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.