Exercise 3.3
During exercise or running, muscles need more oxygen, so breathing becomes faster and blood flow increases (face appears red).
Not cross-checked
NCERT prints no numerical answer for this exercise, so this working has not been cross-checked against the book.
You breathe faster while running because the muscles need more oxygen, and your face turns red because the blood flow increases.
Working muscles use up oxygen quickly, so the breathing rate rises to take in more air.
The rib cage makes this possible: the ribs are joined to the spine at the back and to the sternum in front by flexible cartilage, letting the cage expand and contract so air moves in and out of the lungs.
The nervous tissue is in charge of the adjustment — during exercise the brain signals the heart to beat faster to meet the body's increased oxygen demand.
More blood flowing through the vessels close to the skin of the face is what makes it look red.