The chordae tendinae are strong filaments that attach to the valves in the heart. They keep the valves closed during contraction (i.e. they prevent the valves from opening into the atriums) so that blood does not flow back to the previous chamber. The mitral valve, after the left atrium, is where oxygenated blood arrives when it travels back to the heart from the left and right pulmonary veins. If the chordae tendinae of the mitral valve were damaged, the valve would not function properly and blood could possibly flow backwards towards the pulmonary veins and into the lungs. Since the chordae tendinae keep valves closed during contraction, they would not seal tight enough to keep normal pressure within the left ventricle if they were torn or damaged. The tricuspid valve is on the right side of the heart. So it will not be affected by the chordae tendinae of the mitral valve and in any case blood flow in this part of the heart could not flow back to the pulmonary veins.

The cardiac cycle consists of the filling of the right atrium with venous blood (oxygen-poor blood that has returned from the body to now be pumped into the lungs for oxygenation), and opening of the tricuspid valve to allow transfer of blood to the the right ventricle. The right atrium contracts to do this. Then, the tricuspid valve closes and the right ventricle contracts to pump the blood through the pulmonary valve into the pulmonary arteries, which carry oxygen-poor blood into the lungs to be oxygenated. Pulmonary veins carry oxygen-rich blood from the lungs into the left atrium. The mitral valve opens to allow the flow of blood from the left atrium into the left ventricle, the left atrium contracts to help. The mitral valve closes and the left ventricle contracts, pumping blood through the aortic valve into the aorta, through which oxygen-rich blood is transferred to the whole body. 

The myocardium is the layer of the heart that contains the muscle cells. Remember that "myo" refers to muscle and "cardio" refers to the heart. "Epi" means above or over, "endo" means within or inner; the epicardium and endocardium are serous membranes that comprise part of the pericardium, which protects and lubricates the heart.

Pulmonary arteries leaves the right ventricle and go to the lungs to get oxygenated. This oxygen-rich blood is returned to the left atrium via the pulmonary veins. Umbilical veins take oxygen-rich blood away from the placenta and supply the fetus. The umbilical arteries return the oxygen-poor blood back to the mother, for it to be oxygenated. 

The left ventricle is the chamber responsible for pumping newly reoxygenated blood into the systemic circuit, which makes up the vast majority of the body. Accordingly, it is more muscular and stronger in order to compensate for having to force blood through the largest area. 

Arteries are responsible for carrying blood to organs, traveling away from the heart, while veins are responsible for transporting blood back toward the heart. Arteries generally carry oxygenated blood to deliver oxygen to organs, and veins generally carry deoxygenated blood back to the heart for re-oxygenation. The singular exceptions are the pulmonary arteries and pulmonary veins. The pulmonary arteries still travel away from the heart, but carry deoxygenated blood from the right atrium to the lungs. The pulmonary veins still travel toward the heart, but carry oxygenated blood from the lungs to the left atrium.

Deoxygenated blood returning from the body is carried in the vena cava, which empties into the right atrium. Blood passes through the tricuspid valve and enters the right ventricle. Then, the deoxygenated blood is carried to the lungs by the pulmonary arteries. Once in the lungs, the blood becomes oxygenated.

The oxygenated blood is carried back to the heart by the pulmonary veins. The blood enters the left atrium and passes through the bicuspid (mitral) valve to enter the left ventricle. From the left ventricle it is pumped out of the heart and into the aorta to be circulated through the body and deliver oxygen to the tissues

Always remember that the pulmonary arteries carry DEOXYGENATED blood and the pulmonary veins carry OXYGENATED blood. This is the exception to the rule that states that veins carry deoxygenated blood and arteries carry oxygenated blood.  

A common misconception is that veins always carry deoxygenated blood. Veins always return blood to the heart, but the pulmonary veins are bringing oxygenated blood back to the heart so that it may be pumped to the body's tissues. Deoxygenated blood travels through the right ventricle to the pulmonary arteries. In the lungs it is reoxygenated, then returned to the heart in the pulmonary veins.

Blood is able to flow through the blood vessels due to pressure caused by the pumping of the heart. The pressure is the greatest in vessels just leaving the heart (arteries), and decreases as the blood vessels get further away from the heart. Veins have the lowest pressures out of all the blood vessels. 

Arterial walls have three distinct layers: an inner smooth endothelium, a heavy coat of smooth muscle cells and elastic fibers, and a looser outer layer of connective tissue. Arteries are uniformly recognizable as robust, thick-walled structures compared with nearby veins.

Although some veins have muscular fibers in their walls (for example, those in the legs), this layer of cells is thin and often incomplete. Instead of muscle to counteract gravity, veins usually use valves to prevent blood from flowing backward. Arteries, veins, capillaries, and the heart chambers all have an endothelial layer.