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
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.
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