When you get a shot in the arm, how can you be sure the contents are being released into a blood vessel?


When you donate blood the needle goes into a large, visible vein in your arm or hand but when getting a shot the needle just goes somewhere in your arm.

In: Biology

Many injections don’t have to be directly into a blood vein/artery. Just injecting into muscular tissue alone provides fairly rapid absorption into the bloodstream without the risks of using a vien.

Not all shots need to go directly into a vein.

If something needs to go into a vein they’ll find one, same as when you give blood. This is common for things like intravenous painkillers, general anesthesia and so on.

For a lot of shots they just need to get it into you and it’ll get absorbed in good time.

Those that are given into a muscle are not supposed to go in a vein. Quite the opposite actually – if it gets into a vein, it can cause a huge amount of damage.

The way you can know whether the needle is or isn’t in a vein is simple – you just aspire (pull the piston of the syringe). If some blood appears inside the syringe, then the needle is inside a vein.

There are three ways medication is injected. Most things that are a “shot in the arm” are injected into a large muscle. When they pinch your arm they are locating that muscle to make sure they inject in the right part of your arm. These are called intramuscular injections. Some injections are under the skin which are called subcutaneous injections. Then there are IV or intravenous injections/infusions, these would be the ones that look more like when you give blood. For the intravenous injections there is a needle inserted into a large vein and medication is “pushed” into the blood stream for absorption.

They aren’t trying to get it into your bloodstream, they intentionally inject it into your muscle.

Injecting a vaccine into your bloodstream could be dangerous as the vaccine components would immediately be carried to all the vital organs, possibly causing damage that could make you very sick or kill you without allowing the immune system to develop immunity to the disease.

Injecting into the muscle greatly reduces the spread of the vaccine, keeping it localized to a non-fatal area so the immune system can respond to it and develop immunity to the disease it represents, while also cleaning up and neutralizing the components of the vaccine so they are safer to dispose of via the bloodstream, liver and kidneys.

In animal medicine vaccines are commonly just injected under the skin of the animal, as many animals have much looser skin than people and plenty of subcutaneous space. Human skin hugs our muscles and fat a bit too tight so there’s not really enough space for subcutaneous injection in most cases, so it is much more effective to just inject it into the muscle.

For other types of injections, like medications, intramuscular injections are used to slow the diffusion of the medication into the bloodstream as the effects of the meds can be pretty sudden and severe if injected into a vein. Other types of meds have to be given into a vein because they are too caustic for muscle tissue and could cause severe damage, but vein linings are much tougher and more resilient than muscle tissue.

What would happen if the flu shot was mistakedly injected into a vein?

Routes of drug administration: INJECTION

Subcutaneous route: The drug is injected in the adipose (fat) tissues right beneath the skin. The drug enters the bloodstream through small blood vessels (capillaries) or the lymphatic vessels. Protein drugs (ex. Insulin) are administered subcutaneously because oral administration results in destruction of the protein in the digestive tract.

Intramuscular route: The drug is injected into the muscles (upper arm, thigh or buttock) which lie below the skin and adipose (fat) tissues. It is usually used when larger volumes of drug products are needed. The absorption of the drug into blood depends on the blood supply to the muscle: higher the blood supply, quicker the absorption.

Intravenous route: The drug is directly administered into a vein. It is absorbed completely, and is best way to deliver a precise dose quickly and in a well-controlled manner. It is also used for administering irritating solutions that could damage the tissue if administered subcutaneously or intramuscularly. The drug effects last for a shorter period of time and may require frequent or continuous administration.

Intrathecal route: The drug is administered between two vertebrae in the lower spine and into the space around the spinal cord. The injection site is numbed using an anesthetic prior to drug administration. This route is used to produce rapid/local effects in the brain, spinal cord or meninges (layers of tissue that surrounds brain and spinal cord).

There are four ways medication enters the body through injection.

Intravenously (IV). This is in a vein, usually through a temporarily place access point like and peripheral line or PICC line. Its benefits are rapid absorption because the medications don’t pass the liver from the stomach. This also limits what medications can be given because some need to pass the liver to not be toxic. You can give continuous infusions or just one push slownor fast.

