Body systems & organ function

What you will learn

the levels of organisation: cell $\to$ tissue $\to$ organ $\to$ organ system,
the main human organ systems and what each does,
how structure is related to function (e.g. alveoli in lungs, villi in intestine),
how several systems cooperate during a single activity like running,
the idea of a simple feedback loop that keeps conditions steady.

Worked example 0 Real-world example: what happens when you sprint

You sprint for 100 m. List at least three body systems that cooperate, and what each contributes.

Muscular system — leg and arm muscles contract to move you forward.
Respiratory system — lungs take in extra oxygen and breathe out carbon dioxide.
Circulatory system — the heart pumps oxygen-rich blood to the working muscles.
Nervous system — the brain sends rapid signals to coordinate every movement.
Skeletal system — bones provide the rigid frame the muscles pull against.

Key idea: no single system runs a sprint. Whole-body activity always involves several systems working together.

1. Levels of organisation

Your body is not just a bag of cells. It is organised into neat layers:

\text{cell} \longrightarrow \text{tissue} \longrightarrow \text{organ} \longrightarrow \text{organ system} \longrightarrow \text{organism}.

A tissue is a group of similar cells doing the same job (e.g. muscle tissue, epithelial tissue).
An organ is made of several tissues working together (e.g. the stomach contains muscle, nervous, and glandular tissue).
An organ system is a group of organs cooperating on one big function (e.g. the digestive system).

Worked example 1 Classifying the levels

Place each of these in order from smallest to largest: stomach, digestive system, muscle cell, muscle tissue.

\text{muscle cell} \to \text{muscle tissue} \to \text{stomach} \to \text{digestive system}.

Key idea: each level is built from many copies of the level below.

2. The main human organ systems

System	Main job	Key organs
Digestive	Break food down; absorb nutrients	Mouth, stomach, small intestine, liver
Circulatory	Transport oxygen, nutrients, waste	Heart, arteries, veins, capillaries
Respiratory	Take in O $_2$ , remove CO $_2$	Lungs, trachea, diaphragm
Nervous	Sense the environment; control the body	Brain, spinal cord, nerves
Musculoskeletal	Support and move the body	Bones, muscles, tendons, joints
Excretory (urinary)	Remove liquid waste from blood	Kidneys, bladder, ureters
Reproductive	Produce offspring	Ovaries, testes, uterus
Endocrine	Release hormones that coordinate the body	Pancreas, thyroid, adrenal glands
Immune	Defend against pathogens	White blood cells, lymph nodes, spleen

3. Zoom in: the respiratory system

Simplified diagram of the human respiratory system. Air enters the trachea, branches into the bronchi, and reaches tiny alveoli where gas exchange occurs.

Air travels down the trachea (windpipe), splits into two bronchi, branches into smaller bronchioles, and ends in millions of tiny air sacs called alveoli. Alveoli have very thin walls and a huge combined surface area, which lets oxygen move rapidly into the blood and carbon dioxide move out. The diaphragm is a sheet of muscle below the lungs that pulls down to draw air in and relaxes to push air out.

Worked example 2 Structure matches function

Alveoli have three key features: very thin walls (one cell thick), a huge total surface area, and a rich blood supply. Explain how each feature helps gas exchange.

Thin walls — gases only have to diffuse a tiny distance, so it is fast.
Large surface area — lots of room for many gas molecules to cross at once.
Rich blood supply — blood carries oxygen away quickly, keeping a strong concentration difference across the wall.

Key idea: biology loves to match structure to function — what a thing is shaped like is almost always a clue to what it does.

4. Zoom in: the digestive system

Food travels through a long tube: mouth $\to$ oesophagus $\to$ stomach $\to$ small intestine $\to$ large intestine $\to$ anus. Along the way:

Teeth and saliva begin mechanical and chemical breakdown.
The stomach adds acid and enzymes to break proteins apart.
The small intestine is the main site of absorption — its inner wall is covered in finger-like villi that massively increase surface area.
The liver produces bile to help digest fats; the pancreas adds digestive enzymes.
The large intestine absorbs water, leaving solid waste.

Worked example 3 Why villi?

A student unfolds the small intestine of a sheep and measures its inner surface area as far larger than the outer tube surface. Explain using the idea of villi.

The inner wall is not smooth — it is covered in millions of tiny finger-like projections called villi.
Each villus is itself covered in even smaller microvilli.
This folding multiplies the surface area available for absorbing nutrients into the bloodstream.

Key idea: when a process happens at a surface, biology tends to fold that surface.

5. Cooperation and feedback

Systems never work in isolation. A meal digested by the digestive system is turned into glucose that the circulatory system delivers to every cell. Oxygen from the respiratory system reaches the same cells by the same blood. Carbon dioxide travels back the other way.

Homeostasis is the name for keeping internal conditions steady — temperature, blood sugar, water content. It usually works via a feedback loop:

A sensor detects a change (e.g. body temperature rising).
A control centre (often the brain) processes the signal.
An effector (e.g. sweat glands, blood vessels) acts to restore the normal value.

Worked example 4 Feedback: too hot

You go for a run on a hot day. Describe the feedback loop that prevents you overheating.

Sensor: temperature receptors in your skin and brain detect that body temperature is rising.
Control centre: the hypothalamus in the brain decides action is needed.
Effectors: sweat glands release sweat (cooling by evaporation); blood vessels near the skin widen so heat escapes.
Body temperature returns to about $37^{\circ}$ C.

Key idea: feedback loops act against the change that started them — so-called negative feedback.

Practice: Year 8

Fluency

Systems and organs

List the levels of organisation from cell to organism.
Name the main organ and the main job of (a) the circulatory system, (b) the respiratory system, (c) the excretory system.
Which organ produces bile?
Which system is in charge of rapid communication inside the body?
Name two organs of the digestive system.

Fluency

Structure and function

Name three features of an alveolus and match each to its function.
Why is the small intestine folded into villi?
How does the diaphragm help breathing?
Why does the heart have thick muscular walls?
Red blood cells have no nucleus. Suggest why this is useful for their job.

Reasoning

Explain and connect

Explain how the digestive and circulatory systems cooperate to get glucose to a muscle cell.
A person’s kidneys stop working. Predict two problems this will cause in the body.
Explain why damage to the spinal cord can stop legs from moving even if the legs themselves are healthy.
Use the structure-function principle to explain why bones are hollow rather than solid.

Problem solving

Scenarios

During heavy exercise, heart rate, breathing rate, and sweating all increase. Explain the benefit of each change.
A person cuts a finger. Outline which systems are involved in stopping the bleeding and fighting any infection.
After a big meal, blood sugar rises. Describe a simple feedback loop (using hormones) that returns blood sugar to normal. You may refer to “insulin from the pancreas.”
Explain why an athlete with poor lung function will also struggle to run fast, even if their muscles are strong.

Challenge

Reasoning

Harder reasoning

A patient has a condition where their villi are flattened. Predict the impact on their nutrition, and explain using surface area.
Compare the circulatory systems of a fish (single loop) and a mammal (double loop). Suggest why a double loop supports higher activity levels.
Explain how a failure of negative feedback in blood-sugar regulation causes the symptoms of diabetes.
A scientist claims “the nervous system is fast, the endocrine system is slow, but both are needed.” Give one example of each and explain why both speeds are useful.