Biology-Humans (Science)

"Biology will relate every human gene to the genes of other animals and bacteria, to this great chain of being."-Walter Gilbert (1932-)

The biology of humans is related to that of other animals, but, of course, there are significant differences. Studies of anatomy and physiology have led to a breakdown of the body into interacting systems, each with its own role in making the body and mind function. Some of the systems— circulatory, respiratory, and, of course, reproductive—are quite familiar (but not always completely understood). Others may be less known but are certainly no less important:

♦ The nervous system, the body’s control center, consists of the brain and the network that connects it to senses and muscles.

♦ The gastrointestinal system manages the conversion of food into materials useful to the body and eliminates wastes.

♦ The respiratory system brings in oxygen and eliminates carbon dioxide.

♦ The circulatory system works with the other systems to carry nutrients and oxygen to cells and carries wastes away.

♦ The musculoskeletal system is the framework of bone and muscle that gives structure and motion to the body.

♦ The immune system recognizes and destroys foreign cells and tissues that may be harmful.

♦ The endocrine system produces the hormones that carry signals between systems and regulate many of the body’s functions.

♦ The integumentary system, including the skin, hair, and nails, covers the body and interacts with the outside.

Why are there different types of blood?

During a crisis on a television hospital show, patients stream into the emergency room. Not knowing the patient’s blood type, the doctor calls for O-negative. In most ways, everyone’s blood is the same. It consists of two main types of cells—red and white—suspended in a clear salty liquid—plasma. On a microscopic level, however, the red blood cells are not all alike. If the wrong types are mixed together, they form clumps of cells that can interfere with blood flow, with possibly fatal results. Why do people have different types of blood?

An antigen (anybody generator) is a compound that stimulates a defensive response from the immune system. Most antigens are either proteins, which are chains of amino acid molecules, or polysaccharides, which are chains of sugar molecules. Many antigens are parts of bacteria or viruses.

The difference in blood types is a result of chains of different sugar molecules that are attached to the surface of red blood cells. These attachments, known as antigens, identify the blood cell to the immune system.

Two distinct types of antigens exist in human blood, labeled A and B. If a red blood cell has the A antigen, it is labeled Type A. There are four possible blood types: Type A, Type B, Type AB (both antigens), and Type O (neither antigen). If blood containing one of the antigens is transfused into a person whose cells do not have that antigen, the immune system reacts as if the new cells were an invading infection. As the body attempts to destroy the invaders, the blood forms dangerous clumps.

After the development of the ABO system of classification, a third antigen, called the Rh factor (because it was first observed in the blood of rhesus monkeys) was discovered. Red cells with the antigen are called Rh positive and those without it, Rh negative. Combining this classification with the ABO system gives eight distinct blood types. Because Type O negative blood has none of the antigens, it can be given to a person of any blood type without risk of an immune response, so people with O-negative blood are called universal donors. If you have Type AB-positive blood, your immune system does not respond to any of the antigens, and you are a universal receiver.

The Blood Type Diet is based on the premise that a healthy diet depends on your blood type and recommends specific food groups based on blood type. However, the proponents of the diet do not provide any evidence that clinical trials have been performed to support their claims. Most dieticians and nutritionists consider the diet to be based on pseudoscientific claims.

Blood type is a genetic trait, inherited from your parents. A parent with Type A blood can have children with Type A or Type B, but not Types AB or O. Although it is not certain why the blood types developed, the frequency of particular types of blood varies among populations originating in different places on Earth. Research indicates that blood type may be linked to susceptibility or vulnerability to certain diseases. For example, people with B or O blood have a slightly lower risk of certain cancers, while people with the A or B antigens (or both) have a lower risk of contracting cholera or plague. The antigens may have developed as part of the body’s immune response to diseases.

In 2007, an international research team announced that they had discovered an enzyme that can remove the antigens from red blood cells. Although extensive testing will be necessary before this process becomes acceptable for general use, it may allow any type of blood to become Type O. This would greatly increase the efficiency of blood banks and the safety of transfusions.

Why is it impossible to go without sleep?

You can choose to do many of the things necessary to sustain your body, even though the consequences may not be pleasant. Food is no problem—you can refuse to eat for quite a while and still recover. Refusing water can cause death by dehydration, but it can be done. You can’t refuse to breathe because your body will take over and do it for you, even if you pass out first. And you can’t refuse to sleep. No matter how hard you try to stay awake, it is certain that you will eventually nod off. Why is it impossible to go without sleep?

Experiments with rats have shown that complete sleep deprivation leads to death even faster than starvation. However, it is almost impossible to prevent sleep after a certain point. Even stopping short of death, though, sleeplessness can wreak havoc on the body. Eventually, you become irritable, then forgetful. It becomes impossible to complete even the simplest task, let alone something complex, like driving a car. Extended periods of inadequate rest lead to reduced immune system response, fluctuating blood pressure, and changes in metabolism.

