Blood

composition of plasma

The makeup of plasma varies continuously as cells remove or add substances to the blood. However, assuming a healthy diet, plasma composition is kept relatively constant by various homeostatic mechanisms. For example, when blood protein levels drop undesirably, the liver makes more proteins. When the blood starts to become too acidic (acidosis), both the respiratory system and the kidneys are called into action to restore plasma's normal, slightly alkaline pH. Body organs make dozens of adjustments, day in and day out, to maintain the many plasma solutes at life-sustaining levels.

EPO and sex hormones

The male sex hormone testosterone also enhances EPO production by the kidneys. Because female sex hormones do not have similar stimulatory effects, testosterone may be at least partially responsible for the higher RBC counts and hemoglobin levels seen in males. Also, a wide variety of chemicals released by leukocytes, platelets, and even reticular cells stimulates bursts of RBC production.

regulation and requirements for erythropoiesis

The number of circulating erythrocytes in a given individual is remarkably constant and reflects a balance between red blood cell production and destruction. This balance is important because having too few erythrocytes leads to tissue hypoxia (oxygen deprivation), whereas having too many makes the blood undesirably viscous. To ensure that the number of erythrocytes in blood remains within the homeostatic range, new cells are produced at the incredibly rapid rate of more than 2 million per second in healthy people. This process is controlled hormonally and depends on adequate supplies of iron, amino acids, and certain B vitamins.

Iron and erythropoiesis

The raw materials required for erythropoiesis include the usual nutrients and structural materials--amino acids, lipids, and carbohydrates. Iron is essential for hemoglobin synthesis. Iron is available from the diet and its absorption into the bloodstream is precisely controlled by intestinal cells in response to changing bod stores of iron.

sickle-cell anemia continued

The stiff, deformed erythrocytes rupture easily and tend to dam up in small blood vessels. These events interfere with oxygen delivery, leaving the victims gasping for air and in extreme pain. Bone and chest pain are particularly severe, and infection and stroke are common sequels. Blood transfusion is still standard treatment for an acute sickle-cell crisis, but preliminary results using inhaled nitric oxide to dilate blood vessels are promising.

hemocytoblast

The various formed elements have different functions, but there are similarities in their life histories. All arise from the same type of stem cell, the hemocytoblast, or pluripotent hematopoietic stem cell. These undifferentiated precursor cells reside in the red bone marrow. The maturation pathways of the various formed elements differ, however, and once a cell is committed to a specific blood cell pathway, it cannot change. This commitment is signaled by the appearance of membrane surface receptors that respond to specific hormones or growth factors, which in turn "push" the cell toward further specialization.

1.

1. Its small size and biconcave shape provide a huge surface area relative to volume (about 30% more surface area than comparable spherical cells). The biconcave disc shape is ideally suited for gas exchange because no point within the cytoplasm is far from the surface.

carbaminohemoglobin

About 20% of the carbon dioxide transported in the blood combines with hemoglobin, but it binds to globin's amino acids rather than to the heme group. This formation of carbaminohemoglobin occurs more readily when hemoglobin is in the reduced state (dissociated from oxygen). Carbon dioxide loading occurs in the tissues, and the direction of transport is from tissues to lungs, where carbon dioxide is eliminated from the body.

Albumin

Albumin accounts for some 60% of plasma protein. It acts as a carrier to shuttle certain molecules through the circulation, is an important blood buffer, and is the major blood protein contributing to the plasma osmotic pressure (the pressure that helps to keep water in the bloodstream). (Sodium ions are the other major solute contributing to blood osmotic pressure.)

Anemia

Anemia is a condition in which the blood has abnormally low oxygen-carrying capacity. It is a sign of some disorder rather than a disease in and of itself. Its hallmark is blood oxygen levels that are inadequate to support normal metabolism. Anemic individuals are fatigued, often pale, short of breath, and chilly. Common causes of anemia include the following: (1) an insufficient number of red blood cells, (2) low hemoglobin content, and (3) abnormal hemoglobin.

aplastic anemia

Aplastic anemia's may result from destruction or inhibition of the red marrow by certain drugs and chemicals, ionizing radiation, or viruses. In most cases, the cause is unknown. Because marrow destruction impairs formation of all formed elements, anemia is just one of its signs. Defects in blood clotting and immunity are also present. Blood transfusions provide a stopgap treatment until stem cells harvested from a donor's blood, bone marrow, or umbilical cord blood can be transplanted.

