I can't really add much more as far as a definition goes, but the RBCs (Red Blood Cells) are what transport oxygen to the other cells in your body. They need to be shaped in a manner that allows the oxygen to adhere to the RBC. There are too many examples of diseases caused by sick & diseased RBCs to post here, 'tho Sickle Cell Anemia is an excellent example. A healthy, human RBC looks almost like a donut, ('tho there's no hole, it's just thinner in the middle) the center being where the 2 proteins that make up hemoglobin, (a sort of "glue") that binds with the oxygen provided in your lungs, to be carried throughout the body. HEMOGLOBIN is another part of a CBC (Complete Blood Count) With Sickle Cell Anemia, the RBC is shaped like a sickle (curved, sort of like a quarter moon) which doesn't provide a place for the hemoglobin to adhere the oxygen to (like the healthy "donut" shape which has its own comfy "nesting place" for it does.) So the patient doesn't receive enough oxygen, & there are very painful symptoms associated with this disease. Another abnormal condition in human RBC morphology is called "Rouleaux" where the cells sort of "link" together & form chains of RBCs. Each species has its own characteristics, in fact you would expect to see a Rouleaux condition on a slide of horse's blood, that is natural for them. Dogs & cats RBCs (as well as WBCs) are very similar to human blood. Don't know if this helps clear anything up, but I thought you might find it interesting~ now don't get me started about how fascinating mother kangaroos are, LOL!
Former vet tech, lab tech (human medicine) etc.