Layman D. Biology Demystified: A Self-Teaching Guide

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Revisiting the liver once more, we can say that it has numerous other

critical body functions, in addition to the production of bile. First among

these is detoxiﬁcation (dee-TAHKS-ih-ﬁh-KAY-shun) – ‘‘the process of tak-

ing poison out of ’’ the bloodstream. The liver cells help detoxify (dee-

TAHKS-ih-feye) drinking alcohol, for instance, so that it is broken down

into sugar and water without toxifying (‘‘poisoning’’) the brain! [Study sug-

gestion: When we say that a person is drunk, what does that imply, with

regards to the associated function of the liver?]

Summarizing all of the above, we can say that the duodenum, as the ﬁrst

segment of the small intestine, receives chyme from the stomach, bile from

the liver and cholecyst, and pancreatic juice from the pancreas. As a result,

the chemical digestion of all three basic types of foodstuﬀs – carbohydrates,

lipids, and proteins – is essentially completed within the small intestine. Lying

downstream from the liver and pancreas entry point  the duodenal papilla

 are the jejunum (jeh-JOO-num) and the ileum (IL-ee-um).

The jejunum and ileum basically complete the processes of chemical diges-

tion and absorption of nutrients that began in the duodenum.

We thus have the following simple summary equation:

SMALL INTESTINE ¼ Duodenum þ Jejunum þ Ileum

THE COLON: OUR LARGE INTESTINE

The last major section of the digestive tube is the colon (KOH-lun) or ‘‘large

intestine.’’ The colon (large intestine) is a wide-diameter, folded tube, about 6

feet (2 meters) in length in an average-sized human adult (see Figure 19.5).

The colon begins with the cecum (SEE-kum), a ‘‘blind’’ (cec) or dead-ended

pouch that has the vermiform (VER-mih-form) appendix or ‘‘worm-like

attachment’’ hooked to its base. The vermiform appendix is basically a

solid attachment of modiﬁed lymphatic tissue (Chapter 17) that plays a

minor role in the body’s immune or self-defense system.

Liquid chyme from the ileum of the small intestine pushes through the ileo-

cecal (il-ee-oh-SEE-kul) sphincter. Once within the cecum, the chyme begins

to undergo an extensive drying out process, wherein large amounts of water

and salt (H

O) are absorbed. In addition, there are beneﬁcial bacteria in the

colon that produce a variety of B-vitamins, as well as sulfur-containing

amino acids, which are also absorbed.

Due to this drying out process, chyme is solidiﬁed into feces (FEE-seez)

within the colon. Besides H

O, feces also contain a signiﬁcant percentage of

fecal (FEE-kal) bacteria and dietary ﬁber (actually undigested cellulose mate-

rial).

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3, Order

Looking at Figure 19.5, you might well ask yourself, ‘‘How does chyme/

feces in the cecum, which is way down at the bottom of the large intestine,

move up to the other parts of the colon?’’ Good question! The answer is, by

mass peristalsis (pear-ih-STAHL-sis). Peristalsis literally means ‘‘a constric-

tion’’ (-stalsis) ‘‘around’’ (peri-). Formally deﬁned, then, peristalsis is a con-

striction (narrowing) around a particular point of some tube, due to the

contraction of a circular ring of smooth muscle around the tube. Mass peri-

stalsis is thus the simultaneous constriction or narrowing of a large number

of directly neighboring points along the large intestine wall, such that the

heavy mass of the feces is pushed on to the next section of the colon. [Study

suggestion: Either in your imagination, or for real, get a large tube of tooth-

paste. Turn it upside down, and remove the cap. Now, make a ring around

the upper part of the tube using both your hands. Count one . . . two . . .

three . . . . Constrict your ﬁngers hard around the tube! What do you

observe? This constriction at a single point is a crude model of peristalsis.

Think. How could you use the same basic elements to model mass peristalsis

in the colon? If you phoned up several of your friends, what could you do?]

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338

Fig. 19.5 The human colon (large intestine).

After the cecum, next in sequence are the ascending colon (which goes

upward), transverse colon (which runs sideways), and the descending colon

(which goes downward). Sigmoid (SIG-moyd) means ‘‘S-resembling.’’ Hence,

the sigmoid colon is the S-resembling portion of the large intestine coming

right after the descending colon.

The sigmoid colon snakes down into the rectum (WRECK-tum), a mus-

cular-walled, ‘‘straight’’ (rect) tube that empties feces into the anus. There are

two sphincters within the rectum. The higher one, called the internal anal

sphincter, is not under our conscious control. The internal anal sphincter

tends to open whenever mass peristalsis has moved some feces down into

the upper portion of the rectum.

