Levels of organisation: from cell to organism
A cell is the smallest living unit. Many cells of the same kind make a tissue (for example muscle tissue). Different tissues together make an organ (the heart has muscle, nerve and blood tissue). Organs that share one job make an organ system. All systems together make the organism.
The four main tissue types
- Epithelial: covers and lines (skin, inside of the gut).
- Connective: supports and joins (bone, blood, tendons).
- Muscle: gets shorter to make movement.
- Nervous: carries messages.
The main organ systems and their jobs
| System | Main organs | Main job |
|---|---|---|
| Skeletal | bones, joints | support, protect, store calcium, make blood cells |
| Muscular | skeletal, heart and smooth muscle | movement, posture, heat |
| Circulatory | heart, blood vessels, blood | carry oxygen, food and waste |
| Respiratory | nose, windpipe, lungs | take in O₂, give out CO₂ |
| Digestive | mouth, stomach, intestines, liver | break down and absorb food |
| Excretory (urinary) | kidneys, bladder | remove waste, balance water |
| Nervous | brain, spinal cord, nerves | fast control by electric signals |
| Endocrine | glands (pituitary, thyroid, adrenal, pancreas) | slow control by hormones |
| Immune / lymphatic | white blood cells, lymph nodes | fight germs |
| Integumentary | skin, hair, nails | protect, feel, keep temperature |
| Reproductive | ovaries, testes | make offspring |
Anatomy means the structure (what a part looks like). Physiology means the function (how it works).
How systems work together: movement and exercise
Movement needs a team. The nervous system sends a signal to a muscle. The muscle gets shorter and pulls a bone at a joint, like a lever. Muscles work in pairs: when the biceps shortens, the triceps relaxes, and the arm bends.
Working muscles need oxygen and glucose. So the heart beats faster and the lungs breathe faster and deeper. Muscles make heat, so the skin sweats. Later the kidneys adjust how much water you lose.
Homeostasis, skin, hair and nails, and keeping systems healthy
Homeostasis means keeping the inside of the body steady: temperature near 37 °C, enough water, the right sugar level. The nervous and endocrine systems act like a thermostat: they sense a change and switch on a fix.
Skin, hair and nails
Skin has an outer layer (epidermis) and an inner layer (dermis) with sweat glands, blood vessels and hair roots. Hair and nails are made of a tough protein called keratin; they grow from living cells at the root, but the part you see is dead, so cutting it does not hurt.
When a system goes wrong
Pathology is the study of disease. If one system fails, others suffer: for example, weak lungs mean less oxygen for muscles. Exercise, a balanced diet, sleep and clean water keep all systems healthy.
Key formulas and definitions
- Cell → tissue → organ → organ system → organism
- Anatomy = structure; physiology = function
- Homeostasis = keeping inner conditions steady (about 37 °C)
- Resting heart rate of a teenager ≈ 60–100 beats per minute
- Breathing rate at rest ≈ 12–20 breaths per minute
Worked examples
1. Place in order from smallest to largest: organ, cell, organism, tissue, organ system.
Cell → tissue → organ → organ system → organism. Each level is made from many units of the level before.
2. Name the systems that help a footballer take a penalty kick.
Nervous (brain decides and sends signals), muscular (leg muscles contract), skeletal (bones act as levers at the knee and hip), circulatory and respiratory (supply oxygen and remove CO₂).
3. A girl's heart rate is 72 beats per minute at rest and 150 after sprinting. By how many beats did it rise, and why?
150 − 72 = 78 beats per minute more. Muscles need extra oxygen and glucose, so the heart pumps blood faster to deliver them and remove carbon dioxide.
Common mistakes
- Thinking an organ is made of only one tissue. The stomach has muscle, epithelial, connective and nervous tissue.
- Mixing up excretion and egestion: excretion removes waste made by cells (urine, CO₂); egestion removes undigested food (faeces).
- Thinking the nervous and endocrine systems are the same. Nerves are fast and short-lived; hormones are slower and last longer.
- Believing muscles can push. Muscles can only pull, so they work in pairs.