Vaccari D.A., Strom P.F., Alleman J.E. Environmental Biology for Engineers and Scientists

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TABLE 8.3 Classiﬁcation of Living Members of Phylum Chordates

Subphylum Class Order Family Genus Species

Urochordata (tunicates; sea squirts and salps)

Cephalochordata (lancelets)

Vertebrata

Myxini (hagﬁsh (43 spp.)

Cephalaspidomorphi (jawless ﬁsh, e.g., lamprey)

Chondrichthyes (cartilagenous ﬁsh: sharks and rays, 850 spp.)

Bony ﬁsh (45 living orders, about 30,000 spp.)

Amphibia (3900 spp.)

Caudata (salamanders, 360 spp.)

Anura (frogs and toads, 21 families, 3450 spp.)

Reptilia (13 orders, including the extinct dinosaurs)

Testudines (turtles)

Squamata (lizards and snakes)

Crocodilia (crocodiles and alligators)

Aves (birds) (27 living orders)

Mammalia

Nonplacental mammals:

Monotremes (only 3 spp., including duck-billed platypus);

Marsupials (kangaroos, opossums, koalas, 260 spp.)

Placental mammals:

Insectivora (shrews, hedgehogs, moles, 390 spp.)

Macroscelidea (elephant shrews, 15 spp.)

Dermoptera (ﬂying lemurs, 2 spp.)

Chiroptera (bats, 986 spp.)

Scandentia (tree shrews, 16 spp.)

Xenarthra (anteaters, armadillos, sloths, 30 spp.)

Pholodota (pangolins, 7 spp.)

Lagomorpha (rabbits, hares, pikas, 69 spp.)

Rodentia (gnawing mammals: squirrels, rats, 1814 spp.)

Cetacea (whales, dolphins, porpoises, 79 spp.)

Carnivora (dogs, wolves, cats, bears, weasels, 240 spp.)

Pinnipedia (sea lions, seals, walruses, 34 spp.)

Tubulidentata (aardvark, 1 spp.)

Proboscidea (elephants, 2 spp.)

Hyracoidea (coneys, 7 spp.)

Sirenia (sea cows and manatees, 4 spp.)

Perissodactyla (odd-toed hoofed mammals:

horses, asses, zebras, tapirs, rhinoceroses, 17 spp.)

Artiodactyla (even-toed hoofed mammals: swine, camels, deer,

hippopotamuses, antelopes, cattle, sheep, goats, 211 spp.)

(Perissodactyla and Artiodactyla are called ungulates)

Primates (233 spp.):

prosimians (lemurs, tarsiers), monkeys, apes, humans

Great Apes

Homo

Homo sapiens

Homo habilis (extinct)

Pan (chimpanzees)

Gorilla

Lesser Apes

168

THE ANIMALS

surrounding seawater instead of controlling their internal condition. The lamprey is a

destructive eel-like parasite. It attaches itself to live ﬁsh by a suckerlike mouth and

uses sharp teeth to rasp at the ﬂesh of the victim. Accidental introduction of the sea

lamprey Petromyzon marinus to the Great Lakes in the nineteenth century led to devas-

tation of one commercial species of ﬁsh after another. Larvicides are applied to streams

where they breed. This, combined with ﬁsh stocking programs, are producing a recovery

of the ﬁsheries.

The cartilaginous ﬁsh, which include sharks and rays, surprisingly evolved from

boney ancestors. They track prey by smell and use an interesting lateral line system

of sensors along the side that detect vibrations and bioelectrical ﬁelds produced by all

animals. The electric ray can produce a low-voltage but high-current ﬁeld from large

organs alongside its head. The power output can be several kilowatts and can stun prey

or repel predators.

The boney ﬁsh include most of the familiar fresh- and saltwater ﬁsh. They have

developed ray ﬁns and a swim bladder, a gas-ﬁlled chamber below the spine that the

ﬁsh uses to control buoyancy. The importance of this can be seen by considering

the sharks, which lack a swim blad der and therefore must swim continuously to keep

from sinking to the bottom of the sea.

