Anatomy & Physiology For Dummies, 3rd Ed.

Chapter 15

Change and Development over the Life Span

IN THIS CHAPTER

Unfolding in real time: the drama of development

Summarizing the miracle of reproduction

Changing through life

In the context of anatomy and physiology, development means the pattern of change through an organism’s lifetime. Development is closely related to the specialized branch of biology called ontogeny, which studies an organism’s history within its lifetime. Human development has been a subject of much study for thousands of years, especially for parents and grandparents of young human children. It’s a good thing for babies that people find them so fascinating: The human baby takes a lot of effort to maintain and a long time to mature.

In this chapter, we take a look at human development, from the creation of the zygote through old age. Although you’ve already experienced some of this development, you get a glimpse of some of the changes your body will go through as you age.

Programming Development

One way to think of development is as the unfolding in real time and space of a program for generating a unique biological organism. The program is launched when a new zygote comes into existence. All zygotes are created the same way and then proceed down the path of development encoded in their own species-specific and individual-specific DNA. (Flip back to Chapter 14 if you need a refresher on the zygote.)

The totality of the DNA of a zygote — that is, its genome — comes into existence at the time of fertilization. The DNA in the zygote’s nucleus comprises genes (specific DNA sequences) from both its parents, 50-50, but this particular combination of genes has never been seen before and will never be again.

Most genomes, including all human genomes, have aging and death built into the program. All die sooner or later. A few survive until their program has fully unfolded and reached its end.

Stages of development

Development begins in the zygote and continues until death. There’s no universally agreed-upon definition of the development stages (although two milestone events — birth and, for females, the onset of menstruation — are universally acknowledged), and the age range at which a person passes from one stage to the next is wide. Change is more or less continuous through life, and different organ systems undergo significant changes on their own development timetable. However, conventionally in human biology, the development milestones that mark the stages are based on developments in the nervous and reproductive systems.

Dimensions of development

The structural and physiological changes that happen during human development include an increase in size, the acquisition of some specialized abilities, and the loss of some other specialized abilities continuously throughout life. When all goes well, senescence (aging) is the final stage of development.

The following sections assume an organism for whom all is going well, biologically speaking: no fatal errors in the genome itself and adequate resources to sustain nutrition, thermoregulation, and all the rest of the life-maintaining physiological reactions.

Growth

Part of human development involves an increase in size. Increased size is primarily accomplished by the growth of organs that exist in some form in the embryo: The heart grows larger, the brain grows larger, and the bones get longer and heavier. The organs grow by building more of their own tissues, and tissues get bigger by adding cells or increasing cell size. Everything (well, almost everything — there are always exceptions in biology!) grows together, mostly by adding cells.

However, not everything grows equally. Different stages of development are characterized by different proportions of tissue types. For example, both the brain and the skeletal muscle increase in size from infancy to adulthood, but the proportion of muscle tissue to brain tissue is much higher in adulthood.

When a three-dimensional object such as a living body increases in size, the surface-to-volume ratio decreases. (Or, to put it another way, the volume-to-surface ratio increases — more of your inside parts are dependent on fewer of your outside parts to interact directly with your environment.) The size of a human body strongly influences thermoregulation, fluid balance, and other key aspects of homeostasis.

Differentiation

For humans, the acquisition of new abilities or improvements in existing abilities is part of development. New physiological abilities come about usually because of cell and tissue differentiation (function specialization). Tissue specialization begins in the pre-embryonic stage, as we discuss in the “Development before Birth” section later in this chapter. A newborn has some version of more or less all the cell and tissue types, but many fully differentiated cells must be generated and integrated functionally into tissues at appropriate stages of development. See Chapter 5 for a brief description of the development of a long bone (an organ of the skeletal system) by cell growth and differentiation.

