Layman D. Biology Demystified: A Self-Teaching Guide

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them are in a state of diastole, so that they receive blood through the right

and left atrioventricular valves.

The Cardiac Cycle is one heart beat or one complete cycle of contraction

(systole) plus relaxation (diastole) of all four chambers of the heart. There are

also at least two heart sounds that can be heard through a stethoscope placed

onto the chest or back. The 1st heart sound is commonly described as ‘‘lubb.’’

It is due to closure of both A-V valves, which occurs when the ventricles start

going into systole. The ventricles push the A-V valve ﬂaps shut from below.

As a result, a back-ﬂow of blood from the ventricles, and back up into the

atria, is usually prevented.

The 2nd heart sound is usually represented as ‘‘dupp.’’ This sound occurs

because of the closing of both semilunar valves at the beginning of ventri-

cular diastole. The blood in the common pulmonary artery/aortic arch above

each ventricle starts to fall back downward, due to the force of gravity. As

the blood falls down, it catches the semilunar valve ﬂaps and slams them

shut, thereby preventing a back-leak into the ventricles. [Study suggestion:

Glance back at Fig. 16.3, to help you visualize what’s happening during the

two heart sounds.]

HEART MURMURS

Sometimes the heart valve ﬂaps do not ﬁt tightly together. Consider what

may happen after bacterial endocarditis (en-doh-car-DIE-tis). This disease

involves an ‘‘inﬂammation of ’’ (-itis) the ‘‘inner’’ (endo-) lining of the

‘‘heart’’ (cardi), due to infection with bacteria. During the process of inﬂam-

mation, the ﬂaps of certain valves (especially those of the left A-V valve)

become swollen. When the inﬂammation ﬁnally heals, the valve ﬂaps may

become pulled back and distorted.

Thus, there is a turbulent (TUR-byuh-lunt) back-ﬂow of blood – a back-

ﬂow which is highly disorderly and in a state of ‘‘turmoil’’– through the

distorted and ill-ﬁtting heart valve ﬂaps. The resulting noisy, abnormal

heart sounds are called a heart murmur.

Blood Pressure and Blood Flow

In general, blood pressure (BP) is a pushing force exerted against the blood

and against the walls of the blood vessels (see Figure 16.4). The blood pres-

sure (BP) is at its highest in the major arteries attached directly to the heart

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(such as the aortic arch and common pulmonary artery). The BP then pro-

gressively decreases with greater distance from the heart.

Blood pressure is critically important because it is the main force causing

blood ﬂow. There is a blood pressure gradient (GRAY-dee-unt), a series of

downward ‘‘steps’’ from a region of higher to a region of lower blood pres-

sure. Within the systemic circulation, for example, the BP is highest within

the aortic arch. As the aortic arch progressively branches into smaller and

smaller arteries farther away from the heart, the BP also declines. This creates

a blood pressure gradient or series of downward steps in blood pressure in

vessels farther from the heart. Hence, the blood is pushed down a BP gra-

dient, so that it ﬂows into lower-pressure arteries in a series of steps.

One of these farther arteries is called the brachial (BRAY-kee-al) artery,

which is named for its location within the upper ‘‘arm’’ (brachi). The brachial

artery is the most frequent site used for taking someone’s blood pressure,

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Fig. 16.4 Blood pressure (BP), blood ﬂow, and the brachial artery.

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utilizing a stethoscope and sphygmomanometer (sﬁg-moh-muh-NAHM-uh-

ter). A sphygmomanometer is literally ‘‘an instrument used to measure’’

(-meter) the ‘‘throbbing pulse’’ (sphygmo) at certain ‘‘intervals’’ (mano).

To be sure, one does hear a dull throbbing sensation when the bell of a

stethoscope is placed over the brachial artery and the arm cuﬀ of a sphyg-

momanometer device is inﬂated around it. The sphygmomanometer is

usually marked oﬀ in units of millimeters of mercury, abbreviated as

mmHg (Hg is the chemical symbol for the element, mercury).