Interestingly, for something so important to our well-being, we really don’t understand sleep that well. There is no obvious chemical change, such as the buildup of carbon dioxide when we don’t breathe, to explain the changes. Sleep scientists do know, however, that when our bodies shut down and we sleep, the brain keeps going.

"I think it’s a very valuable thing for a doctor to learn how to do research, to learn how to approach research, something there isn’t time to teach them in medical school. They don’t really learn how to approach a problem, and yet diagnosis is a problem; and I think that year spent in research is extremely valuable to t hem."

In the 1950s, scientists discovered that there are two distinct brain states during sleep— rapid-eye-movement (REM) sleep and non-REM sleep. REM sleep, as the name implies, is characterized by movement of the eyes beneath their lids. During non-REM sleep, the brain appears to go into a slower state, like an idling engine. Breathing and heartbeat are regular and there are few dreams.

During REM sleep, however, the brain is very active and neurons appear to react very similar to the waking state. This is the part of the cycle during which dreams occur. As we dream, the parts of the brain that control motion in the body operate almost as they do during when awake, but the neurotransmitters that carry signals from the brain to the muscles are inhibited, with the exception of those linked to motion of the eyes.

Fast Facts

Although it is not clear whether insects and other invertebrates sleep, researchers have found evidence that mammals, reptiles, and birds all need sleep. At least in mammals, the amount of sleep needed depends on body size. An opossum sleeps about 18 hours every day, while an elephant manages quite well with 4 hours or less. Marine mammals continue swimming while they sleep.

Sleep research is a complicated process but scientists are starting to develop some hypotheses about our need to sleep. Non-REM sleep is a period of lower metabolic rate and lower brain temperature. This appears to be the period during which the brain can undertake repair work inside its cells and restock some enzymes that it needs in order to function properly. During REM sleep, the cells function normally, but the neurotransmitters are turned off. This may be the time during which the key links in the neurons can be restored. Researchers believe that these links play an important role in controlling mood and learning. The brain may also use its sleep time to organize memories and data from the day and to develop the brain itself as it cuts the rest of the body out of the process.

Why doesn’t it hurt to cut your hair and fingernails?

Hair and fingernails both grow but you don’t feel any sensation when you cut them. Are these parts of the body made of cells or something else?

Your hair and your fingernails are made by living cells, but the material itself is not living. Both hair and nails are built from layers of dead cells. The main component of both substances is a fibrous protein, known as keratin, which is also used by many animals to make hooves and horns (unlike antlers, which are made of living bone cells).

Fingernails grow from their base where a living structure, the matrix, produces the nail cells beneath the protective skin of the cuticle. These cells are formed in layers, making the hard plate of the nail. As the nail grows, at a rate of about 3 millimeters per month (half that for toenails), these cells are pushed forward. As they move, the cells die, leaving the layers of hard protein.

Your body produces hair in a similar way. Each hair grows from a follicle that has a cluster of cells that produce hair cells. As in fingernails, the cells die as they move away from the growth point. Because the cells are not living, damaged hair and nails cannot heal or be repaired. Each hair is made of several layers. The keratin fiber is surrounded by the cuticle. The dead cells of the cuticle overlap like roof shingles and protect the protein fibers inside from damage.

There is a belief, fostered by many mystery novels, that hair and fingernails continue to grow after a person dies. This is not the case—the matrix and follicle cells die along with all the others. Fingernails and hair (particularly beard hairs on a shaven face) seem to grow because the skin around them shrinks as it dries. The exposed length increases, although the actual length of the hair or nail does not change.

Why doesn’t stomach acid dissolve the stomach itself?

When you eat a meal, the digestion process starts with your saliva as you chew. The heavy-duty work, breaking down the cells that form plants and animals, falls to the stomach. The acid in your stomach is strong enough to reduce a steak dinner to liquefied mush. So why doesn’t the acid dissolve the stomach itself?

The answer may be surprising—it does. The inside of the stomach is lined with protective cells called epithelial cells. The most common of the epithelial cells is a type that produces mucus—a thick protein covered on the outside with sugar molecules held to it by chemical bonds. The sugars are able to resist the effects of acid much better than protein, so the mucus layer protects the stomach lining from itself.

Even so, acid constantly works its way to and through the layers of the lining. This is where a second layer of protection comes in—and where the stomach eats itself. The outer layer of epithelial cells is constantly being attacked, but new cells move up from beneath to replace damaged cells. In a healthy human, about 500,000 epithelial cells are destroyed each minute. Over the course of about three days, the entire lining of the stomach is replaced in a constant cycle of cell death and birth.

