Layman D. Biology Demystified: A Self-Teaching Guide

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The urocyst empties into the urethra (you-REETH-rah), the tube that

helps a person literally ‘‘make water’’ (urethr) – that is, urinate (YUR-ih-

nayt). Surrounding the upper neck of the urethra is the urinary sphincter.

Much like the external anal sphincter in the digestive pathway (Chapter 19),

the urinary sphincter is a ring of voluntary striated muscle. This means, of

course, that the contraction and relaxation of this sphincter is under our

voluntary control. Thus, after we have been adequately ‘‘potty-trained’’ dur-

ing early childhood, we can voluntarily relax the urinary sphincter whenever

the place and time are right for urination!

Finally, urine exits out of the body through the urinary oriﬁce (OR-ih-ﬁs),

a tiny, ‘‘mouth’’ (or)-like opening.

The Process of (‘‘Making Water’’) or Excreting

Urine

We have, from time to time, been using some very basic descriptive equations

in this book. Let us do so, again. We can state the urinary excretion equation,

for example:

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Fig. 20.2 The urinary pathway.

E ¼ F  R þ S

(This equation is put into visual form within Figure 20.3.)

In this equation, E stands for ‘‘excretion,’’ F for amount ﬁltered, R for the

amount of tubular reabsorption, and S for tubular secretion. The urinary

ﬁltrate (FIL-trayt), or ﬁltration product, comes from the pushing force of

the blood pressure against the walls of the renal capillaries in the glomeruli

(glah-MEHR-you-lie). This quantity of ﬁltrate is huge, averaging about 180

liters of ﬂuid per day, in an average adult! [Study suggestion: Assume that an

adult has a blood volume of about 6 liters. Then, on average, how many

times is this person’s entire blood volume ﬁltered out of his glomeruli, each

day?]

While it may seem wasteful, the huge volume of urinary ﬁltrate (F) acts as

the starting point for the urine. Because there is so much of this ﬁltrate, the

body can adjust many factors to inﬂuence how much urine is actually

excreted, under particular current conditions.

After urinary ﬁltration, one of the chief processes is tubular reabsorption

(R). Reabsorption is the movement of material out of the ﬁltrate, across the

walls of the urinary tubules, and back into the bloodstream. Consider, for

example, the tubular reabsorption of glucose. Under normal conditions,

almost 100% of the glucose that is ﬁltered into the urinary tubules is even-

tually reabsorbed back into the bloodstream. As a result, the urine excreted

from the body is nearly free of glucose. Several hormones also control the

amount of sodium (Na

ions) and water (H

O) molecules that are reab-

sorbed back into the bloodstream. Because the amount of sodium (salt)

and water reabsorbed can vary greatly, the kidneys play a critical role in

regulating the salt–water balance of the human bloodstream.

Under typical conditions, about 99%, or 179 liters, of the urinary ﬁltrate

(mostly water) is reabsorbed. [Study suggestion: If a person becomes extre-

mely dehydrated, as after excessive sweating, then what do you predict will

happen to the amount of H

O reabsorbed? Will the percent (%) reabsorbed

increase above typical conditions, or decrease below it? Why?]

Another process, tubular secretion (S), involves the active (ATP-requiring)

addition of small quantities of particular chemicals from the bloodstream,

into the urinary tubules. Molecules of penicillin (pen-ih-SILL-in) and many

other antibiotics (an-tih-buy-AH-ticks), for instance, are just too large to be

ﬁltered across the walls of the glomeruli. Hence, the epithelial cells lining the

blood vessels actively pump the penicillin into the urinary tubules. Therefore,

penicillin is excreted (E) out of the body, via the urine. Although only a few

milliliters (ml) of ﬂuid are generally secreted each day, they still have an

important inﬂuence.

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Summarizing our previous urinary excretion equation and plugging in

some numbers, we get:

E ¼ F  R þ S

1 L/day of urine excreted ¼ 180 L=day  179 L=day þ (a few ml/day)

Reproduction in Animals: Sexual or Asexual?

Earlier chapters have talked about sexual reproduction occurring in animal

organisms and in non-animal organisms such as in the World of Plants

(Chapter 9). Even in reproducing pine trees, there are sperm cells from the

male (pollen) cones that fertilize egg cells (ova) from the female (seed) cones,

thereby resulting in a zygote. The lowly fungi (Chapter 8) were likewise

shown to engage in sexual reproduction – the fusion of a male and female

gamete (sex cell) to create a zygote. And this zygote in turn develops into an

embryo, within many diﬀerent types of organisms.

