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.

In fetal mammals, oxygenated blood returns to the fetus via the umbilical veins. It then drains into the right side of the heart. There would be no use in sending oxygenated blood into non-aerated lungs, so it is diverted through a channel, the ductus arteriosus, into the aorta of the fetus.

At birth, this structure closes rapidly, allowing blood to pass the lungs of the newborn. A residual band of connective tissue, the ligamentum arteriosum, marks the site of the original structure. The foramen ovale is a developmental aperture between the atria, and it usually closes spontaneously as the heart develops.

The circulatory system of the body is composed of blood vessels. There are three main types of blood vessels: the arteries, the capillaries, and the veins.

Arteries always carry blood away from the heart. Usually the blood is oxygenated; the exceptions are the pulmonary arteries, which carry blood away from the heart to the lungs to become oxygenated. Veins carry deoxygenated blood towards to heart from tissue, with the exception of the pulmonary veins, which carry oxygenated blood from the lungs to the heart. Capillaries allow for exchange between blood and tissue.

Begin by remembering that arteries carry blood away from the heart. Veins carry blood from the body back to the heart. Note that the distinction between the blood in arteries and veins does not lie in the oxygen-content. Rather, it lies in the direction of flow. Arteries always take blood away from the heart, whereas veins always take blood back to the heart.

Arteries carry blood away from the heart. In all but one case, arteries carry oxygen-rich blood. The exception is the pulmonary arteries. They carry oxygen-poor blood away from the heart, to the lungs, to pick up more oxygen. Veins carry blood back to the heart. In all but one case, veins carry oxygen-poor blood. The exception is the pulmonary veins. They carry oxygen-rich blood from the lungs back to the heart.

Veins are the only circulatory vessels that contain valves. These structures contain valves because much of the pumping pressure that originated in the heart has been reduced by the time blood reaches veins, which return deoxygenated blood to the heart. The reduction of force can cause the blood to pool or move very slowly. Since veins rely mostly on gravity, blood flow force (originating from the heart), and muscular contractions to squeeze blood through them, it makes sense for them to have valves to keep blood from flowing backwards in between muscular contractions.