Everyone is unique and will react differently. Some people yell and scream when they are hurt, others will keep quiet. Some will insist that they're perfectly fine and be annoyed by attempts to help. Some people are very squeamish and find the idea of how badly they're hurt more traumatic than the actual injury. Find out how the character you're writing an injury for reacts and stick to it unless you have very good reason not to.
Fainting
Can be caused by pain, fear, surprise, or other emotional stress and is usually not a major problem as long as they wake up within a few seconds. Immediately after fainting a person's pulse would be very slow but recover quickly.
Shock
Can follow many injuries and can be as dangerous or more so than the actual injury. It is not just a case of someone suffering from a nasty fright because they got hurt.
Symptoms include:
Pulse and respiration abnormally fast or slow,
Pale, clammy skin,
Shakiness,
Dilated pupils,
Confusion.
Someone suffering from shock should be lain down and kept warm.
Minor Injuries
Bumps, bruises, cuts and grazes are all inconvenient but not incapacitating.
A blow to a bony part of a limb or to a joint hurts a lot at the time of impact (as anyone who's banged their shin will agree) and may swell and stiffen. The impact may also have the effect of temporarily disrupting the 'power supply' to the limb meaning the person getting hit is likely to lose their grip on anything they're carrying and be unable to move the joint for a few minutes.
Bruises can take anything from a few seconds to over a day to appear and anything from a day to several weeks to fade away again. Soft fleshy areas bruise much more colourfully.
Sprains and torn muscles / tendons and so on will stiffen, swell and become more painful after a few hours. A bad sprain can be every bit as incapacitating as a broken bone.
Head Injuries
Probably the most common injury in fiction. From "let's bash the bad guy over the head to stop him running after us" to those scenes where everyone gets thrown all over the flight deck by the first bit of turbulence and bounce their heads off consoles.
Minor Head injuries
The human skull is pretty robust and designed to take a fair amount of punishment. Consequently the occasional bump won't do all that much damage.
A minor bump on the head may leave a character feeling dazed and suffering from a headache, blurred vision and ringing ears but will clear within a few minutes.
Facial bruising is actually quite uncommon, it takes quite a hard blow or a blow that impacts with the soft tissue around the eyes to leave a mark.
Minor cuts and lacerations on the scalp and face will hurt and bleed out of all proportion to their seriousness.
Medium Head Injuries
A more forceful blow (equivalent to a fall of several feet) can lead to complications of the injury.
Concussion (damage to the brain tissue) is quite common after a hard blow to the head and is often accompanied by temporary unconsciousness. (And it should be very temporary if you don't want your character to be permanently damaged). This can also result in dizziness, nausea and, not surprisingly, a nasty headache.
Medium cuts and lacerations will be painful and messy but not dangerous. There may be scarring.
Severe Head Injuries
A blow to the head resulting in prolonged unconsciousness will almost certainly result in brain damage, possibly a fractured skull and bruising or bleeding within the brain itself. It can be fatal either straight away if the damage is extensive enough or later as the blood from the injury causes pressure to be put on the brain.
Severe cuts can damage muscle and sinew and do permanent damage. The pain from such injuries would have most characters unable to concentrate on much else.
concussion
Symptoms include:
Confusion, disorientation, memory loss,
Dizziness, headache (lingering after the first few hours)
Nausea, vomiting,
Pupils uneven in size and/or reaction,
Sluggish reactions, sleepiness.
Any painkillers given to treat the headache must be non-narcotic and relatively mild. Otherwise it is difficult to tell if sleepiness is caused by a worsening of the injury or by the painkillers.
Someone suffering from a suspected head injury should be watched for at least 24 hours, and woken every few hours if they're asleep, to check for the above symptoms.
Broken Bones
In general they hurt. A lot. Any character with a broken bone (with the possible exception of the ribs which I'll come on to in a minute) is going to know about it and not be very happy. Saying that, it is possible that if there is no displacement they might not hurt much at all, and it may not be immediately obvious that the bone is actually broken.
The initial shock and pain is often enough to cause unconsciousness – Vila is perfectly justified in passing out when having his arm set in Powerplay. Keeping the limb immobile will minimise the pain but any pressure or movement is going to be extremely unpleasant.
