substance in blood and tissues which, in the presence of ionized calcium, aids in the conversion of prothrombin to thrombin. Extrinsic and intrinsic thromboplastin are formed as the result of the interaction of different clotting factors; the factors that combine to form extrinsic thromboplastin are not all derived from intravascular sources, whereas those that form intrinsic thromboplastin are.

• activated partial t. time - see http://www.answers.com/topic/activated-4 partial thromboplastin time.
• extrinsic t. - the prothrombin activator formed as a result of interaction of coagulation factors III, VII, and X which, with factor IV, aids in the formation of thrombin.
• t. generation time (TGT) - evaluates the first stage in blood coagulation by measuring the efficiency of prothrombinase formation.
• intrinsic t. - the prothrombin activator formed as a result of interaction of coagulation factors V, VII, IX, X, XI and XII and platelet factor 3 (PF-3), which, with factor IV, aids in the conversion of prothrombin to thrombin.
• plasma t. antecedent (PTA) - http://www.answers.com/topic/clotting-1 factor XI; deficiency occurs in cattle and dogs, causing mild to severe bleeding tendencies called hemophilia C.
• plasma t. component (PTC) - http://www.answers.com/topic/clotting-1 factor IX; deficiency causes http://www.answers.com/topic/haemophilia-b. Called also Christmas factor, antihemophilic factor B, autoprothrombin II.
• t. time - see http://www.answers.com/topic/activated-4 partial thromboplastin time.
• tissue t. - factor III, a material derived from several sources in the body (e.g. brain, lung), and is important in the formation of extrinsic prothrombin converting principle in the extrinsic pathway of blood coagulation. Called also tissue factor.

Von Willebrand disease is the most common hereditary bleeding disorder.

Von Willebrand disease is caused by a deficiency of von Willebrand factor. Von Willebrand factor helps blood platelets clump together and stick to the blood vessel wall, which is necessary for normal blood clotting. There are several types of Von Willebrand disease.

Von Willebrand disease affects men and women equally. Most cases are mild. Bleeding may occur after surgery or when you have a tooth pulled. Aspirin and other nonsteroidal anti-inflammatory drugs (NSAIDs) can make this condition worse. Bleeding may decrease during pregnancy.

A family history of a bleeding disorder is the primary risk factor. In women with heavy or prolonged menstrual bleeding, von Willebrand is more common in Caucasian women than in African American women. The majority of women with heavy or prolonged menstrual bleeding do NOT have Von Willebrand disease.

• Abnormal menstrual bleeding
• Bleeding of the gums
• Bruising
• Nosebleeds
• Skin rash

Tests that may be done to diagnose this disease include:

• Bleeding time(is prolonged)
• Platelet aggregation test
• Platelet count(may be low or normal)
• Ristocetin cofactor test (the primary assay test used to diagnose von Willebrand disease)
• Von Willebrand factor level (level is reduced)

This disease may also alter the results of the following tests:

• Factor VIII level
• Von Willebrand factor multimers (parts of the von Willebrand factor protein molecule)

Von Willebrand disease may be hard to diagnose. Low von Willebrand factor levels and bleeding do not always mean you have von Willebrand disease.

Medications such as desamino-8-arginine vasopressin (DDAVP) can be given to raise the levels of von Willebrand factor, which will reduce the tendency toward bleeding.

Some types of von Willebrand disease do not respond to DDAVP. Tests should be done to determine a patient's specific type of von Willebrand disease before trauma or surgery occurs. A trial of DDAVP can be done before surgery to test whether von Willebrand factor levels increase.

The drug Alphanate (antihemophilic factor) is approved to decrease bleeding in patients with the disease who must have surgery or any other invasive procedure.

Blood plasma or certain factor VIII preparations may also be used to decrease bleeding.

Women who have this condition generally do NOT have excessive bleeding during childbirth.

This disease is passed down through families. Therefore, genetic counseling may help prospective parents understand the risk to their children.

Bleeding (hemorrhaging) may occur after surgeries or other invasive procedures.

If you have von Willebrand disease, do not take nonsteroidal anti-inflammatory drugs (NSAIDs), such as aspirin or ibuprofen, without talking to your health care provider.

Call your health care provider if bleeding occurs without reason.

If you have von Willebrand disease and are scheduled for surgery or are in an accident, be sure you or your family notify the health care providers about your condition.

Kessler CM. Hemorrhagic disorders: coagulation factor deficiencies. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007:chap 180.

The same question was asked at another site and this was the answer given:

As an example, on average a normal, healthy individual who donates a
unit of blood (about 450 ml or ¾ pint) will fully recuperate from the
blood loss in four to six weeks.

The issue is a bit more complicated than that though. You see, our
blood is made up of a variety of things:

Red blood cells that carry oxygen from the lungs to all other tissues in the body.

White blood cells that provide the body's defenses against disease.

Platelets, specialized cells that plug damaged blood vessel walls and
join other elements in the blood in the production of clots to stop
bleeding.

Plasma, a yellow fluid made up of about 92% water and 7% vital
proteins such as albumin, gamma globulin, and antihemophilic and other
clotting factors.

And other elements consisting of mineral salts, sugar, fats, hormones,
and vitamins.

Red blood cells, white blood cells, and platelets account for about
45% of the volume of blood in the body. The remaining 55% is plasma
including the other extraneous elements I mentioned. The four to
six-week period I indicated a moment ago is the time necessary to
replace ALL these factors, though our bodies actually replace some of
the individual components faster than others. For example, the fluid
(mostly water) lost in a blood donation, is replaced in only a few
hours, provided the donor drinks a moderate amount of water or
healthy, non-alcoholic liquids to replenish the loss. The other
components of the blood, such as platelets and red cells begin to
replenish immediately but take a bit longer to complete; usually
within a few days or weeks depending on how much blood is lost.
Generally speaking, after donating a unit of blood, the body replaces
the fluid and white cells within 24 hours, and platelets and plasma
proteins within 48-72 hours. The red blood cells are completely
replaced in 4-6 weeks.

It would be safe then to say that a blood ?sample? (rather that a
whole unit) or a small amount of loss due to a very minor scrape or
bleed could conceivably be fully replaced in a healthy individual
within hours. How fast the entire blood loss is replenished obviously
depends on the amount of blood lost, the size of the individual (for
every 12-15 pounds of body weight, a human being has about one pint of
blood) and the health of the individual from whom the blood is
removed. If the body is functioning normally and there are no
extenuating circumstances (interfering medication, unhealthy lifestyle
or diet, etc) age alone should have little or no impact on the natural
replacement of blood. A poor diet or sedentary lifestyle can however
have an impact on how fast the body replaces blood because of the
necessary nutrients in a proper diet that enable the cell replacement.
Lack of exercise, for example, can lead to obesity (and ultimately the
need for more blood than normal) and can certainly cause
arteriosclerosis or other venous constrictions than can complicate
blood flow, adding to difficultly or delayed replacement. This of
course is just one hypothetical situation, but there are others.
Replacement is also relative to size and exercise (or lack thereof) If
a pint of blood is a significant portion of a person?s entire blood
volume (as in a newborn for example) it will take longer for the body
to replace half the blood volume. In a 500lb man, a pint of blood is
not that much in terms of overall blood volume, BUT his body may have
a difficult time replacing that pint in the normal range of
expectations because of his morbid obesity and other health factors.
So you can easily see how diet and excercise can affect blood
replacement.

As a rule though, if you are relatively healthy, eat right more often
than not, and even moderately exercise you can expect a fairly
predictable and complete replacement of a non-life threatening blood
loss within the time frame I specified regardless of your age.