how did people discover that the human body has red blood cells

Question: how did people discover that the human body has red blood cells
- Keywords:
  body,
  red blood cell
Asked by 584curm38 to Fiona, Hazel, Jacque, K on 16 Mar 2018.
- Fiona Macfarlane answered on 16 Mar 2018:
  
  I didn’t know this, but I had a look online. Apparently red blood cells themselves were discover by looking at the blood of frogs in a microscope around the year 1658. They were able to see ‘red particles’ in the blood, which turned out to be the red blood cells

Question: how did people discover that the human body has red blood cells