Homeostasis of Erythropoiesis

The number of circulating RBC’s remains relatively constant in a given individual.

NOTE: It is not the concentration of RBC's in the blood that controls the rate of erythropoiesis

Rather the functional ability of cells to transport O2 to tissues in relation to the tissue need for O2

Regulation of RBC’s

Hypoxia stimulates the kidneys to release the glycoprotein hormone erythropoietin

Erythropoietin is the principle factor for erythropoiesis

80-90% of all erythropoietin is formed in the kidneys

Normally trace amounts of erythropoietin circulate in blood at all times to sustain development of proerythroblasts into reticulocytes

Anything that causes cellular oxygen deficiency (hypoxia) will stimulate an increased release of erythropoietin from the kidneys 

Factors that decrease oxygenation

1. low blood volume

2. pulmonary diseases

3. low hemoglobin

4. poor blood flow

5. poor nutrition - need proper A.A.; Iron; and B12

6. anemia

7. aerobic exercise

8. high altitudes 

Hypoxia --> decrease O2 delivered to kidney --> stimulation of receptor cells --> increase erythropoietin secretion --> increase in rapidness of reticulocyte maturation (1-2 days) --> increase in RBC in circulationà increase in O2 delivered to kidney --> return to homeostasis

Destruction of RBCs

RBC last only about 120 days

Related to the wear and tear on their plasma membranes and the inability to synthesize new components (to replace damaged ones).

Worn out RBC are removed from circulation and destroyed by phagocytic macrophages in the spleen and liver.

To maintain normal quantities of erythrocytes, new cells must enter the circulation at the rate of at least 2 million per second

Healthy male: 5.4 million RBC/cubic mm of blood

Healthy female: 4.8 million RBC/cubic mm of blood

Males are higher due to testosterone which stimulates synthesis of erythropoietin
  

Recycling of hemoglobin

1. globin: split from heme, broken down to A.A. and reused for protein synthesis

2. Heme

Broken down into:

a) Iron: stored in the liver or spleen, or transferred back to bone marrow for production of new Hb

b) Bilirubin: pigment

Bilirubin --> blood --> liver cells --> to be excreted into bile --> broken down further by bacteria in the large intestine

Leukocytes 

he only formed elements in the blood that are complete cells with nucleus and organelles

The normal range for WBC count is from 4,000 - 11,000 mm3

They account for only 1% of total blood volume.

WBC’s are formed within the bone marrow and are stored there until they are needed in the circulatory system.

Once released, related to chemotaxic signals, they can travel both within and outside of the vascular system.

When a pathogen enters the body the general function of the WBC is to combat them by phagocytosis or immune responses.

Tissue injury

Results in the release of chemical signals --> Histamine release from mast cells, platelets, and basophils increases permeability and results in the vasodilation of local arterioles

Increase permeability of local capillaries --> localized swelling and inflammation

Normal hydrostatic force pushes out fluid, but now escaped exudate protein creates an osmotic force to pull fluid out

Decrease blood plasma --> increased localized viscosity

Chemotaxis of leukocytes --> increase margination --> diapedesis of neutrophils and monocytes --> phagocytosis of dead cells, infectious agents and affected matrix 

Phagocytosis

Neutrophils and monocytes are the primary phagocytes, ingesting microbes

They form a phagosome and then a phagolysosome 

Granulocytes

Cells with granules in the cytoplasm

Neutrophils: Also called polymorphonuclear leukocytes (PMN)

Irregular lobed nucleus or multinucleate

The most abundant leukocyte in blood (≈70%)

Mature phagocytic cells.

