Group O can donate red blood cells to anybody. It's the universal donor.

O is a universal donor and all blood groups can receive blood group O. Blood group B can take blood from B group.

The hepatic vein is connected to the liver. It transports the deoxygenated blood with urea away from the liver to the inferior vena cava. That's why the hepatic vein contains the highest concentration of urea.

1. During systole, because of heart contraction blood is pumped into arteries.
2. Blood pressure is highest as it leaves the heart through the aorta and gradually decreases as it enters smaller and smaller blood vessels (arteries, arterioles, and capillaries).
3. While during atrial diastole, both atria are relaxing, dilating and filling.
4. Blood pressure drops to the lowest level during this state.