Both bone and muscle require blood in order to live. That is to say, penguin tissues need oxygen and they are delivered by blood vessels. Blood vessels come in intimate contact with almost all parts of the penguin body. Let's take a closer look.

Blood

    In order to get oxygen to a penguin's tissues, something needs to carry that oxygen. In mammals, these cells that carry oxygen are called erythrocytes. You probably know them as red blood cells since they look red when they are carrying oxygen (although the color of cells is ultimately dependent upon what dyes people use to stain cells when they look at them under the microscope. Take a look at the following slides of red blood cells. One slide belongs to a mammal, like us, and the other belongs to a penguin. Can you guess which is which? (HINT: the red blood cells are stained greenish-brown in the top slide and yellow in the bottom one.)

    Have an answer yet? Well, the top slide is of mammal blood and the bottom one is penguin blood. What do you notice immediately about penguin erythrocytes and mammal erythrocytes? (HINT: It's not the color of the stain.) If you noticed that penguin red blood cells have nuclei then you are correct! That's right; mammal red blood cells lose their nuclei late in their development. Penguin and all avian red blood cells retain their nuclei. (The other cells which are stained blue are, oddly, white blood cells or platelets.)

    So now you know about penguin red blood cells and that they carry oxygen. How are these and other blood cells carried through the penguin body?

A: Capillaries B: Superficial plexus C: Subsuperficial plexus D: Intermediate plexus E: Deep plexus     Larger blood vessels give rise to smaller blood vessels, and eventually these give rise to capillaries. As you can see, capillaries are the end of the trip oxygen takes in the blood. When red blood cells reach the capillaries, they travel one-by-one, in single file through those capillaries to deliver their oxygen. Capillaries wind in and out of muscle fascicles and bone marrow cavities throughout the penguin body. Let's take closer look at these capillary beds. The structure to the left has interesting applications in penguin heat regulation. If you want to know how penguin feet keep warm and don't freeze on the ice, check out the FAQ, question 4.3.

    You can see oxygenated blood make its trip through the arteriole (on top, in red). At this point, blood can either enter metarterioles (running down along center) or through the smaller capillaries. Observe the small bulbs along the top arteriole at the beginning of each capillary. These are called pre-capillary sphincters. These are actually small bodies of muscle that control entry of blood into the capillaries. When a penguin isn't using certain tissues -- like when it is at rest or just walking around -- these sphincters contract and divert blood through the metarterioles. Occasionally, the sphincters relax and deliver blood to the tissues through the capillaries. But, let's say this is a capillary bed in the pectoralis major muscle, the primary muscle a penguin uses to flap it's flippers. And let's say the penguin is chasing after an elusive fish or trying to get away from a sea lion. That muscle needs a lot of oxygen. At this point, the pre-capillary sphincters relax and blood flows through the capillaries to perfuse the tissues with lots of blood and, hence, oxygen. When oxygen is released to the tissues, carbon dioxide leaves the tissues and enters the blood. Carbon dioxide then enters the red blood cells. With the delivery of oxygen to tissues, the loss of oxygen makes the blood turn purple in color (it's really not blue as the picture indicates). This is called the venous side of the capillaries. The venous capillaries empty into larger post-capillary venules and then into larger venules. Eventually these give rise to veins and return to the heart. At this point, the trip begins again, after being oxygenated in the lungs. Go onward to learn about penguin lungs.