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Chapter 20 - The Cardiovascular System - Blood

Functions of Blood 1. Transportation of dissolved gases 2. Distribution of nutrients 3. Transportation of metabolic wastes 4. Delivery of enzymes and hormones 5. Stabilization of pH and electrolytes 6. Prevention of fluid losses (clotting reaction) 7. Defense against toxins and pathogens

8. Stabilization of body temperature

Composition of Blood Blood has two components:

1. Plasma The liquid matrix of blood. Plasma contains dissolved proteins and numerous dissolved solutes.

2. Formed Elements a. Red blood cells (rbc's) b. White blood cells (immune system) c. Platelets (clotting mechanism)

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Characteristics of whole blood: Blood is more viscous than water. Plasma is 1.5 times, and whole blood is 5.0 times, the viscosity ofwater. Blood has an alkaline pH being in the range of 7.35 to 7.45. Blood has a temperature slightly higher than core body temperature (38o C vs. 37oC).

Plasma The components of plasma include: 1. Water is the major component (about 92%) and is the solvent for organic and inorganic molecules.

2. Electrolytes include sodium, potassium, chloride, calcium, magnesium, bicarbonate, phosphate andsulfate.

3. Nutrients such as glucose, lipids and amino acids. 4. Blood gases include oxygen, carbon dioxide and nitrogen. 5. Regulatory substances include hormones and enzymes.

6. Plasma proteins. Make up about 7% of plasma and fall into three major classes:

a. Albumins Albumins make up roughly 60% of all plasma proteins and are a major contributor to osmoticpressure. Albumins are the smallest of the three and serve as carriers for hormones, metabolites and drugs.

b. Globulins These proteins make up about 35%. Immunoglobulins are produced by lymphocytes and otherglobulins are produced by the liver. The nonimmune globulins serve as transport globulins for ions, hormones and other compounds.

c. Fibrinogen This is the largest protein but only comprises about 4% of plasma proteins. It participates in theclot formation when it is transformed into fibrin. Blood plasma minus the fibrinogen and other clotting proteins is called serum.

Albumin and globulin form complexes with lipids that are called lipoproteins that enable insolublelipids to be transported throughout the body.

Formed Elements

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 alsocalled volume of packed red cells (VPRC) and packed cell volume (PCV). The normal hematocrit is about 46 (40 to 54) in men, and 42 (37 to 47) in women. Structure of RBCs Rbc's are highly specialized cells that lack a nucleus, mitochondria or ribosomes. Their ability to

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transport O2 and CO2 is due to the presence of hemoglobin and other protein within the cell. The biconcave shape of the rbc's increases the surface area for exchange and enhances the strengthand flexibility of the cells.

The biconcave shape enables rbc's to move more quickly through capillaries by forming stacks calledrouleaux and the ability of the cell to bend upon itself enables the cells to squeeze through smallercapillaries.

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 wearand tear that rbc's experience cause these cells to have a short life span. The typical rbc's live about 120 days before bursting or being phagocytized. RBCs and Hemoglobin Hemoglobin (Hb) makes up 95% of the protein within the rbc's. Hemoglobin is a red pigment that isbright 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 andtwo beta hemoglobin units. Each hemoglobin subunit holds a molecule called heme that has an iron ionthat can reversibly bind with oxygen. The hemoglobin molecule can also reversibly bind with CO2 andabout 23% of CO2 is carried in this way.

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Hemoglobin enables rbc's to pick up O2 and give up CO2 in the lungs and release O2 and pick up CO2in the peripheral tissues.

White Blood Cells (WBCs) a.k.a. leukocytes Leukocytes perform their primary functions in the peripheral tissues where they are present in largenumbers. 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 betweenendothelial cells. They are attracted to specific locations by chemotaxis by which cells use chemicalgradients to guide their movements. Leukocytes defend against invasion by pathogens and help the body remove toxins, wastes anddamaged cells. When there are inadequate numbers of wbc's this condition is called leukopenia; when there is anexcessive number of wbc's this condition is called leukocytosis.

There are two major classes of white blood cells: 1. Granular Leukocytes which have specific granules in their cytoplasm.

