Red blood cells Red Blood Cells The main components of blood include Plasma Red blood cells White blood cells Platelets read more 
, most white blood cells White Blood Cells The main components of blood include Plasma Red blood cells White blood cells Platelets read more , and platelets Platelets The main components of blood include Plasma Red blood cells White blood cells Platelets read more are produced in the bone marrow, the soft fatty tissue inside bone cavities. Two types of white blood cells, T and B cells (lymphocytes Lymphocytes One of the body's lines of defense ( immune system) involves white blood cells (leukocytes) that travel through the bloodstream and into tissues, searching for and attacking microorganisms and... read more 
), are also produced in the lymph nodes and spleen, and T cells are produced and mature in the thymus gland. (See also Overview of Blood Overview of Blood Blood is a complex mixture of Plasma (the liquid component) Red blood cells (the oxygen-carrying component) White blood cells (the cells that defend against infection) Platelets (particles that... read more .)
Within the bone marrow, all blood cells originate from a single type of unspecialized cell called a stem cell. When a stem cell divides, it first becomes an immature red blood cell, white blood cell, or platelet-producing cell. The immature cell then divides, matures further, and ultimately becomes a mature red blood cell, white blood cell, or platelet.
The rate of blood cell production is controlled by the body's needs. Normal blood cells last for a limited time (ranging from a few hours to a few days for white blood cells, to about 10 days for platelets, to about 120 days for red blood cells) and must be replaced constantly. Certain conditions may trigger additional production of blood cells. When the oxygen content of body tissues is low or the number of red blood cells decreases, the kidneys produce and release erythropoietin, a hormone that stimulates the bone marrow to produce more red blood cells. The bone marrow produces and releases more white blood cells in response to infections. It produces and releases more platelets in response to bleeding
Effects of aging on the blood
Aging has some effect on bone marrow and blood cells, resulting in less cell-producing marrow. While this decrease generally does not cause problems, it may when the body experiences an increased demand for blood cells: the marrow of an older person may be less able to meet those increased demands. Anemia Overview of Anemia Anemia is a condition in which the number of red blood cells is low. Red blood cells contain hemoglobin, a protein that enables them to carry oxygen from the lungs and deliver it to all parts... read more is the most common result.
