platelets and homeostasis

three hemostatic mechanisms

: vascular spasm, platelet plug formation, and blood clotting (coagulation); platelets are involved in all three.

coagulation cascade

A deficiency of any clotting factor can stop the ....

spleen

About 25% to 40% of the platelets are stored in the ___________; the remainder circulate freely and live for about 5 to 6 days.

eliminated

After a clot has formed, tissue repair proceeds, and eventually the clot must be ....

degranulation

As platelets combine several elements, they undergo _________________—the exocytosis of cytoplasmic granules and release of factors that promote hemostasis.

formed elements

As these arrive at the injury site, they stick to the fibrin and form a mass of fibrin, blood cells, and platelets.

exertion

Bleeding into the muscles and joints occurs with ______________, and this can lead to terrible pain and eventual joint immobility due to hematomas.

procoagulants

Clotting factors are called ________________, in contrast to anticoagulants; most procoagulants are proteins produced by the liver.

bleeding

Coagulation (clotting) of the blood is the last but most effective defense against...

prothrombin activator

Factor X combines with factors III and V in the presence of Ca2+ and PF3 to produce

blood clot

Factor XIII cross-links fibrin strands to make a dense aggregation that forms the structural framework of a ______________

blood clotting

Far more peoplw die from unwanted _____________ than from clotting failure.

normal level

Half of the normal level of factor VIII prevents the symptoms, and symptoms are mild even in individuals with 30% of the ______________.

simultaneously

In most cases of bleeding, both mechanisms operate ....

megakaryocytes

In terms of form and function, platelets are not cells but small fragments of marrow cells called....

coagulation

In the completion of ________________, once factor X is activated, the remaining events are identical in both mechanisms.

thromboplastin

In the initiation of coagulation, the damaged blood vessel acts through the extrinsic mechanism by releasing tissue _______________ (factor III).

longer-lasting constriction

Injury to smooth muscle of the blood vessel causes....

roman numerals

Many clotting factors are identified by ______________, which indicate the order in which they were discovered, not the order of the reactions.

red bone marrow

Most megakaryocytes live in the _____________ adjacent to blood-filled spaces called sinusoids, lined with a thin simple squamous epithelium called the endothelium

lungs

Much of this breakup is thought to occur when they pass through the small vessels of the...

endothelium

Normally platelets do not adhere to the ____________ because it is smooth and coated with prostacyclin.

positive feedback process

Once a clot begins to form, it launches the __________________.

thrombocytopenia

Platelet deficiency produces a condition called...

second

Platelets are the ______________ most abundant formed elements, after erythrocytes, numbering from 130,000 to 400,000 platelets/μL (averaging 250,000).

Platelet repulsion

Platelets do not adhere to the prostacyclin-coated endothelium of undamaged blood vessels.

nucleus

Platelets have no...

serotonin

Platelets release _______________, a chemical vasoconstrictor.

thrombopoiesis

Production of platelets is a division of hemopoiesis called...

small breaks

Release of these factors activates a positive feedback cycle that can quickly seal....

Dilution

Small amounts of thrombin form spontaneously in plasma, but at normal rates of blood flow it is diluted quickly. If flow decreases, thrombin can accumulate and cause clotting, as can happen in circulatory shock.

megakaryoblasts

Some hemopoietic stem cells produce receptors for the hormone thrombopoietin, which makes them _________________, cells committed to the platelet-producing line.

proplatelets

The blood flow shears off the _______________, which break up into platelets as they travel.

hormone action

The cascade of reactions is similar to the enzyme amplification that occurs in ____________.

damaged blood vessel

The extrinsic mechanism is initiated by clotting factors released by a _____________ _____ _______________ and perivascular tissue. Extrinsic means that these factors come from sources external to the blood.

Anticoagulants

Thrombin formation is suppressed by circulating anticoagulants, such as antithrombin, secreted by the liver, and heparin, secreted by basophils and mast cells.

fibrinogen

Thrombin then converts ________________ into shorter strands of fibrin monomer. These monomers bond to each other end to end and form fibers of fibrin polymer.

inactive people

Thrombosis is especially common in leg veins of ___________ _________ and patients immobilized in a wheelchair or bed.

activated

Through a cascade of reactions, factors XI, IX, and VIII are ______________, in that order, and finally to factor X. This pathway also requires Ca2+ and PF3.

extrinsic and intrinsic

Two reaction pathways lead to coagulation: the ____________ mechanism and the _______________ mechanism.

collagen

Upon contact with ______________ or other rough surfaces, platelets grow spiny pseudopods that adhere to the vessel and to other platelets and then contract, drawing the walls of the vessel together and forming the platelet plug.

platelet plug

When a vessel is broken, collagen fibers of its wall are exposed, causing platelets to adhere and form a...

puesopods

When activated, platelets form _______________ and can perform ameboid movement.

