The Blood

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Compared with vertebrate blood, however, that of the invertebrates has relatively few cells. Among the vertebrates, there are several classes of amoeboid cells white blood cells, or leukocytes and cells that help stop bleeding platelets , or thrombocytes. Oxygen requirements have played a major role in determining both the composition of blood and the architecture of the circulatory system.

In some simple animals, including small worms and mollusks , transported oxygen is merely dissolved in the plasma. Larger and more-complex animals, which have greater oxygen needs, have pigments capable of transporting relatively large amounts of oxygen. The red pigment hemoglobin , which contains iron, is found in all vertebrates and in some invertebrates.

In almost all vertebrates, including humans, hemoglobin is contained exclusively within the red cells erythrocytes. The red cells of the lower vertebrates e. Red cells vary markedly in size among mammals; those of the goat are much smaller than those of humans, but the goat compensates by having many more red cells per unit volume of blood.

The concentration of hemoglobin inside the red cell varies little between species.


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Hemocyanin , a copper -containing protein chemically unlike hemoglobin, is found in some crustaceans. Hemocyanin is blue in colour when oxygenated and colourless when oxygen is removed. Some annelids have the iron-containing green pigment chlorocruorin, others the iron-containing red pigment hemerythrin.

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In many invertebrates the respiratory pigments are carried in solution in the plasma, but in higher animals, including all vertebrates, the pigments are enclosed in cells; if the pigments were freely in solution, the pigment concentrations required would cause the blood to be so viscous as to impede circulation.

This article focuses on the main components and functions of human blood. For full treatment of blood groups, see the article blood group. For information on the organ system that conveys blood to all organs of the body, see cardiovascular system. For additional information on blood in general and comparison of the blood and lymph of diverse organisms, see circulation.

Components of Blood

In humans, blood is an opaque red fluid, freely flowing but denser and more viscous than water. The characteristic colour is imparted by hemoglobin , a unique iron-containing protein. Hemoglobin brightens in colour when saturated with oxygen oxyhemoglobin and darkens when oxygen is removed deoxyhemoglobin. For this reason, the partially deoxygenated blood from a vein is darker than oxygenated blood from an artery. The red blood cells erythrocytes constitute about 45 percent of the volume of the blood, and the remaining cells white blood cells, or leukocytes , and platelets , or thrombocytes less than 1 percent.

The fluid portion, plasma , is a clear, slightly sticky, yellowish liquid. After a fatty meal, plasma transiently appears turbid. Within the body the blood is permanently fluid, and turbulent flow assures that cells and plasma are fairly homogeneously mixed. The total amount of blood in humans varies with age, sex, weight, body type, and other factors, but a rough average figure for adults is about 60 millilitres per kilogram of body weight. An average young male has a plasma volume of about 35 millilitres and a red cell volume of about 30 millilitres per kilogram of body weight.

There is little variation in the blood volume of a healthy person over long periods, although each component of the blood is in a continuous state of flux. In particular, water rapidly moves in and out of the bloodstream, achieving a balance with the extravascular fluids those outside the blood vessels within minutes.

The normal volume of blood provides such an adequate reserve that appreciable blood loss is well tolerated. Withdrawal of millilitres about a pint of blood from normal blood donors is a harmless procedure. Blood volume is rapidly replaced after blood loss; within hours, plasma volume is restored by movement of extravascular fluid into the circulation.

Replacement of red cells is completed within several weeks. The vast area of capillary membrane, through which water passes freely, would permit instantaneous loss of the plasma from the circulation were it not for the plasma proteins—in particular, serum albumin. Capillary membranes are impermeable to serum albumin, the smallest in weight and highest in concentration of the plasma proteins. The osmotic effect of serum albumin retains fluid within the circulation, opposing the hydrostatic forces that tend to drive the fluid outward into the tissues.

The stem cells develop into platelet precursors called megakaryocytes that "shed" platelets into the bloodstream. There, platelets circulate for about 9 days. If they encounter damaged blood vessel walls during this time, they stick to the damaged area and are activated to form a blood clot.

This plugs the hole. Otherwise, at the end of their life span they are removed from the circulation by the spleen. In a diverse number of diseases where the spleen is overactive, e. A complete blood count CBC is a simple blood test that is commonly ordered as part of a routine medical assessment. As the name suggests, it is a count of the different types of cells found in the blood. The test can diagnose and monitor many different diseases, such as anemia, infection, inflammatory diseases, and malignancy.

Table 1 gives an example of CBC values, but note that the reference ranges and the units used may differ, depending upon the laboratory that carried out the test. The number of RBCs and the amount of hemoglobin in the blood are lower in women than in men. This is because of the menstrual loss of blood each month. Below a certain level of hemoglobin, a patient is said to be anemic, suggesting a clinically significant drop in oxygen carrying capacity. Anemia is not a diagnosis but a symptom of an underlying disease that has to be investigated.

Causes of a high MCV include a deficiency of B 12 or folate vitamins in the diet.

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B 12 is found in red meat therefore, a deficiency of B 12 is especially common in vegetarians and vegans. Conversely, folate is plentiful in fresh leafy green vegetables, therefore, a deficiency of folate is common in the elderly, who may have a poor diet.

Blood Components

Low MCV anemia is common and may be a result of hereditary blood disorders, such as thalassemia, but is most often caused by a deficiency of iron. For example, women of reproductive age may lose too much iron through heavy menstrual bleeding and are prone to this form of anemia, known as iron-deficiency anemia.

The hematocrit measures the fraction of the blood that is made up of RBCs. It reflects the combination of the total number of RBCs, and the volume that they occupy.