Among amphibians, the life cycle of frogs comes close to proving a famous biological

principle proposed in the nineteenth century which states that ontogeny (the development

of an individual from embryo to adult) recapitulates (repeats) phylogeny (the evolution-

ary development of a species). This principle, termed recapitulation, is invoked to

explain embryonic features that disappear in the adult, such as gill slits in human

embryos. As new features evolve, they are added to the embryonic development. But

the principle turns out to have too many exceptions to be generally valid. Evolution some-

times deletes characteristics, as well as adding them, so the ontogeny is not an accurate

record of the organism’sevolution. What we observe in frogs is the development of an

aquatic species into a terrestrial species. For example, all frogs hatch from the egg resem-

bling ﬁsh, having gills, a tail, and no limbs. This early stage is called the tadpole.

Gradually, the tadpoles sprout limbs, grow lungs and eyelids, and the tail shortens,

until the adult frog is ﬁnished.

The most abundant frog genus is probably Rana. All frogs, and most adult amphibians,

are carnivores. They feed on anything that moves and that is small enough to swallow

whole. Some amp hibians, such as some salamanders, remain aquatic; others do not

have an aquatic stage. However, all have thin water-permeable skins that require moist

conditions. This may make them among the terrestrial animals that are most vulnerable

to environmental problems. There is considerable anecdotal evidence today that frog

populations in many areas are in serious decline. The cause is a mystery, but some of

the factors that have been blamed include habitat destruction, acid rain, pesticides, and

damage from increased ultraviolet radiation due to destruction of stratospheric ozone.

Reptiles show numerous evolutionary developments. Many of these developments

freed reptiles from dependence on moist conditions. Their dry skin limits evaporative

loss. Internal fertilization and shelled, waterproof eggs eliminates the need for water for

reproduction. In turtles and crocodilians the temperature of incubation of the eggs deter-

mines the sex ratio. More efﬁcient respiration allows greater size, up to the 115-kg

Komodo dragon. Reptile jaws are capable of applying crushing force. They have higher

blood pressure and more efﬁcient circulation. Their lungs are more efﬁcient. Whereas

amphibians force air into their lungs with mouth muscles, reptiles developed the ability

DEUTEROSTOMES (STARFISH, VERTEBRATES, ETC.) 169

to suck air in by expanding the rib cage. They excrete nitrogen as insoluble uric acid,

further conserving water. The nervous system of reptiles is much more complex than

that of amphibians . Crocodilians developed a true cerebral cortex. Plac ement of the

legs below the body instead of at the sides allows better support and motion.

Birds (class Aves) have many distinguishing characteristics other than the obvious ones

of having feathers and forelimbs developed into wings. Their necks are disp roportionately

long. Their skeleton contains air cavities, making them strong, yet light. Their beaks lack

teeth. They have a four-chambered heart and are warm-blooded. They excrete nitrogenous

wastes as uric acid. Fertilization is internal and they produce eggs with a large amount of

yolk. Some birds feed on insects; other invertebrates, such as worms, mollusks, and

crustaceans; and vertebrates. About one-ﬁfth feed on nectar. Many eat seeds. The beak

is specialized for the type of feeding behavior. Bird behavior is highly developed, includ-

ing complex activities such as nest building and social activity. The farther from the

equator one gets, the higher the percentage of birds that migrate for the winter. The

Arctic tern breeds above the Arctic Circle and then migrates to the Antarctic region!

Day length seems to be the factor that stimulates migration. Birds can navigate using

the sun and stars as cues. Amazingly, it has been proven that they can also detect Earth’s

magnetic ﬁeld.

Birds are very sensitive to the use of pesticides in the environment. Some pesticides

tend to biomagnify, increasing in concentr ation as they are passed up the food chain.