ferritin, hemosiderin and transferrin

Approximately 65% of the body's iron supply (about 4000 mg) is in hemoglobin. Most of the remainder is stored in the liver, spleen, and (to a much lesser extent) bone marrow. Free iron ions (Fe2+, Fe3+) are toxic, so iron is stored inside cells as protein iron complexes such as ferritin and hemosiderin. In blood, iron is transported loosely bound to a transport protein called transferrin, and developing erythrocytes take up iron as needed to form hemoglobin. Small amounts of iron are lost each day in feces,urine and perspiration. The average daily loss of iron is 1.7 mg in women and 0.9 mg in men. In women, the menstrual flow accounts for the additional losses.

conditions of reduced RBC count

Conditions that reduce the red blood cell count include blood loss, excessive RBC destruction, and bone marrow failure.

EPO depression

Conversely, too many erythrocytes or excessive oxygen in the bloodstream depresses erythropoietin production. Note that is it not the number of erythrocytes in the blood that controls the rate of erythropoiesis. Instead, control is based on their ability to transport enough oxygen to meet tissue demands.

haptoglobin

Disposal of hemoglobin spilled from red blood cells to the blood (as occurs in sickle-cell anemia or hemolytic anemia) takes a similar but much more rapid course to avoid toxic buildup of iron in blood. Released hemoglobin is captured by the plasma protein haptoglobin and the complex is phagocytized by macrophages

Distribution

Distribution functions of blood include: 1. Delivering oxygen from the lungs and nutrients from the digestive tract to all body cells. 2. Transporting metabolic waste products from cells to elimination sites (to the lungs from elimination of carbon dioxide, and to the kidneys for disposal of nitrogenous wastes in urine). 3. Transporting hormones from the endocrine organs to their target organs.

Bilirubin

Dying erythrocytes are engulfed and destroyed by macrophages. The heme of their hemoglobin is split off from globin. Its core of iron is salvaged, bound to protein (as ferritin or hemosidererin), and stored for reuse. The balance of the heme group is degraded to bilirubin, a yellow pigment that is released to the blood and binds to albumin for transport. Bilirubin is picked up by the liver cells, which in turn secrete it (in bile) into the intestine, where it is metabolized to urobilinogen. Most of this degraded pigment leaves the body in feces, as a brown pigment called stercobilin. The protein (globin) part of hemoglobin is metabolized or broken down to amino acids, which are released to the circulation.

Vitamin B12 and folic acid

2 B-complex vitamins-- vitamin B12 and folic acid--are necessary for normal DNA synthesis. Thus, even slight deficits jeopardize rapidly dividing cell populations, such as developing erythrocytes.

2.

2. Discounting water content, an erythrocyte is over 97% hemoglobin, the molecule that binds to and transports respiratory gases.

3.

3. Because erythrocytes lack mitochondria and generate ATP by anaerobic mechanisms, they do not consume any of the oxygen they are transporting, making them very efficient oxygen transporters indeed.

average volume of blood in the body

Blood accounts for approximately 8% of body weight. Its average volume in healthy adult males is 5 - 6 L (about 1.5 gallons), somewhat greater than in healthy adult females (4 - 5 L)

hematopoisesis

Blood cell formation is referred to as hematopoisesis, or hemopoisis. This process occurs in the red bone marrow, which is composed largely of a soft network of reticular connective tissue bordering on wide blood capillaries called blood sinusoids. Within this network are immature blood cells, macrophages, fat cells, and reticular cells (which secrete the fibers). In adults, red marrow is found chiefly in the bones of the axial skeleton and girdles, and in the proximal epiphyses of the humerus and femur.

blood doping

Blood doping, practiced by some athletes competing in aerobic events, is artificially induced polycythemia. Some of the athlete's red blood cells are drawn off and then reinjected a few days before the event. The erythrocytes are quickly replaced because the erythropoietin mechanism is triggered shortly after blood removal. Then, when the stored blood is reinfused, a temporary polycythemia results. since red blood cells carry oxygen, the additional infusion should translate into increased oxygen, carrying capacity due to a higher hematocrit, and hence greater endurance and speed. Other than the risk of stroke and heart failure due to high hematrocrit and high blood viscosity described earlier, blood doping seems to work. However, the practice is considered unethical and has been banned from the Olympic games.

physical characteristics of blood

Blood is a sticky, Opaque fluid with a characteristic metallic taste. As children, we discover its saltiness the first time we stick a cute finger into our mouth. Depending on the amount of oxygen it is carrying, the color of blood varies from scarlet (oxygen rich blood) to dark red (oxygen poor blood)

density, pH, and temperature of blood

Blood is more dense than water and about five times more viscous, largely because of its formed elements. Blood is slightly alkaline, with a pH between 7.35 and 7.45, and its temperature (38 degrees C or 100.4 degrees F) is always slightly higher than body temperature.