Lots of human body functions aren’t “learned” but “developed.” The ability to digest starch, for example, is acquired during the first year of life, when the body starts producing the necessary enzymes — not when someone teaches a baby how. Toilet training is more about the maturity of the nervous system than the diligence of the parents. The acquisition of a new skill, structure, or process is sometimes accompanied by the loss of existing abilities. A young adult is better at planning than a teenager but has most likely lost some stamina for all-nighters, parties, and road trips. The stages of human development can be characterized by these abilities gained and lost.

Among many aspects of development research, brain research has yielded very interesting data in recent years, aided by advanced imaging technology (see Chapter 1). In the late 1990s, the decades-old doctrine that humans don’t generate any new brain cells after birth was definitively shown to be false. Data from many different kinds of studies since then have indicated that the human brain is plastic (capable of change and development) well into old age. Turn to Chapter 7 for more about brain development.

Senescence

According to recent theories, age-related decline in specialized and even basic physiological functions is built into new genomes right at the start. Structures at the ends of the chromosomes called telomeres, which get shorter and shorter as a genome ages, control the number of times the genome can replicate. Gradually, cells lose the ability to divide. The number of aged and dying cells in a tissue eventually exceeds the number of new cells of their type being made to replace them. The tissue loses its ability to function, which impairs the organism’s survival.

The aging processes are an active area of research in anatomy and physiology. In recent decades, therapies and devices to counter aging’s effects have dominated the medical products marketplace worldwide.

Development before Birth

Human birth is a commonplace miracle: from a single infinitesimal cell to a human baby in less than ten months. The following sections give a brief overview of how it happens.

If you want more detailed information on pregnancy, check out Pregnancy For Dummies, 3rd Edition, by Joanne Stone, Keith Eddleman, and Mary Duenwald (Wiley).

Free-floating zygote to protected embryo

Chapter 14 covers the events leading to the fertilization of a secondary oocyte and implantation of the blastocyst in the uterus from the point of view of female reproductive anatomy and physiology. This section covers those same events from the zygote’s point of view, from the fusion of the haploid genomes of the parent gametes to implantation in the uterus (see Figure 15-1).

Illustration by Kathryn Born, MA

FIGURE 15-1: Early fetal development.

Beginning it all

Fertilization, which takes about a day, begins when a sperm penetrates a secondary oocyte (an egg). After a sperm binds with the receptors in the zona pellucida (see Figure 14-2), it uses enzymes in its acrosome to digest the egg’s protective layer. When the sperm has finally reached the cell membrane of the oocyte, it attaches to receptors there. This triggers two important events:

· The zona pellucida will harden, preventing another sperm from fusing with the egg’s actual cell membrane.

· The oocyte will restart meiosis II. As this happens, the sperm’s nucleus is allowed into the cell. This way, when the nucleus is reforming in telophase II, the DNA contribution of the sperm is incorporated. The cell, now officially considered a zygote, has completed fertilization and ready to begin its journey.

A dangerous journey

The zygote undergoes cleavage (mitotic division) immediately. Over the next few days, the daughter cells (called blastomeres) divide twice more, to a total of 16 blastomeres, all within the rigid wall of the zona pellucida with no increase in overall size.

The mass, now called a morula (mulberry-shaped), leaves the uterine tube and enters the uterine cavity. Cell division continues, still confined within the zona pellucida, and a cavity known as a blastocoel forms in the morula’s center. Around the sixth day after fertilization, the hollow structure, now called a blastocyst, “hatches” from the slowly eroded zona pellucida within the uterine cavity. The outer layer of blastocyst cells secretes an enzyme that facilitates implantation in the endometrium. Angiogenesis (building of blood vessels) begins in the uterus, and diffusion between mother and blastocyst begins. When this diffusion is established, implantation is complete and the pregnancy is established.

The new genome has survived a very dangerous stage of development. Biologists estimate that up to one-half of blastocysts fail to implant, and they die. But the new genome still has challenges ahead.

The embryonic stage

Weeks three through eight after implantation are called the embryonic stage. During these weeks, the embryo’s cells begin to differentiate and specialize.