This has had the greatest impact on birds of prey, such as eagles and osprey. Other

contaminants that affect birds include lead shot discharged by hunters in wetlands. Water-

fowl ingest them, resulting in lead poisoning. The loss of wintering lands in tropical

regions may be a cause of the reduced songbird populations that have occurred in the

last 40 years.

The classiﬁcation of birds and reptiles may soon change. This is because the common

ancestor of all reptiles is also the ancestor of the birds. Thus, they should be classiﬁed

together, or perhaps distinguished as feathered or featherless reptiles. Birds developed

from certain dinosaurs. Their closest relatives today are the crocodilians. For the present,

however, the traditional classiﬁcation is retained.

The main distinguishing characteristic of mammals is the presence of hair or fur and

the production of milk. Hair or fur provides insu lation, giving greater advantage to the

warm-blooded metabolism. It has other functions as well, such as camouﬂage, water-

prooﬁng, and sensing (as by whiskers). Hair is produced from dead cells that leave a

ﬁbrous protein called keratin, the same substance that forms nails, claws, hooves, and

feathers. Mammalian skin contain s a variety of glands, including sweat , scent, and mam-

mary glands. The latter produce the milk to nurture the young. Herbivorous animals

harbor anaerobic bacteria and protozoans in a stomach chamber to ferment cellulose

into sugars, fatty acids, and starches. No vertebrates are able to use cellulose on their

own. Carnivores feed mostly on herbivores. The need for hunting behavior has selected

for intelligence in these animals. The herbivores, on the other hand, have developed keen

senses as protection against predators.

Most mammals have mating seasons, timed to produce young at a favorable season for

rearing. Mating is limited by the female, who is receptive to mating only during a brief

period in the mating season known as estrus. Old World monkeys and humans have a

different cycle, the menstrual cycle. Mammals exhibit three ways of giving birth. Mono-

tremes, such as the duck-billed platypus, is a mammal that lays eggs. Animals that lay

170 THE ANIMALS

eggs are called oviparous. The platypus still nurses its young with milk, however.

Marsupials are viviparous; that is, they give live birth. However, the newborns are

essentially still embryos. For example, the red kangaroo spends about a month in the

uterus. When it is born, it climbs by itself through the mother’s fur to a pouch, where it

attaches itself to a teat to suckle for 235 days until it is capable of living independently.

The third way to give birth is that of the placental mammals, which comprise 94% of

all mammals. Gestation in utero is prolonged. The embryo is nourished by a placenta,a

membrane structure produced by and surrounding the embryo. The placenta grows thou-

sands of tiny ﬁngerlike projections called villi into the lining of the mother’s uterus to

absorb nutrients and oxygen from the maternal blood supply without there being an actual

exchange of maternal and fetal blood. The fetus is connected to the placenta by the umbi-

lical cord. Once born, the mammal may be more or less dependent on parents for a time.

Humans are members of the primate order. The ﬁrst prima te fossils date back about

40 million years. About 8 million years ago, forests in eastern Africa gave way to savan-

nas, giving an advantage to apes that could stand upright. But it was 4 million years ago

that the ﬁrst foss ils that could be called hominids (humanlike) appeared. This species has

been named Australopithecus afarensis. One of the best such fossils, discovered in 1974,

is of a female that has been named ‘‘Lucy.’’ It had a brain the size of a chimpanzee’s,

about 450 mL. The ﬁrst species of our genus was Homo habilis, which lived from 2 million

years ago until about 1.5 million years ago. This species used stone and bone tools and had

a brain size of about 640 mL. This was followed by Homo erectus, which had a brain size

about 1000 mL and lived in social groups. Homo erectus disappeared about 300,000 years

ago and was replaced by our species, Homo sapiens. About 130,000 years ago a subspe-

cies of Homo sapiens appeared, called the Neanderthals. They had a brain size within the

range of modern humans, about 1100 to 1700 mL.