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Fig. 20.3 Processes involved in excretion of urine.

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ASEXUAL REPRODUCTION IN ANIMALS

But we also need to remember that asexual reproduction is likewise common!

In this reproductive strategy, there are no gametes or sex cells involved.

Asexual reproduction, you may remember, is important in the spreading of

the black bread mold, Rhizopus. (Review Figure 8.4, if desired.)

Certain animals (living organisms that are neither plants nor fungi) also

use asexual means to reproduce themselves. Not surprisingly, the animals

engaging in asexual reproduction are generally among the most primitive.

Consider, for example, the invertebrates (Chapter 10). Certain types of star-

ﬁsh reproduce asexually by means of fragmentation (breaking of their body

into fragments), followed by regeneration (the re-growing of lost body parts).

When such a starﬁsh has one of its arms fragmented (broken oﬀ), an exten-

sive series of mitoses (my-TOH-seez) or cell divisions occur within the broken

arm fragment. One such broken arm can thus asexually reproduce (via mito-

sis) a whole new starﬁsh!

Sea anemones (also mentioned in Chapter 10) can asexually reproduce by

means of ﬁssion (FIH-shun) – ‘‘the process of ’’ (-ion) ‘‘splitting or cleaving’’

(ﬁss). In this case, one approximately round sea anemone stretches into two

identical individuals, creating a type of dumbbell-shaped pattern. The two

duplicate individual sea anemones then split or cleave apart.

Finally, some invertebrates such as tunicates asexually reproduce by bud-

ding. A new individual simply grows as a small bud oﬀ the body of a parent.

When the growing bud becomes large enough, it separates from the parent

body.

Basically, all of the above examples of asexual reproduction have two main

features in common:

1. The major mechanism for reproduction is simply an extensive series of

mitoses (cell divisions involving no reduction in the number of

chromosomes present).

2. A group of clones (KLOHNS) – identical copies of a single parent

organism – is created. Like a branch or ‘‘twig’’ (clon) growing from the

trunk of the same tree, a clone has an exact copy of the genes and

chromosomes of its parent.

EXTERNAL FERTILIZATION: SEXUAL REPRODUCTION IN

LOWER VERTEBRATES

In animals engaging in sexual reproduction, the mature ovum of the female is

fertilized by a sperm cell from the male. In certain types of lower (more

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primitive) vertebrates, such as ﬁsh and amphibians, there is external fertiliza-

tion of the ovum ‘‘outside of ’’ the body of the female.

Consider two mating frogs. The larger female frog is mounted by a smaller

male, who clasps her body with his forelimbs. This helps stimulate the female

to release a mass of over 100 eggs into the pond water! Nearly simulta-

neously, the male frog releases a jet of sperm cells, which fertilize many of

the ova externally, right in the surrounding water. After a time of develop-

ment, the fertilized ova develop into swimming tadpoles.

INTERNAL FERTILIZATION: SEXUAL REPRODUCTION IN

HIGHER VERTEBRATES

When one examines the mating of higher vertebrates such as mammals,

sexual reproduction generally occurs via internal reproduction. In this type

of reprodution, the ovum is fertilized by mature sperm cells that have tra-

veled ‘‘within’’ (intern) the female body.

Reproductive System of the Male Mammal

In order to fully understand internal fertilization, we must ﬁrst study the

reproductive system of the male and female mammal (in particular, of

human beings).

Figure 20.4 reveals the basic anatomy of the reproductive system in the

human male. A scrotum (SKROH-tum) or ‘‘leathery bag of skin’’ (scrot)

suspends the two testes (TES-teez) outside of the abdominal cavity. Each

testis (TES-tis) is a rather oval, whitish, ‘‘eggshell’’ (test)-like structure that

contains the seminiferous (sem-ih-NIF-er-us) tubules. The seminiferous

tubules are a collection of tiny, highly coiled tubes that carry out the process

of spermatogenesis (sper-mat-uh-JEN-eh-sis) – the ‘‘production of ’’ (-genesis)

‘‘sperm’’ (spermat) cells.

From the time of puberty (age 12–13 years) onward, mature sperm cells

are continually produced by a germinal (JER-muh-nal) epithelium, which is

located in the thick walls of the seminiferous tubules. This germinal epithe-

lium undergoes a constant process of ‘‘sprouting’’ (germin) new sperm cells

by mitosis, followed by meiosis.

