When blood passes through a capillary bed in the
it downloards O2 and uploads CO2.

Exchange of gases takes place at alveoli sacs of lungs, which are equipped with capillary vessels or bed.

Explanation:

On inspiration or taking air through nostrils, surrounding air enters the lungs and it reaches air sacs or alveoli. Here, the outside air will have more amount of oxygen when compared with oxygen in blood at that site. Hemoglobin in the blood have high bonding capability to oxygen, so it captures oxygen and takes it to heart for pumping into other parts of the body. Likewise, the circulations continues from lungs to heart > hear to body parts > body parts to heart > heart to lungs for purification.

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