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