If the acid manages to breach the mucus layers and the epithelial cells, it can cause dangerous and painful deterioration of the muscles that cause the stomach to work. Ulcers and stomach cancer are two of the most severe consequences of a breach of the protective layers of the epithelial lining.

Fast Facts

Until fairly recently, doctors believed that stomach ulcers are caused by spicy foods and/or stress. In the 1990s, researchers showed that the ulcers are actually the result of a bacterial infection in the stomach lining. The bacterium survives the stomach acid by secreting enzymes that neutralize the acid around it. It then burrows into the mucus of the stomach lining where it is further protected. Ulcers form when the bacteria weaken the lining and allow acid to reach the tissue beyond. Now, the standard treatment for stomach ulcers is an antibiotic.

How many different types of taste can you sense?

Taste is one of our windows into the world around us. Besides providing the ability to enjoy our food, it has a valuable protective function. Many toxic chemical compounds (but certainly not all) have a taste that is universally unappealing or even repulsive. Sometimes taste preferences change with nutritional needs. How does your body register the taste of a food?

The basic sensors for taste are on your tongue and around the other parts of your mouth. The bumps on your tongue contain bundles of taste buds. There are about 10,000 taste buds altogether. Over a two-week period, they will all be replaced as they wear out. Each taste bud consists of dozens to hundreds of taste cells that receive certain types of chemical information and transmit them to the brain through nerve networks that end in the taste buds.

There are a large number of different receptors on the taste buds that bind to specific chemicals, but essentially there are five tastes recognized by human senses:

♦ Sweet

♦ Sour

♦ Salty

♦ Bitter

♦ Umami

These are all familiar terms, except umami, which is sometimes called savory. It is the characteristic taste of a rich, meaty broth or strong cheese.

These five sensations are only part of the story, though. Think about how tasteless food seems when you have a bad cold with a stuffy nose. Taste is a cooperative sense, working with the sensors in your upper nose that determine smell. In fact, based on the number of sensors, the sense of smell may be more important than the taste buds in the perception of the flavor of something. There are about 100 million sensors at the back of each nasal cavity. Molecules travel upward through openings in the roof of your mouth to these sensors. If you cannot smell your food, your taste sensations are very limited.

Remember the map of the tongue that showed up in your high school biology book? It showed which parts of the tongue were able to detect sweet tastes, which could detect salty, and so on. If you ever tried it, you may have found that the map did not match your tongue. Guess what? It doesn’t match anyone’s tongue. It appears that a combination of incomplete research and a poor translation led to the map. Controlled research has shown that all of your taste buds have receptors for all five tastes.

What causes jet lag?

Frequent travelers know the signs: you have traveled across the Atlantic, the sun is coming up, a busy day awaits, and all you want to do is close the curtains and crash. Jet lag has struck again. No matter how many times you travel or how many ways you try to trick your body, you just can’t avoid it. What causes jet lag, and is there any way to avoid it?

Jet lag occurs when you cross multiple time zones in a short time. Just as the clock beside your bed operates on a 24-hour cycle (and must be reset when you change time zones), you have a built-in clock that cycles daily. If you cross several time zones quickly, your internal clock doesn’t match the natural daily clock, based on the rising and setting of the sun. Your body gets confused, giving you a headache and an upset stomach, and making it difficult to concentrate. In general, jet lag occurs with time zone changes of three hours or more, but it is extremely variable from one person to another.

A circadian rhythm, or circa-dian cycle, is an approximately 24-hour internally regulated cycle of functions, including sleeping and waking, growth, and hormone production in an organism. Circadian rhythms have been observed in animals, plants, and bacteria. "Circadian" comes from the Latin for "about a day."

This internal time sense, called a circadian rhythm, is not just a human phenomenon. Animals also function in a day-to-day world and have a time sense that is built into conscious and unconscious schedules. In fact, the subjects of the original studies of the genes that control these biological rhythms were fruit flies. Researchers have even studied fungi that produce chemicals to protect their cells from ultraviolet light just before sunrise, even if the fungus is moved indoors.

In humans, the circadian rhythm is controlled by a tiny part of the brain near the optic nerves. Genes that control our internal clocks produce proteins that break down over time. Time is not the only factor in this breakdown. Changes in exposure to light can reset the clock, so each morning an adjustment takes place, just like someone looking at a watch and setting it against a reference time. Little adjustments each morning keep us in time if our clocks get out of synch. Light striking the retina sends signals to this part of the brain, shutting off the production of melantonin, a hormone that causes people to feel drowsy. Other body systems also respond to signals from the brain, controlling functions such as blood pressure, urine production, and production of other hormones.

So why does jet lag go away after a few days? It turns out that the adjustments triggered by exposure to light are a bit limited and can only move the clock by one or two hours each day. That means a change of six time zones will take about three days of adjustment. In the meantime, your body acts as if you were a few thousand miles east or west. That is jet lag.

