The heart is composed of cardiac muscle. This is the only location in the entire body that contains this type of muscle. Skeletal muscles are those attached to bones, and are under voluntary control. While contraction and relaxation of smooth muscle is involuntary, it is not found in the heart. Cardiac muscle is unique in that the cells are connected via gap junctions, which allows rapid electrical conduction between them, producing wavelike contractions of the heart, essential to its function.

The right side of the heart contains oxygen-poor blood. The right atrium receives oxygen-poor blood from the body through the superior and inferior vena cavae. This blood then flows into the right ventricle and then out the pulmonary valve into the lungs where the blood becomes oxygen-rich. The oxygen-rich blood goes to the right atrium then to the right ventricle where it is pumped to the system to provide oxygen to the cells of the body.

There are four valves in the heart. The tricuspid valve separates the right atrium from the right ventricle. The pulmonary valve separates the right ventricle from the pulmonary arteries. The bicuspid valve separates the left atrium from the left ventricle. The aortic valve separates the left ventricle from the aorta. These valves separate the chambers of the heart, and prevent back flow from one chamber to the other. The tricuspid valve is also known as the right atrioventricular valve, and the bicuspid valve is also known as the left atrioventricular valve, and the mitral valve.

The pulmonary circulation carries oxygenated blood from the lungs to the left atrium via the pulmonary veins. From there it enters the left ventricle, then it is pumped out through the aorta to serve organs of the body.

The right ventricle pumps oxygen-poor blood into the pulmonary circulation, not oxygen-rich blood. This blood leaves the right ventricle through the pulmonary artery and reaches the lungs. All other answer choices are true statements.

The first sound (“lub”) occurs when the mitral and the tricuspid valves close. The second sound (“dub”) occurs when the pulmonary and the aortic valves close. The mitral and tricuspid valves close after both the left and the right atrias have contracted and have released blood into the ventricles. Transitioning towards the second heart sound, the ventricles contract and release blood into the systemic and pulmonary circulatory systems. If an extra noise is heard between the “lub” and the “dub” sounds, then it meant that while the ventricle is contracting, some blood is being pushed back into the atria(s) because the tricuspid valve and/or the mitral valve(s) did not completely close, resulting in regurgitation. 

The “dub” sound occurs when both the aortic and the pulmonary valves close at the same time. When one closes later than the other, then the valve that closed late will create a splitting sound; therefore, creating an extra sound close to the first sound.

The "lub" heart sound occurs when both the mitral and the tricuspid valves close at the same time. If one hears a split sound, then it indicates that one of the two valves closed late.

The activation of inactive bone marrows occur when the increase production of red blood cells is required. An major injury that result massive bleeding will reactive inactive bone marrows. Of the answer choices, a rupture of the abdominal aorta will result in massive internal bleeding. 

Blood is oxygenated in the lungs and pumped through the pulmonary veins to the left atrium of the heart. The blood is then pumped to the left ventricle, which pumps the oxygen-rich blood throughout the body. The oxygen-poor blood returns to the heart by entering the right atrium, and remains oxygen poor as it is pumped into the right ventricle and through the pulmonary arteries until it is oxygenated again in the lungs.