What role does bile play in digestion?

$(a)$ What is the role of mucus in the stomach?
$(b)$ How is the exit of food from the stomach regulated?
$(c)$ Where does food enter from the stomach?

Explain the process of breathing in humans.

Why is blood circulation in the human heart called double circulation?

What is common for $Cuscuta$,ticks,and leeches?

Which organ secretes a hormone when the blood sugar rises? Name a digestive enzyme released by this organ.