So let's take a look at the following diagram to see what exactly we mean. So, we have the exercising cells found within our tissue, and this is the nearby capillary that is carrying our blood plasma that contains the red blood cells and the hemoglobin within those red blood cells. Now, as these exercising cells are carrying out metabolic processes at a higher rate, they produce more thermal energy. And that thermal energy transfers from a higher temperature to a lower temperature via the process of heat. And that's exactly why we sometimes refer to this thermal energy simply as heat. So, as the blood plasma receives more energy from the cells, more thermal energy, it essentially increases in temperature, because the particles and molecules and cells within our blood plasma gain more kinetic energy.