Because mass is constant, while weight depends on gravity.
Explanation:
For example, your mass on Earth will be the same as your mass on planet Mars. Your weight however, will change depending on the force of gravity exerted on you.
Through the process of respiration , in lung the carbon dioxide from the blood is exchanged with oxygen.
The smallest dunctional unit if the lung is the alveoli
At this level the gases are exchange through the capillaries present close to the alveoli.
The concentration gradient ,meaning the greater amount of oxygen present in the lungs and greater amount of carbon dioxide in the blood creates a gradient that allow the movment of oxygen into the blood and carbon dioxide in the alveoli through the membrane .
Partial pressure created allows the easy movement .The carbon dioxide get into the trachea and expelled through nose and mouth.
