Elementary Blood Physiology
Composition of Blood
Blood has two components:
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1. Plasma
| The liquid matrix of blood. |
Plasma contains dissolved proteins and numerous dissolved solutes.
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2. Formed Elements
| a. Red blood cells (rbc’s) |
| b. White blood cells (immune system) |
| c. Platelets (clotting mechanism) |
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Plasma
The components of plasma include:
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1. Water is the major component (about 92%) and is the solvent for organic and inorganic molecules. |
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2. Electrolytes include sodium, potassium, chloride, calcium, magnesium, bicarbonate, phosphate and sulfate. |
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3. Nutrients such as glucose, lipids and amino acids. |
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4. Blood gases include oxygen, carbon dioxide and nitrogen. |
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5. Regulatory substances include hormones and enzymes. |
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6. Plasma proteins.
Make up about 7% of plasma and fall into three major classes:
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a. Albumins
Albumins make up roughly 60% of all plasma proteins and are a major contributor to osmotic pressure.
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| Albumins are the smallest of the three and serve as carriers for hormones, metabolites and drugs. |
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b. Globulins
| These proteins make up about 35%. Immunoglobulins are produced by lymphocytes and other globulins are produced by the liver. |
| The nonimmune globulins serve as transport globulins for ions, hormones and other compounds. |
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c. Fibrinogen
This is the largest protein but only comprises about 4% of plasma proteins. It participates in the clot
formation when it is transformed into fibrin.
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| Blood plasma minus the fibrinogen and other clotting proteins is called serum. |
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Albumin and globulin form complexes with lipids that are called lipoproteins that enable insoluble lipids to be
transported throughout the body. |
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Formed Elements
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Red Blood Cells (RBCs) a.k.a.
erythrocytes
| Rbc’s constitute the largest component of the formed elements (99.9%). |
The hematocrit is the percentage volume of blood contributed by the formed elements. It is also called volume of packed red cells (VPRC) and packed cell volume (PCV).
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| The normal hematocrit is about 46 (40 to 54) in men, and 42 (37 to 47) in women. |
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Structure of RBCs
Rbc’s are highly specialized cells that lack a nucleus, mitochondria or ribosomes. Their ability to transport O2 and
CO2 is due to the presence of hemoglobin and other protein within the cell. |
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The biconcave shape of the rbc’s increases the surface area for exchange and enhances the strength and flexibility of the cells. |
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RBC life span
| The loss of protein making organelles (nucleus, ribosomes) and other organelles (mitochondria) decrease the ability of rbc’s to produce structural proteins and enzymes. This combined with the wear and tear that rbc’s experience cause these cells to have a short life span. |
The typical rbc’s lives about 120 days before bursting or being phagocytized.
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RBCs and Hemoglobin
| Hemoglobin (Hb) makes up 95% of the protein within the rbc’s. Hemoglobin is a red pigment that is bright red (arterial blood) when combined with oxygen but becomes a deep red color (venous blood) when deoxygenated. |
On a molecular level, hemoglobin consists of four protein subunits, two alpha hemoglobin units and two beta hemoglobin units. Each hemoglobin subunit holds a molecule called heme that has an iron ion that can reversibly
bind with oxygen. The hemoglobin molecule can also reversibly bind with CO2 and about 23% of CO2 is carried in this way. |
| Hemoglobin enables rbc’s to pick up O2 and give up CO2 in the lungs and release O2 and pick up CO2 in the peripheral tissues.
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White Blood Cells (WBCs) a.k.a.
leukocytes
| Leukocytes perform their primary functions in the peripheral tissues where they are present in large numbers. The cardiovascular system is primarily a means to travel to the peripheral tissues. |
| Wbc’s leave the circulation by the process of diapedesis that enables them to squeeze between endothelial cells. They are attracted to specific locations by chemotaxis by which cells use chemical gradients to guide their movements. |
| Leukocytes defend against invasion by pathogens and help the body remove toxins, wastes and damaged cells. |
| When there are inadequate numbers of wbc’s this condition is called leukopenia; when there is an excessive number of wbc’s this condition is called leukocytosis. |
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There are two major classes of white blood cells:
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1. Granular Leukocytes which have specific granules in their cytoplasm.