Fortunately, for us, there is also a ring of voluntary striated (cross-striped)

muscle, positioned in the lower portion of the rectum. This muscular ring is

called the external anal sphincter. Its contraction and relaxation is very much

under our conscious control (at least, ever since we were ﬁrst ‘‘potty-

trained’’)! Therefore, we can usually choose the time and place where we

will consciously relax this lower sphincter and carry out defecation (egestion).

Let us end by summarizing in words the parts of the large intestine:

COLON=Cecum+Ascending colon+Transverse colon

þ Descending colon+Sigmoid colon+Rectum

Too Many, or Too Few Calories?

Way back in Chapter 4 we talked about the Chemical Balance of Life, and

its relationship to cellular respiration or metabolism. Recall that in the

catabolism (breakdown) of chemical foodstuﬀs (such as glucose molecules),

free or kinetic energy is released and tied to the production of more cell

ATP. Obviously, these topics are closely related to our current discussion,

since digestion is, in fact, deﬁned as the chemical and physical breakdown

of food. Similarly, nutrition is the process of nourishing or feeding the body

cells.

THE CONCEPT OF CALORIES

The nutrients that humans eat, say, in a candy bar, are all associated with a

certain number of calories (KAL-or-ees). A calorie is technically a unit for

describing the amount of ‘‘heat’’ (calor) that is released during the break-

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down of food. In nutrition, we generally use the ‘‘large calorie’’ or kilocalorie

(KILL-uh-kal-ur-ee) – the amount of heat energy released measured in

‘‘thousands’’ (kilo-) of calories. A candy bar, for instance, may contain 670

kilocalories of heat energy, when it is consumed.

THE CONCEPT OF CALORIC BALANCE

An important principle of Biological Order in humans and other animals is

the notion of caloric (kuh-LOR-ik) balance. Caloric balance is a condition

where the number of kilocalories consumed in food per day exactly equals the

number of kilocalories burned in exercise per day. Summarizing, we have:

CALORIC BALANCE : Number of kilocalories consumed/day

¼ Number of kilocalories burned/day

Caloric balance is desirable, therefore, whenever we wish to maintain our

current body weight. It logically follows that whenever one wishes to either

gain or lose weight, then some degree of Biological Disorder must be inserted

into the scheme, such that the caloric balance is disrupted.

When the number of kilocalories consumed per day signiﬁcantly exceeds

the number burned during exercise, there is a net excess of kilocalories. Some

of these excess kilocalories are stored as extra glycogen deposits in liver and

muscle cells, and some of it is stored as extra lipid within our adipose tissue

cells. The net result, of course, is that we (gulp!) gain weight and often get

fatter!

Conversely, to lose weight, the number of kilocalories burned during daily

exercise must exceed the number consumed in foodstuﬀs (no matter what the

current fad diet books tell you!).

Quiz

Refer to the text in this chapter if necessary. A good score is at least 8 correct

answers out of these 10 questions. The answers are listed in the back of this

book.

1. For humans, eating grass would not be a good way of obtaining

nutrients, because:

(a) Grass contains no nutrient molecules

(b) Blades of grass cannot be digested by any known type of animal

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PART 4 Anatomy and Physiology of Animals

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5, Order

necessary to break down cellulose

(d) Grass can’t even be chewed by human beings!

2. Ingestion is the exact opposite of what process?

(a) Egestion

(b) Digestion

(d) Mastication

3. The movement of nutrients from the interior of the digestive tube into

the blood:

(a) Absorption

(b) Secretion

(d) Ingestion

4. The chemical digestion of carbohydrates begins in the mouth, owing to

the presence of:

(a) Hydrochloric acid

(b) NaHCO

(d) Salivary amylase

5. A thick, soupy mass of partially digested material found in the stomach

and small intestine:

(a) Bolus

(b) Cud

(d) Feces

6. The ﬁrst segment of the small intestine:

(a) Jejunum

(b) Vermiform appendix

(d) Cecum

7. Technical term for the gallbladder:

(a) Gallicule

(b) Cholecyst

(d) Hepatic accessory organ

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8. Regulates the emptying of both the common bile duct and the

pancreatic duct into the duodenum:

(a) Pyloric sphincter

(b) Microvillus

(d) Ileo-cecal sphincter

9. The large intestine begins with the dead-ended _____:

(a) Sigmoid colon

(b) Rectum

(d) Transverse colon

10. A critical structure that allows defecation to be voluntary:

(a) Vermiform appendix

(b) Internal anal sphincter

(d) External anal sphincter

The Giraﬀe ORDER TABLE for Chapter 19

(Key Text Facts About Biological Order Within An Organism)

1. ____________________________________________________________

2. ____________________________________________________________

3. ____________________________________________________________

4. ____________________________________________________________

5. ____________________________________________________________

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The Dead Giraﬀe DISORDER TABLE for Chapter 19

(Key Text Facts About Biological Disorder Within An Organism)

1. ____________________________________________________________

2. ____________________________________________________________

3. ____________________________________________________________

4. ____________________________________________________________

5. ____________________________________________________________

The Spider Web ORDER TABLE for Chapter 19

(Key Text Facts About Biological Order Beyond the Individual Organism)

1. ____________________________________________________________

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344

CHAPTER

Urine and (Gulp!)

