The Blood and Lymph System
The BLOOD and LYMPH are the cell-bearing fluids that nourish and protect the body. Physician specialists who treat conditions of the blood and lymph are hematologists. This section, “The Blood and Lymph,” presents an overview of the structures and functions of the blood and lymph, a discussion of hematologic and lymphatic health and disorders, and entries about the health conditions that can affect the blood and lymph.
Functions of the Blood and Lymph
The BLOOD and the LYMPH are the body’s vital fluids, sharing responsibility for nourishment, cleansing, IMMUNE RESPONSE, and fluid balance. The blood primarily nourishes the cells, and the lymph cleanses and drains the tissues. The lymph derives from as well as returns to the blood. Though the blood and the lymph are unique fluids that circulate through separate networks, they share some structures that allow leukocytes, notably lymphocytes and granulocytes, to move freely between the blood and the lymph.
The rhythm of life: the blood
The adult human body contains about five liters (just under six quarts) of blood that the HEART propels on a continuous circuit through the arteries and veins. Contained within the arteries and veins of the pressurized cardiovascular system, the total blood volume circulates from the heart, through the body, and back to the heart in about a minute. During strenuous activity the blood can pound through six full circuits in a minute, hammering oxygen and GLUCOSE to the cells to fuel their increased energy output.
Though fluid the blood is a living tissue, a mix of cells (45 percent of the blood’s composition) suspended in a watery matrix of PLASMA (55 percent of the blood’s composition). Plasma, which is about 90 percent water, also carries numerous substances dissolved in it including electrolytes, glucose (sugar), hormones, NUTRIENTS, and proteins such as CLOTTING FACTORS and ALBUMIN. A single drop of blood contains roughly:
- 500 million erythrocytes
- 33.5 million platelets
- 830,500 leukocytes
Blood cell production: the bone marrow
The red BONE MARROW, located in cavities within the bones called medullary canals, produces 99 percent of the adult body’s blood cells and all of its erythrocytes. This spongy, somewhat gelatinous substance has two structures, the vascular compartments through which blood circulates and the extravascular compartments that contain the BLOOD STEM CELLS. The red bone marrow is extraordinarily active tissue, releasing into circulation 2 to 3 billion erythrocytes, 2 to 3 billion platelets, up to 100 billion granulocytes, and several hundred million monocytes every 24 hours. The BONE marrow also warehouses minerals it requires for cell synthesis and the bones need for STRENGTH and growth, such as calcium. As well, the bone marrow stores B-cell lymphocytes and plasma cells, leukocytes integral to the body’s IMMUNE RESPONSE.
Oxygen transport: erythrocytes
The erythrocytes, also called red blood cells (RBCs), pick up oxygen molecules in the LUNGS and carry them to the cells. After delivering the oxygen, the erythrocytes then retrieve carbon dioxide molecules, the waste byproducts of cellular METABOLISM, and cart them back to the lungs for elimination from the body through respiration. This OXYGEN–CARBON DIOXIDE EXCHANGE is the foundation of the body’s survival. No cells in the body can survive longer than 10 to 15 minutes (three to five minutes for BRAIN and heart cells) without oxygen.
Erythrocytes acquire their capacity to carry oxygen from the pigmented protein HEMOGLOBIN, which is high in iron. The pigment also gives erythrocytes their red color. The iron hemoglobin contains allows the hemoglobin to bind with the oxygen molecules. A healthy, normal erythrocyte contains about 300 million molecules of hemoglobin; each molecule of hemoglobin can bind with four molecules of oxygen. Iron enters the body from dietary sources. Iron deficiency is the most common cause of ANEMIA, a condition in which the blood cannot meet the body’s oxygenation needs.
Erythrocytes are concave on both sides, giving them the FLEXIBILITY to nearly fold in half to squeeze through the narrowest of the body’s blood vessels, the arterioles, venules, and capillaries. As well, erythrocytes lack nuclei, the “command” structures common to cells that contain deoxyribonucleic acid (DNA). DNA gives the cell its replication instructions; without it a cell cannot reproduce. The absence of a nucleus further aids the erythrocyte’s flexibility, however, which is most important for delivering oxygen deep within the body’s tissues.
Because erythrocytes cannot proliferate, the red bone marrow churns out a steady supply of new ones, releasing them into the circulation at a rate of 2 to 3 million per second. Erythrocytes enter the bloodstream in a slightly immature stage, called reticulocytes. They reach full maturity after about 24 hours in circulation and live in the bloodstream for 110 to 120 days, after which the SPLEEN filters them from the blood and breaks them down (hemolyzes) into their component structures. The LIVER further metabolizes the components of hemolyzed erythrocytes, recycling their ingredients for use in synthesizing new erythrocytes as well as to manufacture BILE and other biochemical substances. Macrophages within the liver, migratory monocytes called Kupffer cells, then consume whatever remains of the erythrocytic waste.