Can phagocytize between 5-20 bacteria before becoming inactivated

Capable of active diapedesis, amoeboid motion, phagocytosis and lysosomal digestion

First cells to appear in exduate in large #'s in early hours of inflammation in response to chemotactic signals

Injured tissue can promote the release of neutrophils from the red bone marrow

Typically the marrow stores 10 x more neutrophils than are circulating in plasma

Neutrophils release the hydrolytic enzymes and powerful oxidizing agents

The functional life span of the PMN is about five days thus they are produced in large numbers

About 100 billion/day

In an acute active infection neutrophil destruction can exceed production and there is an increase in the number of immature neutrophils (band cells) found in plasma. Denoted as a “shift to the left”

Eosinophils (Granulocytes)

. Function in antigen-antibody reactions to destroy antigen-antibody complex

2. Help kill parasitic worms

3. Release histaminase to inactivate the inflammatory effects of histamine

During allergic reactions eosinophils are shown to increase in local affected tissues 

Basophils (Granulocyte)

Circulating basophils are similar to the large mast cells located throughout capillary beds

Granules contain histamine which is released in response to injured tissue, and allergic reactions

Serve to help initiate the acute inflammatory reaction

Agranulocytes

lack visible cytoplasmic granules

Lymphocytes: Mononuclear; spherical nucleus; pale blue cytoplasm

Play role in immunity.

T cells: Acts directly against affected cells

B cells: Develop into plasma cells which produce immunoglobins (antibodies)
 

Monocytes: Develop into macrophages which phagocytize cellular debris and microbes after injury or infection

Much more powerful phagocyte than the neutrophil,

Arrive at site of tissue injury or infection within 10-12 hours

In the tissue they differentiate into macrophages; about 6-8 hours to go from immature monocyte to macrophage

They can persist in this form for months to years unless destroyed while performing their phagocytic function

Actively phagocytize in chronic infections

Monocytes:

Develop into macrophages which phagocytize cellular debris and microbes after injury or infection

Much more powerful phagocyte than the neutrophil,

Arrive at site of tissue injury or infection within 10-12 hours

In the tissue they differentiate into macrophages; about 6-8 hours to go from immature monocyte to macrophage

They can persist in this form for months to years unless destroyed while performing their phagocytic function

Actively phagocytize in chronic infections

Leukopoiesis

Several hematopoietic growth factors, namely colony stimulating factors and interleukins, regulate WBC cell formation and proliferation. Apparently these are activated by specific local chemical signals in the environment due to produced toxins, or foreign bodies.

Some of the main factors that result in leukopoiesis 

1. Tumor necrosis factor – TNF: stimulates the development of the monocytes and neutrophils

2. Interlukin - 1 (IL-1): secreted by antigen presenting cells

Hemocytoblast divide into... 

Lymphoid stem cell  Lymphoblast  Lymphocytes

Myeloid stem cell  Myeloblast  Eosinophils, Basophils and Neutrophils  Monoblast  Monocytes 

Thrombocytes (Platelets) 

Anucleate; Fragments of the megakaryocyte

Functional span of only 5-9 days

before being removed by macrophages in spleen and liver.

Normal adult range:150,000-400,000 per mm3

Platelets primary purpose is the clotting process

Thrombocytes exhibit many glycoprotein receptors and contain two types of intracellular granules

Alpha granules and Dense granules

Normally they do not stick to the endothelium or each other

Endothelial cells prevent platelet adhesion by the release of nitric oxide (NO) and prostaglandin I2 (PGI2)

In response to injury the thrombocytes adhere to the exposed extracellular matrix (ECM)

Once activated thrombocytes release granules which contain chemicals that promote blood clotting.

The subendothelial matrix is highly thrombogenic

When overlying endothelial cells are damaged the thrombocytes adhere to the ECM via interaction of expressed glycoprotein Ib (GpIb) on the surface of the platelet with the von Willebrand factor in the ECM

Binding of the GpIb to the vWF results in thrombocyte activation and degranulation.

GpIIb/IIIa interaction allows platelets to aggregate by binding to fibrinogen molecules

Steps occurring In Hemostasis (1)

Response to vascular injury

1. Vascular spasms: Tissue injury results in transitory vasoconstriction related to:

1. Smooth muscle constriction in vessel wall

smooth muscle splinting may last up to 30 minutes

2. Reflexive nervous system response

Increase of sympathetic vasomotor tone --> vasoconstriction

3. Chemical release by damaged cells

Endothelins released by endothelial cells stimulate smooth muscle contraction

Endothelin is a potent vasoconstrictor released by damaged or injured endothelial cells

Endothelin binds to ET_A receptors on the vasculature smooth muscle leading to vasoconstriction

Endothelin binds to ET_B receptors on other endothelial cells leading to increased nitric oxide production and release