Sex in Animals

Chapter 19 told us all about digestion and nutrition, two essential functions

for the survival of the individual organism. Now, in Chapter 20, we consider

both the urinary (YOUR-ih-nair-ee) and reproductive (ree-proh-DUCK-tiv)

systems. The urinary system literally ‘‘pertains to’’ (-ary) ‘‘urine’’ production,

storage, and excretion from the individual body. The reproductive system in

the male and female, on the other hand, is literally about ‘‘producing’’ a new

organism, ‘‘again’’ (re-). For most animals, this implies using the genital

(JEN-ih-tal) organs to ‘‘beget or produce’’ (genit) sexually.

The Genitourinary (Urogenital)

System Concept

In humans and other mammals, it is appropriate to speak not just of the

urinary and reproductive systems alone but of a combined genitourinary

(JEN-ih-toh-ur-ih-nair-ee) or urogenital (UR-oh-jen-ih-tal) system. This is

because many of the structures of the urinary and reproductive (genital)

organs are shared in common. Consider, for example, the penis (PEA-nis)

in males. The penis is a spongy ‘‘tail’’ (pen)-like structure that serves both to

carry urine out of the body, as well as deliver spermatozoa (sperm cells) to an

ovum (mature egg cell) for reproduction.

Major Urinary Structures in Animals

In all vertebrates, the major organs of urine excretion are the kidneys.In

humans, a pair of bean-shaped kidneys are located along either side of the

vertebral column, deep within the back.

KIDNEY ANATOMY

Figure 20.1 provides an overview of renal (REE-nal) or ‘‘pertaining to’’ (-al)

‘‘kidney’’ (ren) anatomy. The kidney is encased within the renal capsule,a

thin membrane of ﬁbrous connective tissue. The kidney, itself, is subdivided

into three major areas or zones. The outermost zone is called the renal cortex.

Much as the cerebral cortex forms a thin ‘‘bark’’ over the surface of the

cerebrum (Chapter 14), the renal cortex does the same for the kidney. The

‘‘middle’’ (medull) area is the renal medulla (meh-DEW-lah). And the deepest

zone is the renal pelvis (PEL-vis). The renal pelvis is a broad, bowl-shaped sac

that receives the urine as it ﬂows from the renal cortex and medulla. And

carrying the collected urine of the renal pelvis is the ureter (YOUR-eh-ter).

Within the renal cortex are millions of nephrons (NEF-rahns). The

nephrons are the major microscopic functional units of the kidney. It is the

nephrons that are actually responsible for formation of urine from the blood.

Each nephron begins with a glomerulus (gluh-MAHR-yew-lus). The glomer-

ulus is a tiny, red-colored collection of renal capillaries. This structure gets its

name from its resemblance to a little red ‘‘ball of yarn’’ (glomerul). The blood

pressure pushing against the walls of the capillaries in each glomerulus,

causes a ﬁltration of ﬂuid out of the glomerulus, and into the adjoining

group of urinary tubules (TWO-byools) – ‘‘tiny urine tubes.’’

The urinary tubules from each group of neighboring nephrons eventually

empty into a common passageway called a collecting duct. A number of

collecting ducts pass down together through the renal medulla. They create

the renal pyramids, which are pointed at their bottom tips like the rather

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1, Web

2, Order

blunt pyramids constructed by the Aztecs or Inca Indians. The tip of each

renal pyramid drips urine into a renal calyx (KAY-licks), or ‘‘kidney ﬂower

cup.’’ And the urine from each calyx eventually ﬂows into the body of the

renal pelvis, before it leaves the kidney via the ureter.

THE URINARY PATHWAY

Figure 20.2 shows the rest of the urinary pathway, lying beyond the kidney.

The right and left ureters both dump urine into the urocyst (YUR-oh-sist)or

urinary bladder (cyst). The urocyst (urinary bladder) is a hollow, muscular-

walled pouch that temporarily stores the urine before it is excreted.

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Fig. 20.1 An overview of renal anatomy.