You may recall (Chapter 9) that meiosis is literally ‘‘a conditioning of

lessening.’’ This implies that division of a parent cell by meiosis lessens or

reduces the number of chromosomes in each resulting daughter cell by one-

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half. In the human male, each primitive sperm cell contains 46 chromosomes

within its nucleus. But after meiosis, the developing sperm cell has this

reduced by half, to a total of just 23 chromosomes. Eventually, a mature

spermatozoon (sper-mat-uh-ZOH-un) or ‘‘seed’’ (sperm) ‘‘animal’’ (-zoon)

with only 23 chromosomes results.

Thousands of mature spermatozoa (sper-mah-tah-ZOH-ah) or ‘‘seed ani-

mals’’ leave the germinal epithelium of the seminiferous tubules, and are

temporarily stored within the epididymis (eh-pih-DID-ih-mus). The epididy-

mis is a curved, comma-shaped pouch that literally lies ‘‘upon’’ (epi-) each

testis or ‘‘eggshell’’ (didym). The thousands of spermatozoa are ejected from

the epididymis during each ejaculation (ih-JACK-yuh-lay-shun). Ejaculation

is literally the ‘‘throwing out’’ (ejacul) of semen (SEE-mun) and spermatozoa

from the urinary oriﬁce at the tip of the penis.

ACCESSORY REPRODUCTIVE ORGANS IN THE MALE

The semen is a thick, milky, sugar-rich, very basic ﬂuid that suspends the

spermatozoa and gives them nutrition. When the male has an orgasm (OR-

gaz-um), or is literally ‘‘swollen and excited,’’ he ejaculates spermatozoa

suspended in a ﬂuid of semen. The stored spermatozoa are actively sucked

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Fig. 20.4 The male reproductive pathway.

out of the epididymis by strong peristalsis (ring-like muscular contractions)

of the walls of the vas (vas) deferens (DEF-er-enz).

The vas deferens, or ‘‘carrying away’’ (deferens) ‘‘vessel’’ (vas), is not

actually a blood vessel at all. More appropriately, it is alternately called

the ductus (DUCK-tus) deferens or ‘‘carrying away duct.’’ During male

orgasm, the walls of the vas deferens (ductus deferens) powerfully and rhyth-

mically constrict or narrow. This negative pressure (suction) event draws the

stored spermatozoa out of the epididymis, carrying them over the top of the

urinary bladder and out into the urethra.

Semen is added to the spermatozoa from a number of accessory male

reproductive organs. These accessory male reproductive organs include the

two seminal (SEM-ih-nal) vesicles, the two bulbourethral (BUL-boh-you-

REE-thral) glands, and the single prostate (PRAH-state) gland.

Reproductive System of the Female Mammal

In order to achieve internal fertilization, the penis of the male must be

inserted into the vagina (vah-JEYE-nah), a tapered ‘‘sheath’’ (vagin) that

also serves as the birth canal. The vagina leads into the uterus or womb

through a tiny hole in its neck-like cervix (SIR-viks) (see Figure 20.5).

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Fig. 20.5 The female reproductive pathway and internal fertilization.

Attached to the top of the uterus are the right and left oviducts (OH-vih-

ducts) or ‘‘egg ducts.’’ The oviducts are alternately called the Fallopian (fah-

LOH-pea-un) tubes in honor of their discoverer, the Italian anatomist,

Gabriello Fallopio (fah-LOH-pea-oh). The oviducts (fallopian tubes) are a

pair of slender egg ducts that carry ova towards the uterus.

The ova are released from the ovaries (OH-var-eez). There is both a right

ovary and a left ovary, each named for their oval, whitish appearance, much

like a chicken ‘‘egg’’ (ovari).

INTERNAL FERTILIZATION AND THE FEMALE

REPRODUCTIVE PATHWAY

The key to internal fertilization, of course, is both ejaculation by the male and

ovulation by the female. (You may want to review appropriate parts of

Chapter 15, the glands, at this point.) Previously, we learned that increased

secretion of luteinizing hormone was the usual trigger for ovulation. Basically,

the LH (luteinizing hormone) dissolves and weakens the outer surface of the

ovary, allowing a mature ovarian follicle to rupture and release an ovum.

The ovum is usually swept up into a nearby oviduct (Fallopian tube).