Transient vasoconstriction lasts usually a few seconds to 10 min

Then:

A. Vasodilation: related to, among other things, nitric oxide release by endothelial cells

B. Increase capillary permeability: histamine released by mast cells and basophils

Leads to separation of endothelial cells  edema  

Steps occurring In Hemostasis (2)

Platelet Plug Formation:

• Intact vascular endothelium is antithrombogenic

• However, upon injury and exposure of the ECM, platelets undergo:

  • a. Adhesion

  • b. Release reaction

  • c. Aggregation

• Initiated when platelets contact ECM

• Begins a few seconds after injury

  • In the absence of endothelial damage platelets are repelled from each other and the endothelial surface

    • Repelling of platelets is related to prostacyclin (PGI₂) and NO

      • PGI₂ is a derivative of prostaglandins, produced by the endothelium

      • Both PGI₂ and NO are vasodilators and inhibitors of platelet aggregation

  • Upon injury: Platelets adhere to exposed collagen and vWF --> platelet release reaction à platelet aggregation

Steps occurring In Hemostasis (3)

a. Platelet adhesion: Upon injury underlying collagen fibers in the basement membrane are exposed

Damaged endothelial cells retract away from each other

Platelets adhere to the exposed collagen fibers and vWF molecules

Platelets undergo transformation extending cytoplasmic processes and becoming sticky

This process allows them to contact, and stick to, one another

Platelet release reaction

As platelets stick to collagen they undergo degranulation releasing the contents of the alpha granules and dense granules

Alpha granules release:

•  

  • Fibrinogen

    • Allows for formation of fibrinogen bridges between adjacent platelets via GP IIb/IIIa receptors

  • vWF

    • Incorporates into ECM and allows for thrombocyte attachment via GpIb/ vWF interactions

  • Tissue proaccelerin (factor V)

    • Cofactor for activated Stuart-Prower factor (X)

  • Antihemophilic factor (VIII)

    • Cofactor to activate factor X

Dense granules release:

•  

  • ADP

    • Potent activator of platelet aggregation

    • Stimulates the production of Thromboxane A₂ (TxA₂)

      • TxA₂ is a potent vasoconstrictor and aggregating agent

      • TxA₂ is a platelet derived prostaglandin

        • Via the arachidonic acid pathway 

•  

  • Serotonin

•  
  •  

    • Vasoconstrictor and aggregating agent

Ionized calcium

•  
  •  

    • Required for coagulation cascade 

•  

  • Platelet – Activating factor (PAF)

•  
  •  

    • Creates a positive feedback loop by activating more platelets

    • Aggregating agent and increases capillary permeability

    • Along with TxA_{2 ,} PAF stimulates the activation of GP IIb/IIIa receptors on the thrombocyte membrane surface

Prostaglandin synthesis

• Prostaglandins are derived from membrane lipids and are synthesized by most tissue cells; Promote inflammation

  • Phospholipase A2 converts membrane phospholipids into arachidonic acid

  • Cyclooxygenase (COX) converts arachoidonic acid to prostaglandins and compounds such as thromboxane A₂ (TxA₂) and prostacyclin (PGI₂)

• Aspirin inhibits TxA₂ synthesis by irreversibly inhibiting COX -1 enzymes

  • Affected platelets are unable to synthesize new COX-1 enzymes and thus are ineffective for the rest of their functional span (avg. 7-8 days)

This affects the formation of a platelet plug which can lead to increased bleeding time.

Platelet aggregation

Begins within 15 seconds after the injury occurs

• The release reaction stimulates the formation of fibrinogen bridges between adjacent platelets via GP IIb/IIIa receptors

• Note: thromboxane A2 stimulates the activation of GP IIb/IIIa receptors on the thrombocyte membrane surface

• The release reaction stimulates a positive feedback cycle in which platelets continue to degranulate and adhere to the originally activated platelets at the site of injury.

• Platelet aggregation is a positive feedback cycle in which platelets continue to degranulate and adhere to the originally activated platelets at the site of injury.

• This positive feedback system is termed secondary aggregation and results in the formation of a platelet plug in the immediate area of the injured vessel

• Serves to mechanically stop bleeding.