The transition from blastocyst to embryo begins when the implanted blastocyst develops into a two-layer disc. The top layer of cells (epiblast) becomes the embryo and amniotic cavity; the lower layer of cells (hypoblast)becomes the yolk sac that nourishes the embryo. A narrow line of cells on the epiblast, called the primitive streak, signals gastrulation — cells migrate from the epiblast’s outer edges into the primitive streak and downward, creating a new, middle layer. By 14 days or so after fertilization, the embryo, now called a gastrula, has ectoderm, mesoderm, and endoderm layers — the very beginning of tissue formation.

The fetal stage

Following week eight and lasting until birth is the fetal stage. Growth and development occur rapidly during this time. The primary germ layers continue their development as the fetus becomes more recognizable as a baby. The ectoderm develops into the skin and nervous tissues, while the endoderm forms your inner tubes — the alimentary canal and the respiratory tract. The mesoderm develops into everything in between, including the bones and muscles.

The major milestones of fetal development are discussed later in the “Dividing development into trimesters” section.

Forming the placenta

Immediately after implantation, the blastocyst initiates the formation of the placenta, a special organ that exists only during pregnancy that’s made of the mother’s cells in the outer layers and the fetus’s cells in the inner layer. The placenta serves to support the sharing of physiological functions between the mother and the fetus: nourishment (provision of energy and nutrients), gas exchange (a fetus must take in oxygen and eliminate carbon dioxide before birth), and the elimination of metabolic waste. The placenta allows some substances to enter the fetal body and blocks others. It does a good job of delivering nutrients and maintaining fluid balance, but it’s permeable to alcohol, many drugs, and some toxic substances.

The placenta is a dark red disc of tissue about 9 inches (23 centimeters) in diameter and 1 inch (2.5 centimeters) thick in the center, and it weighs about a pound (roughly half a kilogram). It connects to the fetus by an umbilical cord of approximately 22 to 24 inches (56 to 61 centimeters) in length that contains two arteries and one vein. The placenta grows along with the fetus.

Nutrients and oxygen diffuse through the placenta, and the fetal blood picks them up and carries them through the umbilical cord. Then, the wastes that result from the fetus metabolizing the nutrients and oxygen are carried back out through the umbilical cord and diffused into the placenta. The mother’s blood picks up the wastes from the placenta, and her body excretes them. Geez, moms start cleaning up after their kids before they’re even born!

Both the fetus and the placenta are enclosed within the amniotic sac, a double-membrane structure filled with a fluid matrix called amniotic fluid. The fluid keeps the temperature constant for the developing fetus, allows for movement, and absorbs the shock from the mother’s movements.

Dividing development into trimesters

Officially, by convention, Day 1 of a pregnancy is the first day of the woman’s previous menstrual period. Obviously, she wasn’t pregnant on that day, nor for numerous days thereafter. But it’s easier to be sure about the start date of a menstrual period than about the day of ovulation or fertilization or implantation, so that’s the custom that doctors follow. Then, by convention, doctors count ahead 280 days to arrive at the due date, the date on which, if all pregnancies and all babies were alike, the birth would take place. The 280 days, usually expressed as 40 weeks, are the human gestational period (length of pregnancy). This period is divided, again by convention, into three trimesters, though nothing specific marks the transition from one to the next.

The following sections provide an overview of the development of a fetus’s organs through the three stages of pregnancy. See the “Prenatal Development” color plate in the center of the book to get an idea of what a developing fetus looks like.

The first trimester

All the body’s organs begin development in the first trimester. The cardiovascular system forms from small vessels in the placenta three weeks after fertilization. The heart begins to beat at this time as well.

During the second month, the organ systems continue to develop, and the limbs, fingers, and toes begin to form. The embryo starts to move at the end of the second month, although it’s still too small for the mother to feel its movements. Also during the second month, ears, eyes, and genitalia appear, and the embryo loses its tail and begins to look less like a sea horse and more like a human.

At the end of the first trimester, the fetus is about 4 inches long (10 centimeters) and weighs about an ounce (28 grams). The head is large, and hair has begun to grow. The intestines are inside the abdomen, and the urinary system (kidneys and bladder) starts to work.