2. Agranular Leukocytes which lack specific granules in their cytoplasm.

Granular Leukocytes Neutrophils

Neutrophils are the most numerous wbc constituting about 70% of circulating wbc's in a normal

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individual.

The specific granules of neutrophils contain lysozyme and other bacteriostatic and bacteriocidalagents.

The nucleus is dense and contorted with expanded regions connected by slender threads so that thenucleus may appear as beads on a string. Because of this appearance these cells have be calledpolymorphonuclear cells or PMN's.

Neutrophils are highly mobile and are the first to arrive at an injury site. They actively phagocytizebacteria and cellular debris. They die quickly but release chemicals recruit other leukocytes to thetroubled site.

Eosinophils

Eosinophils have specific granules that stain with eosin an acidic red dye and the nucleus isbilobed.

These cells are 2 to 4% of the circulating white blood cells in a normal individual.

Eosinophils phagocytize antigen-antibody complexes and releases chemicals that moderates thedeleterious effects of inflammatory reactions.

The specific granules also have chemicals that are effective against parasites, particularlyhelminthic parasites.

Eosinophil counts increase during allergic reactions and parasitic infections.

Basophils

Basophils are the rarest of the wbc's (less than 1%) and have specific granules that stain with basicdyes.

The specific granules of basophils contain vasoactive chemicals similar to those of mast cells(histamine and heparin).

When released, the chemicals of the specific granules exaggerate the inflammatory response byincreasing vascular permeability.

Basophils are involved with hypersensitivity reactions and anaphylaxis.

Agranular Leukocytes Monocytes

Monocytes constitute about 2 to 8% of wbc's and are the largest wbc (2-3 times the rbc). Theyhave an indented nucleus and lack specific granules.

Monocytes remain in the blood for about 3 days and enter the tissues to become the macrophagesthat 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 themonocytes-macrophage system.

Macrophages arrive at a site of injury or invasion after neutrophils and phagocytizes bacteria, othercells and tissue debris. They also release chemotaxic chemicals that attract other cells of the immunesystem to the site.

Active macrophages also attract fibroblasts to the site to begin repair and the construction of newtissue.

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Lymphocytes

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.

Lymphocytes are responsible for specific immunity or the ability to attack pathogens or foreignproteins on an individual basis.

Three types of lymphocytes are recognized:

T cells T lymphocytes are responsible for cell-mediated immunity. They attack foreign cell directly.A subclass of these cells is attacked by the HIV virus.

B cells B lymphocytes are responsible for humoral immunity. When activated, B cells transform intoplasma cells that produce antibody, also called immunoglobulins.

NK cells (natural killer cell) These cells constitute about 5 - 10% of lymphocytes and are named for their ability to killcertain types of transformed cells. These cells include cells infected with a virus and cancer cells.

Platelets a.k.a. Thrombocytes

Platelets represent small, discoid-shaped fragments of cytoplasm that are involved in thesurveillance of blood vessels, blood clot formation and tissue repair.

Platelets are continually produced in bone marrow by large cells called megakaryoctyes. Plateletsare produced as the cytoplasm of megakaryoctes becomes fragmented and breaks off as individualplatelets.

An abnormal platelet count is called thrombocytopenia and an excessive number of platelets iscalled thrombocytosis.

Platelets participate in the clotting system which is an important aspect of hemostasis

Platelets function as part of the clotting mechanism by: 1. Transporting chemicals involved in the initiation and control of the clotting process. 2. Formation of a temporary patch in the wall of damaged blood vessels.

3. Contraction of the clot after it has formed.

Hemopoiesis Hemopoiesis is the process of blood cell formation from stem cells.

Erythropoiesis

Erythropoiesis refers specifically to the formation of erythrocytes or rbc's. Red bone marrow is theprimary site of erythropoiesis in the adult.

Erythropoiesis requires adequate supplies of amino acids, iron and vitamin B12. (Lack of vitaminB12 causes pernicious anemia.)

Erythropoietin (EPO) is a hormone primarily released by the kidneys that stimulateserythropoiesis. (This hormone is sometimes involved in athletic doping scandals.)

Leukopoiesis

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Leukopoiesis refers to the production of white blood cells from stem cells.