Anatomy & Physiology For Dummies, 3rd Ed.

Chapter 17

Ten Phabulous Physiology Phacts

IN THIS CHAPTER

check Ruminating on your thumbs, hair, and nose

check Making friends with microbes, milk, and your appendix

check Taking in oxygen and transporting it with hemoglobin

This merest smattering of the everyday miracles of the anatomy and physiology of this one species inspires awe at the power of evolution’s forces. Numerous things make us stand out from our mammalian and primate relatives. Be they clearly evolutionary or of unexplained origin, here are a few for your enjoyment.

Unique to You: Hands, Fingers, Thumbs

There they are, down at the end of your arms, one on each side, a matching pair, unique to you in their details. Other humans have them, too, but no other animal has them. Your hands are certainly far removed from the front paws typical of most mammals, and they’re greatly specialized compared even to those of other primates, including man’s closest evolutionary relatives.

One specialization is the opposable thumb, which is a thumb that can touch each finger on the same hand. (Go ahead — try it now!) Along with that, the human thumb is prehensile, meaning capable of grasping. This anatomy underlies the development of manual dexterity and fine motor skills in humans. The prehensile, opposable thumb makes possible tool making, hunting and gathering, textile and metal crafts, art, writing, cooking, and possibly the very existence of human culture.

Nothing’s Better than Mother’s Milk

All the copious research that has been done on the topic over many years has pointed in the same direction: The best nutrition for a human baby is human milk. Human milk is a complex mixture of over 200 different components, and no other substance produced in another animal or yet in a laboratory matches its ability to meet the needs of a human infant. A baby doesn’t necessarily need its own mother’s milk, though. The composition of milk is remarkably consistent — the age, health status, diet, or geographic location of the mother notwithstanding.

Like all foods, the core components of milk are carbohydrates, proteins, and fats. The proportions of these components in the watery matrix and the specific carbohydrate, protein, and fat molecules in a species’ milk are precisely adapted to that species’ infants’ needs. Human milk is the ideal nourishment for a slow-growing, warm-blooded newborn: low in protein (rat’s milk has 12 times as much), high in lactose, a sugar with twice the energy content of glucose, and high in the essential fatty acids required for neural development. (As we discuss in Chapter 15, the configuration of the human female pelvis drives the birth of a baby with a relatively undeveloped brain.)

Human milk contains many other substances that affect nutrition and development in different ways. Milk and its precursor, colostrum, essentially lend the baby part of the mother’s immune system until it can make its own: B cells, T cells, neutrophils, macrophages, and antibodies (see Chapter 13). Lactoferrin and iron-binding protein cause the scant iron in milk to be fully absorbed across the digestive membrane. Milk also has human hormones and growth factors that are believed by some to be required to optimize the development of the brain and other organs.

It’s Apparent: Your Hair Is Different

Along with milk, hair is a defining characteristic of class Mammalia. This class has found hair to be an adaptable accessory and put it to many uses: mechanical protection, UV protection, thermoregulation, sexual selection, social signaling, and waterproofing, among others.

The genus Homo is distinguished by an apparent lack of hair. Evolutionary theorists suggest that the early forebears of Homo were about as hairy as gorillas, who put their hair to use in all the aforementioned ways, making those arrector pili (the tiny muscles that give you goose bumps) all the more useful. What could have driven so drastic a change in so useful an accessory?

Anatomists note that humans haven’t “lost” their hair — the skin is covered with hair follicles at about the same density as other apes. But the hair itself is different. Most of it is short and fine, and in some cases barely visible. Head hair is longer and coarser than body hair. Head hair and body hair may be curly. (No other primate has curly hair anywhere.) It may be lightly pigmented or apigmented. How does Homo escape a predator or thermoregulate under that? Maybe he runs away on his long, hairless legs, cooled by a steady stream of water from newly evolved glands on his hairless chest and arms. Evaporative cooling would be quite the benefit for a hunter’s life on the hot, dry, equatorial savannah.

A typical mammal has dense hair covering the epidermis. As far as thermoregulation goes, wearing a warm blanket is good for retaining heat but bad for dissipating it. Many mammals, including many large predators, rely on panting and certain patterns of behavior to thermoregulate. For example, on hot days, some mammals lie in the shade near the watering hole. The hunter who could be active when others were stressed escaped predation and ate well — not to mention didn’t have to worry as much about lice or ticks.

But what about the cold nights? An agile thinker could put his prey’s skin and fur to its rightful purpose, keeping a mammalian body warm and safe from abrasion and mechanical shock.

The Only Thing You Have to Fear Is …

The amygdalae are paired structures in the middle brain almost exactly the size and shape of almonds, whence they get their name. They’ve been attracting attention in neuropsychiatry for 60 years — which is pretty much the entire history of neuropsychiatry.

