Why Is Milk White? (3 page)

As we learn more and more about life and about chemistry, the sciences of biology and chemistry have more and more to do with each other.

While biology is the study of living things, physiology is the study of how living things work. Although many aspects of physiology have little to do with chemistry, it is hard to study how living things work without getting into chemistry somewhere.

You might be studying how light is focused in the eye, but eventually you get down to chemistry when you ask how the light gets turned into electrical signals to be sent to the brain. You might be studying how bacteria and viruses cause disease, but without a knowledge of chemistry you won't get very far.

Almost all of the Nobel Prizes in physiology and medicine were awarded for discoveries or inventions that deal with physiology at the molecular level in some respect.

Before studying physiology in medical school, it is strongly recommended that students first take chemistry courses, because so much of physiology depends on understanding how living things function at the molecular level.

Some of us don't. And most of us don't have hair on the bottoms of our feet, on our lips, on the palms of our hands, or on our eyelids, except along the very front edge, where we have eyelashes.

Each hair on your body grows in three stages. There is a growth stage, usually lasting two to three years (but sometimes as many as eight). This is followed by a two- to three-week period during which the hair stops growing and is cut off from its blood supply and from the cells that make new hair. After this comes a resting stage, lasting about three months. Then the cycle begins again, and
a new hair pushes the old, dead hair out, and it falls away. Each hair has its own cycle, so only a few hairs are shed each day.

The timing of these three stages determines how long your hair will be. If the hair grows for a long time before it rests, the hair will be longer. The hair on your eyebrows only grows for about four to seven months and rests for about nine months, so the hairs there are not as long as those on the top of your head. The same timing of the growth and rest phases determines the length of the other hairs on your body.

Each person has different hair growth and rest cycles, determined by genetics and environmental factors. Some people can grow their hair very long, while others will find that their hair never gets much past their shoulders, even if they never cut it. Women can usually grow longer hair than men, because the hair growth cycles are affected by sex hormones.

Boys and girls are actually about the same height on average—until they reach the age of 12 or 13. After that age, girls' growth starts to level off. Boys continue growing until age 17 or 18.

Boys and girls grow about two inches per year until they reach puberty. Then they each have a growth spurt that lasts a year or two. During the growth spurts, boys grow a little faster than girls. Girls reach puberty (on average) about a year before boys do. Girls also end puberty earlier than boys. The differences in when they reach puberty, how fast they grow during it, and how long puberty lasts are what make boys average about five inches taller than girls by the time they stop growing.

To estimate how tall you will be when you are fully grown, you can use the knowledge that men are on average five inches taller than women. If you are a girl, subtract five inches from your father's height. Then add your mother's height, and then divide by two. This will give you the average height your parents would have if they were both women.

If you are a boy, add five inches to your mother's height, then add your father's height, and divide by two. This gives you the average height your parents would be if they were both men.

The problem with these estimates is that they will only let you guess your height within about four inches. That is a lot of variation. On average, siblings of the same sex will be within four inches of one another.

There is another way to estimate your adult height that indicates just how unreliable height estimation is. If you know how tall you were when you were two years old, you can double that to estimate your adult height. Notice that this method does not ask whether you are a boy or a girl. Remember that boys and girls grow at about the same rate until puberty, and that there is about a five-inch difference on average between men and women's height. So this estimate may easily be up to five inches off.

Because they don't want dry skin. Skin acts as a barrier, preventing bad things from getting into the body but also preventing water in the body from evaporating away or leaking out.

The outermost layer of skin is made of dead cells. If these dead cells dry out too much, they can crack and separate, making holes in the protective barrier. Even if there are no cracks, the dry cells don't function as well, allowing moisture loss from the living cells below them.

Moisturizers generally have three types of ingredients.
Humectants
are molecules that absorb water from the air. Glycerin and urea are humectants, and you may see one or both of them on the label of your favorite lotion.

Emollients
lubricate the skin and fill in the spaces between the dead cells to help make a better moisture barrier. They are generally oils, such as mineral oil, petroleum jelly, or lanolin. They also change the look and the feel of the skin in the same way that putting oil on paper makes the dry paper look transparent and wet.

The third type of ingredient will generally be some kind of preservative that prevents bacteria from degrading (damaging) the emollients and humectants.

Oil-free moisturizers are often used for areas like the face, which already have plenty of oil and where adding oil might cause acne. Moisturizers for knees and elbows will more likely have more oil than moisturizers for the face.

Some moisturizers improve the skin's natural barrier by adding a layer of oil, sunscreen, or antioxidants. These protect the skin from damage by sun, wind, and abrasion.

There are a number of supplements marketed to runners and cyclists to improve performance during races. How much they actually help may be more of an indication of the diet before the race or during training and of the mental state of the athlete than of any special powers of the ingredients. In other words, if the athlete is missing something, adding it back should help. And if the athlete believes the supplement will help, then the
placebo effect
(deceiving her into feeling better) may also improve performance.

