What is Vampire ? part 3

Physiology

Vampire (Edward) displaying the enhanced muscle structure.

After the change, vampires' physical appearance and muscle mass is enhanced and perfected, eye color changes, and they no longer need to breathe, though they often still do so out of habit, and feel uncomfortable without their sense of smell - the first warning of an impending threat or the smell of potential prey. The ability to cut off airflow from their lungs allows them to remain submerged underwater for as long as they wish, and contributes greatly to their excellent swimming capabilities.
Vampires have uncannily powerful senses. As they grow older and more disciplined, vampires can learn to smell differences between the scents of various humans' blood, as well as scents of other things (like fabrics - the unique scent of denim). The range of their senses can be increased (thought only fractionally) through concentration.
They can hear sounds coming from several miles away, and even with their eyes closed, they can be sure that there are multiple people in a room thanks to their varied breathing patterns. They can hear the sound of a heartbeat, a pulse thudding in a vein, and words spoken too quietly or too fast for humans to detect.
Their enhanced vision allows them to see the smallest details, and extends into the ultraviolet region of the light spectrum. Vampire vision is unhindered by darkness, and is so acute that they view humans as blind, or "mud-eyed", in comparison. Bella describes her sight as being better than an eagle's. Vampires find wearing glasses or contact lenses to be uncomfortable or irritating because they see the miniature imperfections (ex. scratches).
Their bodily scent is based on their venom, and is pleasant, not only to humans, but to other vampires as well.
Their body's natural changes no longer occur. They cannot age or grow (things such as fingernails, toenails, and hair will no longer be able to grow - if they are cut they will never grow back. However, if an arm, leg, nose, or other such appendage should be removed [still painfully], it can be easily reattached. Since the venom is adhesive, coating the severed end with it will help it heal faster). They show no physical or mental illnesses and defects, and their voice becomes more alluring, melodic, and seductive. Their heart no longer beats.
The venom, and venom based liquids replace all of the natural chemical processes and functions in the body. It wets and pools in the mouth instead of saliva. It coats the eyes, burning through contacts after only a few hours, protecting them from damage and thus eliminating all necessity and reason to produce tears. It also lubricates them, allowing them to move around more easily within their sockets. A similar venom based liquid lubricates between the vampires cells allowing their hard stone bodies to move. If a vampire were to remain motionless over thousands of years, dust will actually begin to petrify in response to the venom based liquids, turning their skin powdery and similar to shale in texture, as well as their eyes which develop a milky film over their red irises making the eyes appear pink in color. This can be seen with the ancient Volturi members. These imperfections, however, do not affect the vampires impenetrability or compromise their vision, a vampire that stays motionless would be just as indestructible and sharp as a vampire remaining active.
As vampires describe themselves, they are unchanging, "living stone" beings frozen exactly as they were when they were transformed. (Though to another vampire, the granite-skin feels smooth, soft, and warm and not the icy-hard touch a human would feel). Requiring little or no change to produce sperm, males can still breed, while female vampires cannot, their bodies no longer accommodate the changes related to pregnancy. In male vampires, the venom takes on a form so similar to seminal fluid that can bond with a human ovum, making possible the creation of a human-vampire hybrid. The underlying biochemistry and physiology is currently unknown.
Vampires, although essentially immortal and indestructible, can be killed. Even a human could accomplish it under the right circumstances - venom and venom-based fluids are very flammable. However, humans are unable to compete with vampires in strength and speed, therefore they are unlikely to succeed. If a human were to succeed in lighting a vampire on fire , the vampire could simply take a quick high-speed bound to put it out. This is why it is necessary to rip a vampire's body to pieces before incinerating them into ashes, to prevent them from putting out the fire before it can do serious damage. This is the only known way to kill a vampire.
Thankr to their much greater physical abilities, Children of the Moon and shape-shifters are instead much better equipped to kill a vampire.

Lifestyles

Though there are a range of vampire lifestyles, the most common is nomadic. The majority of vampires move frequently, never settling permanently in one place. This pattern is partly an attempt to hide from the notice of humans. If too many humans disappear from one area, suspicion might be aroused and the Volturi might take notice. Also, if a vampire interacts regularly with humans, eventually the humans will notice that the vampire isn't aging. Another aspect is boredom. Vampire lives are so long that many keep up a continuous search for novelty.
A few covens are exceptions to this rule, and maintain semi-permanent or permanent homes. Doing this requires a great deal of subterfuge - if the coven wants to avoid suspicion and keep the Volturi from becoming involved - and most vampires don't care for the hassle.