If you’re counting weeks and feel you’re losing track, remember that the trimesters of pregnancy are measured from Day 1 of the mother’s last menstrual period. This date is around two weeks earlier than the date of fertilization. The embryonic stage is the second and third month of pregnancy.

The second trimester

The fetus, with all its systems in place, continues programmed development in the second trimester. Ultrasound imaging shows the skeleton, head details, and external genitalia. Bone begins to replace the cartilage that formed during the embryonic stage. At the end of the second trimester, the fetus is about 12 to 14 inches (30 to 36 centimeters) long and weighs about 3 pounds (1.4 kilograms).

The third trimester

The fetal development program speeds up in the third trimester. The fetus, with its systems developed, continues to grow in size. Subcutaneous fat is deposited, which serves as a critical energy reserve for brain and nervous system development.

Near the end of the third trimester, the fetus positions itself for birth, turns its head down, and aims for the exit. When the fetus’s head reaches the ischial spines of the pelvic bones (see Chapter 5), the fetus is said to be engagedfor birth. (See Figure 15-2.)

Illustration by Kathryn Born, MA

FIGURE 15-2: A fetus late in the third trimester.

The Human Life Span

Mammals have a general pattern of development from birth to senescence, and humans closely follow that pattern in most ways. In addition to following a typical sequence of events (live birth of dependent young, late development of the reproductive system, hairiness increasing with age, and so on), the mammalian pattern has some “rules” that tie the size of the animal with the pace of development. Generally, the larger the mammal (typical adult size range), the longer the development period. Humans are right where you would expect them to be on this curve.

Every species of mammal has a species-specific version of the pattern, of course, encoded in the species genome. The human species genome encodes a long infant dependency period and, for females, an extraordinary prolongation of life beyond her reproductive years.

Changes at birth

The timing of birth is a compromise between the anatomical needs of a large brain and those of a bipedal gait. The fetus is born a little earlier than would probably be ideal from the point of view of its development, but the pelvis of the adult female has grown narrower and less flexible to support a redistribution of weight and bipedal mobility. Evolution has favored a compromise: a period of a few weeks when the baby, though fully separate from the mother, is still developing in ways that other mammals have completed before birth. Evolution continues to support this compromise relentlessly.

The newborn undergoes a number of changes at birth to allow it to survive outside the womb and adapt to life in a new cold, dry, and terribly immediate environment.

· The first breath: The fetus exchanges oxygen and carbon dioxide through the placenta. At birth, the newborn’s lungs are not inflated and contain some amniotic fluid. Within about ten seconds after delivery, the newborn’s central nervous system reacts to the sudden change in temperature and environment by stimulating the first breath. The lungs inflate and begin working on their own as the fluid is absorbed by the lungs or coughed out.

· Thermoregulation: Almost as quickly as the lungs start functioning, temperature receptors on the newborn’s skin mediate the generation of metabolic heat by muscle action (shivering) and the burning of stores of brown fat.Very little brown fat is present in adults, but it’s prevalent in infants, generating heat by “burning” the lipids stored in it.

· Digestive system: The newborn’s digestive system starts to work in a limited way immediately after birth. A newborn can digest colostrum and breast milk. Even so, the digestive system can take several weeks to settle down to efficient functioning.

In the fetus, the liver acts as a storage site for sugar (glycogen) and iron. After birth, the liver begins to takes on its other functions. It begins breaking down waste products such as excess red blood cells.

· Urinary system: The fetus’s kidneys begin producing urine by the end of the first trimester of pregnancy. The newborn usually urinates within the first 24 hours. The capabilities of the kidneys increase sharply through the first two weeks after birth. The kidneys gradually become able to maintain the body’s fluid and electrolyte balance.

· Immunity: The immune system begins to develop in the fetus and continues to mature through the child’s first few years of life. The development of immunity is an active area of research; there is still much we don’t understand. Mothers pass their antibodies on through fetal circulation and when nursing. Breast milk also contains components shown to promote the development of an infant’s own immune system.

