The blood consists of cells and cell fragments, called formed elements, and water with dissolved molecules, called blood plasma (see Table 1).
Erythrocytes, or red blood cells (RBCs), transport oxygen (O 2) and carbon dioxide (CO 2) in the blood. Erythrocytes contain the protein hemoglobin to which both O 2 and CO 2 attach.
Mature erythrocytes lack a nucleus and most cellular organelles, thereby maximizing the cell's volume and thus its ability to carry hemoglobin and transport O 2.
Erythrocytes are shaped like flattened donuts with a depressed center (rather than a donut hole). Their flattened shape maximizes surface area for the exchange of O 2 and CO 2 and allows flexibility that permits their passage through narrow capillaries.
Hemoglobin contains both a protein portion, called globin, and nonprotein heme groups. Globin consists of four polypeptide chains, each of which contains a heme group. The heme group is a red pigment that contains a single iron atom surrounded by a ring of nitrogen‐containing carbon rings. One oxygen atom attaches to the iron of each heme group, allowing a single hemoglobin molecule to carry four oxygen atoms. Each erythrocyte contains about 250 million hemoglobin molecules.
Oxyhemoglobin (HbO 2 ) forms in the lungs when erythrocytes are exposed to oxygen as they pass through the lungs. Deoxyhemoglobin (Hb) forms when oxygen detaches from the iron and diffuses into surrounding tissues.
Carbaminohemoglobin (HbCO 2 ) forms when CO 2 attaches to amino acids of the globin part of the hemoglobin molecule. About 20–25 percent of the CO 2 transported from tissues to lungs is in this form.
Carbonic anhydrase, an enzyme in erythrocytes, converts CO 2 and H 2O in the cells to H + and HCO 3–. About 70 percent of the CO 2 collected from tissues travels in the erythrocytes as HCO 3–. About 10 percent of the carbon dioxide stays in the plasma and is transported in the circulatory system as the bicarbonate ion.
Because they lack cellular organelles and thus the physiology to maintain themselves, erythrocytes survive for only about 120 days. Degenerated erythrocytes are broken down in the spleen and liver by macrophages (phagocytic white blood cells) as follows:
The globin and heme parts of the hemoglobin are separated. The globin is reduced to amino acids, which are returned to the blood plasma.
Iron is removed from the heme group and bound to the proteins ferritin and hemosiderin, which store the iron for later use (because unbound iron is toxic). Iron is also attached to transferrin, which enters the bloodstream. Transferrin may be picked up by muscles or liver cells, where it may be stored as ferritin or hemosiderin or picked up by bone marrow, where the iron is used to produce new erythrocytes.
The remainder of the heme group is broken down into bilirubin (a yellow‐orange pigment), which enters the bloodstream and is picked up by the liver. Liver cells incorporate bilirubin into bile, which enters the small intestine during the digestion of fats. Bilirubin is then converted into urobilinogen by intestinal bacteria. Finally, most urobilinogen is converted to the brown pigment stercobilin, which is eliminated with the feces (and which gives feces its brown color). A small amount of urobilinogen is absorbed into the blood, converted to the yellow pigment urobilin, picked up by the kidneys, and eliminated with the urine (contributing to the yellow color of urine)
Leukocytes, or white blood cells (WBCs), protect the body from foreign microbes and toxins. Although all leukocytes can be found in the bloodstream, some permanently leave the bloodstream to enter tissues where they encounter microbes or toxins, while other kinds of leukocytes readily move in and out of the bloodstream. Leukocytes are classified into two groups, granulocytes and agranulocytes, based on the presence or absence of granules in the cytoplasm and the shape of the nucleus. Leukocytes have just one nucleus, but some leukocytes have a multilobed nucleus, making them look like they have several nuclei.
