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.

Erythrocytes and leukocytes are both produced and mature in bone marrow, with the exception of T cells, a type of white blood cell that matures in the thymus. Unlike leukocytes, red blood cells contain no nuclei or mitochondria, which could interfere with transporting oxygen, carbon dioxide, and nutrients.

Because Jack has blood type B, he will form antibodies against blood types A and AB, as they contain a foreign antigen that his body will reject. Furthermore, he cannot accept any blood types that are Rh⁺, as this antigen will also seem foreign to his body. He can thus only accept from the blood types B^- and O^- (the universal donor type).

Arteries have thick, muscular walls that allow for constriction and flow direction, while veins have thin walls to carry blood.

Capillaries have extremely thin walls to allow exchange of oxygen, carbon dioxide, and nutrients with tissues, resulting in both oxygenated and deoxygenated blood in these vessels. Pressure in the arteries is always higher than in veins so that blood can be continuously pushed forward, negating the need for valves to prevent backflow. Such valves are present in veins and help to counteract gravity when returning blood to the heart.

Hemoglobin will have varying affinity for oxygen depending on its environment. For example, hemoglobin will have a very high affinity for oxygen in the lungs, where most oxygen is loaded onto the hemoglobin molecules. Once hemoglobin goes to the tissues of the body, there is a much lower oxygen tension. This decreased oxygen causes hemoglobin to have a lower affinity for oxygen and release the oxygen to the tissues.

Hemoglobin is comprised of two alpha and two beta proteins and uses iron to facilitate oxygen transportation. Some variations of hemoglobin, such as fetal hemoglobin, contain gamma proteins that changes the shape of the protein. Consistent with the theme that structure determines function, fetal hemoglobin has a higher affinity for oxygen than does adult hemoglobin. This is necessary since fetuses lack lungs; they obtain all of their oxygen from the hemoglobin of their mothers.

Capillaries are the smallest blood vessels, which allow transport of oxygen and other small molecules. There are capillaries involved in gas exchange in the lungs, but it does not involve transfer of oxygen to the cells. Rather, it involves uptake of oxygen by red blood cells from the air inside the alveoli, and removal of carbon dioxide from the blood into the air in the alveoli to be exhaled. All other blood vessels have walls that are too thick to allow transport of any substances across them. The heart is the muscular pump of the circulatory system, which provides the pressure required to drive blood flow.

Arteries carry blood away from the heart, while veins transport blood towards the heart. Because the pulmonary arteries transport blood from the right ventricle towards the lungs to exchange carbon dioxide for oxygen, they contain deoxygenated blood.

The aorta, however, transports oxygenated blood from the left ventricle to the rest of the body for circulation. The pulmonary vein carries oxygenated blood from the lungs to the left ventricle and the vena cavae return deoxygenated blood to the right atrium.

As fluid moves through the capillary, the hydrostatic pressure decreases from the arteriole end to the venule end (fluid exits the capillary along the gradient). The osmotic pressure in the interstitium is relatively constant, and will be stronger than capillary hydrostatic pressure near the venule end. As a result, an increase in the interstitial osmotic pressure would cause less fluid to enter the interstitium, because there is less area in the bed where the capillary hydrostatic pressure is greater than the interstitial osmotic pressure.

The capillary is the site of fluid exchange with the body's tissues. This fluid transfer is moderated by two factors: hydrostatic pressure and osmotic pressure. Hydrostatic pressure is the "pushing" force on water due to the presence of more fluid in one region than another. In general, larger fluid volumes generate higher hydrostatic pressure. Osmotic pressure is the "pulling" force on water due to the presence of solutes in solution. Albumin proteins are the main source of osmotic pressure in capillaries, pulling water into the blood.

At the arteriole end of the capillary, the hydrostatic pressure is stronger than the interstitial osmotic pressure and fluid is forced into the interstitium. Osmotic pressure remains relatively constant over the length of the capillary, but hydrostatic pressure drops sharply as it nears the venule end due to the initial loss of fluid volume. At that point, the interstitial osmotic pressure becomes stronger than the capillary's hydrostatic pressure. This forces fluid back into the capillary.