Primary Hemostasis - Pt 1

What is hemostasis?

The combined properties of several different elements in the circulatory system to keep blood fluid during circulation yet also prevent blood loss if the system is compromised;

What does hemostasis involve?

Involves blood vessels, platelets, and various plasma proteins.

What are the three major parts of hemostasis?

• Primary hemostasis (initial response to make a platelet plug),

• secondary hemostasis (stabilizes the platelet plug and makes a fibrin clot),

• fibrinolysis (dissolution of the clot with healing of the initial injury).

Slide 3: Quick Summary of Primary Hemostasis

What is the sequence of events in primary hemostasis after a vessel injury?

After an injury to the blood vessel, the vascular system initiates the process…

Vasoconstriction slows blood flow to the damaged area →

damaged endothelial cells release substances involved in platelet plug creation →

platelets arrive and produce a primary platelet plug by interacting with the damaged vessel wall and with each other.

Slide 5: The Vascular System

Where is hemostasis initiated, and what are the first two events?

In the vascular system when there is an injury, vasoconstriction occurs first, followed by arrival and activation of substances involved in hemostasis.

What is Vascular intima

the vessels have different layers

What are the three layers of blood vessels?

Tunica intima (interior),

tunica media (middle),

tunica adventitia/externa (outer).

What makes up the inner lining of blood vessels and its support structure?

Inner lining: endothelial cells;

and have the support of endothelial cells, including collagen and elastin.

Slide 6: Endothelial Cells — Structure

What is the structural role of endothelial cells in blood vessels?

A monolayer of cells that line the lumen of the blood vessel;

normally have a smooth surface so blood flows easily without any obstructions.

What supports endothelial cells, and what is in the subendothelial connective tissue?

Supported by a basement membrane, elastin-rich lamina, and collagen;

Collagen, fibroblasts, and smooth muscle cells are in the subendothelial connective tissue to support blood vessels.

Slide 7: Endothelial Cells — Normal (Antithrombotic) Roles

What is the role of endothelial cells?

• roles in immune response

• vascular permeability

• hemostasis

  • Healthy intact vasculature should resist the clotting process

  • active in both the clotting process and in avoiding clots

What is the normal state of healthy endothelial cells in relation to clotting?

Healthy, uncompromised endothelial cells are nonreactive —

• They have a smooth surface for easy blood flow,

• keep platelets and coagulation proteins from activating,

• provide a barrier between collagen supporting the vessel and circulating platelets.

What do glycoproteins on endothelial cells do?

Glycoproteins cover endothelial cells because their negative charge keeps cells from binding to adhesion molecules.

What is Prostacyclin (PGI2), and what are its two functions?

A substance produced and secreted by endothelial cells;

• it promotes vasodilation

• prevents unnecessary platelet activation (since it is a thromboxane A2 antagonist).

What is nitric oxide produced by endothelial cells, and what are its two functions?

A substance secreted by endothelial cells that induces smooth muscle relaxation and vasodilation, and inhibits platelet activation.

What do healthy endothelial cells promote

they promote angiogenesis

What other anticoagulant substances do healthy endothelial cells produce?

• TFPI (tissue factor pathway inhibitor),

• thrombomodulin, and heparan sulfate — to inhibit thrombin formation. (making the clot itself)

Slide 8: Endothelial Cells — If Damage Occurs (Procoagulant Roles)

What procoagulant events (roles) occur when endothelial cells are damaged?

• Vasoconstriction slows blood flow through the damaged portion;

• Exposed collagen leads to platelet activation;

• VWF (von-wilibrons factor) is secreted from Weibel-Palade bodies along with ADAMTS13;

• P-selectin is secreted to promote WBC and platelet binding —other adhesion molecules (ICAMs and PECAMs) are also secreted;

• Damage exposes tissue factor, promoting coagulation.

What fibrinolysis roles do damaged endothelial cells play?

• They secrete tPA to activate fibrinolysis

• Provide inhibitors to regulate the process (PAI-1 and TAFI).

Slide 10: Resting vs. Damaged Endothelium Summary

What three types of substances does resting endothelium secrete to inhibit hemostasis?