Granulocytes contain numerous granules in the cytoplasm and have a nucleus that is irregularly shaped with lobes. Each of the three types of granulocytes is named after the type of stain that its granules absorb:
Neutrophils, the most numerous of granulocytes, have an S‐ or C‐shaped nucleus with three to six lobes. Their granules, which are small and inconspicuous, poorly absorb both basic and acidic stains (neutral pH preference), producing a pale, lilac color. Because the shape of the nucleus is so variable, neutrophils are referred to as polymorphonuclear leukocytes (PMNs), or polys. Young neutrophils that are shaped like rods are called band neutrophils. Older neutrophils, with a segmented nucleus, are called segs. Neutrophils are the first leukocytes to arrive at a site of infection, responding (by chemotaxis) to chemicals released by damaged cells. The neutrophils, by phagocytosis, actively engulf bacteria, which are then destroyed by the various antibiotic proteins (such as defensins and lysozymes) contained within the granules. The neutrophils, usually destroyed in the process, contribute, together with other dead tissue, to the formation of pus.
Eosinophils have a bilobed nucleus (two lobes connected by a narrow strand of chromatin). Their granules, which stain red with acid (eosin) dyes, contain digestive enzymes. Their granules are therefore considered to be lysosomes. Eosinophils actively phagocytize complexes formed by the action of antibodies on antigens (foreign substances). Numbers of eosinophils increase during parasitic infection and allergic reactions.
- Basophils have a U‐ or S‐shaped nucleus with two to five lobes connected by a narrow strand of chromatin. Their granules, which stain blue‐purple with basic dyes, contain histamine, serotonin, and heparin. Basophils release histamine in response to tissue damage and to pathogen invasion (as part of the inflammatory response). Basophils resemble mast cells, which are similar in appearance and function to basophils, but found only in connective tissues. Many times, there are so many granules in a basophil that you cannot see the nucleus.
Agranulocytes, the second group of leukocytes, do not have visible granules in the cytoplasm and the nucleus is not lobed. There are two types of these leukocytes:
- Lymphocytes, often classified as small, medium, and large, have a roughly round nucleus surrounded by a small amount of blue‐staining cytoplasm. Lymphocytes are the only leukocytes that return to the bloodstream, circulating among the bloodstream, tissue fluids, tissues, and lymph fluid. There are two major groups of lymphocytes, which vary based upon their role in an immune response. T lymphocytes ( T cells), which mature in the thymus gland, and upon exposure to thymosin attack aberrant cells (such as tumor cells, organ transplant cells, or cells infected by viruses). B lymphocytes (B cells), which mature in the bone marrow, respond to circulating antigens (such as toxins, viruses, or bacteria) by dividing to produce plasma cells, which in turn produce antibodies.
Monocytes have a large, kidney‐shaped nucleus surrounded by ample blue‐gray‐staining cytoplasm. When monocytes leave the bloodstream and move into tissues, they enlarge and become macrophages, which engulf microbes and cellular debris.
Platelets ( thrombocytes) are fragments of huge cells called megakaryocytes. Megakaryocytes fragment as they pass from the bone marrow into the bloodstream. Platelets lack a nucleus and consist of cytoplasm (with few organelles) surrounded by a plasma membrane. Platelets adhere to damaged blood vessel walls and release enzymes that activate hemostasis, the stoppage of bleeding.
Plasma is the straw‐colored, liquid portion of the blood. It consists of the following:
- Proteins (8 percent). Albumin, the most common protein, is produced by the liver and serves to preserve osmotic pressure between blood and tissues. Other proteins include alpha and beta globulins (proteins that transport lipids and metal ions), gamma globulins (antibodies), fibrinogen and prothrombin (clotting proteins), and hormones.
- Waste products (urea, uric acid, creatinine, bilirubin, and others).
- Nutrients (absorbed from the digestive tract).
- Electrolytes (various ions such as sodium, calcium, chloride, and bicarbonate).
- Respiratory gases (O 2 and CO 2).
Serum is the liquid material remaining after blood‐clotting proteins have been removed from plasma as a result of clotting.