Blood Composition

•     Body’s only fluid tissue

•     Composed of liquid plasma and formed elements

•     Formed elements include:

–    Erythrocytes, or red blood cells (RBCs)

–    Leukocytes, or white blood cells (WBCs)

–    Platelets

•     Plasma

–    92% water

–    8% proteins

•   Albumins

•   Globulins

–   Immunoglobulins

–   Transport globulins

•   Fibrinogen

Components of Whole Blood

Functions of Blood

•     Transportation

–    Substance distribution

–    Blood transports:

•   Oxygen from the lungs and nutrients from the digestive tract

•   Metabolic wastes from cells to the lungs and kidneys for elimination

•   Hormones from endocrine glands to target organs

•     Regulation

–    Regulate blood levels of particular substances

–    Blood maintains:

•   Appropriate body temperature by absorbing and distributing heat

•   Normal pH in body tissues using buffer systems

•   Adequate fluid volume in the circulatory system

•     Protection of body

–    Blood prevents blood loss by:

•   Activating plasma proteins and platelets

•   Initiating clot formation when a vessel is broken

–    Blood prevents infection by:

•   Synthesizing and utilizing antibodies

•   Activating complement proteins

•   Activating WBCs to defend the body against foreign invaders

Formed Elements

Red Blood Cells

•     Also known as erythrocytes

•     Biconcave disc

•     Anucleate

–    RBCs have no nuclei or organelles

•     Filled with hemoglobin (Hb), a protein that functions in gas transport

•     Contain the plasma membrane protein spectrin and other proteins that:

–    Give erythrocytes their flexibility

–    Allow them to change shape as necessary

•     Large surface area/volume

–    Biconcave shape has a huge surface area relative to volume

Hemoglobin

•     Hematocrit

–    The percentage of RBCs out of the total blood volume

•     RBCs are dedicated to respiratory gas transport

•     Erythrocytes are more than 97% hemoglobin

•     Hb reversibly binds with oxygen and most oxygen in the blood is bound to Hb

•     ATP is generated anaerobically, so the erythrocytes do not consume the oxygen they transport

•     Hb is composed of the protein globin, made up of two alpha and two beta chains, each bound to a heme group

•     Each heme group bears an atom of iron, which can bind to one oxygen molecule

•     Each Hb molecule can transport four molecules of oxygen

Structure of Hemoglobin

Hemoglobin (Hb)

•    Oxyhemoglobin – Hb bound to oxygen

–  Oxygen loading takes place in the lungs

•    Deoxyhemoglobin – Hb after oxygen diffuses into tissues (reduced Hb)

•    Carbaminohemoglobin – Hb bound to carbon dioxide

–  Carbon dioxide loading takes place in the tissues

RBC Formation

•     Erythropoiesis

•     Circulating erythrocytes – the number remains constant and reflects a balance between RBC production and destruction

–   Too few RBCs leads to tissue hypoxia

•   Hypoxia is when tissues are exposed to low levels of oxygen

–   Too many RBCs causes undesirable blood viscosity

•     Erythropoiesis is hormonally controlled and depends on adequate supplies of iron, amino acids, and B vitamins

Hormonal Control of Erythropoiesis

•    Erythropoietin (EPO) release by the kidneys is triggered by:

–  Hypoxia due to decreased RBCs

–  Decreased oxygen availability

–  Increased tissue demand for oxygen

•    Enhanced erythropoiesis increases the:

–  RBC count in circulating blood

–  Oxygen carrying ability of the blood

Erythropoietin Mechanism

RBC Recycling

•     The life span of an erythrocyte is 100–120 days

•     Old RBCs become rigid and fragile, and their Hb begins to degenerate

•     Dying RBCs are engulfed by macrophages

•     Hb has to be broken apart

–    Heme and globin are separated and the iron is salvaged for reuse

•   Transferrin

–    Globulins

•   Globin is metabolized into amino acids and is released into the circulation

–    Heme

•   Heme is degraded to a yellow pigment called bilirubin

•   The liver secretes bilirubin into the intestines as bile

•   The intestines metabolize it into urobilinogen

•   This degraded pigment leaves the body in feces, in a brown pigment called stercobilin

Erythrocyte Disorders

•    Anemia – blood has abnormally low oxygen-carrying capacity

–  It is a symptom rather than a disease itself

–  Blood oxygen levels cannot support normal metabolism

–  Signs/symptoms include fatigue, paleness, shortness of breath, and chills

Anemia: Decreased Hemoglobin Content

•     Iron-deficiency anemia results from:

