Blood collects oxygen from the lungs and transports it throughout the body. On its return trip to the heart, blood collects carbon dioxide and brings it back to the lungs to be exhaled. Blood also delivers electrolytes, nutrients and vitamins, hormones, clotting factors and proteins to cells throughout the body.

An adult human has about 5 liters of blood, which accounts for 7 to 8 percent of total body weight. About 55 percent of blood (about 2.75 to 3 liters) is plasma (or the liquid portion of blood); the rest is made up of red blood cells (erythrocytes), white blood cells (leukocytes), and platelets (thrombocytes). Red blood cells carry oxygen from the lungs, white blood cells help to fight infection and platelets enable the blood to clot.

Most blood cells are created in bone marrow, the spongy substance found inside a bone’s structure. There are two types of marrow, called red and yellow; both contain blood vessels and veins that transport nutrients and waste in and out of bones. Yellow marrow is composed mostly of fat and resides in the hollow centers of long bones, such as thigh bones. Red marrow is found in the center of flat bones such as the ribs and shoulder blades and actively produces blood cells.
Read more about what part of the body makes blood.

The production of blood cells in the skeleton changes as we age. At birth, all human marrow is red, allowing the body to produce more blood cells, which the body needs to grow. As the body matures, some of the red marrow is replaced with yellow marrow. In fully grown adults, the amount of red and yellow marrow is about equal. The bones that make blood cells are those with high concentrations of red marrow: the spine, sternum, ribs, pelvis and small parts of the upper arm and leg.

The process by which the body produces blood is called hematopoiesis. Bone marrow produces 200 billion red blood cells, 10 billion white blood cells and 400 billion platelets each day. All three types of blood cells come from the same type of cells, called pluripotential hematopoietic stem cells, which have the potential to form any of the different types of blood cells and also to self-replicate.

Blood cells begin life as stem cells. As these cells mature, they divide and either create more stem cells or evolve into progenitor cells, which will then further develop into red or white blood cells or platelets. (Once progenitor cells form, their future cell type is determined.) Some of these stem cells travel to other parts of the body and develop further while others remain and mature in the bone marrow.

As the most abundant type of blood cell in a healthy body, red blood cells distribute oxygen and essential nutrients throughout the body. They make up about 40 to 45 percent of the blood and provide its red color. This percentage is known as the hematocrit and is frequently measured by doctors in what is known as a complete blood count (CBC) test. The normal ratio is 600 red blood cells to one white blood cell and 40 platelets.

Red blood cells are structured differently than other cells. They are round and flat biconcave discs that look somewhat like a shallow bowl. A red blood cell has no nucleus, and it can change shape without breaking, enabling it to squeeze through capillaries.

The largest of the three types of blood cell, white blood cells regularly circulate in the bloodstream, so they are ready to leave the bloodstream and enter other tissues when an infection is detected. While most white blood cells are produced in the body’s red marrow, they also can be produced in special glands in other parts of the body when more are needed. An increase in the white blood cell count is typically a sign of infection; these cells are able to quickly reproduce to better fight off foreign objects in the system.
Read more about the difference between red and white blood cells.

There are five main types of white blood cells: lymphocytes, neutrophils, monocytes, eosinophils and basophils. Eosinophils and basophils contain digestive enzymes in granules in their cells and are also known as granulocytes. Each of the different types plays its own role, depending on the type of infection: bacterial, viral, fungal or parasitic. They also ingest unnecessary matter (such as dead cells, tissue debris and old red blood cells), protect against foreign bodies such as allergens and protect against mutated cells such as cancer.

Lymphocytes direct the body's immune system; unlike other white blood cells, they can recognize and remember invading bacteria and viruses. Neutrophils kill bacteria through a process known as phagocytosis. Monocytes enter the tissue, become larger and turn into macrophages where they can phagocytize bacteria in the body. (They also destroy old, damaged and dead cells in the body.) These macrophages are found in the liver, spleen, lungs, lymph nodes, skin and intestine. Eosinophils kill parasites and basophils fight allergic reactions.

Platelets, or blood cell fragments, form a platelet plug to seal small cuts or breaks in the blood vessel walls. They help blood to clot, which stops the body from losing too much blood. Like red and white blood cells, they are created in bone marrow, where very large cells called megakaryocytes break up into cellular fragments called platelets. These cells do not have a nucleus and do not reproduce.

Sometimes bone marrow does not produce enough healthy red or white blood cells. This can lead to fatigue and infection. This failure may be triggered by external factors such as chemicals, radiation or certain viral infections, or by other unknown stimuli that provoke the body’s own immune system into destroying stem cells. In other rare cases, bone marrow failure syndromes may be genetic.

Too few platelets can lead to spontaneous or uncontrolled bleeding. When the number of red blood cells is less than normal, less oxygen is delivered to the body’s cells, causing a condition known as anemia. While anemia is not necessarily a dangerous condition, it may indicate a more serious disorder or even cancer.

In aplastic anemia, the bone marrow stem cells are damaged, and normal blood production slows down or even stops. Although production levels drop, the cells that are produced are normal. Aplastic anemia is most commonly seen in those 20 to 25 years old and people older than 60 years, affecting about four out of every 1 million people in the United States each year. When it occurs in children, it is most likely genetic and caused by abnormal chromosomes.

Myelodysplastic syndrome (MDS) generally involves the production of defective stem cells. Instead of developing into healthy red or white blood cells or platelets, these cells die in the bone marrow. In some cases, this develops into leukemia, a type of blood cancer. MDS affect more than 15,000 people in the United States each year and typically affects those between 70 and 80 years.

Lymphoma, which starts in lymph nodes, and multiple myeloma, a cancer that starts in white blood cells, are both cancers that can spread to bone marrow and interfere with the production of blood cells. These diseases may be treated with radiation or chemical treatments or with stem cell or bone marrow transplants.