Breathing is not respiration

Activity 9.3 puts equal amounts of fresh lime water in two test tubes. Into A, ordinary air is pushed with a syringe or pichkari. Into B, you blow your breathed-out air through a straw.

The lime water in B turns milky; A does not. Lime water turns milky with carbon dioxide. So the air we breathe out has more carbon dioxide than the air we breathe in.

The book gives the numbers:

• Breathed-in air: about 21% oxygen, about 0.04% carbon dioxide.
• Breathed-out air: about 16–17% oxygen, about 4–5% carbon dioxide.

Notice that not all the oxygen gets used up.

Where does the swap happen? Fresh air fills the alveoli. Their walls are very thin and wrapped in fine tubes full of blood. Blood brings carbon dioxide from the body to the alveoli and lets it go into the air; at the same moment, oxygen passes from the alveoli into the blood, which carries it all round the body.

Now the key idea. When we eat, the body breaks food down into simple substances like glucose. Oxygen helps break down glucose to release energy. This is respiration:

• Glucose + Oxygen → Carbon dioxide + Water + Energy

So breathing and respiration are not the same:

• Breathing brings oxygen in and takes carbon dioxide out. It is a physical process — just moving air.
• Respiration uses that oxygen to break down food and release energy, inside the body. It is a chemical process.

Both are essential. The energy from respiration lets us walk, run, play and even think.

The body also needs a way to deliver food, oxygen and other things everywhere: the circulatory system — the heart, blood and blood vessels. The heart pumps blood through the vessels, delivering what the body needs and carrying waste away.