A single cell can do a lot, but a large animal needs teamwork. Think of a hospital: individual workers (cells) form departments of people doing the same job (tissues), departments combine into units such as an emergency ward (organs), and the units work together as a whole hospital (organ systems and the organism). In animals, specialized cells group into tissues, tissues build organs, organs work together in organ systems, and the systems together make a living organism.
| Level | Definition | Example |
|---|---|---|
| Cell | The basic unit of life | A cardiac muscle cell |
| Tissue | A group of similar cells that work together to do one job | Cardiac muscle tissue |
| Organ | A structure made of two or more tissue types working together | The heart |
| Organ system | A group of organs that work together to carry out a major function | The circulatory system (heart, blood vessels, blood) |
| Organism | A complete living thing made of interacting organ systems | A human |
The four animal tissue types
| Tissue type | Main job | Examples |
|---|---|---|
| Epithelial | Covers body surfaces and lines organs and cavities; protects, absorbs and secretes | Outer layer of skin; lining of the mouth, stomach, intestines, blood vessels and air sacs |
| Connective | Supports, connects and protects other tissues; cells are spread out in a material (matrix) they make | Bone, cartilage, tendons, ligaments, fat, and blood (with liquid plasma as its matrix) |
| Muscle | Contracts to produce movement | Skeletal muscle (moves bones, under your control), smooth muscle (walls of the stomach, intestines and blood vessels), cardiac muscle (heart only) |
| Nervous | Detects changes and carries electrical signals to coordinate the body | Brain, spinal cord, nerves |
- Attached to bones by tendons
- Voluntary: you decide when it contracts
- Long, striped cells with several nuclei
- Tires with heavy use
- Smooth: in the walls of hollow organs; pushes food along, narrows blood vessels
- Cardiac: only in the heart; contracts rhythmically for a lifetime
- Both involuntary: they work without conscious control
- Cardiac cells are branched and linked so the heart contracts as a unit
Organs: tissues working together
The stomach shows how one organ uses all four tissue types. Epithelial tissue lines the inside and secretes acid, enzymes and a protective layer of mucus. Layers of smooth muscle contract to churn food. Connective tissue holds the layers together and carries blood vessels that supply the organ. Nervous tissue signals the muscles and glands when food arrives. Remove any one tissue and the organ fails. The heart is similar: cardiac muscle does the pumping, nerves adjust its rate, connective tissue forms the tough valves, and a thin epithelial lining lets blood flow smoothly.
Organ systems and how they interact
| Organ system | Main organs | Main function |
|---|---|---|
| Digestive | Mouth, esophagus, stomach, intestines, liver, pancreas | Breaks down food and absorbs nutrients |
| Circulatory | Heart, blood vessels, blood | Transports oxygen, nutrients, wastes and hormones |
| Respiratory | Nose, trachea, bronchi, lungs | Takes in oxygen and removes carbon dioxide |
| Nervous | Brain, spinal cord, nerves, sense organs | Detects changes and coordinates responses |
| Muscular and skeletal | Muscles, bones, tendons, ligaments | Movement, support and protection |
| Excretory (urinary) | Kidneys, bladder | Filters blood and removes liquid wastes |
| Endocrine | Glands such as the pancreas, thyroid and adrenals | Releases hormones that regulate body processes |
| Immune and lymphatic | White blood cells, lymph nodes, spleen | Defends against disease |
| Integumentary | Skin, hair, nails | Protection and temperature control |
| Reproductive | Ovaries, testes and related organs | Produces offspring |
No system works alone. When you sprint, your nervous system signals skeletal muscles to contract. The muscle cells need much more oxygen and glucose, so your respiratory system breathes faster and deeper, your circulatory system pumps blood faster to deliver oxygen and remove carbon dioxide, and your digestive system is the original source of the glucose. Your skin sweats to shed the extra heat. If a vital system such as the circulatory or respiratory system fails, the others cannot keep the body alive for long. Dissections of a frog, fish or earthworm (real or computer-simulated) make these links visible: for example, the blood vessels that run through the wall of the intestine show where the digestive and circulatory systems meet.