Taking a lot of supplements before a race can actually harm performance if they cause digestive upset or other problems, and many of the supplements used have several negative side effects. A lot of coaches and professionals recommend doing nothing special for a race, only what you normally do during training.

Without making any recommendations or encouraging the use of supplements to improve performance, I can describe some of the supplements used and their claimed benefits.

Sodium phosphate or sodium bicarbonate is taken to buffer the lactic acid that builds up in muscles that are working without sufficient oxygen. Loading up on these before a race can cause digestive problems that lower performance.

Caffeine is taken as a stimulant. Drinking a couple cups of coffee is safe, but caffeine is a urinary stimulant, and stopping to pee will affect your race time.

Antioxidants such as vitamin E, beta carotene, selenium, and vitamin C are safe (though you
can
overdose on selenium, which affects insulin) and may help prevent some damage caused by strenuous exercise, but they won't help your performance in the race itself. And using too much may cause stomach upset.

Some people take supplements that relax the artery walls (
vasodilators
) under the impression that opening up the arteries increases blood flow to the muscles. But your body regulates your blood pressure carefully to make sure there is enough oxygen to the brain, and fainting during a race will affect your time. Being lightheaded generally does not improve performance in any activity, except possibly sleep. Combining vasodilators with dehydration is not a good idea.

Getting adequate salt for a long race can help prevent cramping. Some racers add magnesium and calcium as well. Loss of these electrolytes through sweat is one cause of cramps.

If you break your leg, it should be able to heal just fine without any chemical assistance other than a good diet. But you might feel better if you also take some acetylsalicylic acid (aspirin).

Other health problems can benefit more from a chemist's help. If you have a bacterial infection, some antibiotic chemicals that stop bacteria from growing, or kill the bacteria, might be a lot of help.

Knowledge of nutrition, which is the chemistry of what we eat, can also save lives and help people live longer. If you don't get a balanced diet or enough sun, then you might benefit from a chemist's knowledge of vitamins and minerals. Soap, toothpaste, acne medications, sunscreens, iodine, and chlorinated water all help keep you clean and free of infections.

But where chemistry really helps is when you have a serious health problem. Chemicals such as the insulin needed to treat diabetes or the antivirals used to treat HIV are lifesavers. Drugs to treat cancer, heart disease, and stroke can prolong lives by years.

On the other side of the coin, knowing which chemicals are dangerous can also save lives. The chemistry of poisonous substances
(toxicology)
helps keep us safe, but it also helps control pests that can affect our health, such as mosquitoes, rats and mice, cockroaches, and flies. Pesticides also increase crop yields, allowing more people to get the nutrition they need.

We are made of chemicals. Understanding chemistry helps us understand our bodies and keeps us healthy.

Your bones are made of calcium phosphate. Your blood has iron in it to carry oxygen. The oxygen is needed for your cells to burn the glucose fuel they use for energy. The cells are enclosed in membranes made up of phospholipid molecules, and they function using enzymes made up of protein. More protein makes up your muscles, skin, and hair. Chemistry happens all throughout your body.

You can change the chemistry of your body. For example, the proteins in your hair can be changed to have more connections between them. This is done by adding chemicals to the hair to get a “permanent” wave to curl the hair. You can add fluorine to your teeth to make them less prone to cavities.

Benadryl contains the antihistamine diphenhydramine. The brain cells that are most concerned with wakefulness are the ones that release histamine, and these are the same brain cells that are targeted by antihistamines.

Histamine has many effects in the body. In the brain, it acts as a
neurotransmitter
, a chemical that sends signals between nerve cells.

It also causes the passageways in the lungs to constrict. It dilates blood vessels, allowing more blood to flow through them and lowering blood pressure. It is part of the immune system and is the cause of many of the symptoms you get when fighting off disease.

To help fight infections, histamine makes the small blood vessels leaky, so that white blood cells and antibodies can leak from the blood into the tissues to fight bacteria and viruses.

A side effect of leaky blood vessels is water leaking into tissues from the blood, causing runny noses and watery eyes. The enlarging of the small blood vessels causes swelling of tissue in places like the nasal passages, which then become blocked—a stuffy nose.

And histamine's other action, as a neurotransmitter, causes side effects when you have a cold. The nerves that detect sensation in your nose get stimulated, and you sneeze.

The action that makes the cells in the blood vessels loosen and leak can also affect skin cells, causing hives. The pain and itching of insect bites is due to histamine's effects on the skin and blood vessels as the body recognizes foreign proteins and attacks them.

To fight the symptoms of colds, allergies, and insect bites, we use antihistamines. But since histamine is a neurotransmitter that regulates wakefulness, a side effect of relieving cold symptoms is drowsiness.

There are four different types of histamine receptors in the body, and they control different reactions to histamine. Some new antihistamines target only one or two of these receptors, so they can relieve allergy symptoms without causing drowsiness.

The main pigment in your skin is
melanin.
It protects your skin from damage caused by the rays of the sun. When you get more sun, your skin compensates by producing more melanin, and you get a tan.