In hemophilia

a family of hereditary deficiencies, one factor or another is lacking. Because most are deficiencies sex-linked recessive, hemophilia occurs mostly in males.

Thrombin

also activates plasmin, which indirectly promotes the formation of more kallikrein.

Fibroblasts

also invade the clot and divide.

The intrinsic mechanism

also leads to activation of factor X; when platelets degranulate, they release factor XII.

Most strokes and heart attacks

are due to thrombosis—abnormal clotting in an unbroken vessel.

release of factors

are serotonin; adenosine diphosphate (ADP), and thromboxane A2, an eicosanoid.

Various laboratory tests

are used to evaluate the efficiency of clotting; blood in a clean test tube should clot within 15 minutes.

Bleeding

can be relieved for a few days by transfusion of plasma or purified clotting factors.

Clotting deficiencies

can result from malnutrition, leukemia, and gallstones.

Platelet counts

can vary greatly under different physiological conditions and even in different places in the body.

inactive form

clotting factors are always present in the plasma in _____________ _______, but activation of one factor sets off a reaction cascade, in which one reaction leads to the next.

Kallikrein

converts plasminogen into plasmin, a fibrin-dissolving enzyme.

Most venous blood

flows to the heart and then to the lungs, so clots arising in the limbs commonly lodge in the lungs and cause pulmonary embolism, which may result in hypoxia and death.

Platelets

have a complex internal structure that includes lysosomes, mitochondria, microtubules and microfilaments, granules, and a system of channels that open to the surface called the open canalicular system.

Clotting disorders

include clotting deficiencies and inappropriate clotting.

platelets

internalize and destroy bacteria.

The extrinsic mechanism

is a "shortcut" to coagulation; because it has fewer steps, it takes 15 seconds to form a clot, compared to 3 to 6 minutes via the intrinsic pathway.

The cascade of reactions

is an amplifying mechanism that ensures rapid clotting of blood.

Vascular spasm

is maintained long enough for the other two hemostatic mechanisms to begin.

Thrombosis

is more likely to occur in veins than in arteries because blood flows more slowly in veins.

Bleeding time

is most accurately measured using the Ivy method—inflating a blood pressure cuff on the arm to 40 mm Hg, making a 1 mm deep incision in the forearm, and measuring the time it takes to stop bleeding.

Coagulation

is one of the most complex body processes, involving over 30 chemical reactions.

Platelet-derived growth factor (PDGF)

is released by platelets and endothelial cells, stimulating fibroblasts and smooth muscle cells to divide.

Hemostasis

is the cessation of bleeding. Although hemostatic mechanisms may not stop hemorrhage from a large vessel, they are quite effective for smaller ones, and platelets play major roles

Fibrinolysis

is the dissolution of a clot and is achieved by a reaction cascade with a positive feedback component.

Vascular spasm

is the most immediate protection against blood loss.

A thrombus (clot)

may grow large enough to obstruct a small vessel, or may travel in the blood stream as an embolus.

An embolus

may lodge in an artery and block blood flow, causing infarction (tissue death).

chromosomes

megakaryocyte has a huge multilobed nucleus and multiple sets of...

gigantic cell

megakaryocyte is a ___________ up to 150 μm in diameter and visible to the naked eye.

Clot retraction

occurs when spinous pseudopods of platelets adhere to fibrin and retract, drawing the edges of a broken vessel together like a drawstring.

adhere

plateles ______________ to each other to form temporary platelet plugs that seal small breaks in injured blood vessels.

initiate

platelets ______________ the formation of a clot-dissolving enzyme that dissolves clots that are no longer needed. neutrophils and monocytes platelets secrete chemicals that attract _____________ _______ ______________ to sites of inflammation.

vasoconstrictors

platelets secrete ___________________________ that stimulate spasmodic constriction of broken vessels to reduce blood loss.

mitosis

platelets secrete growth factors that stimulate ________________ in fibroblasts and smooth muscle to help maintain and repair blood vessels.

procoagulants (clotting factors)

platelets secrete....

small

platlets are so _________, 2 to 4 μm in diameter, that they contribute even less than WBCs to the blood volume.

Precise controls

prevent coagulation when it is inappropriate.

A megakaryocyte

sprouts long tendrils called proplatelets that protrude through the endothelium into the blood of the sinusoid.

Injury

stimulates pain receptors that cause short-term vessel constriction.

Thrombin

works with factor V to accelerate production of prothrombin activator, creating more thrombin.

atherosclerotic plaques

The intrinsic mechanism uses only clotting factors found in the blood itself, such as when platelets adhere to....

platelets

The last four procoagulants in Table 18.7 are called platelet factors (PF1 through PF4) because they are produced by ....

DNA

The megakaryoblast duplicates ____________ repeatedly without nuclear or cytoplasmic division, becoming a megakaryocyte.

fibrin

The objective of clotting is to convert the plasma protein fibrinogen into ___________, a sticky protein that adheres to vessel walls.