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One of the changes seen in pregnancy is a drop in hematocrit. This occurs because although the production of RBCs does not change greatly, the plasma volume increases, i. Alternatively, a low hematocrit can reflect a drop in RBC production by the bone marrow. This may be attributable to bone marrow disease damage by toxins or cancer or due to a decrease in erythropoietin, a hormone secreted by the kidney that stimulates RBC production. A high hematocrit value may truly reflect an increase in the fraction of RBCs e.

An increased number of WBCs is most commonly caused by infections, such as a urinary tract infection or pneumonia. It may also be caused by WBC tumors, such as leukemia. A decreased number of WBCs is caused by the bone marrow failing to produce WBCs or by an increased removal of WBCs from the circulation by a diseased liver or an overactive spleen. Bone marrow failure may be caused by toxins or by the normal bone marrow cells being replaced by tumor cells. Finding out the count of each type of WBC gives more information about the underlying problem.

For example, in the early stages of an infection, most of the increase in WBCs is attributable to the increase in neutrophils. As the infection continues, lymphocytes increase. Worm infections can trigger an increase in eosinophils, whereas allergic conditions, such as hay fever, trigger an increase in basophils. Normally, one cubic millimeter of blood contains between , and , platelets. If the number drops below this range, uncontrolled bleeding becomes a risk, whereas a rise above the upper limit of this range indicates a risk of uncontrolled blood clotting.

Hemoglobin is the oxygen-carrying protein that is found within all RBCs.

Formation of Blood Cells

It picks up oxygen where it is abundant the lungs and drops off oxygen where it is needed around the body. Hemoglobin is also the pigment that gives RBCs their red color. As its name suggests, hemoglobin is composed of "heme" groups iron-containing rings and "globins" proteins. In fact, hemoglobin is composed of four globin proteins—two alpha chains and two beta chains—each with a heme group.

The heme group contains one iron atom, and this can bind one molecule of oxygen. Because each molecule of hemoglobin contains four globins, it can carry up to four molecules of oxygen. See hemoglobin structure in Albert's Molecular Biology of the Cell. In the lungs, a hemoglobin molecule is surrounded by a high concentration of oxygen, therefore, it binds oxygen. In active tissues, the oxygen concentration is lower, so hemoglobin releases its oxygen. This behavior is much more effective because the hemoglobin——oxygen binding is "co-operative".

This means that the binding of one molecule of oxygen makes it easier for the binding of subsequent oxygen molecules. Likewise, the unbinding of oxygen makes it easier for other oxygen molecules to be released. This means that the response of hemoglobin to the oxygen needs of active tissues is much quicker. Aside from the oxygen saturation of hemoglobin, other factors that influence how readily hemoglobin binds oxygen include plasma pH, plasma bicarbonate levels, and the pressure of oxygen in the air high altitudes in particular. The molecule 2,3-disphosphoglycerate 2,3-DPG binds to hemoglobin and lowers its affinity for oxygen, thus promoting oxygen release.

In individuals who have become acclimatized to living at high altitudes, the level of 2,3-DPG in the blood increases, allowing the delivery of more oxygen to tissues under low oxygen tension. Fetal hemoglobin differs from adult hemoglobin in that it contains two gamma chains instead of two beta chains. Fetal hemoglobin binds oxygen with a much greater affinity than adult hemoglobin; this is an advantage in the womb because it allows fetal blood to extract oxygen from maternal blood, despite its low concentration of oxygen.

Old or damaged RBCs are removed from the circulation by macrophages in the spleen and liver, and the hemoglobin they contain is broken down into heme and globin. The globin protein may be recycled, or broken down further to its constituent amino acids, which may be recycled or metabolized. The heme contains precious iron that is conserved and reused in the synthesis of new hemoglobin molecules.

During its metabolism, heme is converted to bilirubin, a yellow pigment that can discolor the skin and sclera of the eye if it accumulates in the blood, a condition known as jaundice. Instead, the plasma protein albumin binds to bilirubin and carries it to the liver, where it is secreted in bile and also contributes to the color of feces.

Jaundice is one of the complications of an incompatible blood transfusion.

Circulatory and pulmonary systems

This occurs when the recipient's immune system attacks the donor RBCs as being foreign. The rate of RBC destruction and subsequent bilirubin production can exceed the capacity of the liver to metabolize the bilirubin produced. Hemoglobinopathies form a group of inherited diseases that are caused by mutations in the globin chains of hemoglobin.

Sickle cell anemia is the most common of these and is attributable to a mutation that changes one of the amino acids in the hemoglobin beta chain, producing hemoglobin that is "fragile". When the oxygen concentration is low, RBCs tend to become distorted and "sickle" shaped. These deformed cells can block small blood vessels and damage the organs they are supplying.

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This can be very painful, and if not treated, a sickle cell crisis can be fatal. Another inherited anemia that particularly affects individuals of Mediterranean descent is thalassemia. A fault in the production of either alpha or beta globin chains causes a range of symptoms, depending on how many copies of the alpha and beta genes are affected. Some individuals may be carriers of the disease and have no symptoms, whereas if all copies of the genes are lost, the disease is fatal.

The porphyrias are a group of inherited disorders in which the synthesis of heme is disrupted. Depending upon the stage at which the disruption occurs, there are a range of neurological and gastrointestinal side effects. Karl Landsteiner, Nobel Laureate from Nobelprize. Turn recording back on.

National Center for Biotechnology Information , U. Show details Dean L. Search term. Chapter 1 Blood and the cells it contains. This chapter introduces the components of blood. Blood contains cells, proteins, and sugars. Box Meet the blood cells. Red blood cells transport oxygen. White blood cells are part of the immune response WBCs come in many different shapes and sizes. Neutrophils digest bacteria. Monocytes become macrophages. Lymphocytes consist of B cells and T cells.

Platelets help blood to clot.



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