About 30,000 years ago Neanderthal man was replaced by a new subspecies, Homo

sapiens sapiens—us! These ancestors of ours were distinct from the Neanderthals in a

number of ways. Although Neanderthals made stone blades, those of sapiens were of

much higher quality. Sapiens buried their dead with ritual objects, suggesting a belief

in an afterlife. Finally, their vocal apparatus and related portions of the brain show

increased development. This suggests that early humans may have been capable of

language. Language capability confers the ability to do abstract reasoning. Thus, language

is the basis of intellect, the one thing that most decisively distinguishes us from the other

animals. It is interesting to realize that modern humans coexisted with a distinct sub-

species not long before the beginning of recorded history. Many suspect that Neanderthal

man was wiped out in conﬂicts with our ancestors. Others think that the two groups

merged by interbreeding.

Some interesting information on our more recent origins has been obtained from

studies of mitochondrial DNA. Although we inherit the DNA in our cells’ nuclei from

both parents, there is also DNA in our mitochondria that we inherit from our mother

alone. Mitochondrial DNA can be used to trace genetic heritage because it is not

confounded by recombination and the mixing due to sexual reproduction. The diversity

in mitochondrial DNA from different ethnic populations has been compared with the

divergence that would have occurred due to the rate of mutation from natural effects.

From this it has been possible to establish that if the assumptions used in the analysis

are reasonable, all humans in the world today are descendents of a single individual

that lived in Africa 200,000 years ago.

DEUTEROSTOMES (STARFISH, VERTEBRATES, ETC.) 171

PROBLEMS

8.1. Which phyla consist of organisms with no specialized excretory organs and therefore

would be most susceptible to toxic water pollutants? What other mechanisms would

these organisms have for excreting toxins?

8.2. The most common type of organism on land is virtually absent from the sea, and vice

versa. What are these two groups, and what do they have in common?

8.3. What class of vertebrates, other than the ﬁsh, are most susceptible to water pollution,

and why?

REFERENCE

Hickman, C. P., Jr., L. S. Roberts, and Allan Larson, 1997. Integrated Principles of Zoology, Wm. C.

Brown, Dubuque, IA.

172

THE ANIMALS

THE HUMAN ANIMAL

Now we begin a detailed examination of the physiology and anatomy of the human ani-

mal. Homo sapiens has adapted itself for life under the widest conditions of any animal

species, from the equator to the poles, from the bottom of the ocean to the top of the atmo-

sphere and beyond. The phenomenal success of this animal has not been without negative

effects on other organisms. Its activities have resulted in extinction of other species and

loss of habitat for many more. It may be the only species that by itself is responsible for

global changes in the environment. It produces chemicals that deplete stratospheric ozone,

with effects that are yet to be well understood. Its activities produce CO

fast enough to

result in a doubling of the atmospheric concentration by the mid-twenty-ﬁrst century, and

which is expected to alter Earth’s climate. Homo sapiens has turned a good portion of the

world’s biological productivity toward its own use, but it is at risk of depleting its own

resources. It has the greatest ability to plan of any species by far, including the capability

of controlling its own resource consumption to prevent the risk of depletion. However, it

remains to be seen whether it will actually do so to prevent catastrophe in the future.

We will study the biological functioning of the human body, stopping short of human

behavior, which is beyond the scope of this book. We focus in turn on each of the 11 major

organ systems. An understanding of the structure and function of the human body will

help in understanding the effect of toxins.

The cells in the body are differentiated into about 200 different kinds. These are

classed into four basic types, which also correspond to the four basic types of tissues:

1. Muscle cells and tissues generate mechanical force and movement. They are

derived from the mesoderm. There are only three types of muscle tissue: smooth, skeletal,

and cardiac.

Environmental Biology for Engineers and Scientists, by David A. Vaccari, Peter F. Strom, and James E. Alleman

173

2. Nerve cells and tissues produce and conduct electri cal signals over large distances

within the organism and at great speed. They can stimulate response from other cells, such

as in muscles or glands. They are derived from the ectoderm.