Early research found that neural circuits through the amygdalae connected the midbrain, among the most primitive brain structures, to the frontal cortex, the most advanced. These circuits are part of the limbic system and are thought to be critical in regulating emotion and in guiding emotion-related behaviors.

The amygdalae have been associated clinically with a range of mental and emotional conditions, including depression, autism, and even “normalcy.” Physicians have widely and publicly discussed one case in particular — a woman whose amygdalae are partly nonfunctional. This patient is incapable of experiencing the emotion of fear. The doctors have tried everything, not just for research purposes but because a total lack of fear is a maladaptive trait; it threatens her well-being and survival. This patient has been injured and victimized in situations that normal, healthy fear would have kept her far away from.

Some neuropsychiatric researchers theorize that the amygdalae evolved as part of a protective mechanism. Probably very early on in the evolution of vertebrates, the amygdalae reacted to change in the chemical environment, moving the organism away from toxic substances. New organisms adapted this functionality to perceive and respond to new stimuli in the environment. Stepping back from a chemical spill is still a model of appropriate, fear-based reflexive behavior. The circuit between the frontal lobes and the amygdalae is what generates such survival-enhancing behavior.

You Smell Well!

It is often said that, compared with other animals, the human nose is poor at gathering the information available from volatile molecules in the environment. Just how does the human olfactory sense compare with that of other animals? Take a look at the evidence.

As with other mammals, the human olfactory structures are located at the interface of the brain and the airway. Specialized neurons called olfactory neurons, actually protuberances from the brain, sit right on the border of the nasal passages, behind and slightly above the nostrils.

An olfactory neuron bears olfactory receptors on its plasma membrane. An olfactory receptor recognizes a certain chemical feature of an odor molecule, but that feature is present in numerous kinds of odor molecules. The receptor can bind any odor molecules that have that feature. Thus, humans don’t have a single receptor for “coffee” or “lavender” or “wet dog.” They have many receptors for many kinds of molecules released into the air and drawn into the nose. The brain assembles its olfactory perception of the environment by aggregating the signals from the various receptors. The process is similar to that of vision. Odor recognition is like object recognition based on aggregating many different impulses from the retina. The combination of receptors communicating with the brain gives us our recognition of around 50,000 different scents.

Molecular biology experiments in the early 1990s led to the identification and cloning for research purposes of a large family of olfactory receptors. The experiments showed that the family of genes coding these receptors is the largest in the mammalian genome. Some animals have more than 1,000 different receptors; humans have about 450. One out of every 50 human genes is for an odor receptor!

Based on information from the complex set of olfactory receptors, the brain can determine the concentration of an environmental odor and distinguish a new odor signal from the background odor noise, which is how you get “used to” a smell.

The 450 olfactory receptors give humans their very sophisticated palates, allowing them to enjoy the possibly hundreds of different flavors of a savory meal. Apart from entertainment, this faculty may have helped humans discover new food sources as they moved into new climates and environments.

Microbes: We Are Their World

For thousands of millions of tiny creatures, your gut is the only universe they know. They live and die in that warm, moist, nutrient-rich, immune-protected environment. They work almost every minute of their lives, providing a service to their community and their universe and abiding by the laws of thermodynamics. These good citizens of the gut are adapted specifically to that environment, in the way of symbiotic organisms, and can survive nowhere else.

The internal tissues — blood, bone, muscle, and the others — are normally free of microbes. But the surface tissues — the skin, the digestive and respiratory tracts, and the female urogenital tract — have distinctive colonies of symbiotic microorganisms. The term symbiosis (adjectival form, symbiotic) describes a more or less cooperative and reciprocal relationship between organisms and species. Symbioses are, by definition, good for all. That brings us back to your supporting role as Mr. or Ms. Universe.

The microbe colonies derive from the “host” (a particular human body) a steady supply of nutrients, a stable environment, and protection. The host gets help with some tricky digestive tasks, stimulation of the development and activity of the immune system, and protection against colonization by other, less-well-adapted (pathogenic) microbes. Truly, you’d have difficulty digesting and gaining nutrients from the typical human diet without them.

Considering only cell number, you are more bacteria than human, with bacterial cells outnumbering your own by at least ten times. More than 200 species of bacteria are commonly found in one or another of the human symbiotic colonies. The number and species in a microbe colony is influenced by various characteristics of the host, including age, sex, diet, and genetic makeup. So, a question to ponder: If these microbes are, literally, vital to your survival and you are, just as literally, vital to their survival, does that change your understanding of “you” and “them”?

The Pesky Appendix

Your poor little appendix gets quite the bad rap — labeled with words like pointless or vestigial (the proper term for an organ with no function). Although the appendix is much smaller than it was in our hunter-gatherer ancestors (in modern humans, it’s about 4 inches, located where the small intestine meets the large intestine), calling it useless just isn’t quite fair.

