|MadSci Network: Anatomy|
Karen: Unfortunately you have been badly misled by the previous answer that you have seen on Mad Sci. Young Mr. Carlson is under the misapprehension that osmosis moves water into the human body. While nothing could be farther from the truth, this is a common mistake among new biologists (or old ones who don't think). One of the main purposes of the skin is as a water barrier. It keeps water in and out of the human body. Water moves into the dead layer of flattened skin cells that make up the stratum corneum of the skin (the keratin that Mr Carlson referred to) by capillary action, the same way it soaks into a paper towel. No doubt it first seeps into the spaces between the flakes of dead skin. Then the dead skin cells become rehydrated and the layer swells and becomes more expansive, thereby increasing it's surface area. But... the stratum corneum is attached to the underlying layers of living skin cells in the epidermis which are in turn attached to connective tissue in the dermis. In order to accommodate its new surface area the stratum corneum must wrinkle. It is able to do so because the tissues to which it is bound are somewhat elastic and stretch to accommodate the larger surface. The resulting interaction between the swollen dead skin layer and the underlying, slightly elastic tissues to which is its connected produces the corrugated appearance which we sometimes call "prune fingers". Some biology instructors may not be prepared to accept this explanation. It is a sad fact of modern science education that many teachers (even university professors who do very advanced research) do not question their initial, simplistic explanations for ordinary phenomena outside their area of expertise. The prevailing notion among biology teachers that osmosis accounts for ALL water movement is as rampant, in my experience, as is the popular but unfounded notion that we use only 10% of our brain. To the best of my limited knowledge, water (on Earth, at least) moves in response to four different forces. 1. It responds to gravity, as when water is poured from one container into another. This is called "bulk flow" of water. It is responsible for the often observed phenomenon that "water seeks its own level". 2. Water will also respond to pressure, as when it is pumped from one location to another. Such pressure is called "hydrostatic pressure" and is also responsible for some biological water movement, as when water is lost from capillary blood vessels near the arterial (pressurized) end. 3. Water will move as a result of electrostatic intermolecular forces; the repulsion and attraction generated by partial charges on the surface of the water molecule. A common type of this movement is called "capillary action". This is the movement of water up the sides of tubes in apparent defiance of gravity or into small spaces of all kinds. This type of water movement is responsible for the fact that water from a puddle on a table will soak into our misplaced shirt sleeve. Also, it contributes in no small way to the movement of water up the stems (trunks) of large trees. 4. Osmosis is, like its parent concept diffusion, powered by the thermal energy inherent in the system of which the water is a part (reflected in the temperature of the surroundings). Osmosis is the diffusion of water molecules through a semipermeable membrane. A semipermeable membrane is a thin membrane that will let water molecules through but will hold back large solute molecules dissolved in the water. Such a membrane must separate two water compartments in order to facilitate the diffusion of water. In each of the four examples the same explanatory motif can be invoked: water moves from where it is in greater concentration to where it is in lesser concentration. We pour water from a full jug into an empty glass. We pump water from the well into the empty sink. Water moves from the puddle on the counter top to the many, small empty spaces in the paper towel. Water moves from a solution with a lower concentration of solutes (where the water is in higher concentration) to a solution with a higher concentration of solutes (where the water is in lower concentration). The slipshod definition of diffusion as "the movement of a substance from higher to lower concentrations", when applied to osmosis, leads to much confusion stemming from the reverse nature of the relationship between the water concentration and that of the solutes. What is most often missing in the typical presentation of osmosis is an explanation of the energy source. Typically, students walk away from a lecture on osmosis with the idea that it requires no energy -- that it is energy free! This, of course, is not the case. Osmosis (like diffusion) depends on thermal energy which is manifested in the incessant motion of the molecules. The point that many instructors are trying to make (perhaps without much reflection given to it) is that no energy needs to be supplied. It is already present in the system. Unfortunately, this is usually carelessly conveyed as "no energy is required for osmosis". The notion that osmosis has any potential (at least on the short term) to affect an immersed living human body completely covered by a relatively thick layer of dead cells, which does not constitute a semipermeable membrane separating two water compartments, is suspect. The fact that even extended immersion seems to have no effect on the actual water balance of a living human body needs some sort of explanation, no doubt. A thick yet porous membrane may, given extended time of immersion in fresh water (perhaps much longer than a typical bath), emulate a semipermeable membrane. Of this and its results I am not sure. I am sure that the "prune fingers" effect cannot be attributed to osmosis. The $64 question, for those inclined to tackle it, is: "Why does the wrinkled effect only appear on the skin of the finger tips (or toe tips) and not on the palms (or soles) and other parts of the skin?" Mr. Carlson's explanation does not hold water (Just a little joke!) for the reasons I have given. The answer, I think, lies in the fact that the stratum corneum on the palms and soles is so thick that we don't notice the (large) wrinkles and that on other parts of the body is so thin it can't pull effectively against it's connection with the dermis; it can't stretch the elastic, so to speak. Bear in mind, however, that my answer is just an educated guess (just like Mr. Carlson's). I may be wrong. In fact, there is another hypothesis. Quite frankly, I don't buy it but it is worth considering. The idea is that water does actually get into your body when you soak and the the wrinkling response is from nerve activity attempting to keep the water out. Here is a news release about this idea: http://www.hindustantimes.com/news/181_827772,00500011.htm I have not found an original source for this study so am reserving my judgment. My main source of skepticism about the study is the statement: "The test is based on the fact that water seeps in easily through the pores in the hand, diluting the body's own liquids and changing the electrolyte values." This statement, as any professor of human anatomy and physiology will tell you, does not have the ring of truth. If "water seeps in easily" then it would be dangerous to get into the water. What "pores" are they referring to? If a slight dilution of the body's own liquids is what produces this effect, then why don't our fingers get wrinkled whenever we become edematous (have excess water in our systems)? Don't get me wrong, I am willing to admit and accept that there are reasonable answers to these questions but we must have them before we can rest with this hypothesis. I hope that my answer has cleared up this question for you without undue confusion. Science is like that. There are few pat answers. Most everything is in the 'this-is-the-best-we-can-do-just-now' stage. Here are some sources that agree with me: I sort-of got the last word in on this one! http://biowww.clemson.edu/biolab/wrinkle.html This one references the previous discussion (which comes up easily on Google) but cites someone else who agrees. http://www.islandscene.com/body_knowledge/1999/990707/fingers/ This next one is purported to come from the Library of Congress. http://www.loc.gov/rr/scitech/mysteries/wrinkles.html Here is a good one. http://www.seed.slb.com/en/watch/bathroom/fingers.htm C. Susan Brown, Ph.D. agrees with me. http://www.birch.net/~lindabrown/000150.htm Here is another one of Mr. Carlson's answers. He has (quite admirably) done some experiments but still has to explain away facts (my fingers wrinkle in the ocean). Further, his answer is based on the assumption the the human body interacts osmotically with it's environment, an assumption which I do no accept. http://www.madsci.org/posts/archives/feb99/917326845.Gb.r.html
Try the links in the MadSci Library for more information on Anatomy.