Blood Health Article

Red blood cells

The human body contains an estimated 25 trillion red blood cells; approximately 4.8 million to 5.4 million are found in every microliter of blood. The structure of a red blood cell is eminently suited to its primary function, the transport of oxygen from the lungs to body tissues. Red blood cells are very small (about 6 nanometers wide), shaped like a disk, and contain a small depression on either side. Their small size allows them to squeeze through the tiniest of blood vessels (capillaries). In addition, their size allows a greater diffusion of oxygen across the blood cells' plasma membranes than if the cells were larger—because blood contains so many of these small cells, their combined surface areas translate into an extremely large surface area for the diffusion of oxygen. The disk shape and the depressions on either side also contribute to a greater surface area.

TRANSPORT OF OXYGEN. Red blood cells are unusual in that they do not contain nuclei or mitochondria, the cellular organelles in which aerobic metabolism (the breakdown of nutrients that requires oxygen) is carried out. Instead, red blood cells acquire energy through metabolic processes that do not require oxygen. The lack of nuclei and mitochondria therefore allow the red blood cell to function without depleting its cargo of oxygen, leaving more oxygen for the body tissues.

The molecule that binds oxygen in red blood cells is called hemoglobin. Hemoglobin is a large, globular protein consisting of four protein chains surrounding an iron core. Hemoglobin is densely packed inside the red blood cell; in fact, hemoglobin accounts for a third of the weight of the entire red blood cell. Each red blood cell contains about 250 molecules of hemoglobin.

In the lungs, oxygen diffuses across the red blood cell membrane and binds to hemoglobin. As blood circulates to the tissues, oxygen diffuses out of the red blood cells and enters tissues. The waste product of aerobic metabolism, carbon dioxide, then diffuses across red blood cells and binds to hemoglobin. Once circulated back to the lungs, the red blood cells discharge their load of carbon dioxide, which is then breathed out of the lungs. However, only 7% of carbon dioxide generated from metabolism is transported back to the lungs for exhalation by red blood cells; the majority is transported in the form of bicarbonate, a component of plasma.

HEMOPOIESIS. Red blood cells are formed in red bone marrow from precursor cells called pluripotent stem cells. The process of red blood cell formation is called hemopoiesis (alternatively, hematopoiesis). In adults hemopoiesis takes place in the marrow of ribs, vertebrae, the breastbone, and the pelvis. On average, a red blood cell lives only three to four months. Constant wear and tear on the red blood cell membrane, caused by squeezing through tiny capillaries, contributes to the red blood cell's short life span. Worn out red blood cells are destroyed by phagocytic cells (cells that engulf and digest other cells) in the liver. Parts of red blood cells are recycled for use in other red blood cells, such as the iron component of hemoglobin.

White blood cells

White blood cells are less numerous than red blood cells in the human body; each microliter of blood contains 5,000 to 10,000 white blood cells. The number of white blood cells increases, however, when the body is fighting off infection. Their numbers are maintained until the immune system detects the presence of a foreign invader. When the immune system is activated, chemicals called lymphokines stimulate the production of more white blood cells.

White blood cells function in the body's defense against invasion and are key components of the immune system. They usually do not circulate in the blood vessels, and are instead found in the interstitial fluid and in lymph nodes. Lymph nodes are composed of lymphatic tissue and are located at strategic places in the body. Blood filters through the lymph nodes, and the white cells present in the nodes attack and destroy any foreign invaders.

TYPES OF WHITE BLOOD CELLS. The human body contains five types of white blood cells: monocytes, neutrophils, basophils, eosinophils, and lymphocytes. Each type of white blood cell plays a specific role in the body's immune defense system.

Under a microscope, three kinds of white blood cells appear to contain granules within their cytoplasm. These three types are the neutrophils, basophils, and eosinophils. Together, these three types of white blood cells are called granulocytes. The granules are specific chemicals that are released during the immune response. The other two types of white blood cells, the monocytes and lymphocytes, do not contain granules. These types are known as the agranular leukocytes.

Monocytes, which comprise 3% to 8% of the white blood cells, and neutrophils, which comprise 60% to 70% of white blood cells, are called phagocytes. They ingest and digest cells, including foreign microorganisms such as bacteria. Monocytes differentiate into cells called macrophages. Macrophages can be fixed in one place, such as in the brain and lymph nodes, or can "wander" to areas where they are needed, such as the site of an infection. Neutrophils have an additional defensive property: they release granules of lysozyme, an enzyme that destroys cells.

Basophils comprise 0.5% to 1% of the total composition of white blood cells and function in the body's inflammatory response. Allergies are caused by an inflammatory response to relatively harmless substances, such as pollen or dust, in sensitive individuals. When activated, basophils release various chemicals that cause the characteristic symptoms of allergies. Histamines, for instance, cause the runny nose and watery eyes associated with allergic reactions; heparin is an anticoagulant that slows blood clotting and encourages the flow of blood to the site of inflammation, inducing swelling.

Eosinophils, which comprise 2% to 4% of the total composition of white blood cells, are believed to counteract the effects of histamine and other inflammatory chemicals. They also phagocytize bacteria tagged by antibodies.

Lymphocytes, which comprise 20% to 25% of the total composition of white blood cells, are divided into two types: B lymphocytes (also called B cells) and T lymphocytes (also called T cells). The names of these lymphocytes are derived from their origin. T lymphocytes are named for the thymus, an organ located in the upper chest region where these cells mature; and B lymphocytes are named for the bursa of Fabricus, an organ in birds where these cells were discovered.

T lymphocytes play key roles in the immune response. One type of T lymphocyte, the helper T lymphocyte, activates the immune response when it encounters a macrophage that has ingested a foreign microorganism. Another kind of T lymphocyte, called a cytotoxic T lymphocyte, kills cells infected by foreign microorganisms. B lymphocytes, when activated by helper T lymphocytes, become plasma cells, which in turn secrete large amounts of antibodies.

All white blood cells arise in the red bone marrow. However, the cells destined to become lymphocytes are first differentiated into lymphoid stem cells in the red bone marrow. These stem cells undergo further development and maturation in the spleen, tonsils, thymus, adenoids, and lymph nodes.