Infancy and childhood

The long human infancy and childhood is one of the wonders of the biological world.

All organ systems grow and develop in infancy almost as rapidly as during fetal development. (All except the reproductive system. See the upcoming “Adolescence” section.) The physical development milestones in the first year alone would take a full book this size to describe. Just looking at a year-old infant and then at the infant’s photos at birth shows you a lot.

Most infants double or triple their birth weight in the first year. Besides adding size to all tissues and organs, the new cells differentiate elaborately and add functionality, according to the individual’s genomic scheme. The baby’s skeleton changes size, proportion, and composition and takes on all the attributes of bipedalism. The mouth acquires the subtle muscle control to form words and kisses. The baby starts applying the characteristically human opposable thumb to everyday tasks (picking up toys and throwing them down again) in the second half of the first year. The brain grows in size and, just as important, the number and complexity of connections grow astronomically.

A toddler child (between ages 1 and 3) develops sphincter control. Social life begins. Vigorous play coordinates the development of the musculoskeletal and nervous systems. The mechanisms of homeostasis gradually strengthen.

Overall, from infancy to adolescence, the human child becomes bigger, stronger, and smarter every day. The child steadily gains control of the body on a conscious and physiological level. The child is typically fluent in at least one spoken language by age 6. A fully human degree of hypersociability is frequently evident in preadolescents (about age 10 to puberty).

And yet the juvenile’s dependency goes on and on. Although most children are weaned before age 2 and can walk fairly long distances by age 10, they are, for the most part, hopeless at obtaining food and shelter. Their caregivers do that for them.

Their physical and mental development is devoted, instead, to mastering the unique aspect of human life called “culture.” That takes these brilliant human children about 20 years of intense study. During the period of human evolution, the survival of any individual was dependent primarily on the survival of the individual’s kin group. Participating effectively in the culture has always been the best way for humans to increase the likelihood of their own survival and the survival of those who carry their genes.

Adolescence

One organ system doesn’t undergo much development in infancy and childhood: the reproductive system. It remains in a state of suspended animation from early in fetal development until puberty, the first part of the development stage called adolescence (adol- means “adult”).

During puberty, the reproductive systems of males and females emerge from the suspended state. This usually happens sometime between age 11 and 14 for girls; in boys, it begins a couple of years later. Puberty ends when the reproductive system is mature enough to produce viable gametes. (That is, reproduction becomes physically possible.) Hormones play a very large part in this complex process, and getting them all to function smoothly together typically takes a few years.

The hormones produced during adolescence cause physical and neurological changes in female and male bodies. The frequent hormone surges cause acne, among other miseries, and they often trigger emotionally uncomfortable mood swings. The brain’s “executive” functions (judgment, impulse control, and risk-assessment) are frequently impaired.

Growth spurts are common during puberty, and growth continues through adolescence. Bones lengthen, muscle mass increases, and all organs reach near-adult size. The primary and secondary sex organs grow and mature. Fat and muscle are redistributed. Adolescents can maintain high levels of physical activity, fueled by the output of millions of new mitochondria. Adolescent sleep-wake cycles may be strikingly different from those of children and adults.

Female puberty

In females, the ovarian and uterine cycles begin (see Chapter 14), which becomes evident when menstruation starts. After the female is ovulating, pregnancy is possible. The female breast develops during puberty.

Other changes that occur in females during puberty include growth of hair in the axillary (armpit) and pubic regions, and the development of the female fat distribution pattern: more on the hips, thighs, and breasts.

Male puberty

Hormones from the anterior pituitary in boys allow them to produce testosterone, and as a result, to begin to develop sperm regularly. Testosterone has certain effects, such as initiating the growth of facial and chest hair, building lean muscle, causing hair to develop in the axillary and pubic regions, and making hair on arms and legs dark and coarse. The vocal cords thicken and lengthen, which causes deepening of the voice. The penis and testes enlarge. Males develop broader shoulders and narrower hips than females.

