Liquids and gases expand when heated, which causes question 19 options: heat transfer by conduction. convection currents in fluids due to constant temperatures. convection currents in fluids due to changing densities. convection currents in fluids due to changing masses.

Red cells contain a special protein called hemoglobin, which helps carry oxygen from the lungs to the rest of the body and then returns carbon dioxide from the body to the lungs so it can be exhaled. Blood appears red because of the large number of red blood cells, which get their color from the hemoglobin.