Where the Needle Stops

a shot, from the needle to the heart.

how an injection works: why medicine is injected rather than swallowed, and why a shot into fat, muscle or a vein takes hours, minutes or seconds

Some medicines cannot be swallowed: insulin is a protein, and stomach acid and digestive enzymes break it down like food. So it is injected, and where the needle stops decides how fast it works. A medicine leaves the place it was put only when it reaches a blood vessel, and the blood carries it away. Fat under the skin has few vessels, so a subcutaneous shot is absorbed over hours; muscle is packed with capillaries, so an intramuscular one takes minutes (which is why most vaccines go into the shoulder); into a vein it is in the blood at once. At rest the heart pumps about 5 litres a minute, roughly all the blood you have, so it is round the body in about a minute. Pages 6 to 8 are a toy model of that one rule: the drop's specks wander until one touches a vessel. "In the blood" is the share of those specks, not a dose, and the model's clock is fast; the real times depend on the drug. The last page runs the same toy model, on a faster clock still, in all three places at once: only the vessels differ, so the vein fills at once, muscle over a few seconds, and fat is still creeping. That is why each is used as it is: insulin goes under the skin into fat, for a slow and steady supply; most vaccines go into muscle; and in emergencies, when it has to work now, a medicine goes straight into a vein. A typical shot is 0.5 mL, about ten drops of 0.05 mL.

Concepts: circulation, anatomy, biology.

An interactive you play with one thumb, on Wundr.