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2. Agranular Leukocytes which lack specific granules in their cytoplasm. |
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Granular Leukocytes
| Neutrophils |
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Neutrophils are the most numerous wbc constituting about 70% of circulating wbc’s in a normal individual.
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The specific granules of neutrophils contain lysozyme and other bacteriostatic and bacteriocidal agents. |
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The nucleus is dense and contorted with expanded regions connected by slender threads so that the nucleus may appear as beads on a string. Because of this appearance these cells have be called polymorphonuclear cells or PMN‘s. |
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Neutrophils are highly mobile and are the first to arrive at an injury site. They actively phagocytize bacteria and cellular debris. They die quickly but release chemicals recruit other leukocytes to the troubled site.
Neutrophil phagocytosis |
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Eosinophils |
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Eosinophils have specific granules that stain with eosin an acidic red dye and the nucleus is bilobed. |
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These cells are 2 to 4% of the circulating white blood cells in a normal individual. |
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Eosinophils phagocytize antigen-antibody complexes and releases chemicals that moderates the deleterious effects of inflammatory reactions.
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The specific granules also have chemicals that are effective against parasites, particularly helminthic parasites. |
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Eosinophil counts increase during allergic reactions and parasitic infections. |
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Basophils |
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Basophils are the rarest of the wbc’s (less than 1%) and have specific granules that stain with basic dyes.
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The specific granules of basophils contain vasoactive chemicals similar to those of mast cells (histamine and heparin). |
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When released, the chemicals of the specific granules enhance the inflammatory response by increasing
vascular permeability. |
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Basophils are involved with hypersensitivity reactions and anaphylaxis.
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Agranular Leukocytes
| Monocytes |
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Monocytes constitute about 2 to 8% of wbc’s and are the largest wbc (2-3 times the rbc). They have an indented nucleus and lack specific granules. |
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Monocytes remain in the blood for about 3 days and enter the tissues to become the macrophages that are found in various tissues including histiocytes in connective tissues, osteoclasts in bone, Kupffer cell in liver and macrophages in various other tissues. Hence, they form a part of the monocytes-macrophage system.
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Macrophages arrive at a site of injury or invasion after neutrophils and phagocytizes bacteria, other cells and tissue debris. They also release chemotaxic chemicals that attract other cells of the immune system to the site.
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Active macrophages also attract fibroblasts to the site to begin repair and the construction
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Lymphocytes |
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Lymphocytes form about 20 to 30% of wbc in the normal individual and most have a large, purple-staining nucleus surrounded by a thin halo of cytoplasm. |
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Lymphocytes are responsible for specific immunityor the ability to attack pathogens or foreign proteins on an individual basis. |
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Three types of lymphocytes are recognized:
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TCells
T lymphocytes are responsible for cell mediated immune response.
They attack foreign cell directly. A subclass of these cells is attacked by the HIV virus. |
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B cells
B lymphocytes are responsible for humoral immunity. When activated, B cells transform into plasma cells that produce antibody, also called immunoglobulins. |
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NK cells (natural killer cell)
These cells constitute about 5 – 10% of lymphocytes and are named for their ability to kill certain types of transformed cells. These cells include cells infected with a virus and cancer cells. |
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Platelets a.k.a. Thrombocytes
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Platelets represent small, discoid-shaped fragments of cytoplasm that are involved in the surveillance of blood vessels, blood clot formation and tissue repair. |
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Platelets are continually produced in bone marrow by large cells called megakaryoctyes. Platelets are produced as the cytoplasm of megakaryoctes becomes fragmented and breaks off as individual platelets. |
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An abnormal platelet count is called thrombocytopenia and an excessive number of platelets is called
thrombocytosis. |
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Platelets participate in the clotting system which is an important aspect of hemostasis |
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Platelets function as part of the clotting mechanism by:
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1. Transporting chemicals involved in the initiation and control of the clotting process. |
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2. Formation of a temporary patch in the wall of damaged blood vessels. |
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3. Contraction of the clot after it has formed. |
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