Lesson 2: Human Body Systems

Your body is a collection of organ systems working together to keep your internal conditions stable — a state called homeostasis. This lesson tours the major human systems tested at GCSE: the heart and circulation, the lungs and gas exchange, the digestive system, and the nervous and hormonal control systems. You will learn how each is structured for its job, how they cooperate, and how things go wrong — from coronary heart disease to diabetes. Six-mark questions in this topic almost always ask you to explain adaptations or describe sequences of events, so precise, ordered explanations are the skill to practise.

The Heart and Circulation

The heart is a double pump made of muscle. The right side pumps deoxygenated blood to the lungs; the left side pumps oxygenated blood around the body. Blood enters the atria (the upper chambers) and is pumped out by the ventricles (the lower chambers) — the left ventricle wall is much thicker because it must generate enough pressure to push blood around the entire body. Valves prevent backflow: blood flows atrium → ventricle → artery, never backwards. The natural resting heart rate is controlled by a group of cells in the right atrium acting as a pacemaker.

There are three blood vessel types. Arteries carry blood away from the heart at high pressure; they have thick muscular walls and a narrow lumen to withstand the pressure. Veins return blood at low pressure; they have thinner walls, a wider lumen, and valves to stop backflow. Capillaries are one cell thick, with a huge total surface area and slow blood flow — perfect for the exchange of oxygen, nutrients and waste between blood and tissues. Learn to link each structural feature to its function; that link is where the marks are.

Coronary heart disease (CHD) is a major killer in the UK. Fatty deposits build up in the coronary arteries, narrowing them and reducing blood flow to the heart muscle. A clot can block the artery completely, causing a heart attack. Risk factors include smoking, a diet high in saturated fat and salt, lack of exercise, and excessive alcohol. Treatments include stents (to hold arteries open), statins (to lower cholesterol), and in severe cases heart transplants or artificial hearts. Lifestyle changes reduce risk — exam questions often ask you to evaluate treatments, weighing benefits against risks and costs.

The Lungs and Gas Exchange

Breathing moves air in and out; gas exchange is the diffusion of oxygen and carbon dioxide in the alveoli. The adaptations of the lungs are a favourite exam topic: millions of tiny alveoli give an enormous surface area (about the size of a tennis court); alveolar walls are one cell thick (short diffusion distance); each alveolus is wrapped in capillaries giving a rich blood supply; and ventilation constantly brings in fresh air and removes stale air, maintaining steep concentration gradients. Quote all four adaptations with explanations for full marks.

The breathing mechanism: the diaphragm contracts and flattens and the intercostal muscles contract, pulling the ribs up and out — this increases the volume of the chest cavity, lowering the pressure inside, so air rushes in (inhalation). For exhalation the muscles relax, volume decreases, pressure rises, and air is pushed out. You can measure breathing rate, and a classic data question asks you to interpret how exercise increases the rate and depth of breathing to supply more oxygen for respiration and remove more carbon dioxide.

Smoking damages this system in well-documented ways: tar causes cancer and coats the alveoli; nicotine is addictive and narrows blood vessels; carbon monoxide binds to haemoglobin more strongly than oxygen, reducing oxygen transport. Emphysema destroys alveolar walls, reducing surface area for gas exchange. When answering "explain the health risks of smoking" questions, name the harmful substance, describe its effect, and state the consequence — that three-step chain earns the marks.

The Digestive System

Digestion breaks large insoluble food molecules into small soluble ones that can be absorbed into the blood. It happens in stages along the gut: the mouth (chewing and salivary amylase on starch), the stomach (churning and protease in acid conditions), the small intestine (most digestion and all absorption), and the large intestine (water absorption). Three key enzymes: amylase breaks down starch to sugars, protease breaks down proteins to amino acids, and lipase breaks down lipids to fatty acids and glycerol. Bile, made in the liver and stored in the gall bladder, neutralises stomach acid and emulsifies fats into tiny droplets with a large surface area for lipase.

Enzymes are biological catalysts — proteins that speed up reactions without being used up. Each enzyme has an active site with a shape complementary to its substrate (the lock-and-key model). Temperature and pH affect enzyme action: too hot and the enzyme denatures as its active site changes shape; each enzyme has an optimum pH (stomach protease works in acid, intestinal enzymes in alkaline conditions). The required practical investigates how pH affects amylase — you will use iodine to test when starch has disappeared and should be able to describe a method, identify variables, and process results.

The small intestine is adapted for absorption: the villi (and microvilli on their surface) massively increase surface area; walls are one cell thick; there is a rich capillary network to carry away absorbed molecules and maintain the gradient; and glucose is absorbed by active transport. Absorbed food molecules travel in the blood to cells for respiration, growth and repair. A common 6-mark question: "Explain how the structure of the small intestine is adapted for the absorption of digested food" — plan four adaptations, each with the "how it helps" explanation.

The Nervous System and Reflexes

The nervous system enables rapid responses. It has three neurone types: sensory neurones carry impulses from receptors to the central nervous system (CNS — brain and spinal cord); relay neurones connect neurones within the CNS; motor neurones carry impulses from the CNS to effectors (muscles and glands). Impulses cross synapses — gaps between neurones — via chemicals called neurotransmitters. Myelin sheaths insulate axons and speed up transmission.

