What will happen if platelets were absent in the blood?

Why is it necessary to separate oxygenated and deoxygenated blood in mammals and birds?

In which of the following vertebrate group/groups,does the heart not pump oxygenated blood to different parts of the body?

$(i)$ Why are the valves needed in the heart?
$(ii)$ Leakage of blood from vessels reduces the efficiency of the pumping system. How is the leakage prevented?

How are oxygen and carbon dioxide transported in human beings?

$(a)$ 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?