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Blood


The Cardiovascular System: An Introduction

The cardiovascular system

Provides a mechanism for rapid transport of nutrients, waste products, respiratory gases and cells


Functions and Composition of Blood

Blood

Fluid connective tissue

Functions include

Transporting dissolved gases, nutrients, hormones, and metabolic wastes

Regulating pH and ion composition of interstitial fluids

Restricting fluid loss at injury sites

Defending the body against toxins and pathogens

Regulating body temperature by absorbing and redistributing heat

 

The composition of blood

Plasma and formed elements comprise whole blood

Red blood cells (RBC)

White blood cells (WBC)

Platelets

Can fractionate whole blood for analytical or clinical purposes

 

Hemopoiesis

Process of blood cell formation

Hemocytoblasts are circulating stem cells that divide to form all types of blood cells

Whole blood from anywhere in the body has roughly the same temperature, pH and viscosity

 

Plasma

Accounts for 46-63% of blood volume

92% of plasma is water

Higher concentration of dissolved oxygen and dissolved proteins than interstitial fluid

 

Plasma proteins

more than 90% are synthesized in the liver

Albumins

60% of plasma proteins

Responsible for viscosity and osmotic pressure of blood

 

Additional Plasma Proteins

Globulins

~35% of plasma proteins

Include immunoglobins which attack foreign proteins and pathogens

Include transport globulins which bind ions, hormones and other compounds

Fibrinogen

Converted to fibrin during clotting

Removal of fibrinogen leaves serum


Red Blood Cells

Abundance of RBCs

Erythrocytes account for slightly less than half the blood volume, and 99.9% of the formed elements

Hematocrit measures the percentage of whole blood occupied by formed elements

Commonly referred to as the volume of packed red cells

 

Structure of RBCs

Biconcave disc, providing a large surface to volume ration

Shape allows RBCs to stack, bend and flex

RBCs lack organelles

Typically degenerate in about 120 days.

 

Hemoglobin

Molecules of hemoglobin account for 95% of the proteins in RBCs

Hemoglobin is a globular protein, formed from two pairs of polypeptide subunits

Each subunit contains a molecule of heme which reversibly binds an oxygen molecule

Damaged or dead RBCs are recycled by phagocytes

 

RBC life span and circulation

Replaced at a rate of approximately 3 million new blood cells entering the circulation per second.

Replaced before they hemolyze

Components of hemoglobin individually recycled

Heme stripped of iron and converted to biliverdin, then bilirubin

Iron is recycled by being stored in phagocytes, or transported throughout the blood stream bound to transferrin

 

RBC Production

Erythropoeisis = the formation of new red blood cells

Occurs in red bone marrow

Process speeds up with in the presence of EPO (Erythropoeisis stimulating hormone)

RBCs pass through reticulocyte and erythroblast stages

 

Blood types

Determined by the presence or absence of surface antigens (agglutinogens)

Antigens A, B and Rh (D)

Antibodies in the plasma (agglutinins)

Cross-reactions occur when antigens meet antibodies


The White Blood Cells

Leukocytes

Have nuclei and other organelles

Defend the body against pathogens

Remove toxins, wastes, and abnormal or damaged cells

Are capable of amoeboid movement (margination) and positive chemotaxis

Some are capable of phagocytosis

 

Types of WBC

Granular leukocytes

Neutrophils 50 to 70 % total WBC population

Eosinophils phagocytes attracted to foreign compounds that have reacted with antibodies

Basophils migrate to damaged tissue and release histamine and heparin

 

Types of WBC

Agranular leukocytes

Monocytes - become macrophage

Lymphocytes includes T cells, B cells, and NK cells

 

Differential count

Indicates a number of disorders

Leukemia = inordinate number of leukocytes

WBC Production

Granulocytes and monocytes are produced by bone marrow stem cells

Divide to create progenitor cells

Stem cells may originate in bone marrow and migrate to peripheral tissues

Several colony stimulating factors are involved in regulation and control of production

 

Platelets

Flattened discs

Circulate for 9-12 days before being removed by phagocytes

 

Platelet functions

Transporting chemicals important to clotting

Forming temporary patch in walls of damaged blood vessels

Contracting after a clot has formed

 

Platelet production (thrombocytopoiesis)

Megakaryocytes release platelets into circulating blood

Rate of platelet formation is stimulated by thrombopoietin, thrombocyte-stimulating factor, interleukin-6, and Multi-CSF

 

Hemostasis

Prevents the loss of blood through vessel walls

Three phases

Vascular phase

Platelet phase

Coagulation phase

 

Hemostasis

Vascular phase

Local blood vessel constriction (vascular spasm)

Platelet phase

Platelets are activated, aggregate at the site, adhere to the damaged surfaces

 

Coagulation phase

Factors released by platelets and endothelial cells interact with clotting factors to form a clot

Extrinsic pathway

Intrinsic pathway

Common pathway

Suspended fibrinogen is converted to large insoluble fibrin fibers

 

Clot retraction

Final phase of healing

Platelets contract and pull the edges of the vessel together

 

Fibrinolysis

Clot gradually dissolves through action of plasmin

Activated form of plasminogen

Clotting can be prevented through the use of drugs that depress the clotting response or dissolve existing clots

Anticoagulants include heparin, coumadin, aspirin, dicumarol, t- PA, streptokinase, and urokinase