Blood is produced in bone marrow. Substances inside the bone marrow make various kinds of blood cells. All bones produce blood cells in bone marrow until adulthood. Bones that still produce the blood cells, such as the spine and ribs, are called red marrow. Bones that don’t produce blood anymore are called yellow marrow. All blood is produced through the same stem cell, through the stem blood can be changed into a different kind of blood cell such as a white or red blood cell. The body also knows when to make more blood cells. If oxygen is low the body will signal the stem cell to make more blood cells.

The human heart is a vital organ that uses cardiac cycles to pump blood through the body. Systemic circulation transfers blood with oxygen from the left side of the heart to body tissues. The left ventricle pumps blood through the arteries. It passes through peripheral tissues before returning to the right atrium of the heart. Blood enters the right ventricle and is pumped through the pulmonary artery to the lungs. The pulmonary veins return deoxygenated blood to the left atrium of the heart. Pulmonary circulation carries blood that is deprived oxygen to the lungs from the right side of the lungs. After the blood receives oxygen it returns to the left side of the heart. Blood vessels allow transportation of blood throughout the body. These vessels make sure blood flows efficiently and quickly to everywhere in the body from the heart. Distribution of blood is important because the whole body needs blood.