Long ago, our diet was full of foliage — lots of leafy greens, nuts, berries, even bark. We don’t make, nor have we ever made, an enzyme to break down cellulose, which is the main carbohydrate in plant structures. Because of this, when the chyme (digested food) reached the large intestine, it was bulkier, requiring a larger cecum (pouchlike first segment of the large intestine). The appendix, also much larger at the time, housed bacteria that made cellulase, the enzyme that breaks down cellulose. They got a meal and we got a stool that was easier to pass.

Over time, we started growing our own food and cooking it. We stopped eating so many cellulose-rich foods and started using heat to help break things down. (Compare eating a raw carrot to a cooked one.) The cecum shrunk but maintained its function because it’s still the first part of the large intestine. The appendix apparently lost its purpose, as evidenced by its tiny size and lack of any secretions. Hence, its classification as a vestigial structure.

Seemingly supporting this is the lack of complications from appendix removal. Because the appendix is a dead-end tube, it’s easy for bacteria to get trapped in there. Especially if it’s not one of our normal gut flora, it starts replicating and you have an immune response: inflammation (known as appendicitis). The inflammation may resolve on its own, but its isolation increases the likelihood of rupture, allowing your soon-to-be-considered feces into your abdominal cavity. Not good. That’s why the most common treatment for appendicitis is removal of the organ.

Recent research has shown, though, that the appendix is composed of lymphatic tissue, indicating an immune function. Further investigation has shown that the appendix acts very much like a “safe-house” for good bacteria. Any time you take an antibiotic or suffer from lower-intestinal distress, you lose some of that beneficial bacteria. If there are always some just hanging out in your appendix, they can easily repopulate your large intestine before a dangerous one can take hold. So, although you can clearly survive just fine without it, your appendix does, in fact, have a function.

Talkin’ about Breath Control

You don’t have to think about breathing. The steady in-and-out continues while you sleep and go about your daily business. The depth and rhythm adjust to your level of effort. Just climb those stairs; breathing will take care of itself. Many of us would have died young if breathing required constant attention.

Humans are also capable of controlling their breath. Cetaceans (whales and dolphins) can, too; must, in fact, and some also use breath control to sing. Other animals can’t — or at least don’t appear to. Canines in chorus are not really controlling their breath.

Humans use breath control to generate speech. Humans can make finely controlled exhalation pass over the vocal cords while the length and thickness of the cords is changing to generate different frequencies of sound. The lips, tongue, glottis, and other structures shape the vibration, allowing you to make those finely distinguished sound symbols called “words” and “syllables.” Singing, closely related to talking, requires even finer breath control. We doubt that such a hypersocial, hypercommunicative species as Homo sapiens could have evolved as far as it has without speech and singing.

Various religious practices and breathing disciplines take breath control in another direction. Conventional physiological thinking is skeptical of the idea that the conscious brain can reach “down” and exert control over the quintessentially autonomic processes of breathing. However, brain imaging studies and other experimental results have shown that some people with a long history of regular meditation practice have important differences in their neural systems. Some people believe that systematic breath control exercises can provide benefits to many organ systems: the cardiovascular, digestive, neurological, and endocrine systems for a start.

Taking Your First Breath

You spent your fetal development with fluid in your lungs. Not a problem because your mother “breathed” for you (not that you had access to air anyway). The fluid is squeezed out during delivery and coughed out shortly after. Some of it is absorbed by the lung tissue itself. So, instantly, with that first scream at birth, the infant’s lungs fill with air and begin gas exchange. Why, then, are respiratory issues the main concern for babies born prematurely?

The problem is water. Alveoli that are filled with fluid can’t perform gas exchange, but they do require moisture. The nasal cavity warms (to get the oxygen molecules moving faster) and moistens the air as we bring it in. That’s why your nostrils feel drier in the winter, the air is drier and pulls more moisture from the lining. The moist air contributes to the thin layer of water that lines the inner wall of the alveoli, allowing for the oxygen and carbon dioxide to move through the walls (between the cells) and into the capillaries. Unfortunately, water has a high surface tension; the individual molecules are highly attracted to each other. Because alveoli are spherical and lined with water, this creates an inward pull from all directions, collapsing them.

A collapsed alveolus obviously won’t bring in air for gas exchange. In order to counteract the water’s surface tension, thereby keeping the alveoli nice and spherical, the cells secrete surfactant. The production of surfactant is one of the final steps in fetal development. In fact, one of the protein components of surfactant is thought to be a trigger for the onset of labor. Babies born prematurely don’t yet make surfactant, so they can’t breathe on their own. Though the presence of surfactant was discovered in the 1950s, it wasn’t until the 1990s that researchers found an effective way to administer it. Since then, the number of premature babies who die from respiratory distress has been cut in half.

