Why do veins have thin walls as compared to arteries?

How do the guard cells regulate the opening and closing of stomatal pores?

Liver is the largest organ as well as the largest gland of the body.

$(a)$ 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 $Xylem$,minerals in solution form move in both directions (upward and downward).

How does aerobic respiration differ from anaerobic respiration?