Alveoli

Inside the lungs, the air passage divides into smaller and smaller tubes. Each tube ends in balloon-like alveoli. One of them is called an alveolus. Their walls carry a dense network of blood vessels.

Blood brings carbon dioxide from the body and gives it up into the alveoli. It takes up oxygen from the air inside them. Haemoglobin, the respiratory pigment in red blood cells, then carries that oxygen to the rest of the body.

A small example

Follow one breath in. We lift our ribs and flatten the diaphragm. The chest cavity becomes larger. Air is sucked into the lungs and fills the alveoli. The lungs always keep some air in them, even after we breathe out. That gives oxygen time to be taken up and carbon dioxide time to leave.

Where marks go

Question 16 of CBSE's 2026-27 Science sample paper is a 5-mark question with an internal choice. Option B shows two diagrams and asks which one shows breathing in, and how it happens. The board's Science marking scheme for 2026-27 gives 1 mark for picking the right diagram. Then it gives half a mark for each of four points:

• The ribs are lifted.
• The diaphragm is flattened.
• The chest cavity becomes larger.
• Air is sucked in.

An answer that skips a step loses that half mark. The version for visually impaired students adds 1 mark for how the lungs give a large area: many alveoli, richly supplied with blood. Read more on how step marking works.

About the author

Deepti Sardana

Founder Member & Academic Lead

An IIT alumna (M.Sc., M.Phil.) with over 25 years in teaching, child psychology and learning assessment. As Academic Lead at EduBrahma, she leads the method TRUtesting uses to find where and why a child loses marks.

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