Is Blood Really Blue?

Nearly every diagram of blood vessels printed in color shows the arteries in red and the veins in blue. People with fair skin can look down at their wrists and see very clear, blue veins. So surely that means the blood in the arteries is red and the blood in the veins is blue, right? Not so much.

First, your blood vessels aren’t see-through. They have several layers of tissues. The fact that your veins appear blue and your arteries are red (though they’re too deep to be seen through the surface of the skin) is related to the color of the tissue layers as well as the color of the blood inside. And although venous blood is a different color from arterial blood, it’s definitely not blue.

Hemoglobin is the predominant protein in the red blood cells (RBCs). Specialized for the transport of the blood gases, hemoglobin has the capacity to transport molecular oxygen (attached to the heme group) and carbon dioxide (attached to the globin portion) simultaneously.

When it’s carrying oxygen molecules, it’s called oxyhemoglobin and is bright red. When it is not full of oxygen, it’s called deoxyhemoglobin and is dark red. Between the darker shade of red and the structures of the walls of the veins, you see a blue color through the skin.

Body System

Change

Implications

Cardiovascular system (see Chapter 9)

Heart increases in size.

There is an increased risk of thrombosis (clotting) and heart attack.

 

Fat is deposited in and around the heart muscle.

Varicose veins develop.

 

Heart valves thicken and stiffen.

There is a rise in blood pressure.

 

Resting and maximum heart rates decrease.

 
 

Pumping capacity declines.

 
 

Arteries decrease in diameter and lose elasticity.

 

Digestive system (see Chapter 11)

Teeth may be lost.

There is an increased risk of hiatal hernia, heartburn, peptic ulcers, constipation, hemorrhoids, and gallstones.

 

Peristalsis slows.

Rates of colon cancer and pancreatic cancer increase in the elderly.

 

Pouches form in the intestines (in a condition known as diverticulosis).

 
 

Liver requires more time to metabolize alcohol and drugs.

 

Endocrine system (see Chapter 8)

Glands shrink with age, decreasing hormone release.

Numerous homeostatic mechanisms are disrupted.

   

The metabolic rate decreases.

Lymphatic system (see Chapter 13)

Thymus gland shrinks with age.

Cancer risk increases.

 

Number and effectiveness of T lymphocytes decrease with age.

Infections are more common in elderly.

   

Autoimmune diseases (such as arthritis) increase.

Integumentary system (see Chapter 4)

Epidermal cells are replaced less frequently.

The skin loosens and wrinkles.

 

Adipose tissue in face and hands decreases.

Sensitivity to cold increases.

 

There is a loss and degeneration of fibers in dermis (collagen and elastin).

The body is less able to adjust to increased temperature.

 

Fewer blood vessels and sweat glands are present.

Hair grays and skin becomes paler.

 

Melanocytes decrease.

Hair thins.

 

Number of hair follicles decreases.

 

Muscular system (see Chapter 6)

Muscle tissue deteriorates and is replaced by connective tissue or fat.

The muscles lose strength.

 

Fewer mitochondria are in muscle cells.

Endurance decreases due to fewer mitochondria.

 

Neuromuscular junction degenerates.

There is a decrease in response and overall function.

Nervous system (see Chapter 7)

Brain cells die and are not replaced.

Learning, memory, and reasoning decrease.

 

Cerebral cortex of the brain shrinks.

Reflexes slow.

 

There is decreased production of neurotransmitters.

Alzheimer’s disease occurs in elderly people.

   

There is a loss of sensory input (smell, vision, hearing, and so on).

Reproductive system (see Chapter 14)

Females: Menopause occurs between 45 and 55 years of age and causes cessation of ovarian and uterine cycles, so eggs are no longer released, and hormones such as estrogen and progesterone are no longer produced.

Osteoporosis and wrinkling of skin occur, and there is an increased risk of heart attack.

 

Males: Possible decline in testosterone level after age 50; enlarged prostate gland; decreased sperm production.

Impotence and decreased sex drive occur.

Respiratory system (see Chapter 10)

Breathing capacity declines.

There is decreased efficiency of gas exchange.

 

Thickened capillaries, loss of elasticity in muscles of rib cage.

Risk of infections such as pneumonia increases.

Skeletal system (see Chapter 5)

Cartilage calcifies, becoming hard and brittle.

Bones become thinner and weaker.

 

Bone resorption occurs faster than creation of new bone (loss of bone matrix).

More time is required for bones to heal if they break.

   

Osteoporosis risk increases.

Urinary system (see Chapter 12)

Kidney size and function decrease.

Wastes build up in the blood.

 

There is decreased bladder capacity.

Incontinence occurs.

 

The prostate gland in men is enlarged.

The risk of kidney stones increases.

   

The urge to urinate is more frequent.

   

Urinary tract infections are more likely.