A team of systems
Your body has about eleven organ systems. Each has its own job, but none works alone. They constantly communicate and depend on each other to keep you alive.
The major systems
| System | Main organs | Main job |
|---|---|---|
| Skeletal | Bones, cartilage | Support, protection, blood cell production |
| Muscular | Skeletal, cardiac, smooth muscle | Movement |
| Circulatory | Heart, blood vessels, blood | Transport oxygen, nutrients, wastes |
| Respiratory | Lungs, trachea, diaphragm | Gas exchange |
| Digestive | Stomach, intestines, liver | Break down and absorb food |
| Nervous system | Brain, spinal cord, nerves | Fast control and communication |
| Endocrine system | Pituitary, thyroid, pancreas, adrenal glands | Slow control using hormones |
| Excretory system | Kidneys, bladder | Remove wastes; balance water |
| Immune system | White blood cells, lymph nodes, spleen | Fight infection |
| Integumentary | Skin, hair, nails | Protection, temperature control |
| Reproductive | Ovaries, testes | Produce offspring |
Two control systems
The body has two ways of sending messages:
| Nervous system | Endocrine system | |
|---|---|---|
| Messenger | Electrical impulses along nerves | Hormones in the blood |
| Speed | Very fast (milliseconds) | Slower (seconds to hours) |
| Effect lasts | Briefly | Longer |
| Example | Pulling your hand from a hot stove | Growth hormone controlling height |
Homeostasis
Homeostasis is keeping internal conditions stable, even when the outside world changes. Systems work together to control:
- Body temperature (about 37 °C)
- Blood sugar
- Water balance
- Oxygen and carbon dioxide levels
Example: When you get too hot, your nervous system detects it. Your skin makes sweat, and blood vessels near the skin widen to release heat. When you’re cold, you shiver (muscles make heat) and blood vessels narrow.
How systems work together
Getting oxygen to cells
- The respiratory system takes in oxygen; gas exchange happens in tiny air sacs called alveoli.
- The circulatory system carries oxygen in red blood cells.
- Cells use oxygen to release energy from food.
- Carbon dioxide travels back to the lungs to be breathed out.
Using food
- The digestive system breaks food into nutrients like glucose.
- Nutrients enter the blood through the small intestine.
- The circulatory system delivers them to cells.
- The pancreas (endocrine system) releases insulin to help cells take in glucose.
- The excretory system removes wastes such as urea through the kidneys.
Fight or flight
When you’re frightened:
- the nervous system signals the adrenal glands
- they release the hormone adrenaline
- your heart beats faster and you breathe faster
- more blood flows to your muscles so you can react quickly
Fighting disease
The immune system defends the body:
- First line: skin, mucus and stomach acid block germs.
- Second line: white blood cells attack invaders and cause inflammation.
- Third line: specific defences make antibodies and remember germs, so you fight them faster next time. Vaccines train this memory.
When systems fail
Diseases often affect more than one system. Diabetes is an endocrine problem (not enough insulin, or cells don’t respond to it), but it can damage the circulatory system, kidneys, eyes and nerves.
Common mistakes
- Thinking the heart’s only job is pumping. It depends on nervous signals and hormones to change speed.
- Mixing up excretion and digestion. Digestion breaks down food; excretion removes wastes made by cells.
Try it yourself
Choose an activity, such as running a race, eating a sandwich or getting a cold. List at least four systems involved and explain what each one does.
Quick recap
- The body has about eleven organ systems that depend on each other.
- The nervous system sends fast signals; the endocrine system uses slower hormones.
- Homeostasis keeps temperature, blood sugar and water levels stable.
- Respiratory and circulatory systems work together to deliver oxygen.
- The immune system defends the body in three lines.