The heart is a muscular organ that pumps blood. blood that contains waste materials is brought to the heart by blood vessels. this blood enters the __1__ and then moves down. the presence of the __2__prevets the backflow of blood inside the heart 1. right atrium left atrium right ventricle 2. atrioventricular valve valves in the aorta valves in the veins

Answer:

The blood enters the right artrium.The Presence of valves prevents backflow of blood.

Explanation:

The heart is a muscular organ, blood enters the heart through two large veins that is superior and inferior vena cava. Valves opens and close tightly, as heart pumps the blood, the valves prevent the blood to stop flowing in the backward direction. There are two main valves which is tricuspid valve and bicuspid valve. It helps in preventing back flow. The tricuspid valve is present in between right atrium and ventricle. The biscuspid valve is present in between left ventricle and left atrium.

the correct answer, according to e2020, is the second choice there is an occluded front in region a and a cold front in region b.