Link lungs, digestive organs and kidneys to cellular needs.
You breathe, eat, drink and pass urine. Each action links the outside world to the needs of cells inside your body.
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Alveolus
A tiny air space in a lung, beside a network of capillaries.
For example
Its thin wall shortens the distance gases must cross.
Read all notes for this section
Alveolus
A tiny air space in a lung, beside a network of capillaries. Its thin wall shortens the distance gases must cross.
Digestion / absorption
Digestion breaks food down. Absorption moves suitable products across the gut lining into the body. Breaking starch into sugars is digestion; taking sugars into blood is absorption.
Metabolism
All the chemical reactions in cells. Catabolism breaks molecules down; anabolism builds them. Cells break down fuel to release energy and build proteins for repair.
Excretion
Removal of wastes produced by the body’s metabolism. Lungs excrete carbon dioxide. Kidneys remove urea in urine.
Bring oxygen in
Breathing moves air into and out of the lungs. Air travels down the windpipe (trachea), through branching airways (bronchi), to tiny air sacs called alveoli. Oxygen crosses their thin walls into blood; carbon dioxide crosses from blood into the air to be breathed out.
Turn food into usable materials
Digestion breaks food into small components. Absorption is the next step: those components cross the gut lining into blood or lymph. Most nutrient absorption happens in the small intestine. The liver makes bile to help handle fats; the pancreas supplies enzymes, substances that help break food down.
Remove what cells no longer need
Cells use nutrients in chemical reactions called metabolism. These reactions also produce wastes. Lungs remove carbon dioxide. Kidneys remove dissolved wastes from blood and adjust water and salts while making urine. Follow the urine: kidneys → ureters → bladder → urethra → outside. The bladder stores urine; it does not make it.
Explore the illustration
Look at the whole view first. Then select one label and connect it to the idea you just read.
Airways, lungs and alveoli
The branching airway ends in many tiny exchange surfaces. The circular inset is magnified, not to the same scale.
More controls
01 / 7 Trachea
Inhalation: the diaphragm descends and the lungs expand. Exhalation: the diaphragm relaxes upwards and air leaves. Motion is slowed and exaggerated to show direction. Reduced motion is enabled. Use the arrows to study one step at a time, or choose Play.
Trachea
The trachea conducts air and divides into right and left main bronchi. Its cartilage supports the airway.
Adapted for this lesson: labels removed, artwork cropped or recoloured, and interactive teaching overlays added. Adapted artwork retains its source licence.
Go deeper
In-depth reading
Follow this example step by step, then try explaining it yourself. The extra reading is optional: you can return to it after exploring the illustration.
Worked example: from a bite of bread to a working cell
Chewing breaks bread into pieces. Digestive enzymes break its starch into smaller sugars, including glucose. This is digestion; the sugars are still inside the digestive tube.
Glucose crosses the small-intestinal lining into blood. This is absorption. Blood carries it first towards the liver and eventually to other tissues. Cells can use glucose and oxygen in reactions that release usable energy.
Carbon dioxide produced by cells travels back to the lungs. Undigested or unabsorbed material leaves the gut in faeces. These are different routes: breathing out removes a metabolic waste, while passing faeces removes intestinal contents.
Explain it yourself
Is breaking food down the same as getting it into the blood?
Show the explanation
No. Digestion breaks food down; absorption moves useful products across the intestinal lining. A nutrient must cross that lining before blood can carry it elsewhere.
More detail — when you are ready
These notes expand the mechanisms and vocabulary. Read one topic at a time and find the structures in the illustration.
Air movement is not the same as gas exchange
Ventilation moves air along conducting passages into and out of the lungs. Alveoli provide a large exchange surface next to pulmonary capillaries.
Oxygen diffuses towards blood and carbon dioxide towards alveolar air because of differences in their partial pressures, a measure of each gas’s contribution to pressure. Each gas diffuses from higher to lower partial pressure.
Blood then transports gases between lungs and tissues. Cells use oxygen in aerobic reactions, meaning reactions that require oxygen, to release energy from nutrients.
Carbon dioxide is one waste product of these reactions. A clear airway alone does not guarantee adequate tissue oxygen delivery: the exchange surface, circulation and oxygen-carrying blood must also function.
Follow food from ingestion to absorption
The alimentary canal runs through mouth, pharynx, oesophagus, stomach, small intestine, large intestine, rectum and anus. Mechanical processing and digestive enzymes break food into smaller components.
Absorption then moves usable components across the intestinal lining. Salivary glands, liver and pancreas are accessory organs: food does not pass through them, but their secretions reach the canal.
Bile helps disperse fat; it is not a digestive enzyme. Nutrients provide energy and materials for growth and repair.
Catabolism breaks molecules down, while anabolism builds larger molecules.
Compare three routes of waste removal
Lungs remove carbon dioxide; kidneys remove dissolved metabolic wastes while adjusting water, electrolytes and acid–base balance. Kidneys form urine, ureters transport it, the bladder stores it and the urethra conducts it out.
These roles should not be interchanged. Defaecation removes intestinal contents that include unabsorbed material and microbes.
It differs from renal excretion of urea made during nitrogen metabolism. The common destination outside the body does not make all three mechanisms identical.
Trace the origin of a waste before naming its route of removal.
Further reading — optional
Free external resources in English. Internet connection required to open the links. Choose the topic you want to explore; these readings are not required to complete the section.
Use the lung and tissue diagrams to follow oxygen and carbon dioxide in opposite directions. The pressure calculations are a later step.
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Sources and illustration notes
Bilingual teaching notes organised from your supplied anatomy reference pages. Medical SVG artwork is credited beneath each illustration; guided sequences and process overlays support slow, step-by-step study. Anatomy and translation await expert review.