• Untoward thrombus formation is controlled by release of:

• 1. Prostacyclin (PGI₂) and N.O. from the endothelial cells inhibits platelet aggregation along healthy endothelium.

• 2. Adenosine diphosphatase

  • Produced by endothelial cells and serves to degrade ADP

  • Circulating plasma proteins also degrade ADP

Platelet contraction 

• Contraction of the thromboytes creates a fused mass of platelets and forms a hemostatic plug

  • Contraction serves to approximate the edges of damaged vessels

• The coagulation cascade (discussed next) will stabilize the plug with fibrin to produce a clot 

Summary

• Vessel injured à subendothelial ECM exposed to blood

• Platelet adhesion to collagen and vWF

• Release reaction

  • ADP stimulates the production of TxA₂ --> activation of GP IIb/IIIa receptors --> cross linking of fibrinogen

• Secondary aggregation

• Platelet plug

  • Platelet contraction

    • Platelets contain actin and myosin molecules

      • Squeezes out serum and leaves the gel

      • Approximates torn endothelium

• Blood clot (Platelet plug + Fibrin)

  • The blood clot contains platelets and often red blood cells, held together in a fibrin mesh

  • Forms within 3-6 minutes

Coagulation (clotting) (3)

Begins about 30 seconds post-injury

Complex sequence of steps leading to the conversion of fibrinogen into fibrin

The clot consists of a network of insoluble protein fibers called fibrin in which the formed elements of blood are trapped.

Clotting factors must be activated in order to go from a platelet plug to a blood clot

Ca ²⁺ and 11 different proteins

Clotting is a cascade of reactions in which coagulation factors activate each other, once the process is initiated it results in positive feedback to form a large quantity of product.

The process of coagulation involves several enzymes and clotting factors that ultimately results in a growing fibrin mesh that covers the surface of the platelet plug reinforcing it and sealing the damaged area

Role of Calcium ions in the coagulation process

• Ca²⁺ ions are released from the platelets

  • Stored in the residual RER and golgi apparatus

• Ca²⁺ is a cofactor needed throughout the enzyme cascade

  • In the absence of Ca²⁺ blood will not clot

Two pathways activate the production of fibrin and thus clot formation. 

• The intrinsic pathway

  • Begins with the activation of proenzymes and PF-3, a platelet factor released by aggregating platelets

  • Can occur outside the body

    • Intrinsic to blood

      • contained in plasma is everything needed to form fibrin

    • Allows blood drawn from a vessel and placed in a test tube to clot within 8 – 18 minutes

• The extrinsic pathway

  • Begins with the release of tissue factor by damaged tissues and endothelial cells

  • Allows for a more rapid formation of fibrin

    • Allows blood to clot within 1 – 4 minutes 

Intrinsic pathway

XII --> XIIa --> XI --> XIa --> IX --> IXa -(Ca++ and PF3, a phospholopid component of the platelet membrane released during aggregation) --> VIII VIIIa --> X --> Xa

Extrinsic pathway

tissue thromboplastin  --> VII + Ca++ --> VIIa --> X --> Xa  

Xa + Ca++ --> prothrombin + Ca++ --> thrombin(enzyme) 

--> fibrinogen --> fibrin monomer -(XIII)-> fibrin polymer
 

Prothrombin is the precursor to the enzyme thrombin

• It catalyzes the formation of fibrin strands from fibrinogen

• thrombin also activates factor XIII, which serves to strengthen the clot by binding the fibrin strands together

• Note: thrombin, along with tissue plasminogen activator, converts plasminogen into plasmin which is incorporated into the clot and ultimately results in fibrinolysis

BLOOD TYPING 

Agglutinogens (isoantigens): Genetically determined antigens on the surfaces of erthrocytes

Agglutinins (isoantibodies): Contained in blood plasma react against RBC carrying ABO antigens not in persons own blood

Hemolysis: When blood is mismatched the agglutinins in the recipient’s blood reacts with the agglutinogens in the incompatible donor blood

e.g. Agglutinin b responds to Type B agglutinogens by activating complement  lysis of donor RBC

People with Type O blood are considered universal donors (Can give blood to all four blood groups)

People with Type AB blood are considered universal recipients