a. Ventricles pump blood to long distances for example- right ventricle pumps blood to lungs for oxygenation and left ventricle pumps blood to the whole body. So walls of ventricles are thicker as they have to pump blood to long distance, so they contract more and have to withstand more pressure.

b. Breathing cycle is rhythmic which involves inhalation and exhalation of air. But the exchange of gases is a continuous process which takes place in alveoli between blood capillaries and air sacs. Even though after forceful exhalation, some residual air is left inside air sacs, so the exchange of this air takes place between air sacs and blood capillaries. Since cells continuously take in oxygen for oxidation of glucose to release energy and carbon dioxide is given out hence exchange of gases occurs continuously.

c. During day time, green plants perform photosynthesis so the carbon dioxide produced in respiration is used by green plants for photosynthesis. As carbon dioxide is an important raw material for photosynthesis process. So in green plants during the day carbon dioxide is generated in respiration but is not released out.