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Intro to Anatomy 7: The Integumentary System
Intro to Anatomy 7: The Integumentary System
The Lone Ranger
Published by The Lone Ranger
01-07-2007
Default The Subcutaneous Layer

The Subcutaneous Layer (Hypodermis):
The hypodermis (hypo – “under” + dermis – “skin”) or subcutaneous (sub – “below” + cutaneous) layer lies below the skin and is made up of loose connective and adipose tissues. There is no distinct boundary between it and the lower portion of the dermis.

The hypodermis binds the skin to underlying organs while allowing the skin to move somewhat independently of underlying structures. Adipose tissue in the hypodermis provides padding and shock-absorption that helps to protect underlying tissues from damage; it is also important in insulating against loss of body heat. Because the subcutaneous layer contains numerous blood vessels but no vital organs, it is a near-ideal place to inject drugs. This is why so many drugs are administered through subcutaneous injection by a hypodermic (hypo – “under” + dermic – “skin”) needle.

Babies and young children have extensive deposits of “baby fat” in the hypodermis, which helps provide additional insulation against heat loss. This is important for young children, because the smaller a warm-blooded animal is, the faster it tends to lose body heat across its skin surface. As children grow larger and become less vulnerable to heat loss, these fat layers (hopefully) become thinner.

Of course, even adults have considerable amounts of adipose tissue in the subcutaneous layer. Interestingly, body fat tends to be distributed differently in men and women. In men, subcutaneous fat accumulates primarily in the neck, arms, and lower back, above the buttocks, and in the abdominal region (the “paunch”). Subcutaneous fat tends to be more evenly distributed in women. Women generally have proportionately more body fat than do men, and their subcutaneous layers are generally thicker – this is one reason why women tend to have softer skin than do men. In women, subcutaneous fat is especially prone to accumulate in the breasts, buttocks, hips, and thighs.

Most marine mammals have thick layers of subcutaneous fat (called blubber) that provide insulation against loss of body heat to the surrounding water. Because women tend to have thicker subcutaneous layers than do men, this means that they’re better insulated against heat loss in some ways. (This tendency may have evolved because women, being smaller than men generally, are more vulnerable to heat loss.) There are some interesting consequences of this.

Perhaps you’ve heard of the Donner Party? The party consisted of 89 people. During the winter of 1846/1847, 81 members of the Donner Party were trapped by snow while trying to cross the Sierra Nevada Mountains (several people died before the party actually became trapped), and they soon ran out of food. Ultimately, 41 people died. The interesting thing is that 2/3 of the men died while 2/3 of the women survived. Why?

As a general rule, men have higher metabolic rates than do women, and so they produce more body heat. So long as enough food is available to keep their internal fires going then, men tend to feel the cold less than women do. On the other hand, being better insulated on average, a woman will lose body heat less rapidly than will a man of the same size. What’s more, since the average woman has more stored body fat than does a man of the same size, she has more “fuel” to survive on than does the average man.

This means that the average woman will survive longer than the average man under starvation conditions. That’s especially true when it’s cold-enough for hypothermia to be a serious concern.

In the cold waters off the coasts of Korea and Japan, divers known as ama have been making their livings diving for shellfish for centuries. They dive to 30 meters (100 feet) or more without diving equipment, and stay underwater for 2 minutes or longer. Traditionally, an ama wears either a simple loincloth or no clothing at all while diving. (In modern times, the ama have become a tourist attraction; where tourists are likely to be watching, the ama generally wear thin cotton garments.) The word “ama” translates as “sea woman”; in Korea, all ama are women, though some Japanese ama are male.

Why are the ama almost always women? It’s thought that women, since they’re better-insulated than are most men, are less prone to loss of body heat, and so are less susceptible to hypothermia.

As an aside, the ama traditionally hyperventilate themselves and then give a low whistle just before plunging into the water. Though it’s doubtful that many of the ama know what functions the whistle serves, it is nonetheless important, because it does two vital things.

The deeper you dive, the more pressure the surrounding water exerts on your body. Some ama dive so deeply that their lungs are compressed to less than 2/3 of their volumes at the surface. When they whistle just before diving, the ama empty some of the air from their lungs. The delicate tissues inside the lungs simply aren’t equipped to handle the internal pressure that would be exterted by a lungful of air compressed to 2/3 of its original volume, and so diving to that depth with your lungs completely inflated could cause serious injury.

The whistling has a subtler function as well. You’re surely aware that if you hold your breath (without first hyperventilating yourself) for long enough, you’ll eventually be compelled to take a breath. Most people are under the impression that the urge to take a breath is triggered by low blood oxygen levels, but that’s incorrect. It’s high blood CO2 levels that are responsible. So what?

Well, hyperventilation before diving, does not raise your blood O2 levels significantly, contrary to what most people think. What hyperventilation does is lower your blood CO2 levels. Since it’s high CO2 levels that trigger the breathing response, hyperventilation can indeed increase the length of time that you can hold your breath, but it’s a very dangerous thing to do when diving. If you hyperventilate before a long dive, you can exhaust your available oxygen supply while underwater without realizing it, because the blood CO2 levels never rise to a high-enough level to cause you to feel the urge to breathe. When the brain runs out of oxygen, you lose consciousness. Needless to say, if you happen to be underwater at the time, the result will probably be fatal.

The long, low whistle the ama make just before diving has the effect of preventing them from blowing off too much CO2, and so greatly reduces the likelihood that they’ll lose consciousness while beneath the surface and drown.


The ama have been diving for pearls, seaweed, and shellfish in the cold waters off Japan and Korea for centuries.

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Ensign Steve (12-13-2008)
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