Components of blood- formed elements and plasma

Blood is the only fluid tissue in the body. It appears to be a thick, homogeneous liquid, but the microscope reveals that blood has both cellular and liquid components. Blood is a specialized type of connective tissue in which living blood cells, called formed elements are suspended in a nonliving fluid matrix called plasma . The collagen and elastic fibers typical of other connective tissues are absent from blood, but dissolved fibrous proteins become visible as fibrin strands during blood clotting

Blood plasma

Blood plasma is a straw-colored, sticky fluid. Although it is mostly water (about 90%), plasma contains over 100 different dissolved solutes, including nutrients, gases, hormones, wastes and products of cell activity, ions, and proteins.

Functions of blood

Blood preforms a number of functions, all concerned in one way or another with (1) distributing substances, (2) regulating blood levels of particular substances, or (3) protecting the body.

bloodborne EPO

Bloodborne erythopoietin stimulates red marrow cells that are already committed to becoming erythrocytes, causing them to mature much more rapidly. One to two days after erythropoietin levels rise in the blood, a marked increase in the rate of reticulocyte release (and hence reticulocyte count) occurs. Notice that hypoxia (oxygen deficit) does not activate the bone marrow directly. Instead it stimulates the kidneys, which in turn provide the hormonal stimulus that activates the bone marrow.

# of RBCs produced

Each type of blood cell is produced in different numbers in response to changing body needs and different regulatory factors. As blood cells mature, they migrate through the thin walls of the sinusoids to enter the bloodstream. On average, the marrow turns out an ounce of new blood containing some 100 billion new cells each and every day.

erythropoiesis

Erythrocyte production, or erythropoiesis begins when a hemoctyoblast descendant called a myeloid stem cell is transformed into a proerythroblast. Proerythroblasts, in turn, give rise to the early (basophilic) erythroblasts that produce huge numbers of ribosomes. During these first 2 phases, the cells divide many times. Hemoglobin is synthesized and iron accumulates as the early erythroblast is transformed into a large erythroblast and then a normoblast. The "color" of the cell cytoplasm changes as the blue-staining ribosomes become masked by the pink color of hemoglobin. When a normoblast has accumulated almost all of its hemoglobin, it ejects most of its organelles. Additionally, its nuclear functions end and its nucleus degenerates and is pinched off, allowing the cell to collapse inward and eventually assume the biconcave shape. The result is the reticulocyte (essentially a young erythrocyte), so named because it still contains a scant reticulum (network) of clumped ribosomes.

Hemoglobin

Erythrocytes are completely dedicated to their job of transporting respiratory gases (oxygen and carbon dioxide). Hemoglobin, the protein that makes red blood cells red, binds easily and reversibly with oxygen, and most oxygen carried in blood is bound to hemoglobin. Normal values for hemoglobin are 14-20 grams per 100 milliliters of blood (g/100 ml) in infants, 13-18 g/100 ml in adult males, and 12-16 g/100 ml in adult females.

RBC viscosity

Erythrocytes are the major factor contributing to blood viscosity. Women typically have a lower red blood cell count than men [4.3 - 5.2 million cells per micro-liter (1 ul = 1 mm) of blood versus 5.1 - 5.8 million cells/ul]. When the number of red blood cells increases beyond the normal range, blood viscosity rises and blood flows more slowly. Similarly, as the number of red blood cells drop below the lower end of the range, the blood thins and flows more rapidly.

% of blood volume

Erythrocytes normally constitute about 45% of the total volume of a blood sample, a percentage known as the hematocrit. Normal hematocrit values vary. In healthy males the norm is 47% + or - 5%; in females it is 42% + or - 5%. Leukocytes and platelets contribute less than 1% of blood volume. Plasma makes up most of the remaining 55% of whole blood.