Substances that inhibit platelet activation (PGI2, NO, ADPase), inhibit coagulation (heparan sulfate, thrombomodulin, TFPI), and activate fibrinolysis (tPA, uPA).

What three types of substances does damaged endothelium produce to promote hemostasis?

Substances that activate platelets (TXA2, PAF, ET) and bind them to the vessel wall (VWF), activate coagulation (tissue factor for fibrin formation), and inhibit fibrinolysis (PAI-1).

Slide 12: Platelets — Basic Characteristics

What are platelets, and where do they come from?

Small, anucleate disc-like fragments derived from megakaryocyte cytoplasm;

role is in the initial defense against blood loss after injury.

What is the size and lifespan of platelets?

2–3 µm; they circulate for 7–10 days if not activated and used in hemostasis.

How are platelets distributed in the body?

2/3 are in circulation (peripheral blood); 1/3 are sequestered in the spleen and available on demand.

Slide 13: Platelet Numbers

What is the normal platelet count range?

150–450 × 10⁹/L.

• if there is numbers below or above this range, there may be problems

What is thrombocytopenia, and at what count is it defined?

Decreased platelets; <150 × 10⁹/L.

What is thrombocytosis, and at what count is it defined?

Increased platelets; >450 × 10⁹/L.

What is the Mean Platelet Volume (MPV), what does it measure, and what is its normal range?

The size/volume of platelets is measured by a hematology cell counter;

Normal range is 7.0–12.0 fL.

What are reticulated platelets (Immature Platelet Fraction)?

Immature platelets released from bone marrow that contain more RNA than mature platelets.

Slide 14–15: Where Do Platelets Come From?

What is megakaryopoiesis?

The process of platelet production,

which is part of hematopoiesis (the process of producing various blood cells from a common hematopoietic stem cell).

What are megakaryoblasts, and where are they produced?

Large cells with a multilobulated nucleus and lots of granular cytoplasm;

produced in the bone marrow.

What is a megakaryocyte, and how many platelets can come from each one?

The product of a megakaryoblast; megakaryocytes shed their cytoplasm to produce 2,000–4,000 platelets each. (biggest cell in bone marrow) (found in low power field)

Slide 18: Key Terms in Megakaryopoiesis

What is polyploid?

Cells containing multiple copies of chromosomes;

normal DNA is 2N but polyploid cells can be up to 64N.

What is endomitosis?

A form of mitosis where DNA content replicates without actual cell division — there is no telophase or cytokinesis.

What is thrombopoietin (TPO)?

A platelet growth factor that works synergistically with other cytokines to induce megakaryocyte production.

What is terminal differentiation?

As the megakaryocyte matures, it continues the process until it reaches the mature platelet stage.

Slide 19: Maturation Stages of Megakaryopoiesis

What cant early stage megakaryocyte progenitors do?

They cant be microscopically differentiated

What are the three megakaryocyte progenitor stages?

• BFU-Meg (Burst forming unit megakaryocyte — diploid cells that undergo mitosis),

• CFU-Meg (Colony forming unit megakaryocyte — diploid cells that undergo mitosis), and

• LD-CFU-Meg (Light density CFU-Meg — progenitor cell that undergoes endomitosis).

What are the megakaryocyte precursor stages?

May be referred to as MK-I, MK-II, MK-III — or Stage 1, Stage 2, Stage 3, Stage 4.

Slide 20: Growth Factors in Megakaryopoiesis

What growth factors influence proliferation of stem and progenitor cells in megakaryopoiesis?

Flt ligand, SCF, GM-CSF, and IL-3.

What growth factor influences ALL stages of megakaryocyte production, and what factors support maturation specifically?

Thrombopoietin influences all stages; IL-6 and IL-11 support primarily maturation.

Slide 23: Identifying Megakaryocytes by Markers

What markers are used to identify megakaryoblasts and megakaryocytes?

• MPL (TPO receptor),

• CD34 (stem cell receptor),

• CD41,

• CD42,

• PF4,

• VWF,

• fibrinogen.

What methods are used to measure megakaryocyte markers?

measured by Flow cytometry or immunostains