Microscopic View of a group of Red Blood Cells.
They have no inner structures
and are simply "bags of hemoglobin."
(Photo Credit: CDC Public Health Image Library)

Our story begins with hemoglobin, the iron-containing protein which binds oxygen in our lungs, carries it to our tissues, and releases it there. In mammals, the red blood cell is basically a small bag of hemoglobin. The red blood cell has no internal structures; it basically contains hemoglobin and transports it around the body. A normal canine red blood cell lives 120 days and dies when it either becomes too stiff to fold itself through the tiny capillaries of the circulation (and it simply bursts trying to do so) or it cannot generate enough energy to keep itself alive.

The spleen is responsible for removing old red blood cells. The spleen possesses many tortuous, winding blood vessels where red blood cells may break if they are not supple enough to pass through.

The spleen then collects the hemoglobin for recycling but since hemoglobin by itself is not safe for release into the bloodstream, a conversion is necessary. The hemoglobin is broken into a protein part (which is broken apart even further and recycled), iron (also broken off and recycled) and heme which is converted into a substance called bilirubin for transport. Bilirubin is sent to the liver to be chemically processed for removal (the process is called "conjugation"). This process enables the bilirubin to dissolve in water so that it can be excreted in bile and disposed of through the intestinal tract. Bilirubin is responsible for the brown color of feces.

If too many red blood cells are being destroyed at once, the systems described above get overwhelmed.

In 1982, the US Government began minting pennies made of zinc wafers coated in copper rather than out of pure copper. Canadian pennies minted after 1996 are also copper-clad zinc wafers. As it is not uncommon for young animals to swallow pennies (and other foreign objects), it was not long before zinc toxicity became recognized. Larger dogs (those over 27 lbs) seem able to pass pennies out their GI tract before toxicity results; a smaller dog's stomach cannot pass the coin and the zinc begins to leach off the coin in the acidity of the stomach. Zinc toxicity is mostly a problem for smaller breed dogs and puppies.

Other zinc sources include nuts, bolts, zippers, and zinc oxide based skin creams (such as diaper rash cream and sun screen). One may see a penny or other radio dense object of about the right size on a radiograph or there may be a known history of penny eating. It is important to rule out immune-mediated hemolysis as an alternative explanation as this is a common condition requiring specific (and very different) treatment.

This dog has swallowed a coin
(readily visible as a white disk in the stomach).
(Photo Credit: Jennie Marvelle, DVM
Boothbay Animal Hospital Boothbay, ME)