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www.GOALias.blogspot.comOn an average, an adult human beingat rest breathes in and out 15–18times in a minute. During heavyexercise, the breathing rate canincrease upto 25 times per minute.While we exercise, not only do webreathe fast, we also take deep breathsand thus inhale more oxygen.supplied to our cells. It speeds upthe breakdown of food and moreenergy is released. Does this explainwhy we feel hungry after a physicalactivity?When you feel drowsy, does yourbreathing rate slow down? Does yourbody receive sufficient oxygen?Activity 10.3Figure 10.3 shows the various activitiescarried out by a person during a normalFig. 10.3 Variation in the breathing rate duringdifferent activitiesPaheli wants to know whywe yawn when we aresleepy or drowsy.day. Can you say in which activity, therate of breathing will be the slowest andin which it will be the fastest? Assignnumbers to the pictures in the order ofincreasing rate of breathing accordingto your experience.10.3 HOWDO WE BREATHEREATHE?Let us now learn about the mechanismof breathing. Normally we take in airthrough our nostrils. When we inhaleair, it passes through our nostrils intothe nasal cavity. From the nasal cavity,the air reaches our lungs through thewindpipe. Lungs are present in thechest cavity (Fig. 10.4). This cavity issurrounded by ribs on the sides. A large,muscular sheet called diaphragm formsthe floor of the chest cavity (Fig. 10.4).Breathing involves the movement of thediaphragm and the rib cage.During inhalation, ribs move up andoutwards and diaphragm moves down.This movement increases space in ourchest cavity and air rushes into thelungs. The lungs get filled with air.During exhalation, ribs move down andinwards, while diaphragm moves up toits former position. This reduces the sizeof the chest cavity and air is pushed outof the lungs (Fig. 10.5). Thesemovements in our body can be felt111RESPIRATIONIN ORGANISMS111