Severe breaks (compound fractures) can cause part of the bone to protrude through the skin, this will also cause blood loss, which can be severe enough to be dangerous. Nerves and blood vessels can also be permanently damaged.
Smaller bones are obviously more likely to break than larger ones but I can assure you from personal experience that they hurt every bit as much.
Distinguishing between breaks/sprains is not always easy with just 'it hurts to go on but as a guide... Lots of pain but some movement is a relatively good thing -- it indicates 'just' a tear. Less pain but very limited movement is a worry, because it can mean you've snapped something, and the joint becomes useless without surgery.
Broken Ribs
All sorts of nasty complications can arise here. For a start, though a character who has just broken a rib will feel winded and uncomfortable, the initial discomfort will pass quickly and they may feel fine for some hours afterwards. Possibly they may not even realise that they had broken anything.
After a few hours it will start to hurt badly and breathing may be impaired and painful. Problems can occur when the injured person is breathing only shallowly because of the pain and not expanding their lungs fully, lungs can collapse as a result, causing pneumonia. (painkillers and deep breathing excercises is all my evil doc prescribed me which I think is common now.) Interesting in a morbid kind of way is that the breathing difficulties of a collapsed lung aren't what gets you - it's the air pressure that builds up in the chest cutting of the blood flow to the heart.
Broken ribs can also puncture a lung or even the heart with fatal results. A punctured lung would result in death within 3-15 minutes if untreated.
Dislocated Joints
Hurt just as much as broken bones. Can be forced back into place without medical facilities but it's not recommended and will hurt a lot, probably enough to cause unconsciousness. On-the-hoof treatment is the same as for broken bones – imobilise and support the limb.
There are a few dislocations which can be life-threatening -- the sterno-clavicular joint (where the collarbone joins the breastbone) is one. It requires a lot of force to pop it (most people's collarbones will break before the joint goes), and the collarbone usually goes outwards, but if it displaces inwards, it can compress the airways. This joint can dislocate if you get slammed very hard into something like a wall and take the impact on the point of the shoulder. I can also say it hurts very badly and for a very long time.
Cutting and Piercing
Getting stabbed or cut open is Not Nice and most human beings come equipped with a healthy set of defensive reflexes to avoid it. If at all possible they will try to put something else (like hands) in the way of the blow. Most people injured in a stabbing have injuries on their hands and arms as well from trying to ward off their assailant.
The severity of the injury depends a great deal on its location.
Limbs
The arms and legs are not protected by much flesh so even a shallow cut or piercing injury here may damage bone and muscle and render the limb effectively useless.
Severe blood loss can occur if the major blood vessels in the inside of the upper arm and inner thigh are damaged.
Abdomen
Piercing injuries will bleed a lot and can easily do fatal damage, although unless a main artery is hit then it's not going to be a quick death. A piercing more than 2 inches deep starts to get dangerous.
If the main descending aorta is hit, the character has seconds to live.
The femoral or renal arteries will lose a fatal amount of blood in 2 – 3 minutes.
Injury to internal organs would cause bleeding, infections and a nasty slow death if left untreated. Bleeding from the spleen or liver would cause death within 20 minutes. Less major damage to internal organs would cause death either from blood loss over several hours or up to several days later from infection and other complications.
Relatively slight cuts to the stomach area would affect breathing and damage muscles, More major cuts to this area can damage nerves and muscles, meaning the injured character would have no control over their legs. Not nice, when you're trying to get away from the nutter who's just sliced you up and suddenly your legs don't work...
Extensive cuts here can also mean the insides are suddenly outside. Not pretty, not comfortable and, untreated, leaves the character with about 15 minutes to live and they're going to wish it was much less. Quite apart from the pain (which is pretty horrific) the sight of their own insides tends to make most people quite hysterical.
Blood Loss
Major blood loss will result in a fast weak pulse and accelerated respiratory rate.
For an average healthy person about a litre of blood lost is enough for shock to set in.
Loss of approximately a litre and a half to two litres or more will require transfusion.
Loss of more than 2 and a half litres will probably result in unconsciousness and, if transfusion is not given, death.
Severe blood loss, I've defined as a wound larger than a fist or that caused by a compound fracture.
Blunt Trauma
Getting hit...