–   Inadequate intake of iron-containing foods

–   Impaired iron absorption

•     Pernicious anemia results from:

–   Deficiency of vitamin B₁₂

–   Lack of intrinsic factor needed for absorption of B₁₂

–   Treatment is intramuscular injection of B₁₂ or application of Nascobal (gel)

Other Erythrocyte Disorders

•    Sickle-cell anemia – results from a defective gene coding for an abnormal Hb called hemoglobin S (HbS)

–  This defect causes RBCs to become sickle-shaped in low oxygen situations

•    Polycythemia – excess RBCs that increase blood viscosity

–  Blood doping

•  Banned from Olympic Games

White Blood Cells

•     Also know as leukocytes

•     The only blood components that are complete cells

–   Possess nuclei and organelles

•     Protect body against invasion

•     Characteristics

–   Amoeboid movement

–   Can leave capillaries via diapedesis

•   Move through tissue spaces

–   Positive chemotaxis

–   Some are capable of phagocytosis

•     Leukocytosis – WBC count over 11,000 / mm³

–   Normal response to bacterial or viral invasion

Granulocytes

•     Are all phagocytic cells

•     Eosinophils

–   Lead the body’s counterattack against parasitic worms

•     Basophils

–   Are functionally similar to mast cells

–   Have large, purplish-black (basophilic) granules that contain histamine

•   Histamine – inflammatory chemical that acts as a vasodilator and attracts other WBCs (antihistamines counter this effect)

•     Neutrophils

–   Are our body’s “bacteria slayers”

Agranulocytes

•     Lack visible cytoplasmic granules

•     Lymphocytes

–   Two types:

•   T cells function in the immune response

•   B cells give rise to plasma cells, which produce antibodies

•     Monocytes

–   Largest leukocytes

–   They leave the circulation, enter tissue, and differentiate into macrophages

–   Macrophages

•   Are highly mobile and actively phagocytic

•   Activate lymphocytes to mount an immune response

Percentages of Leukocytes

PLATELETS

•     Fragments of megakaryocytes (red bone marrow cells)

•     Function in the clotting mechanism

–    Transport chemicals

–    Form a temporary platelet plug that helps seal breaks in blood vessels

–    Contraction after clot formation

HEMOSTASIS

•    A series of reactions for stoppage of bleeding

•    During hemostasis, three phases occur in rapid sequence

1) Vascular spasm

–  Vascular spasms – immediate vasoconstriction in response to injury

–  Release signals

HEMOSTASIS

2) Platelet plug formation

–  Platelets do not stick to each other or to blood vessels

–  Upon damage to blood vessel endothelium platelets:

•  With the help of von Willebrand factor (VWF) adhere to collagen

•  Stick to exposed collagen fibers and form a platelet plug

•  Release serotonin and ADP, which attract still more platelets

HEMOSTASIS

3) Coagulation (blood clotting)

•     Follows intrinsic and extrinsic pathways

–    Clotting factors

–    Each pathway cascades toward factor X

•   Once factor X has been activated, it complexes with factor V to form prothrombin activator

–    Extrinsic pathway

•   Tissue factor (III) à X + V à prothrombin activator

–    Intrinsic pathway

•   XII à X + V à prothrombinase

–    Common pathway

•   Prothrombin activator is formed

•   Prothrombin activator catalyzes the transformation of prothrombin to the active enzyme thrombin

–   Prothrombin à thrombin (Prothrombin is converted into thrombin)

•   Fibrinogen à fibrin (Thrombin catalyzes the joining of fibrinogen into a fibrin mesh)

•   Insoluble fibrin strands form the structural basis of a clot

Detailed Events of Coagulation

HEMOSTASIS

•     Calcium ions & vitamin K

–   Fibrin in the presence of calcium ions strengthens and stabilizes the clot

•     Clot retraction

–   Stabilization of the clot

•     Fibrinolysis

•   Plasminogen à plasmin

Blood Types

•     RBC membranes have glycoprotein antigens on their external surfaces

•     These antigens are:

–   Unique to the individual

–   Recognized as foreign if transfused into another individual

–   Promoters of agglutination and are referred to as agglutinogens

•     Presence or absence of these antigens is used to classify blood groups

•     The antigens of the ABO and Rh blood groups cause vigorous transfusion reactions when they are improperly transfused

ABO Blood Groups