Red Blood Cells

Erythrocytes or red blood cells (RBCs) are small cells, about 7.5 um in diameter. Shaped like biconcave discs --- flattened discs with depressed centers -- they appear lighter in color at their thin centers than at their edges. Consequently, erythrocytes look like miniature doughnuts when viewed with a microscope. Mature erythrocytes are bound by a plasma membrane, but lack a nucleus (are anucleate) and have essentially no organelles. In fact, they are little more than "bags" of hemoglobin (Hb), the RBC protein that functions is gas transport. Other proteins are present, such as antioxidant enzymes that rid the body of harmful oxygen radicals, but most function mainly to maintain the plasma membrane or promote changes in RBC shape.

Spectrin

For example, the biconcave shape of an erythrocyte is maintained by a network of proteins, especially one called spectrin, attached to the cytoplasmic face of its plasma membrane. The spectrin net is deformable, giving erythrocytes flexibility to change shape as necessary -- to twist, turn, and become cupped shaped as they are carried passively through capillaries with diameters smaller than themselves --- and then to resume their biconcave shape.

Heme and Globin

Hemoglobin is made up of the protein globin bound to a red heme pigment. Globin consists of 4 polypeptide chains -- 2 alpha (a) and 2 beta (B) --- each binding a ringlike heme group. Each heme group bears an atom of iron set like a jewel in its center. A hemoglobin molecule can transport 4 molecules of oxygen because each iron atom can combine reversibly with 1 molecule of oxygen. A single red blood cell contains about 250 million hemoglobin molecules, so each of these tiny cells can scoop up about 1 billion molecules of oxygen!

hemorrhagic anemia

Hemorrhagic anemia's result from blood loss. In acute hemorrhagic anemia, blood loss is rapid (as might follow a serve stab wound); it is treated by blood replacement. Slight but persistent blood loss (due to hemorrhoids or an undiagnosed bleeding ulcer, for example) causes chronic hemorrhagic anemia. Once the primary problem is resolved, normal erythropoietic mechanisms replace the deficient blood cells.

centrifuge and blood

If we spin a sample of blood in a centrifuge, the heavier formed elements are packed down by centrifugal force and the less dense plasma remains at the top. Most of the reddish mas at the bottom of the tube is erythrocytes, the red blood cells that transport oxygen. A thin, whitish layer called the Buffy coat is present at the erythrocyte-plasma junction. This layer contains leukocytes, the white blood cells that act in various ways to protect the body, the platelets,cell fragments that help stop bleeding.

formed elements- RBC

If you examine a stained smear of human blood under the light microscope, you will see disc-shaped red blood cells, a variety of gaudily stained spherical white blood cells, and some scattered platelets that look like debris. Erythrocytes vastly outnumber the other types of formed elements.

hemolytic anemia

In hemolytic anemia's, erythrocytes rupture, or lyse, prematurely. Hemoglobin abnormalities, transfusion of mismatched blood, and certain bacterial and parasitic infections are possible causes.

Sickle-cell anemia

In sickle-cell anemia, the havoc caused by the abnormal hemoglobin, hemoglobin S (HbS), results from a change in just one of the 146 amino acids in a beta chain of the globin molecule! This alteration causes the beta chain to link together under low-oxygen conditions, forming stiff rods so that hemoglobin S becomes spiky and sharp.This, in turn, causes the red blood cells to become crescent shaped when they unload oxygen molecules or when the oxygen content of the blood is lower than normal, as during vigorous exercise and other activities that increase metabolic rate.

Flow of blood in the body

In this chapter, we describe the composition and functions of this life-sustaining fluid that serves as a transport "vehicle" for the organs of the cardiovascular system. To get started, we need a brief overview of blood circulation, which is initiated by the pumping action of the heart. Blood exits the heart via arteries, which branch repeatedly until they become tiny capillaries. By diffusing across the capillary walls, oxygen and nutrients leave the blood and enter body tissues, and carbon dioxide and wastes move from the tissues to the bloodstream. As oxygen-deficient blood leaves the capillary beds, it flows into veins, which return it to the heart. The returning blood then flows from the heart to the lungs, where it picks up oxygen and then returns to the heart to be pumped throughout the body once again.

iron-deficiency anemia

Iron-deficiency anemia is generally a secondary result of hemorrhagic anemia's, but it also results from inadequate intake of iron-containing foods and impaired iron absorption. The erythrocytes produced, called microcytes, are small and pale. The obvious treatment is iron supplements, but if chronic hemorrhagic is the cause, red cell transfusions may also be needed.

erythrocyte disorders

Most erythrocyte disorders can be classified as anemia's or polycythemias. We describe the many varieties and causes of these conditions next.

