Are there any parts of the human body that get oxygen directly from the air and not from the blood?
Published: June 25, 2015
Yes. Upper-layer skin cells and the cells in the front surface of the eyes get a significant amount of oxygen directly from the air rather than from the blood. Human bodies have a huge demand for oxygen. As a result, the oxygen that is able to passively diffuse into the body directly from the air is not nearly enough to run the whole body. Fortunately, we have lungs that can actively pull in oxygen and transfer it to the blood, allowing the body to transport oxygen to the cells by using the blood like a fleet of delivery trucks. Most of our cells rely on the blood delivery service. However, the cells in the outer layers or our skin and eyes are in direct contact with the atmosphere and can efficiently get their oxygen right from the air. Let's look at the eyes first.
For the eyes, it is especially important that there be no blood in the front parts. The parts at the front of the eye need to be transparent in order to let light shine into the eye, thus enabling vision. However, blood is an opaque red color. If blood flowed directly to the front parts of the eye, we would be blinded. As shown in the diagram at the right, the human eye consists of a round, tough white shell called the sclera which envelops a clear gel-like fluid called the vitreous humor. Light travels through the front parts of the eye, through the vitreous humor, and then strikes an array of light-detecting cells on the back of the eye which is called the retina. The front parts of the eye have the job of letting the light inside and focusing the light into images. Therefore, these parts must be transparent (except for the iris and the supporting structures along the edges) and must collectively form a lens shape. The main front parts consist of the lens as well as a lens-shaped pocket of fluid called the aqueous humor and the outer surface which is called the cornea. The cornea is in direct contact with the air. It's job is to contain the aqueous humor and give it a lens-like shape.
The aqueous humor is mostly water and contains very few cells. In contrast, the cornea and lens consist of living cells which must be supplied with oxygen to stay alive. At the same time, they must also stay transparent in order to be able to focus light through. The human body solves this problem in two ways. First, it uses a clear fluid to deliver the oxygen rather than red blood. The aqueous humor itself is the clear fluid that delivers oxygen to the cells in the lens and back side of the cornea. Without red blood cells present to actively clamp on to oxygen molecules and transport them, the aqueous humor must rely on the less-efficient mechanism of simple diffusion to deliver the oxygen. Secondly, our bodies get oxygen into the cells in the front surface of the cornea by simply absorbing it from the air.
Similarly, the outer layers of the skin absorb oxygen directly from the atmosphere. It's true that the skin does not have to be transparent like the cornea, so it can receive oxygen from the blood, which it indeed does. However, since skin is exposed to the air, it makes sense from an efficiency standpoint that the skin would get its oxygen both from the blood and directly from the air. In fact, according to a study performed by Markus Stucker and his collaborators, as published in The Journal of Physiology, "the upper skin layers to a depth of 0.25-0.40 mm are almost exclusively supplied by external oxygen, whereas the oxygen transport of the blood has a minor influence." The amount of oxygen that makes it beyond the skin is negligible, so that most of the cells in our body must get their oxygen from the blood. Interestingly though, the skin itself is able to absorb much of its oxygen directly from the air.