3. Epithelial cells and tissues, or epithelium, form a lining over the organs, line body

cavities such as the inside of the intestines or the lungs, and cover the organism as a

whole, as the outer part of the skin. Thus, almost all absorption of pollutants must ﬁrst

pass through the epithelium. In keeping with this role, its cells are specialized to be

selective in their absorption and secretion of ions and organic compounds, enabling them

to act as a barrier to the passage of chemicals into and within the body. Epit helium is

formed from ectoderm, mesoderm, and endoderm. Epithelial tissue never has its own

blood supply; instead, it receives nutrients by diffusion from the underlying connective

tissue. Cells that secrete subst ances such as hormones or digestive enzymes are epithelial

cells called gland cells.

4. Connective cells and tissues connect and support parts of the body. Examples

include bones, cartilage, fat storage cel ls, blood cells, and lymph ﬂuid. Connective tissue

includes a number of cells that respond to tissue injury. These include a variety of

leukocytes, or white blood cells, which prot ect against infection, and mast cells, which

release the compounds histamine and heparin, which are responsible for the inﬂammation

response. Connective tissue underlies most epithelial layers. Connective cells are often

embedded in a large amount of extracellular material. For example, bone cells are

surrounded by a crystalline matrix. Tendons and ligaments consist almost completely of

collagen, a tough, ropelike ﬁbrous protein that resists stretching. Some ligaments have a

stretchy ﬁber called elastic ﬁber, which is made up of the protein elastin.

The organs themselves are made of combinations of different kinds of the four basic

types of tissues. In a typical example, blood vessels have an inner lining of epithelium,

surrounded by a layer of connective tissue, and then by another layer of muscle tissue. The

muscle layer controls the movement of blood by constricting or dilating. For example,

when the body is cold, the muscle layer surrounding the blood vessels near the skin con-

stricts, reducing the transport of heat to the skin surface. Blushing occurs when the oppo-

site happens: The capillary muscles of facial skin dilate (open wi der), bringing more

blood to the surface. Adipose tissues are the fatty deposits below the skin. They can be

an important storage site for toxins.

9.1 SKIN

The integumentary system consists of the skin and associated structures, such as hair,

nails, and glands. The skin is one of the body’s main protections against toxins in the

environment. Looking at this from a different point of view, the skin is also an important

route of exposure for many toxins. The skin consists of two main layers, the dermis and

the epidermis (Figure 9.1). The dermis, the inner layer, is made up primarily of connec-

tive tissue, plus sweat glands, hair follicles, nerve endings, blood vessels, and the small

muscles that make your hair ‘‘stand up on end.’’ The dermis contains a large amount of

intracellular collagen ﬁbers and elastic ﬁbers. The elastic ﬁbers give the skin the ability to

stretch. The collagen ﬁbers limit the stretching and give skin strengt h.

The epidermis, the outer layer, is about 1 mm thick. The epidermis does not have its

own blood supply; it is supplied from capillaries in the dermis. The inne rmost part of the

epidermis is the basal cell layer, a single layer of cells that replicate to replace lost

174 THE HUMAN ANIMAL

epidermal cells. These new cells push up as other new cells are produced below. Once

they are several cells away from the basal cell layer, they start producing large amounts

of keratin, a tough ﬁbrous protein, and the cells gradually die off and dehydrate. This

process produces the outermost layer of the epidermis, called the stratum corneum.

This layer consists of dead epidermis cel ls ﬁlled with keratin and sandwiched between

phospholipid membranes. These cells are sloughed off continuously and regenerated by

the epidermis. It takes about 15 to 30 days for a cell to go from the basal layer to the

stratum corneum, and another 14 before it is shed.