Fertilization usually occurs in the ﬁrst (outer) 1/3 of the oviduct. Fusion of

sperm and ovum creates a zygote. As the zygote moves through the oviduct

and towards the uterus, it undergoes a series of mitoses (my-TOH-seez).

Embryo Development Leading to Birth

The single-celled zygote becomes a solid mass of cells called a morula (mor-

OO-luh), whose name comes from the Latin for ‘‘little mulberry.’’ The mor-

ula passes out of the oviduct and enters the body (main hollow cavity) of the

uterus. Here it becomes a blastula (BLAS-chew-lah), alternately called a

blastocyst (BLAS-toh-sist). The blastula (blastocyst) is a ‘‘little sprouter’’

(blastul) or ‘‘hollow sprouting bladder’’ (blastocyst) that embeds itself within

the endometrium (en-doh-ME-tree-um), the ‘‘inner’’ (endo-) epithelial lining

of the ‘‘uterus’’ (metr).

After the blastula (blastocyst) implants itself into the uterine (YOU-ter-in)

wall, it starts a process of cellular diﬀerentiation (the cells becoming diﬀerent

from each other and specialized). This leads to a gastrula (GAS-true-lah) or

hollow ‘‘little stomach’’ (gastrul). As Figure 20.6 clearly displays, the gastrula

gives rise to the three primary germ layers of the human embryo.

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The gastrula does, indeed, have some of the same characteristics as a

miniature version of the real stomach! Besides having a hollow interior, the

gastrula (like the real stomach), has multiple layers of cells within its walls.

These cell layers are technically called derms – various layers of ‘‘skin.’’

The endoderm (EN-doh-derm), or ‘‘inner skin,’’ for instance, eventually

creates much of the inner lining of the main body cavities in the mature

adult. The mesoderm (ME-so-derm) – ‘‘middle skin’’ – diﬀerentiates to

become bone and muscle tissue. And the ectoderm (EK-toh-derm) specializes

to ultimately become much of the skin and nervous system. It is the skin and

nervous system (Chapter 14), after all, that lies at or near the body surface,

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Fig. 20.6 From embryo to fetus.

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reacts to environmental stimuli, and communicates extensively about such

stimuli.

To summarize the above information, we have:

THREE PRIMARY GERM LAYERS OF THE EMBRYO ¼

Endoderm (‘‘Inner skin’’) þ Mesoderm (‘‘Middle skin’’) þ

Ectoderm (‘‘Outer skin’’)

FROM EMBRYO TO FETUS

The embryo, in general, represents a ‘‘sweller.’’ In humans, the embryo repre-

sents all of the stages of body development that occur during the ﬁrst 3

months of development. This starts with the zygote stage and includes the

morula, blastula (blastocyst), and gastrula.

The embryo body keeps ‘‘swelling’’ by extensive mitoses, adding thousands

and thousands of new tissue cells. The embryo body becomes surrounded by

an amnion (AM-knee-un). The amnion is a protective membrane forming a

sac around the embryo of ‘‘little lambs’’ (amnions), human beings, and other

mammals. An amnion also protects the embryos of other types of higher

vertebrates such as birds and reptiles. The amnion also encloses a quantity

of amniotic (am-knee-AH-tik) ﬂuid. This ﬂuid creates a watery cushion and

shock absorber for the embryo, as well as keeping the body wet and moist.

A yolk sac provides nourishment for the early embryo. Limb buds develop

and mark the locations of the future body limbs. An umbilical (um-BILL-ih-

kal) cord stretches from the ‘‘central pit’’ (umbilic) area present in the mid-

section of the embryo, out to the placenta (plah-SEN-tah). The placenta is a

‘‘ﬂat cake’’ (placent)-like organ that supplies nourishment to the later embryo

and fetus as the yolk sac progressively shrinks in size.

The fetus is the stage of the ‘‘oﬀspring’’ from the third month after ferti-

lization, up to the time of birth. The ‘‘newborn’’ child is literally a neonate

(KNEE-oh-nayt).

Sexual Dysfunctions and Reproductive Failure

A sexual dysfunction (dis-FUNK-shun) is a type of ‘‘bad, painful, or diﬃcult’’

(dys-) failure of reproduction. There are many speciﬁc types of sexual dys-

function – some involving the male reproductive pathway, some involving the

female reproductive pathway – so that successful production of a zygote is

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