Stop the bleeding: platelets
The smallest cell elements in the blood, platelets, are encased in protein coatings that become adhesive (sticky) when chemical messengers released at the site of bleeding enter the bloodstream. The chemicals activate PLATELET AGGREGATION, in which platelets swarm to the site of bleeding and stick to each other as well as to the collagen fibers at the site to form a hemostatic plug. This activation also enables platelets to change shape, elongating or contracting as necessary to bridge the gaps among the collagen fibers to form a weblike structure that ensnares other cells and substances. As the coagulation cascade unfolds the plug expands and hardens, eventually forming the clot that halts the bleeding. On the surface of the SKIN, this clot is a scab. Within a blood vessel, it is a thrombus.
Platelets arise from the largest cells in the red bone marrow, megakaryocytes, and actually are fragments of megakaryocytic cytoplasm rather than independent cells. They are irregularly shaped and loosely defined, a structure ideally suited to their purpose. Platelets also lack nuclei and live in the circulation for about 10 days. Roughly a third of the body’s total platelet volume resides in the spleen, which releases them into the circulating blood in response to bleeding.
Defend and protect: leukocytes
The leukocytes, also called white blood cells (WBCs), are the foundation of the body’s IMMUNE RESPONSE. They take one of three forms: LYMPHOCYTE, MONOCYTE, or GRANULOCYTE. Each has specialized functions within the immune response. Lymphocytes attack invading pathogens, and monocytes and granulocytes consume the remains of the pathogenic invaders. Lymphocytes circulate primarily in the lymph. Monocytes circulate in the blood for about 24 hours after the bone marrow releases them and then migrate into the tissues where they establish themselves as fixed defenders called macrophages. Granulocytes circulate in the blood and in the lymph and also take up residence in the lymph structures and the general body tissues. The three subtypes of granulocytes—basophils, eosinophils, and neutrophils—have specified roles in the body’s inflammatory response and are responsible for hypersensitivity reactions and allergies. The bone marrow primarily manufactures leukocytes, with assistance from the lymph tissues and spleen when necessary to meet the body’s INFECTION control needs.
Flow with the body: the lymph
In contrast to the force of the blood’s circulation, the lymph channels through the body at a gentle flow of about 100 milliliters per hour. Gravity and the body’s movements (MUSCLE contractions) massage lymph through the lymph vessels that roughly parallel the arteries and veins. The lymph vessels are thin-walled, originating with cul-de-sac structures arising in the epithelial spaces, the lymphatic capillaries, that join increasingly larger channels that carry lymph into the central body and ultimately into the circulating blood.
Slightly more watery than blood (92 percent), lymph carries a suspension of primarily lymphocytes and monocytes as well as dissolved proteins and other substances. Clear and only slightly more viscous than water, lymph drains from the spaces between cells into the lymphatic capillaries, microscopic channels comprised of a single thickness of cells overlapped like backward shingles. This construction encourages fluid to seep under the cells and into the lymph capillaries. The lymph capillaries collect the droplets of lymph, pooling them into microscopic trickles that eventually merge with larger lymphatic vessels—the CISTERNA CHYLI, THORACIC DUCT, and RIGHT LYMPHATIC DUCT-that carry the lymph toward the subclavian veins where it rejoins the bloodstream.
The lymph carries leukocytes, proteins, antibodies, and other materials directly to the cells. While erythrocytes in the blood can carry oxygen molecules into the CAPILLARY BEDS, the capillaries eventually become too narrow even for the flexible erythrocytes to make further passage. So the erythrocytes off-load their oxygen molecules into the lymph, which floats them through the capillary walls and into the interstitial spaces (the space between the cells of the tissues). Lymph flows through the interstitial spaces, bathing the cells, which then withdraw the nutrients, including oxygen and glucose, that they require. Cells also discharge their metabolic wastes into the lymph.
Critical passengers in the lymph are the leukocytes, predominantly neutrophils and lymphocytes. These protective cells vigilantly patrol the interstitial spaces on the alert for invading pathogens. When they detect pathogenic invaders leukocytes secrete chemicals, called CYTOKINES, into the lymph that initiate or activate specific immune responses. Some of these responses recruit additional lymphocytes and granulocytes (notably neutrophils) into circulation both in the lymph and in the blood. As agents of immune response, granulocytes and lymphocytes have the ability to migrate between the blood and the tissues, bolstering the body’s defenses as needed.
The lymph also transports pathogens, such as viruses and bacteria, to the lymph nodes where masses of lymphocytes, macrophages (tissuebound monocytes), and granulocytes wait to dispose of them. The lymph nodes often swell when they are busy fighting infections (LYMPHADENOPATHY) and may themselves become infected (LYMPHADENITIS). The lymph also offers a route of transport for cancer cells that leave the original tumor site. The lymph network can unfortunately carry cancer cells that enter its flow to any location within the body, facilitating METASTASIS (spread of the cancer).