[MikeL (OP)]

I just read an interesting article about turtles breathing through their bottoms during periods of hibernation underwater, even though they have lungs. The density of blood vessels around the cloaca facilitates sufficient oxygen exchange to maintain metabolism.

It got me thinking about vasodilatation during exercise. The superficial veins of the limbs dilate during exercise. We learn this to facilitate heat exchange with the environment. I was wondering if we might not also be picking up oxygen in trace amounts during this process and releasing CO2. Is the diffusion barrier too high or is it feasible that gaseous exchange across the epidermis occurs? It is after all, the oxygen poor/ CO2 rich veins that are dilating.

Here's the turtle article if you're interested. https://www.livescience.com/61018-turtles-breathe-through-butt.html?utm_source=notification [nofollow]

[evan_au]

Human lungs have an area of 50 - 75 square metres, compared to 1.5-2 square metres of skin for an adult human. So they would make a small contribution by area.

In the lung, the fine blood vessels (capillaries) are separated from the air by a very thin layer, which transfers oxygen and carbon dioxide very efficiently. However, in the skin, the blood vessels are separated from the air by several layers of cells, including fat, hair follicles and several layers of dead cells, which means that transfer of oxygen and carbon dioxide would be very slow.

Unlike turtles, humans are warm-blooded, and can't put their metabolism in low gear (and then reliably get it going again).

See:
https://en.wikipedia.org/wiki/Human_skin
https://en.wikipedia.org/wiki/Lung#Microanatomy

[Colin2B]

Interesting article.
Vasodilatation of blood vessels during exercise does have a big effect on oxygen usage, but as @evan_au  says, not in the lungs.
During exercise the muscles release byproducts such as adenosine and carbon dioxide, which make the blood vessels in that area expand. This vasodilation allows more oxygenated blood to be delivered to the muscles. In a healthy body, vasodilation also takes place in the coronary blood vessels that surround your heart.
Another way the increased demand for oxygen in your exercising muscles is met is by redirecting blood from the less critical areas of your body, such as your abdomen and kidneys. The sympathetic nervous system stimulates the blood vessels in these areas to constrict reducing blood flow and oxygen to those tissues.

[Bored chemist]

From time to time, the blood flow to a limb gets interrupted by some sort of accident.
The limb doesn't generally do well.
So we know that diffusion via the skin isn't sufficient to supply the minimal oxygen needs of that limb.
A limb that's not doing any work has, on a  weight for weight basis, relatively low oxygen requirements compared to the body's other organs (brains, kidneys etc).
So the oxygen supplied via the skin can't be a significant supply to the body as a whole.

[chris]

Thanks for raising this interesting question. There are some excellent answers and perspectives above. I'd like to add another point that hasn't really been mentioned.

Turtles, like other reptiles, are cold-blooded (the scientific term is poikilothermic). As such, their metabolic rate is at the mercy of the local temperature of the environment. This is because metabolism is based on chemical reactions and the rate of a chemical reaction is proportional to the temperature. Roughly, for every ten degree increase in temperature, the reaction rate doubles.

This means that the supply of oxygen required to support metabolic rate is not constant. When the animal is in cold conditions, like hibernation, then metabolic rate, and hence oxygen demand, plummets. As such, there can be sufficient uptake of oxygen through thin mucous membranes to sustain life.

As homeotherms (warm blooded animals), humans could not manage this. Our metabolic rate is permanently much higher, and we are very ill-adapted to swings in metabolic output and low oxygen tension.

More significant is that, when we exercise, there is peripheral vasodilatation of skin arterioles to divert hot blood towards the body surface in order to shed excess heat. In fact, this robs us of considerable performance potential, because blood that could be perfusing muscles, delivering oxygen and carrying away waste is instead being used to help with heat control. Athletic performance is not being limited by how much oxygen is in blood - which is pretty much 100% saturated all the time - but instead by the rate at which oxygen-rich blood can be delivered to the muscles.

Adding a bit more oxygen to the blood returning in the venous circuit via the skin will not be at all helpful under these conditions, because the lungs would achieve that level of oxygenation when that blood gets back there anyway. And the blood that is in the skin is not destined for a muscle before it's been back to the lungs.

Ergo, we're doing the best we can with what we have got, and it's temperature that is the enemy, rather than oxygen.

[Bored chemist]

Interesting.
I wondered if that effect - at high temps we can't lose heat so well so we shouldn't be able to run so fast- was apparent and it seems that it's a measurable effect.
http://www.runningstrong.com/temperature.html

[MikeL (OP)]

Thanks for your input guys. There's a lot of talk of oxygen demand, but not so much for CO2 clearance, which definitely has a rate limiting effect on performance. The partial pressure gradient is much higher for CO2 as well. As carbon chains break down and CO2 builds up, even a small diffusion gradient through the skin during strenuous exercise should confer a performance advantage. There are a lot of capillary networks coming up out of the dermis. It's just enough to make me wonder.

[Bored chemist]

Broadly, if the lungs can manage enough  gas exchange to draw in oxygen then they can manage to expel CO2.
There are two places the blood can go:
the skin- which is designed to keep stuff (particularly water) in or the lungs
which are designed to have really good gas exchange properties.

That's not a contest the skin is going to win.

[chris]

As carbon chains break down and CO2 builds up

CO₂ doesn't build up, at least in a healthy person. Venous blood does have a high CO₂ burden, but the first place this sees is the lung, which gets rid of it. Pulmonary venous blood (returning from the lungs for re-issue to the body via the left side of the heart) does not contain an increased CO₂ load in an exercising athlete, unless they have a lung or vascular problem.

What augments respiration during severe exercise is not entirely CO2 either; anaerobic respiration leads to the production of lactic acid as glucose is burned anaerobically; the resulting pyruvic acid is converted to lactate and dumped into the blood for processing by the liver. (By turning the pyruvate into lactate cells prevent a build up of the end-product of the glycolysis pathway, which could impair energy production.) This constitutes a metabolic acidosis, which tickles chemoreceptors and increases respiratory drive.