Aside from the obvious risk of getting smacked upside the head or breaking bones (see above) there are assorted other injuries and complications which can arise.
Due to the elasticity of the ribcage getting smacked in the chest can cause a person to fly backwards some distance. Of course this means they can bounce off of something else and hurt themselves that way. At best they're going to be winded and have difficulty breathing, which causes a certain amount of panic in most people. And it looks rather alarming.
Heavy blows to the back can damage the spine resulting in possible paralysis and death. Kidney injuries are also common when someone is hit in the small of the back. They can bleed and may shut down altogether. Kidney failure means the body can't clear certain waste products from its system, if the waste products build up too far then coma and death can result.
Internal organs such as the liver and spleen can also be damaged by blunt trauma and bleed as detailed above. Other organs which may be injured are the pancreas and the intestines.
If the pancreas is damaged it may spill digestive enzymes which start to digest the person's own insides. Obviously this is rather painful and unpleasant.
Damage to the intestines can result in blockages (causing pain, nausea and vomiting), bleeding, and the release of bacteria into the bloodstream resulting in septic shock (high fever followed by sudden drop in temperature and blood pressure – fatal if not treated) This can take 24 hours or more.
Usual treatment for internal injuries is IV feeding, antibiotics, painkillers and sometimes surgery.
Burns
Burns are classified into degree by their seriousness.
1st degree burns – Red, sensitive skin, like a sunburn.
2nd degree burns – Blistering on the first layer of skin (the epidermis) only.
3rd degree burns – Damage to both the epidermis and dermis (the first two layers of skin), visible scars.
Burns over more than 70% of the body are life threatening due to dehydration and the risk of shock, kidney failure and infection.
Electrical shock
Hurts! Physical marks can vary from none at all to severe tissue damage depending on the severity of the shock. Internal damage can be done by electrical current traveling along the nerves and blood vessels.
Hostile Environments
As it's not uncommon for characters to find themselves stranded somewhere with less than ideal conditions...
Cold
There are the obvious risks of hypothermia and frostbite.
The time for these to set in can vary drastically with temperature and windchill.
In both cases warming should be done slowly or risk blistering of the skin and other complications such as shock.
Like burns, frostbite is classified into degrees of severity.
First degree frostbite – Numbness and whitening of skin.
Second degree frostbite – Outer layer of skin frozen, blistering likely when warmed.
Third degree frostbite – Skin is white or blue and blotchy. Skin and the tissue beneath it is hard and cold.
Heat
Risk of heat exhaustion or heat stroke.
Heat exhaustion is caused by insufficient water and salt intake. It is easy to develop without realising it.
Heat stroke is more severe and often follows untreated heat exhaustion. It is extremely dangerous and can be fatal.
Symptoms of heat exhaustion include:
Headache,
Dizziness, faintness,
Confusion and irritability,
Thirst,
Fast and weak pulse and breathing,
Clammy skin and cramps.
Additional symptoms of heatstroke are:
Hot, dry, flushed skin,
Sweating stops altogether,
Nausea,
Disorientation up to and including hallucinations,
Abnormal blood pressure,
Elevated temperature,
Unconsciousness.
Treatment focuses on lowering the body temperature and rehydrating the person as quickly as possible.
Oxygen Deprivation
Apart from the obvious – lack of oxygen is rather bad for you (6-8 minutes for permanent brain damage) – the reason for the lack can have side effects of its own. For example smoke, chemical or water inhalation can do physical damage to the lungs making breathing difficult even when the person is removed to a safe place.
Exposure to Vacuum
There is a very complete webpage covering this which I've referenced in the links section. However in short your major problems would be...
Oxygen deprivation,
Exposure to extreme cold,
Expansion of gases within the body – meaning that holding your breath would be a Big Mistake, but not one you'd live to regret for very long.
In a sudden decompression there would also be the risk of decompression sickness (the bends) and of getting hit by flying debris during the decompression itself.
Assuming you survived the initial decompression you'd have about 10 seconds of consciousness to do something about it and about one and a half minutes to live.
Parts of the body exposed would suffer from swelling, frostbite, and interrupted circulation.
Leaky spaceships...
50% atmospheric pressure is enough to have people suffering from hypoxia (oxygen deprivation). 15% and you more or less, may as well be in vacuum.