why do some people have small veins
Some people, genetically, are born with weaker or smaller veins than others - and some people are born with larger, and more flexible vein tissues. Due to genetics, some people just have poor, weak veins. This only affects the large return veins you can see however - the veins which transport the rest of your blood are likely fine. Regardless of the vein size, blood can still be drawn. I personally have smaller and weaker veins, and can still donate, but sometimes it takes a few tries.
Here's how to enlarge your veins and arteries for plasma donation: 1  Heat up your arm. When veins become overheated, they expand, and draw closer to the skin. 2  Hold off on drinking and smoking. Both smoking and drinking alcoholic beverages makes veins and arteries smaller and harder. 3  Hold off on the caffeine.