oxyhemoglobin an deoxyhemoglobin

Oxygen loading occurs in the lungs, and the direction of transport is from lungs, oxygen diffuses from the air sacs of the lungs into the blood and then into the erythrocytes, where it binds to hemoglobin. When oxygen binds to iron, the hemoglobin, now called oxyhemoglobin, assumes a new three-dimensional shape and becomes ruby red. In the tissues, the process is reversed Oxygen detaches from iron, hemoglobin resumes its former shape, and the resulting deoxyhemoglobin, or reduced hemoglobin, becomes dark red. The released oxygen diffuses from the blood into the tissue fluid and then into the tissue cells.

pernicious anemia

Pernicious anemia is due to a deficiency of vitamin B12. Because meats, poultry, and fish provide ample amounts of the vitamin, diet is rarely the problem expect for strict vegetarians. A substance called intrinsic factor, produced by the stomach mucosa, must be present for vitamin B12 to be absorbed by intestinal cells. In most cases of pernicious anemia, intrinsic factor is deficient. Consequently, the developing erthrocytes grow but do not divide, and large pale cells called macrocytes result. Pernicious anemia is an autoimmune disease in which the stomach mucosa atrophies, and it most often affects the elderly. Treatment involves regular intramuscular injections of vitamin B12 or application of a B-12 containing gel (nascobal) to the nasal lining once a week.

Blood plasma continued

Plasma proteins are the most abundant plasma solutes, accounting for about 8% by weight of plasma volume. Except for hormones and gamma globulins, most plasma proteins are produced by the liver. Plasma proteins serve a variety of functions, but they are not taken up by cells to used as fuels or metabolic nutrients as most other plasma solutes, such as glucose, fatty acids, and amino acids.

Polycythemia

Polycythemia is an abnormal excess of erythrocytes that increases blood viscosity, causing it to sludge, or flow sluggishly. Polycythemia vera, a bone marrow cancer, is characterized by dizziness and an exceptionally high RBC count (8-11 million cells/ul). The hematocrit may be as high as 80% and blood volume may double, causing the vascular system to become engorged with blood and severely impairing circulation.

conditions of abnormal hemoglobin

Production of abnormal hemoglobin usually has a genetic basis. 2 such examples, thalassemia and sickle-cell anemia, can be serious, incurable, and sometimes fatal diseases. In both diseases the globin part of the hemoglobin is abnormal and the erythrocytes produced are fragile and rupture prematurely.

Protective

Protective functions of blood include: 1. Preventing blood loss. When a blood vessel is damaged, platelets and plasma proteins initiate clot formation, halting blood loss. 2. Preventing infection. Drifting along in blood are antibodies, complement proteins, and white blood cells, all of which help defend the body against foreign invaders such as bacteria and viruses.

Life and Death of RBCs

Red blood cells have a useful life span of 100 to 120 days. Their anucleate condition carries with it some important limitations. Red blood cells are unable to synthesize new proteins, to grow, or to divide. Erythrocytes become "old" as they lose their flexibility and become increasingly rigid and fragile, and their contained hemoglobin begins to degenerate. They become trapped and fragment in smaller circulatory channels, particularly in those of the spleen. For this reason, the spleen is sometimes called the "red blood cell graveyard".

Regulation

Regulatory functions of blood include: 1. Maintaining appropriate body temperature by absorbing and distributing heat throughout the body and to the skin surface to encourage heat loss. 2. Maintaining normal pH in body tissues. Many blood proteins and other blood-borne solutes act as buffers to prevent normal cell activities. Additionally, blood acts as the reservoir for the body's "alkaline reserve" of bicarbonate atoms. 3. Maintaining adequate fluid volume in the blood proteins act to prevent excessive fluid loss from the bloodstream into the tissue spaces. As a result, the fluid volume in the blood vessels remains ample to support efficient blood circulation to all parts of the body.

Reticulocytes

Reticulocytes account for 1 - 2% of all erythrocytes in the blood of healthy people. Reticulocyte counts provide a rough index of the rate of RBC formation -- reticulocyte counts below of above this percentage range indicate abnormal rates of erythrocyte formation.

secondary polycythemia

Secondary polycythemia's result when less oxygen is available or EPO production increases. The secondary polycythemia that appears in individuals living at high altitudes is a normal physiological response to the reduced atmospheric pressure and lower oxygen content of the air in such areas. RBC counts of 6-8 million/ul are common in such people. serve polycythemia is treated by blood dilution, in other words, removing some blood and replacing it with saline.

