1 Red blood cells (RBCs) are also called erythrocytes. They make up almost half of blood. Red blood cells are filled with hemoglobin. That's a protein that picks up oxygen in the lungs and brings it to cells all around the body.  White blood cells (WBCs) are also called leukocytes.
Pain originating in the stomach or esophagus is often felt in the upper abdomen and can be due to heartburn, gastroesophageal reflux disease (GERD), or hiatal hernia (weakening in the diaphragm that allows the stomach to protrude into the chest). You may also have gastritis (inflammation of the stomach lining) or ulcers.
As you wait to see your doctor, you can spend some time educating yourself about your gut and the most common digestive problems. Typically, symptoms that occur in the upper part of your abdomen indicate a problem with organs in the upper part of your digestive system, including your esophagus and stomach. 1  Symptoms lower down indicate problems with the organs in the lower part of your digestive system. These include the pancreas, gallbladder, intestines, and rectum.