Reflex actions are rapid, automatic responses that do not involve conscious thought — the pathway is called a reflex arc: stimulus → receptor → sensory neurone → relay neurone (in spinal cord) → motor neurone → effector → response. Pulling your hand from a hot surface is the classic example. Because the impulse bypasses conscious brain processing, the response is faster, reducing harm. The required practical measures reaction time with the ruler-drop test, and you should know how to reduce error: repeat, calculate a mean, exclude anomalies, use the same person dropping the ruler.

The brain has specialised regions: the cerebral cortex (consciousness, intelligence, memory, language), the cerebellum (muscle coordination), and the medulla (unconscious actions like breathing and heartbeat). Studying the brain is difficult — it is delicate, damage is risky, and electrical stimulation studies are ethically limited — so much evidence comes from patients with brain damage or from brain scans (CT, PET, MRI). Higher-tier questions may ask you to evaluate the difficulties and ethics of brain research.

Hormones and Homeostasis

While nerves give fast, short-lived responses, hormones give slower, longer-lasting control via the blood. Key glands: the pituitary (the "master gland", controlling others), thyroid (thyroxine — metabolic rate), pancreas (insulin and glucagon — blood sugar), adrenals (adrenaline — the fight-or-flight response), ovaries and testes (reproductive hormones). Hormones are slower than nerves but their effects last longer and can act on many organs at once.

Blood glucose is controlled by negative feedback — the hallmark of homeostasis. When blood sugar rises (after eating), the pancreas releases insulin, which causes liver and muscle cells to take up glucose and convert it to glycogen for storage. When blood sugar falls, glucagon triggers the liver to break glycogen back into glucose. Type 1 diabetes is caused by the pancreas failing to produce insulin (treated with insulin injections); Type 2 is caused by cells no longer responding to insulin, strongly linked to obesity (treated with diet, exercise and drugs). Exam questions often present blood-glucose graphs to interpret.

Temperature, water and ion content are also controlled. The thermoregulatory centre in the brain monitors blood temperature: if too hot, sweat glands release sweat (evaporative cooling) and blood vessels near the skin dilate (vasodilation); if too cold, muscles shiver, hairs stand up, and vessels constrict (vasoconstriction). The kidneys control water and ion balance by filtering blood and reabsorbing what the body needs — ADH from the pituitary controls how much water is reabsorbed. In every homeostasis answer, use the pattern: receptor detects change → control centre coordinates → effector corrects → levels return to normal (negative feedback).

Data analysis — the enzyme and pH required practical

In the amylase investigation, the time for starch to disappear was measured at five pH values. Rate is calculated as 1 ÷ time: at pH 5 the time was 240 s (rate 0.0042 s⁻¹); pH 6, 120 s (0.0083 s⁻¹); pH 7, 60 s (0.0167 s⁻¹); pH 8, 90 s (0.0111 s⁻¹); pH 9, 300 s (0.0033 s⁻¹). The peak rate at pH 7 identifies the optimum: below it, fewer successful collisions occur; above it, the high pH breaks bonds in the enzyme, the active site changes shape, and the enzyme denatures.

A strong evaluation adds: temperature must be controlled with a water bath because it also affects rate (a confounding variable); buffer solutions hold pH constant far better than single drops of acid or alkali; and using the same starch and amylase concentrations and volumes in every tube keeps the test fair.

Common mistakes

• Bile is not an enzyme. Bile emulsifies fats and neutralises acid — it contains no enzyme and digests nothing chemically.
• Insulin and glucagon swapped. Insulin lowers blood glucose (glucose → glycogen); glucagon raises it (glycogen → glucose).
• Amylase products. Amylase breaks starch into sugars (maltose, then glucose) — not into amino acids or fatty acids.

Exam Experience

Question 1. Name the enzyme that breaks down starch, and state where it is produced in the human body. [2 marks]

Question 2. A healthy person's blood glucose concentration rises after a meal containing starch. Describe the sequence of events that brings it back down, naming the hormone and the organ that releases it. [4 marks]

Question 3. The lungs are adapted to maximise the rate of gas exchange. Explain how their structure achieves a fast rate of oxygen uptake, using the principles of diffusion in your answer. [6 marks]

Mark scheme

Q1: amylase (1); salivary glands or pancreas (1).

Q2: glucose absorbed into the blood (1); the pancreas detects the rise and releases insulin (1); body cells take up glucose and the liver and muscle convert glucose to glycogen for storage (1); negative feedback returns concentration to normal (1).

Q3: Level 3 (5–6 marks): full chain of linked reasoning — millions of alveoli give a very large combined surface area, so more diffusion per unit time; alveolar and capillary walls are each one cell thick, giving a short diffusion distance; a dense capillary network maintains a steep concentration gradient by continuously carrying oxygenated blood away and bringing deoxygenated blood in; ventilation refreshes the air in the alveoli, keeping the gradient steep from the air side. Level 2 (3–4 marks): some structural features linked to faster diffusion, but gradient maintenance or both sides of the exchange missing. Level 1 (1–2 marks): features listed ("alveoli are small and round") with no explanation of their effect on the rate.