit of timing and composition, not of kilojoules, and it is deliberately not a judgement exercise.

Then two conclusions: one change you would make and the physiological reason for it, and one thing this record cannot tell you. The second is the more important skill, and Lesson 15 will build on it.

A 17-year-old has increased training over four months and is committed and conscientious. They report: performances plateaued then declined; constant tiredness; three colds this term; disturbed sleep; irritability; and recent persistent shin pain. They have also lost weight, which they and their coach regard as a positive sign.

Work through it as a systems problem rather than a motivation problem:

• Which systems from this module could explain each symptom? Several symptoms have more than one candidate explanation — note where the module gives you two.
• What single underlying cause would account for the whole pattern at once? An explanation covering all of it is stronger than six separate ones.
• Why is the shin pain the most concerning symptom, given what you know from Lesson 1 about bone and from this lesson about endocrine function?
• Why did the weight loss get read as success? This is where the Module 1 material on how health is measured, and on what a single indicator hides, becomes directly relevant.
• What would you do? Be explicit about what is a coaching decision and what requires medical assessment, and say who should be involved.