ANATOMY AND CULTURE

Just about everything about human development is more or less “typical” for mammals: The young are born alive, ready to breathe air and suck milk from their mothers’ mammary glands. As for size, human infants are within the very wide range of normal for a mammalian infant (between a shrew and a whale). Some mammalian infants are ready to run with their herd at one day of age, but lots of mammal babies are pretty helpless.

Only one thing is very odd about human development (ontogeny), and that’s the exceptionally long duration of immaturity. Compared with any other known species, even humans’ closest evolutionary relatives, human babies take a long time to grow up. The evolution of this remarkable trait has been the subject of much scientific research and speculation at least since Darwin in the fields of anthropology, evolutionary biology, psychology, and genetics.

The consensus is that the long human childhood extends the brain’s development, giving it time to absorb the culture’s complexities. Humans are a hypersocial species, and learning culture is an absolute requirement for survival. The period between weaning and adulthood is strongly devoted to learning spoken language, nonverbal communication, and other aspects of culture. The very food that humans eat must be prepared within the social group.

Anatomy and culture have been developing together for a few million years. All organ systems have adapted to culture, and the human animal is fully hypersocial. Humans have never been independent of the social group.

Young adulthood

Typically, young adulthood is a time of good health and resilience. Many people complete their parents’ bid for evolutionary success during this stage. (That is, they become parents themselves.) Energy levels can remain high all through the 30s, 40s, and 50s for some people. However, after growth is completed, energy requirements decrease, and adults must decrease their caloric intake to avoid accumulating body fat that can put long-term stress on several organ systems.

The gradual physical decline of senescence actually begins during these years. Influenced by genetic and environmental factors, arteries begin to accumulate damage, slightly more bone is lost than is made, and the same thing happens with the structural proteins of the skin. Muscle mass declines slowly but not imperceptibly. Damage accumulates from repetitive injuries, bad habits, and bad genes.

Middle age

For most people, young adulthood ends sometime in the 40s or 50s. The cellular cycles that replace cells and repair tissues slow down. Loss of bone and muscle mass accelerates. However, most of the time, these losses aren’t critical and can be mitigated by medical therapies or lifestyle adjustments (diet, exercise, sunscreen, and so on).

For men, reproductive capacity diminishes. For women, it disappears altogether at menopause, usually around age 50. Production of some hormones diminishes, triggering anatomical and physiological changes great and small.

The brain, however, continues to develop, cognitively and in many other ways. Some recent brain research shows that an older brain thinks better about some things than a younger brain does, including making financial decisions, exercising social judgment (intuitive judgments about whom to trust), and recognizing categories. The older brain is better at seeing the proverbial forest and following the gist of an argument. These are typically the peak years of professional or occupational achievement. In addition, across all occupations and ethnicities, a sense of well-being peaks as people reach middle age.

An extraordinary aspect of human development is the length of this period. Human females commonly outlive their fertility by three decades — a third of the life span in a stage of life that almost no other animal experiences! Many researchers see a matrix of evolutionary cause-and-effect conjoining natural selection and cultural evolution in the very existence of your grandma.

Growing creaky

Life expectancy beyond the reproductive years is dependent to a great extent on genes.

The developments of senescence (growing old) are gradual and diffuse. Body parts don’t work quite as well as they used to, and they just keep getting worse. For some people, this is a brief stage of life after a long, healthy middle age. Others aren’t so lucky.

In particular, the immune system doesn’t work as well as it used to, and malformed cells that would have been immediately eliminated at age 35 now may evade immune surveillance and become cancerous.

Age-related changes in the large arteries, as well as cumulative damage to the smaller vessels, can bring about problems with blood pressure.

Inactivity and chronic overconsumption of calories have their worst effects now, in all the major systems.

Still, the brain continues to develop. Research has been shown repeatedly that, under the right conditions, the brain continues to produce new cells and make new connections among neurons in adults as old as 100.

Table 15-1 lists some of the common age-related changes to the body’s systems.

TABLE 15-1 Age-Related Changes to the Body’s Systems and Associated Health Implications

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.