What prevents backflow of blood inside the heart during contraction?

$(a)$ What happens to the heart when muscles work harder?
$(b)$ Which body system is directly affected when a person has heart disease?
$(c)$ Which cells increase in number during infection?

What will happen if platelets were absent in the blood?

What are the components of the transport system in highly organised plants?

Single circulation,i.e.,blood flows through the heart only once during one cycle of passage through the body,is exhibited by:

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