Psychology

When a human becomes a vampire, all of his natural behavior, needs and characteristics are frozen within him forever, though they are also heightened at the same time. From the moment a vampire is made, his interests, dislikes and personality is permanently petrified. For instance, if a human with a loving character becomes a vampire, his or her passion is magnified, allowing him or her to love others even more intensely. Another example is if the human was ambitious and cunning, those characteristics become magnified as a vampire, thus making him a ruthless killer. The only aspect that does change is a vampire's outlook on the world.
Aside from their original personalities, they also have ferocious instincts and a compelling drive for consuming blood. The moment they give in to their thirst, all of their characteristics disappear and they could risk hurting one another by competing for the prey. The longer they abstain from blood completely, the harder it becomes for them to resist, and eventually they will give in to their thirst.
Once a vampire falls in love, that feeling never fades and that love is bound for eternity. As a general rule, only the bond between mates is strong enough to survive the competitive drive for blood. Larger covens are less stable, and usually end because of internal violencd. It is possible for a vampire to fall in love after he'd been turned, and that love will be just as permanent as any other.

Driven by fury and grief at James's death, Victoria prepares to attack Bella and Edward.

Another prevalent vampire trait is that of a vengeful nature. Related again to their unchanging state, vampires are not forgiving; they do not move past an insult or injury. The most common example of vampire vengeance is the aftermath of the loss of a mate. When a vampire loses his mate, he never recovers from the pain. He cannot rest until the party responsible is eradicated. Centuries can pass without lessening the ferocity of his need for vengeance. However, it is possible to suppress that desire if they compel themselves to focus.
The second bonding force, and one that is able to keep a large coven stable, is ambition. Vampires are competitive by nature, and some vampires have been able to evolve this competitive disposition into a greater cause capable of uniting many individuals into a secure whole. The pursuit of power is that bonding cause for vampires. The Romanians were the first to do this successfully for a significant length of time.
The third, and rarest, bonding force is the vampire conscience. Very few vampires are born with or develop a value for human life. The consumption of human blood is such a known aspect of vampire life that few ever question it; the driving thirst for human blood seems irresistible. However, those who do learn to value human life in spite of this reality are able to subsist on animal blood. Vampires who live this way are sometimes referred to as "vegetarians". Animal blood is unappealing to vampires, and thus a difficult diet to maintain. Those who make the sacrifice, though, experience related benefits. In the absence of human blood, the competitive drive disappears. Vampires are able to form bonds of love in addition to the bond between mates. The weak coven alliance is replaced by a strong, family-style union. The Olympic and Denali covens are the only two groups to havd successfully adjusted to this diet.

Transformation

"The warmth inside my heart got more and more real, warmer and warmer. Hotter. The heat was so real it was hard to believe I was imagining it. Hotter. Uncomfortable now. Too hot. Much, much too hot."

―Bella

Esme Cullen being transformed into a vampire by Carlisle Cullen.

In Twilight, transformation from human to vampire is described as being "the sharpest memory they have of their human life." A human is bitten, and venom from glands inside the vampire's mouth is injected into the bloodstream.
Depending upon "how much venom is in the bloodstream, and how close the venom is until it enters the heart", the transformation could last anywhere from 2-5 days. During this time, the human will endure indescribable pain. Once the venom is injected, it is described as being "the beginning of the greatest pain you will ever understand". The venom then has to make its way all through the body. Next through the heart, and it will pump again and again in the heart until it starts meeting itself in the veins. Then it will burn all the veins until the heart stops beating. It moves slower than blood because it is thicker, which makes the transformation long-lasting. Each beat of the heart can only push it so far. The changing/burning process is slow. The venom has to saturate every cell in the body before the process can be completed.
There is no medicine that is strong enough to numb the pain; the best one could do is to immobilize the body.

Weaknesses

"We have to kill him. Rip him apart and burn the pieces."

―Edward

Victoria is beheaded.

In the Twilight universe, vampires are not known to be killed by human effort, are unharmed by sunlight, garlic, holy water, silver, any kind of stake, cross, and can enter homes without permission. Also, any kind of human weapon, including bullets, bounces off of them without any pain or harm done. The only known way to kill a vampire is to dismember his or her body and burn the remains before it can reconstruct itself. Thus their weakness is the strength of their kind or something as strong, which cannot be copied by humans, and fire that consumes them.
In the films, vampires are more vulnerable as they can be killed by severing their heads or biting it off their necks. However, fire is still the only known method to destroy them completely.