why does blood turn brown after it dries up out of the human body?

Answer 1

Loss of color from no oxygen , most everything looks darker when it dries up, and has lost its life, ( oxygen) Did you know that blood is blue before it hits oxygen, Then it turns red.

Answer 2

When blood vessels are cut or damaged, the loss of blood from the system must be stopped before shock and possible death occur. This is accomplished by solidification of the blood, a process called coagulation or clotting.

A blood clot consists of :
a plug of platelets enmeshed in a
network of insoluble fibrin molecules.
Platelet aggregation and fibrin formation both require the proteolytic enzyme thrombin. Clotting also requires:
calcium ions (Ca2+)(which is why blood banks use a chelating agent to bind the calcium in donated blood so the blood will not clot in the bag).
about a dozen other protein clotting factors. Most of these circulate in the blood as inactive precursors. They are activated by proteolytic cleavage becoming, in turn, active proteases for other factors in the system.
By tradition, these factors are designated by Roman numerals. I find this somewhat confusing and will use Arabic numerals instead.

Initiating the Clotting Process:

Damaged cells display a surface protein called tissue factor (TF)
Tissue factor binds to activated Factor 7.
The TF-7 heterodimer is a protease with two substrates:
Factor 10 and
Factor 9
Let's follow Factor 10 first.
Factor 10 binds and activates Factor 5. This heterodimer is called prothrombinase because it is a protease that converts prothrombin (also known as Factor II) to thrombin.
Thrombin has several different activities. Two of them are:
proteolytic cleavage of fibrinogen (aka "Factor I") to form:
soluble molecules of fibrin and a collection of small
fibrinopeptides
activation of Factor 13 which forms covalent bonds between the soluble fibrin molecules converting them into an insoluble meshwork — the clot.
(Thrombin and activated Factors 10 ("Xa") and 11 ("XIa") are serine proteases. Link to discussion.)

Answer 3

Why not?