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Human blood is red because of the iron in the haemoglobin. This gives red blood cells their red colour. Veinous blood is much darker than arterial blood, and your teacher is right that oxygenated (arterial) blood is 'redder' than deoxygenated (veinous) blood. However, human blood is not blue. Some animals do have blue blood though.
You know what blood is â it's that red stuff that oozes out if you get a paper cut. The average person has about 1 to 1Â½ gallons (4-6 liters) of it. But what is blood, really, and where does it come from? How Does the Body Make Blood? It's not made in a kitchen, but blood has ingredients, just like a recipe. To make blood, your body needs to mix: 1  red blood cells, which carry oxygen throughout the body. 2  white blood cells, which fight infections. 3  platelets, which are cells that help you stop bleeding if you get a cut. 4  plasma, a yellowish liquid that carries nutrients, hormones, and proteins throughout the body.