How Blood Cells Are Made
All blood cells are produced in the bone marrow. As children, most of our bones produce blood. As we age this gradually diminishes to just the bones of the spine (vertebrae), breastbone (sternum), ribs, pelvis and small parts of the upper arm and leg. Bone marrow that actively produces blood cells is called red marrow, and bone marrow that no longer produces blood cells is called yellow marrow. The process by which the body produces blood is called hematopoiesis.
All blood cells (RBCs, WBCs and platelets) come from the same type of cell, called the pluripotential hematopoietic stem cell. This group of cells has the potential to form any of the different types of blood cell and also to reproduce itself. This cell then forms committed stem cells that will form specific types of blood cells.
During formation, the RBC eventually loses its nucleus and leaves the bone marrow as a reticulocyte. At this point, the reticulocyte contains some remnants of organelles. Eventually these organelles leave the cell and a mature erythrocyte is formed. RBCs last an average of 120 days in the bloodstream. When RBCs age, they are removed by macrophages in the liver and spleen.
A hormone called erythropoietin and low oxygen levels regulate the production of RBCs. Any factor that decreases the oxygen level in the body, such as lung disease or anemia (low number of RBCs), increases the level of erythropoietin in the body. Erythropoietin then stimulates production of RBCs by stimulating the stem cells to produce more RBCs and increasing how quickly they mature. Ninety percent of erythropoietin is made in the kidney. When both kidneys are removed, or when kidney failure is present, that person becomes anemic due to lack of erythropoietin. Iron, vitamin B-12 and folate are essential in the production of RBCs.
White Blood Cells
White blood cells (WBCs), or leukocytes, are a part of the immune system and help our bodies fight infection. They circulate in the blood so that they can be transported to an area where an infection has developed. In a normal adult body there are 4,000 to 10,000 (average 7,000) WBCs per microliter of blood. When the number of WBCs in your blood increases, this is a sign of an infection somewhere in your body.
There are five main types of WBCs:
Neutrophils, eosinophils and basophils are also called granulocytes because they have granules in their cells that contain digestive enzymes. Basophils have purple granules, eosinophils have orange-red granules and neutrophils have a faint blue-pink color.
What They Do
Neutrophils are the one of the bodyís main defenses against bacteria. They kill bacteria by actually ingesting them (this is called phagocytosis). Neutrophils can phagocytize five to 20 bacteria in their lifetime. Neutrophils have a multi-lobed, segmented or polymorphonuclear nucleus and so are also called PMNs, polys or segs. Bands are immature neutrophils that are seen in the blood. When a bacterial infection is present, an increase of neutrophils and bands are seen.
Eosinophils kill parasites and have a role in allergic reactions.
Basophils are not well understood, but they function in allergic reactions. They release histamine (which causes blood vessels to leak and attracts WBCs) and heparin (which prevents clotting in the infected area so that the WBCs can reach the bacteria).
Monocytes enter the tissue, where they become larger and turn into macrophages. There they can phagocytize bacteria (up to 100 in their lifetime) throughout the body. These cells also destroy old, damaged and dead cells in the body. Macrophages are found in the liver, spleen, lungs, lymph nodes, skin and intestine. The system of macrophages scattered throughout the body is called the reticuloendothelial system.
Neutrophils and monocytes use several mechanisms to get to and kill invading organisms. They can squeeze through openings in blood vessels by a process called diapedesis. They move around using ameboid motion. They are attracted to certain chemicals produced by the immune system or by bacteria and migrate toward areas of higher concentrations of these chemicals. This is called chemotaxis. They kill bacteria by a process called phagocytosis, in which they completely surround the bacteria and digest them with digestive enzymes.
Lymphocytes are complex cells that direct the bodyís immune system. T lymphocytes (T cells) are responsible for cell-mediated immunity. B lymphocytes are responsible for humoral immunity (antibody production). Seventy-five percent of lymphocytes are T cells. Lymphocytes are different from the other WBCs because they can recognize and have a memory of invading bacteria and viruses.
There are many types of T cells that have specific functions, including:
B cells become plasma cells when exposed to an invading organism or when activated by helper T cells. B cells produce large numbers of antibodies (also called immunoglobulins or gamma globulins). There are five types of immunogloulins (abbreviated Ig): IgG, IgM, IgE, IgA and IgD. These are Y-shaped molecules that have a variable segment that is a binding site for only one specific antigen. These bind to antigens, which causes them to clump, be neutralized or break open. They also activate the complement system.
- Helper T cells - Helper T cells have proteins on their cell membranes called CD4. Helper T cells direct the rest of the immune system by releasing cytokines. Cytokines stimulate B cells to form plasma cells, which form antibodies, stimulate the production of cytotoxic T cells and suppressor T cells and activate macrophages. Helper T cells are the cells the AIDS virus attacks -- you can imagine that destroying the cells that direct the immune system has a devastating effect.
- Cytotoxic T cells - Cytotoxic T cells release chemicals that break open and kill invading organisms.
- Memory T cells - Memory T cells remain afterwards to help the immune system respond more quickly if the same organism is encountered again.