Intramuscular (IM). This is a rapid injection of a small amount of medication into a large muscle like the deltoid, or ventralglutral (butt) muscle. The benefits are similar to IV, the medication does not pass the liver first so it is quicker onset. The limit here is volume of medication, generally less than 3 ml and some medications are toxic to the muscle tissue and can’t be used.

Subcutaneous (SubQ). This is in the fat with a much smaller needle. This is a bit slower than IV or IM but is good for medications that need a more steady absorption, like insulin. Also needs to be a small amount, usually less than 1 ml.

Subdermal. This is below the skin between skin and muscle or fat. Think the TB test or lidocaine. Takes a while to absorbs nd usually reserved for things that stay local and don’t get used by the whole body.

I’m seeing a frightening amount of misinformation and half-truths that seem specifically written to scare people away from getting injections. Take everything including my post with a grain of salt and listen to doctors for medical advice.

The aim of any drug administration is to get the drug into your bloodstream so it can be delivered to wherever in your body it needs to go. How does it know where to go? The drug is designed to bind specifically in that place and will travel around your body via the bloodstream and bind to its target as it passes through. Does that mean that not all drug gets there? Yes, but that’s fine, the dosing takes that into consideration.

All drug administration eventually leads to getting into your bloodstream and then gets filtered out by your kidneys or metabolized by your liver. The different routes (oral, IV, intramuscular(what OP is asking about) need differing amounts of time to make it into your bloodstream. IV is instantaneous, oral takes one of the longest, and intramuscular is in the middle. If you need a drug that needs to work immediately, like adrenaline, then IV. If you want something that spends a longer time in your body before leaving (which is not a bad thing) then pick a slower route. Blood vessels and capillaries in your muscles absorb the drug absorbed there at a slower rate meaning the full dose doesn’t enter circulation all at once. If it did it might not work at all since it would be processed through your body too quickly.

Source: Pharmaceutical Engineer

True ELI5 to your question. Imagine your body as a giant, highly populated country. There are roads leading almost every where. Big Veins are like the Interstate or large highway, they carry lots of stuff “cars” in and out of the major City’s “muscle, tissue”.There are micro veins “roads” running all through the city. If you inject a bunch of cars “medicine” in the downtown area they will all eventually make there way to the major highways.

Tissue like muscle have lots of roads. Tissue like fat has much less roads but still a decent amount. Tissue like cartilage and other connective tissue have very little or no roads. So some connective tissue that done have roads need off road vehicles “osmosis, cell wall transport” to get to the destination.
Edit/add more info: others have mentioned that some medicine needs to absorb slowly like Epinephrine for example. If you are having allergic reaction we give a small amount 0.3 mg into the muscle. Your heart will start beating faster slowly as the epi reaches your heart. If your heart stops and we are doing CPR, we will give epinephrine in your large vein, it ~instantly reach your heart. I have seen someone get IV epi while awake, patient said their “head felt like it was going to explode” because the meds did absorb slowly. They were fine by the way. If they were older or had heart problem could have gone a different direction.


~~Well when you get shot in the arm, the bullet can fragment into little tiny pieces but since your blood vessels will be damaged, it’s highly unlikely the fragments can travel through them.~~

Nurse here, normally when you get a shot in the arm, you don’t want the contents to be released directly into the blood vessel. Normally we inject into your arm muscle tissue and retract the plunger, looking for blood return. There shouldn’t be any blood return since muscle tissue isn’t saturated with free flowing blood. Otherwise, if you do want contents to be released into the blood stream directly then you go for the veins.

There are a lot of drugs that can go multiple routes to get in your body (into a vein, into a muscle, into fat, oral, sublingual, etc). There are also a lot of drugs that have to go a specific route because of how the body breaks them down or the effect they can have if they are particularly harsh.

Like you give insulin into fatty tissue because of how the body can use it. You can also give insulin straight into a vein during emergency situations because it works faster that way.

You can give certain nausea medicine (phenergan) into a big vein and into the muscle, but you can’t give in fat because it is too harsh and will tear up your fatty tissue.

Vaccines are designed to go into muscle. But typically the muscles we use for vaccines aren’t particularly vein-y. If you were to accidentally give a vaccine in a vein, it would likely be okay, it just might mess with the effectiveness of the vaccine.