Human Body Systems
The Major Body Systems
The human body contains 11 major organ systems. Each has a specific role, but no system works alone — they are all interconnected.
| System | Main organs | Key function |
|---|---|---|
| Digestive | Mouth, oesophagus, stomach, intestines, liver, pancreas | Break down food; absorb nutrients |
| Circulatory | Heart, arteries, veins, capillaries, blood | Transport oxygen, nutrients, hormones, and waste |
| Respiratory | Lungs, trachea, bronchi, diaphragm | Gas exchange (O2 in, CO2 out) |
| Nervous | Brain, spinal cord, nerves, sense organs | Control, coordination, and response |
| Skeletal | Bones, joints, cartilage | Support, movement, protection, blood cell production |
| Muscular | Skeletal, smooth, and cardiac muscles | Movement, posture, generating heat |
| Immune | White blood cells, lymph nodes, spleen, thymus | Defence against pathogens and disease |
| Excretory/Urinary | Kidneys, ureters, bladder, urethra | Filter waste from blood; produce urine |
| Endocrine | Pituitary, thyroid, adrenal glands, pancreas | Produce hormones to regulate body processes |
| Reproductive | Ovaries/testes, uterus | Produce gametes; reproduce offspring |
| Integumentary | Skin, hair, nails, sweat glands | Protection, temperature regulation, sensation |
The Digestive System in Detail
Digestion is both mechanical (physical breakdown) and chemical (enzymes breaking molecules apart):
- Mouth — teeth chew food; saliva contains amylase to begin starch digestion
- Oesophagus — muscular tube carrying food to the stomach via peristalsis
- Stomach — churns food; hydrochloric acid kills bacteria; protease enzymes digest protein
- Small intestine — most digestion and absorption occurs here; villi increase surface area
- Large intestine — absorbs water from undigested material; forms faeces
- Liver — produces bile to emulsify fats; processes nutrients from blood
- Pancreas — secretes digestive enzymes into the small intestine
The Circulatory System in Detail
The circulatory system is a double circulatory system in humans:
- Pulmonary circulation: heart → lungs → heart (picks up oxygen, drops off CO2)
- Systemic circulation: heart → body → heart (delivers oxygen and nutrients)
Blood vessels:
- Arteries — carry blood away from the heart; thick muscular walls; high pressure
- Veins — carry blood towards the heart; have valves to prevent backflow
- Capillaries — microscopic vessels where exchange of gases, nutrients, and waste occurs
How Systems Work Together
No system operates in isolation. Here are key examples:
During exercise: Muscles demand more glucose and oxygen → the circulatory system speeds up (heart beats faster) → the respiratory system increases breathing rate → waste CO2 is removed more quickly.
After eating: The digestive system breaks down food into glucose, amino acids, and fatty acids → the circulatory system carries nutrients to all cells → the endocrine system (insulin from pancreas) regulates blood glucose levels.
Fighting infection: The immune system detects pathogens → white blood cells are produced and transported by the circulatory system → the lymphatic system drains fluid and filters out pathogens.
Homeostasis
Homeostasis is the maintenance of a stable internal environment despite changes in external conditions. It is essential for cells to function properly.
Key variables kept stable:
- Body temperature — regulated at 37°C by the skin, muscles (shivering), and sweat glands
- Blood glucose — regulated by insulin and glucagon from the pancreas
- Water balance — regulated by the kidneys (osmoregulation)
- Blood pH — maintained around 7.4 by the lungs and kidneys
Homeostasis works through negative feedback loops: a change is detected, a corrective response is triggered, and the variable returns to its set point.
Key Terms
- Organ — a group of tissues working together for a specific function
- Organ system — a group of organs working together
- Homeostasis — maintenance of a stable internal environment
- Peristalsis — wave-like muscle contractions that move food through the digestive tract
- Negative feedback — a control mechanism that reverses a change to restore balance
- Villi — finger-like projections in the small intestine that increase surface area for absorption
Common Mistakes
- Confusing the excretory system (removes metabolic waste from blood) with the digestive system (processes food)
- Thinking the liver is only part of the digestive system — it also detoxifies blood and is part of the endocrine system
- Forgetting that the heart is a muscle and part of both the circulatory AND muscular systems
- Saying the respiratory system delivers oxygen to cells — it is the circulatory system that delivers it
Tips and Tricks
- Learn organ systems by their key function: Digest (digestive), Transport (circulatory), Breathe (respiratory), Control (nervous/endocrine), Support (skeletal), Move (muscular)
- Homeostasis questions often follow the pattern: stimulus → receptor → control centre → effector → response
- Remember: arteries carry blood Away from the heart (both start with A)
- Veins have valves; arteries do not — this is why veins bulge when you clench your fist