How wounds can kill you.
As per this thread, the Useful Notes criteria for stuff like Instant Death Bullet is being split off so it doesn't clutter up the main page. Overview:
Organic Life Support.What keeps a living thing alive. Of course by living thing we generally mean people, various land dwelling animals, and quite a few water dwelling animals as well. There are exceptions based on biology but the majority of animals will share the major systems that keep them alive. Severe enough damage to any of the below systems can result in death. Keep in mind all of these systems are tied together to form a fully functioning living organism. Any number of injuries whether from accidents, predators, weapons, diseases, environmental factors or other sources of damage can injure any of these systems and cause death. These systems are effectively the same in any land dwelling animals aside from bugs. They are also common to quite a few water dwelling animals like fish and water based mammals. Instant death from injury to these support systems is uncommon and unreliable but not impossible. What is more common is for the victim to become incapacitated and die without some form of medical intervention.
The Brain and Upper Spine also known as the Central Nervous System.This batch of tissues and specialized organs includes the brain and it's various structures, the spinal cord and it's associated nerves, and various other nerve clusters and nerve groups that are dispersed through out the body. The brain controls autonomous functions like natural breathing and the beating of the heart. Damage to the brain or central nervous system can disrupt or stop the brains ability to keep the body alive. A simple way to think to of the brain and the attached nervous system components is like that of a computer and a series of wires. The brain (computer) makes and relays commands to the rest of the body via the nerves (wires). This includes nerves that tell your organs like your heart and lungs to work as well your bodies other organs. Injuries that destroy the back portion of the brain located in the back of the head and the brain stem tend to be instantly lethal. Not only does it shut down your organs it shuts down your bodies ability to move any of it's muscles. But you don't need to hit that portion of the brain to kill someone. While it is possible to survive damage to certain portions of the brain it is uncommon. Even light or moderate damage to the brain can be lethal. The brain also contains blood vessels fed by the circulatory system and severe bleeding can result in death by blood loss. The brain may also swell and put pressure on portions of the brain by pressing against the skull and cutting off blood flow or pinching of nerves. Anyone lucky or unlucky, depending on your perspective, to survive traumatic brain injury can result in a vegitative state, dramatic changes in personality, coma, memory loss, and other ailments.
Criculatory SystemComprised of the heart and the blood vessels that contain and pump blood through out the body. The circulatory system in humans and many other animals is a closed system (meaning blood stays in the system) and is under pressure due to the heart beating. Nutrients, water, waste, and other substances enter and leave the blood by diffusing through the body into the blood stream. The heart is the mechanism which moves blood around our body via fluid pressure. The blood carries oxygen from the lungs to the brain and other parts of the body. Blood that has lost it's oxygen is pumped back to the lungs. The key part of blood is that it keeps oxygen going to the brain. Without that oxygen supply the brain stops working. Death from blood loss occurs when not enough blood is reaching the brain. This can be caused by injury, disease, physical defects, or problems with the heart itself. The body has many blood bearing "tubes" that carry blood all over the body. Every from your skin to your organs has blood flowing to and from it. Bleeding occurs when you damage the tissues carrying the blood causing it leak out of the body until it is clotted by the body or stopped by outside means. Most injuries to the torso are very dangerous not just because of the organs but because of the number of blood vessels in and around that part of the body. Severe damage often leads to rapid blood loss, drop in blood pressure, and eventually death. The torso is not the only part of the body that has large blood vessels. The head arms and legs all have a large number of blood vessels. The head contains large blood vessels that feed the brain and other organs in the head like the eyes and ears. Even minor injuries to the head tend to bleed quite a bit. If a person loses even their hand they are at risk of bleeding to death if the bleeding is not staunched or a tourniquet is applied. Blood loss can happen quickly if medical attention is not received. Most people will lose consciousness. Many organs contain blood vessels of some variety or have a lot of blood bearing tissue like the liver. Damage to organs usually results in severe bleeding.
Respiratory SystemThe lungs, nose, throat, and mouth the air intake and out put of the body. The lungs are a muscle operated pressurized gas bag system. Air containing oxygen is breathed in passing down the throat into the respiratory system where blood gets oxygenated in the lungs. Carbon dioxide is given off as a by product and exhaled. Air comes in through the nose and mouth and through the airways bringing in vital air and exhales waste air out of the body into the environment. Damage or obstruction of any part of the Respiratory system can lead to death by oxygen starvation of the brain. If the lungs are injured they may not fill with air and release carbon dioxide properly. IF they fill with fluid they stop work. It is also possible for the lung to lose its shape or collapse preventing proper inflation.
Altogether Now.The Central Nervous System tells the heart to beat leading to blood flowing through the circulatory system. The brain also tells the lungs to breath in and out getting oxygen to the blood. In turn the blood with oxygen gets to the brain keeping it functioning. All three components are absolutely essential being alive. Severe injury or trauma to any of them can lead to death.
Other important systems. The body is of course comprised of more then the just three key organ groups. Other components like the bones, muscles, and connective tissues are also important.
The Skeletal StructureThe support structure of the body. It serves as framework for organs and body parts to sit in and give protection to our "squishy bits". The bones protect the brain, and most of our internal organs from damage or injury. At the same time the bones allow us to walk, talk, eat, move, and handle thing. The bones are held together by connective tissues and are moved with the muscles. When a bone breaks it is usually rather painful as the bone has nerves attached to it. The bone can be dangerous as when it breaks it can produce sharp shards and edges that can puncture or cut organs, break the skin, or even cut through a blood vessel. Loss of support of the skeleton often leas to severely impaired movement to varying degrees.
The Locomotive PartsThe muscles, tendons, ligaments, and other connective tissues provide the layer that covers most of our bones along with fat. This allows our various body parts to move, allows us to breath, eat, and talk as well. Muscles have a fair amount of blood flow to and from them meaning injury to the muscle tends to bleed a fair amount. They also help provide tension that keeps the body upright or permits actions like walking or holding things. Damage may cause the muscles to suddenly relax or lose tension like cutting a rope. The other connective tissues work with the muscles to help us move around connect the parts of our skeleton. Injury to these tissues is often painful and impairs function of the body part.
Other PartsAn assorted list of body bits. The fat layer on our body serves to both provide limited insulation but also as a temporary storage for "food sources" for the body as well as frequently containing water. Other important bits are our eyes, ears, nose and mouth. The eyes give us visual input about the world around us, the nose acts as an air intake, filter, and olfactory sensor (smells things), the mouth takes in food, air, and permits communication. Injury to these body parts doesn't usually result in death but can result in loss or lessening of senses and other problems.