Working a rotating shift exerts the same effects on the body as changing time zones. The risks of disruption of circadian cycles also seem to increase with age and they can be severe. Research indicates that women whose jobs cause chronic changes in schedule for longer than 15 years have higher levels of breast and colorectal cancers.

Just above your large intestine is a small worm-shaped organ, about 3 inches long.You seldom hear about the topic unless it becomes inflamed. Then there is a risk of the organ bursting, spreading bacteria throughout the abdomen and causing severe pain and, sometimes, death. An inflamed topic can be removed with no apparent effect on the body. If it causes problems, and we can do without it.

Until recently (and possibly even now), most doctors would have told you that the topic has no useful function. In fact, it is almost a hazard. About 7 percent of Americans have an appendectomy during their lifetime and appear to suffer no ill effects.

In some mammals, such as rabbits, the topic helps digest cellulose, a major component of grass and the stems and leaves of many plants. Specialized bacteria live in the topic and break down the cellulose into compounds that can be absorbed by the animal’s digestive system. Primates, however, do not eat grass. They live on insects, meats, and plant foods that contain starches and sugars instead of cellulose. According to the hypothesis, the topic was once a useful organ to our far distant (preprimate) ancestors.Therefore, there is nothing that would lead to its disappearance.

Recent research, however, suggests that the answer is not that simple and that the topic does have a use, after all. The immunologists who performed the study point to the number of bacteria in our intestines that are needed for our survival. These bacteria play a major role in breaking down food so that it can be used by our bodies. Researchers have found evidence that the topic provides a place for these essential bacteria to live. Sometimes diseases that are accompanied by severe diarrhea cause such extreme emptying of the intestines that the populations of bacteria are substantially depleted. These researchers have formed a hypothesis that the topic then serves as a reservoir of "good" bacteria, releasing them into the intestines and speeding recovery. Because the reservoir is seldom used, and it is possible to recover without it.

The tonsils at the back of the throat are lymph nodes that provide a first line of protection against infection. However, like the topic, they are prone to become infected themselves. A generation or two back, removing the tonsils was almost a routine part of childhood, with no obvious decrease in immunity. Pediatricians today tend to recommend a tonsillectomy only when chronic infections interfere with the general health of a patient.

Why do women live longer than men?

Along with medical advances and increased availability of basic needs, the average life span of humans has increased significantly over time. There is a significant variation among different populations but, with only a few exceptions, women tend to live longer than men. For example, in the United States, the average for women is 80.1 years, while for men, it is 74.8 years. How can we explain the correlation of life expectancy to gender?

This question is not easy to answer, mainly because there are many factors to be considered and, in general, they are not easily controlled in an experiment. Part of the answer seems to be tied to societal roles. For example, in most societies, men are much more likely to fight wars, work in physically risky jobs, and generally be more aggressive, leading to death in conflicts. However, there is some evidence that this explanation does not account for all of the difference. Part of the explanation may be biological, which would not be surprising considering the number of physiological differences based on gender.

Additional evidence that biology explains part of the difference comes from studies of animals. Females have a greater average life span among elephants, mice, and even fruit flies. Several explanations have been proposed and researchers are looking for evidence that one or more of them can explain the longevity difference in animals and to determine how they relate to humans.

One difference is fairly obvious. Sex hormones play a large role in the differences between male and female. Estrogen, for example, helps to eliminate cholesterol, while testosterone increases the levels of low-density lipoproteins (the so-called "bad cholesterol"). This could account for part of the difference in mortality due to heart disease. Women also have a slower metabolism, on average, than men. Many studies of animals have related slower metabolism with longer life span.

Another major difference between men and women is genetic. In many animals, including humans, females have two X chromosomes, while males have one X and one Y chromosome (see Chapter 11). A number of diseases—hemophilia, for example— are much more common among men than women because a gene on the second X chromosome blocks the defective gene that causes the disease. The X chromosome also includes a gene that is used in the repair of damaged DNA. Because a male has only one copy of this gene, if it is defective he may experience more of the effects of aging and disease as unrepaired mutations accumulate during his lifetime.

Fast Facts

Research shows that reducing food intake to the minimum amount needed for proper nutrition extends the life spans of many animals, including monkeys, rats, mice, and fruit flies. Interestingly, calorie restriction shows no effect on the life span of the housefly. Researchers do not know if humans can increase the length of their life by cutting calories to the bare minimum, although there are some indications that, even in nonobese people, reducing calories improves some indicators of health.

It is important, though, to keep in mind that all of these explanations address the average longevity. For any individual, the potential to live a long and healthy life is increased by a healthy lifestyle.

"The brain is the last and grandest biological frontier, the most complex thing we have yet discovered in our universe. It contains hundreds of billions of cells interlinked through trillions of connections. The brain boggles the mind."

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