# Human Body Systems Review
ACT Science passages sometimes present experiments or data related to human physiology. Understanding the major body systems helps you interpret these passages quickly. This guide reviews the systems most commonly featured.
1. Circulatory System
Components
- Heart: 4 chambers (2 atria, 2 ventricles); pumps blood
- Arteries: Carry blood away from the heart (high pressure, thick walls)
- Veins: Return blood to the heart (low pressure, contain valves)
- Capillaries: Tiny vessels where exchange occurs (thin walls, one cell thick)
Blood Flow
Body → Vena cava → Right atrium → Right ventricle → Pulmonary artery → Lungs (gas exchange) → Pulmonary vein → Left atrium → Left ventricle → Aorta → Body
Blood Components
- Red blood cells: Carry oxygen (haemoglobin)
- White blood cells: Immune defence
- Platelets: Blood clotting
- Plasma: Liquid; transports dissolved substances
2. Respiratory System
Gas Exchange
- Air enters through trachea → bronchi → bronchioles → alveoli
- Alveoli: tiny air sacs with thin walls and rich blood supply
- O₂ diffuses from alveoli → blood; CO₂ diffuses from blood → alveoli
Breathing Mechanics
- Inhalation: Diaphragm contracts (moves down), intercostal muscles contract (ribs move up and out) → volume increases → pressure decreases → air flows in
- Exhalation: Diaphragm relaxes (moves up), intercostal muscles relax → volume decreases → pressure increases → air flows out
Key Measurements
- Tidal volume: Normal breath (~500 mL)
- Vital capacity: Maximum air exhaled after maximum inhalation
- Breathing rate: Typically 12–20 breaths/min at rest
3. Nervous System
Components
- CNS: Brain + spinal cord (processing centre)
- PNS: Sensory and motor neurons connecting CNS to body
Neuron Structure
- Dendrites → Cell body → Axon → Axon terminals
- Signal: Electrical impulse along axon; chemical (neurotransmitter) across synapse
Reflex Arc
Stimulus → Receptor → Sensory neuron → Relay neuron (spinal cord) → Motor neuron → Effector (muscle/gland) → Response
- Reflexes are fast, automatic, and protective
4. Digestive System
Mechanical and Chemical Digestion
- Mouth: Teeth (mechanical); salivary amylase (starch → maltose)
- Stomach: Churning (mechanical); pepsin + HCl (protein digestion, pH ~2)
- Small intestine: Main site of digestion and absorption
- Pancreatic enzymes: lipase, protease, amylase
- Bile (from liver): Emulsifies fats
- Villi and microvilli increase surface area for absorption
- Large intestine: Water absorption; formation of faeces
Key Enzymes
| Enzyme | Substrate | Product | Location |
|---|---|---|---|
| Amylase | Starch | Maltose/Glucose | Mouth, small intestine |
| Pepsin | Protein | Peptides | Stomach |
| Lipase | Fats | Fatty acids + glycerol | Small intestine |
| Trypsin | Protein | Peptides | Small intestine |
5. Endocrine System
Key Hormones
| Hormone | Gland | Function |
|---|---|---|
| Insulin | Pancreas | Lowers blood glucose (promotes storage) |
| Glucagon | Pancreas | Raises blood glucose (promotes release) |
| Adrenaline | Adrenal glands | Fight-or-flight response |
| Thyroxine | Thyroid | Regulates metabolic rate |
| Oestrogen/Testosterone | Ovaries/Testes | Sexual development |
Homeostasis
- Negative feedback: Response counteracts the change (e.g., blood glucose regulation)
- Blood glucose rises → insulin released → glucose stored → blood glucose falls
- Blood glucose falls → glucagon released → glucose released → blood glucose rises
6. Worked Example
Q: A graph shows blood glucose levels after a meal. The level rises from 80 mg/dL to 140 mg/dL within 30 minutes, then returns to 90 mg/dL by 2 hours. Explain the hormonal control involved.
A: After eating, glucose is absorbed from the small intestine into the blood, raising blood glucose to 140 mg/dL. The pancreas detects this and releases insulin, which stimulates cells to take up glucose and the liver to store glucose as glycogen. This negative feedback mechanism brings blood glucose back down to near-normal levels (~90 mg/dL) within 2 hours.
7. Practice Questions
Q1. Why does heart rate increase during exercise?
A1. During exercise, muscles need more oxygen and glucose for aerobic respiration, and more CO₂ must be removed. The brain detects increased CO₂ levels and sends signals to increase heart rate, pumping more blood to deliver O₂ and remove CO₂. Adrenaline also contributes to the increased heart rate.
Q2. A patient has damage to their motor neurons. What symptoms would you expect?
A2. Loss of voluntary movement (paralysis) and/or loss of reflex responses in the affected area. The patient could still feel sensations (sensory neurons intact) but could not respond with movement because the signal cannot reach the muscles.
Want to check your answers and get step-by-step solutions?
Summary
- Know the basic structure and function of the major body systems
- Understand gas exchange, blood flow, nerve impulse transmission, and digestion
- Homeostasis and negative feedback are frequently tested concepts
- On the ACT, you'll typically interpret data about these systems rather than recall detailed anatomy