- Suppressor T cells - Suppressor T cells suppress the immune response so that it does not get out of control and destroy normal cells once the immune response is no longer needed.
The complement system is a series of enzymes that help or complement antibodies and other components of the immune system to destroy the invading antigen by attracting and activating neutrophils and macrophages, neutralizing viruses and causing invading organisms to break open. Memory B cells also remain for prolonged periods, and if the same antigen is encountered it causes a more rapid response in producing antibodies.
This is the average percentage of each type of WBC in the blood:
- Neutrophils - 58 percent
- Bands - 3 percent
- Eosinophils - 2 percent
- Basophils - 1 percent
- Monocytes - 4 percent
- Lymphocytes - 33 percent
The Life and Times of a WBC
Most WBCs (neutrophils, eosinophils, basophils and monocytes) are formed in the bone marrow. T lymphocytes start in the bone marrow from pluripotent hematopoietic stem cells, then travel to and mature in the thymus gland. The thymus is located in the chest between the heart and sternum (breastbone). B lymphocytes mature in the bone marrow.
When a granulocyte (neutrophil, eosinophil and basophil) is released into the blood, it stays there for an average of four to eight hours and then goes into the tissues of the body, where it lasts for an average of four to five days. During a severe infection, these times are often shorter.
Monocytes stay in the blood for an average of 10 to 20 hours and then go into the tissues, where they become tissue macrophages and can live for months to years.
Lymphocytes continually pass back and forth between lymph tissue, lymph fluid and blood. When they are present in the blood, they stay for several hours. Lymphocytes can live for weeks, months or years.
Platelets (thrombocytes) help blood to clot by forming something called a platelet plug. The other way that blood clots is through coagulation factors. Platelets also help to promote other blood clotting mechanisms. There are approximately 150,000 to 400,000 platelets in each microliter of blood (average is 250,000).
Platelets are formed in the bone marrow from very large cells called megakaryocytes, which break up into fragments -- these cellular fragments are platelets. They do not have a nucleus and do not reproduce. Instead, megakaryocytes produce more platelets when necessary. Platelets generally last for an average of 10 days.
Platelets contain many chemicals that assist clotting. These include:
- Actin and myosin, to help them contract
- Chemicals that help the coagulation process to begin
- Chemicals that attract other platelets
- Chemicals that stimulate blood vessel repair
- Chemicals that stabilize a blood clot
Plasma is a clear, yellowish fluid (the color of straw). Plasma can sometimes appear milky after a very fatty meal or when people have a high level of lipids in their blood. Plasma is 90-percent water. The other 10 percent dissolved in plasma is essential for life. These dissolved substances are circulated throughout the body and diffuse into tissues and cells where they are needed. They diffuse from areas of high concentration to areas of lower concentration. The greater the difference in concentration, the greater the amount of material that diffuses. Waste materials flow in the opposite direction, from where they are created in the cells into the bloodstream, where they are removed either in the kidneys or lungs.
Hydrostatic pressure (blood pressure) pushes fluid out of blood vessels. Balancing this is something called oncotic pressure (caused by proteins dissolved in blood), which tends to keep fluid inside the blood vessels.
Proteins make up a large part of the 10 percent of material dissolved in plasma and are responsible for oncotic pressure. Protein molecules are much larger than water molecules and tend to stay in blood vessels. They have more difficulty fitting through the pores in capillaries, and therefore have a higher concentration in blood vessels. Proteins tend to attract water to keep their relative concentration in blood vessels more in line with fluid outside the blood vessels. This is one of the ways the body maintains a constant volume of blood.
Plasma contains 6.5 to 8.0 grams of protein per deciliter of blood. The main proteins in plasma are albumin (60 percent), globulins (alpha-1, alpha-2, beta, and gamma globulins (immunoglobulins)), and clotting proteins (especially fibrinogen). These proteins function to maintain oncotic pressure (especially albumin) and transport substances such as lipids, hormones, medications, vitamins, and other nutrients. These proteins are also part of the immune system (immunoglobulins), help blood to clot (clotting factors), maintain pH balance, and are enzymes involved in chemical reactions throughout the body.
Electrolytes are another large category of substances dissolved in plasma. They include:
- Sodium (Na+)
- Potassium (K+)
- Chloride (Cl-)
- Bicarbonate (HCO3-)
- Calcium (Ca+2)
- Magnesium (Mg+2)
These chemicals are absolutely essential in many bodily functions including fluid balance, nerve conduction, muscle contraction (including the heart), blood clotting and pH balance.
Other materials dissolved in plasma are carbohydrates (glucose), cholesterol, hormones and vitamins. Cholesterol is normally transported attached to lipoproteins such as low-density lipoproteins (LDLs) and high-density lipoproteins (HDLs). For more information on cholesterol, read How Cholesterol Works.
When plasma is allowed to clot, the fluid left behind is called serum. When blood is collected from a patient it is allowed to clot in a test tube, where the cells and clotting factors fall to the bottom and the serum is left on top. Serum is tested for all the numerous items discussed above to determine if any abnormalities exist.
There are four major blood types: A, B, AB, and 0. The blood types are determined by proteins called antigens (also called agglutinogens) on the surface of the RBC.