What is in blood?
Blood is composed of cells and cell fragments in an aqueous solution (plasma). Blood makes up about 7-8% body weight for an adult human being. Plasma constitutes almost 55% of the blood volume and contains almost 90% water. Red blood cells (or erythrocytes or RBC) make up for (pretty much) the rest of the blood volume and constitute about 45% of our blood. White blood cells (or WBC or leukkocytes) constitute only 0.7% of our blood. The RBCs transport oxygen (due to presence of hemoglobin) and carbon dioxide (to a lesser extent as compared to oxygen transport). WBC are part of our immune system and help us fight infections.
An interesting fact about blood: our blood is red in color, while that of some other species may have some other color. For example, insects have yellow or green blood.