Question

Why does the heart have valves at all the apertures? Write about the bicuspid and tricuspid valves.

Answer 1

The circulation of blood through the human heart occurs in only one direction.
The heart pumps blood by going through a series of regular contractions and relaxations (cardiac cycle). And with each contraction of the ventricles the blood is pushed into the arteries. So, there is a constant change in pressure in between the heart chambers (between the auricles and ventricles) and in between the blood vessels and heart chambers.

Complete answer:
The valves present in the heart prevent the backflow of blood. Valves are present in between the auricles and ventricles and in the pulmonary artery and aorta.
During contraction of the ventricle (systole), the auricles are in a relaxed state (diastole). So, as the pressure rises in the ventricles, blood may leak out into the auricles. This is prevented with the help of valves. Similarly, in the pulmonary artery and aorta, a pair of valves are present that prevent the backflow of blood into the ventricles during ventricular diastole.
The valve present between the right auricle and right ventricle is known as a tricuspid valve. It has three flaps or cusps. The valve present between the left auricle and ventricle is known as a bicuspid valve. It has two flaps or cusps.
Special fibrous cords known as Chordae Tendinae are attached to the flaps of bicuspid and tricuspid valves at one end and their outer ends are attached to the ventricular walls with special muscles known as papillary muscles. The chordae tendineae prevent the bicuspid and the tricuspid valve from collapsing back into the atria during powerful ventricular contractions.
Three semilunar valves are found at the points where the pulmonary artery leaves the right ventricle and in the aorta which emerges from the left ventricle. The valves are crescent, half-moon shaped, and hence they are named so.

Figure showing the four valves of the heart.

Note:
It is worthy to note that the veins also have valves. This is due to the fact that the pressure in the veins is very much low as compared to the arteries and so blood may not pass through the veins and accumulate in the capillaries, especially in the limbs. Thus, the valves in veins prevent this backflow and accumulation. A disease called varicose veins occurs in the legs of people who stand all day long. In this case, the valves in the veins are stretched beyond the point where they become dysfunctional. Thus, blood accumulates in the legs, the legs eventually become swollen and immense pain occurs.