Why do veins have thin walls as compared to arteries?

On which type of food does salivary amylase act in the buccal cavity,and what is the name of the initial product formed due to the action of this enzyme?

Name the cell organelle in which the breakdown of pyruvate to give carbon dioxide,water,and energy takes place.

Oxygen liberated during photosynthesis comes from

$(a)$ Name the following:
$(i)$ Part where air is filtered by fine hair and mucus.
$(ii)$ Part which terminates in balloon-like structures.
$(iii)$ Balloon-like structures where exchange of gases takes place.
$(iv)$ Part which separates the chest cavity from the abdominal cavity.
$(b)$ Why is the rate of breathing in aquatic organisms much faster than in terrestrial organisms?

In which part of the alimentary canal is food finally digested?