ECG and red blood cell. Computer artwork of an electrocardiogram (ECG) trace and a red blood cell (erythrocyte). Red blood cells are biconcave, giving them a large surface area for gas exchange, and highly elastic, enabling them to pass through narrow capillary vessels. The cell's interior is packed with haemoglobin, a red iron-containing pigment that has an oxygen carrying capacity. The main function of red blood cells is to distribute oxygen to body tissues and to carry waste carbon dioxide back to the lungs. The ECG trace shows the electrical activity of the heart. Contractions are caused by electrical signals between the upper (atrial) and lower (ventricular) chambers. The tall peak indicates the pumping action of the ventricles.