The stratum corneum is resistant to water and other substances, although pores in the

skin provide a route for substanc es such as drugs or toxins to enter. The solvent dimethyl

sulfoxide (DMSO) easily crosses the skin and enters the circulation. Substances that other-

wise do not easily cross the skin may do so if dissolved in DMSO. Melanin is the dark

pigment in the skin of black people and of Caucasians with substantial exposure to sun-

light (a tan). It is produced by cells in the basal layer called melanocytes. The melanin

protects skin by absorbing ultraviolet light from the sun. Ultraviolet radiation from sun-

light is a common cause of skin cancer.

9.2 SKELETAL SYSTEM

The skeletal system consists of the bones, cartilage, and joints. Its functions are to pro-

vide support, protection, movement, mineral storage, and the production of red and white

Figure 9.1 Layers of the skin. (From Van de Graaff and Rhees, 1997. # The McGraw-Hill

Companies, Inc. Used with permission.)

SKELETAL SYSTEM 175

blood cells. Of environmental interest, the bones may be a signiﬁcant storage site for toxic

divalent heavy metals such as lead, radium, or strontium.

Bone is a living tissue, although most of its mass consists of calcium salt crystals

and collagen ﬁbers. Almost two-thirds of the mass of the bone is hydroxyapatite,

(PO

)

(OH)

, plus sodium, magnesium, and ﬂuoride salts. About one-third of bone

mass is made up of collagen ﬁbers. Living cells make up only 2% of the bone’s mass.

The cells include blood vessels and bone cells, called osteocytes. The combination of

crystalline hardness and collagen toughness give bone structural properties comparable

to high-quality steel-reinforced concrete. Bone structure often consists of a solid compact

bone exterior, with a strong, yet light spongy bone layer inside. Many bones have an

internal cavity containing marrow, which is the bone tissue that produces red and

white blood cells and that stores fat reserves.

Bones are dynamic tissues. Some osteocytes, called osteoblasts, produce new bone;

others, called osteoclasts, dissolve bone material. For example, when a long bone

grows in diameter, the osteoblasts produce bone on the outside while osteoclasts dissolve

bone on the intererior to enlarge the marrow cavity. Physical stress, such as from exercise,

causes bones to grow at the point of attachment of the muscle tendons. (Exception: In

children, excessive strenuous activity can damage sites of bone lengthening and result

in shortened stature.) Baseball pitchers and tennis play ers have 35 to 5 0% more bone

in their playing arm than in the other arm. Inactivity, on the other hand, can cause a

loss of bone mass. Weightlessness causes astronauts to lose signiﬁcant bone mass even

if they exercise regularly. Older women are vulnerable to bone loss due to hormonal

changes, a condition called osteoporosis. However, this problem can be at least partially

forestalled by physical activity and/or hormonal therapy.

Humans have about 206 bones. The exact number varies because a variable number

fuse together during development. The head contains 29 bones: 22 facial bones, 8 cranial

bones, 6 bones in the ears, plus the hyoid bone. The latter is the only bone in the body not

connected to the skeleton. It is located just under and behind the tongue, and is involved in

swallowing. The spine consists of 26 bones: 24 are vertebrae, divided into the cervical,

thoracic, and lumbar regions. At the bottom of the spine are the sacrum, which connects to

the pelvis and is formed from ﬁve fused bones, and the coccyx, a vestigial tail formed

from three to ﬁve fused bones. The rib cage plus the sternum makes another 25 bones.

The upper and lower extremities plus the pelvic and pectoral girdles makes up another

126.

Cartilage is a clear, glassy mater ial composed of cartilage cells surrounde d by a mesh

of collagen ﬁbers. It lines the moving surfaces of the skeletal joints. Cartilage has few

blood vessels, a reason that injury to it is slow to heal. Tendons are bands of dense tissues

containing parallel ﬁbers of collagen that connect muscles to bones. Ligaments are like

tendons, but form connections from one bone to another. Some ligaments, such as those in

the spine, contain more elastic ﬁbers than collagen, making them springy.

9.3 MUSCULAR SYSTEM

Muscles are organs containing cells, called muscle ﬁbers, which are specialized to con-

tract. There are three types of muscle tissue. Cardiac muscle is present only in the heart.