Treatments for sickle-cell anemia

Several treatment approaches focus on preventing RBCs from sickling. Fetal hemoglobin (HbF) does not "sickle" even in those destined to have sickle-cell anemia. Hydroxyurea, a drug used to treat chronic leukemia switches the fetal hemoglobin gene back on. This drug dramatically reduces the excruciating pain and overall severity and complications of sickle-cell anemia (by 50%). Another drug, clotrimazole, reduces sickling by blocking ion channels in the RBC membrane, keeping ions and water inside the cell. Other approaches being tested include oral arginine to stimulate nitric oxide production and dilate blood vessels, stem cell transplants, and gene therapy to deliver genes for synthesizing normal beta chains.

occurrence of sickle-cell anemia

Sickle-cell anemia occurs chiefly in black people who live in the malaria belt of Africa and among their descendants. It strikes nearly one of every 400 black newborns in the US. Globally, 300-400 million people are infected with malaria and more than a million die each year. While individuals with 2 copies of the sickle-cell gene have sickle cell anemia, individuals with only one copy of the gene (the sickle-cell trait) have a better chance of surviving in regions where malaria is present. Their cells only sickle under abnormal circumstances, most importantly when they are infected with malaria. Sickling appears to reside the parasites ability to survive and enhances macrophages ability to destroy infected RBCs and the parasites they contain.

Thalassemia

Thalassemia's are typically seen in people of Mediterranean ancestry, such as Greeks and Italians. One of the globin chains is absent or faulty, and the erythrocytes are thin, delicate, and deficient in hemoglobin. There are many subtypes of thalassemia. classified according to which hemoglobin chain is affected and where, ranging in severity from mild to so severe that monthly blood transfusion is required.

EPO and HIF

The direct stimulus for erythrocyte formation is provided by erythropoietin (EPO), a glycoprotein hormone. Normally, a small amount of EPO circulates the blood at all times and sustains red blood cell production at a basal rate. The kidneys play a major role in EPO production, although the liver produces some. When certain kidney cells become hypoxic (ie. have inadequate oxygen), oxygen-sensitive enzymes are unable to carry out their normal functions of degrading an intracellular signaling molecule called hypoxia-inducible factor (HIF). As HIF accumulates, it accelerates the synthesis and release of erythropoietin.

EPO formation (3 reasons)

The drop in normal blood oxygen levels that triggers EPO formation can result from 1. Reduced numbers of red blood cells due to hemorrhage or excessive RBC destruction. 2. Insufficient hemoglobin per RBC (as in iron deficiency). 3. Reduced availability of oxygen, as might occur at high altitudes or during pneumonia.

time of maturation of RBCs

The entire process from hemocytoblast to reticulocyte takes about 15 days. The reticulocytes, filled almost to bursting with hemoglobin, enter the bloodstream to begin their task of oxygen transport. Usually they become fully mature erythrocytes within 2 days of release as their ribosomes are degraded by intracellular enzymes.

RBC oxygen transport

The erythrocyte is a superb example of complementarity of structure and function. It picks up oxygen in the capillary beds of the lungs and releases it to tissue cells across other capillaries throughout the body. It also transports some 20% of the carbon dioxide released by tissue cells back to the lungs. 3 structural characteristics contribute to erythrocyte gas transport functions:

Why hemoglobin is contained in RBCs

The fact that hemoglobin is contained in erythrocytes, rather than existing free in plasma, prevents it (1) from breaking into fragments that would leak out of the bloodstream (through the rather porous capillary membranes) and (2) from contributing to blood viscosity and osmotic pressure.

formed elements

The formed elements of blood, erythrocytes, leukocytes,and platelets, have some unusual features. (1) two of the three are not even true cells: Erythrocytes have no nuclei or organelles, and platelets are cell fragments. Only leukocytes are complete cells. (2) Most of the formed elements survive in the bloodstream for only a few days. (3) Most blood cells do not divide. Instead, they are continuously renewed by division of cells in red bone marrow, where they originate.

athlete's anemia

When athletes exercise vigorously, their blood volume expands and can increase by as much as 35% over time. Since this increased volume effectively dilutes the blood components, a test for the iron content of the blood at such times would indicate iron-deficiency anemia. However this illusion of iron deficiency, is called athlete's anemia, is revered as blood components return to physiological levels within a week or so after the athlete resumes a normal level of activity.

nutritional anemia

When hemoglobin molecules are normal, but erythrocytes contain fewer than the usual number, a nutritional anemia is always suspected.