Smooth muscle is responsible for involuntary actions, such as the contractions that move

food through the digestive system. It is also the type of muscle that surrounds the blood

176 THE HUMAN ANIMAL

vessels, controlling their contraction and dilation. Both of these types of muscle make up

about 3% of the body. However, the great majority of the body’s muscle is of the third

type: skeletal muscle, which makes up about 40% of the body’s weight. We focus on this

type of muscle.

Each skeletal muscle is made up of muscle ﬁbers, each one a single cell that can

extend the length of the muscle (up to 20 cm) and 10 to 100 mm in diameter. Each of

these giant cells has hundreds of nuclei. Within each muscle ﬁber are many parallel bun-

dles called myoﬁbrils, which run the length of the cell. Finally, within each myoﬁbril are

numerous thick ﬁlaments composed of bundles of the protein myosin, and thin ﬁlaments

made of actin and other proteins. Muscle contraction is caused by the sliding ﬁlament

mechanism, in which the myosin ﬁlaments pull on the actin ﬁlaments like a person pull-

ing on a rope.

The ATP for muscle contractions can come from four sources: aerobic oxidation, or

glycolysis with fermentation to lactic acid, creatine phosphorylation, and free ATP.

Under resting conditions, muscle cells produce ATP from the oxidation of fatty acids

via aerobic respiration. Because it requires oxygen, this mechanism can provide energy

only as fast as the required amount of oxygen can be provided to the muscle by the blood-

stream. Although this is most efﬁcient, aerobic respiration does not provide ATP fast

enough for continuous contractions such as those during athletic performances. It can, how-

ever, support moderate continuous activity such as hiking or, literally, ‘‘aerobic’’ exercise.

Recall from Section 5.4.1 that glycolysis, which occurs in the cytoplasm, can produce a

few ATPs from a partial oxidation of glucose to pyruvate. A fermentation step then con-

verts the pyruvate to lactic acid. The glucose itself is stored in the muscle in the form of

glycogen. Glycolysis provides enough energy for vigorous activity lasting 1 to 3 minutes,

such as an 800-m run. It has the advantage that it does not require oxygen. However, the

lactic acid accumulates in the muscle ﬁber, reducing the pH and disrupting the cell’s activ-

ity. This is the ‘‘burn’’ felt after heavy exercise that forces us to rest to recover from the

exertion. Muscle cells cannot oxidize the lactic acid. They must release it to the blood-

stream, where it will reach the liver. Only the liver can eliminate lactic acid. The liver will

gradually oxidize about 30% via respiration and use the resulting ATP to convert the rest

of the lactic acid back to glucose. Thus, the rate of oxygen utilization is elevated for a time

after the exertion stops, to repay the accumulated oxygen debt .

For very brief, explosive activity, such as a single leap or a throw, the muscle cell has

two immediate sources of ATP. One is the small amount of free ATP always p resent in

cells. This can support about 2 seconds of contractions. The second immediate source is a

high-energy compound called creatine phosphate (CP). The muscle ﬁbers store about ﬁve

times as much CP as they do ATP.

An analogy for these sources of energy is in sources of money. Free ATP is like change

in the pocket. It’s ready in an instant, but it doesn’t last long. If you take a moment more,

you can ﬁsh for paper money in your wallet or purse. This is like the creatine phosphate.

Glycolysis is like using a credit card. It lasts longer but produces uncomfortable side

effects (ﬁnancial debt, in this case), which must be paid back. Finally, the reactions of

respiration are like getting a job to obtain money. It comes in slow and steady, although

not fast enough to sustain a marathon shopping spree. To extend the analogy, fats can be

compared to ownership of stocks. Although they can be a signiﬁcant reserve, they must be

converted to cash (liquidated, in the ﬁnancial parlance) in order to be useful.

The muscles contain sensors that provide information to the central nervous system

about the state of contraction of the muscles. This gives us a sense of body position and

MUSCULAR SYSTEM 177