As the air is slowly let out of the inﬂated arm cuﬀ, the ﬁrst dull throbbing

noise one hears through the stethoscope is called the systolic (sis-TAHL-ik)

blood pressure,orSBP. The systolic blood pressure (SBP) is the pressure

created by the systole (contracting and emptying phase) of the left ventricle

of the heart. A slug of blood is pushed out of the left ventricle with its con-

traction. Flowing progressively down a BP gradient, this slug of blood ﬁnally

enters the brachial artery. The blood bulges out the artery somewhat as it

passes through it, thereby creating a thumping sound. For a resting adult, the

systolic blood pressure (SBP) is usually recorded as about 120 mmHg.

The diastolic (DIE-ah-stahl-ik) blood pressure,orDBP, is the blood pres-

sure associated with the diastole (relaxing and ﬁlling) phase of the left ven-

tricle. Upon thoughtful reﬂection, you might well ask, ‘‘Why isn’t the

diastolic BP just 0 mmHg? Isn’t diastole the resting and ﬁlling phase, when

the ventricle isn’t even contracting or creating any blood pressure? So, why

should there even be any diastolic BP, at all?’’

Good question! Glance back at Figure 16.4. Note that the brachial artery

bulges out with the force of the systolic BP against its walls. Now, when the

ventricle stops contracting, and diastole begins, there is a powerful elastic

recoil, or snapping back force, created by the stretched brachial artery wall

coming back to its non-stretched shape. This force of elastic recoil or snap-

ping back of the stretched brachial artery wall is what creates the diastolic

BP. The diastolic blood pressure is, then, in a sense, a residual or left-over

blood pressure, the force of blood pressure temporarily ‘‘stored’’ in the

bulged-out walls of the brachial artery during systole. Because it is a residual

BP, the diastolic BP is normally signiﬁcantly lower than the systolic BP. It

averages about 80 mmHg in a resting adult.

The stethoscope and sphygmomanometer measure a resting adult’s blood

pressure as about 120/80 mmHg. The SBP is 120, the DBP is 80. The diastolic

BP is usually recorded as the level of blood pressure where the dull thumping

sound heard through the stethoscope just disappears. [Study suggestion: Ask

yourself, ‘‘Why does the thumping sound disappear just below the recorded

level of the diastolic BP?’’ Hint: Think about what is happening to the wall of

the brachial artery.]

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Hypertension, Hypotension, and

Arteriosclerosis

We have been talking about the normal blood pressure, which has an average

or set-point level of about 120/80 mmHg. The normal BP can rise to an upper

limit of about 140/90 mmHg. And it can fall down to a lower limit of about

100/60 mmHg. The distance between these upper and lower normal readings

creates the normal range for blood pressure, a condition that can technically be

called normotension (NOR-moh-TEN-shun). We use this term because blood

‘‘pressure’’ represents the amount of tension exerted against the blood. Figure

16.5 (A) illustrates a state of normotension: that is, a state of relative con-

stancy or homeostasis of blood pressure within its normal range.

Unfortunately, however, blood pressure doesn’t always remain within its

normal range. Say that a lumberjack accidentally cuts his brachial artery with

a chainsaw while felling a large oak tree. The blood spurts out in hot red jets,

resulting in a severe and possibly fatal hemorrhage (HEM-uh-rij) – a ‘‘burst-

ing out’’ (-orrhage) of ‘‘blood’’ (hem). When so much blood is lost, there isn’t

much blood left to press against the arterial wall. Thus, blood pressure

steeply declines. It may even reach a state of hypotension (HIGH-poh-

TEN-shun). Hypotension is a condition of ‘‘below normal or deﬁcient’’

(hypo-) blood ‘‘pressure’’ (tens). Speciﬁcally, hypotension is a blood pressure

signiﬁcantly less than 100/60 mmHg (Figure 16.5, B).

At the opposite extreme lies hypertension (HIGH-per-TEN-shun) – an

‘‘excessive or above normal’’ (hyper-) blood pressure. Hypertension is a

blood pressure signiﬁcantly above the upper normal limit of approximately

140/90 mmHg (Figure 16.5, C).

‘‘So what if a person has hypotension or hypertension?’’ a skeptic might

inquire. With hypotension, the person may easily faint. And if the condition

is severe, there may be circulatory shock or coma, due to a lack of blood

pressure pushing blood up to feed the brain. In the case of hypertension, the

chronically above-normal pressure may overstretch and thin out the walls of

arteries, creating aneurysms (AN-yuh-riz-ums). Aneurysms are abnormally

‘‘widened up’’ arteries, which are highly susceptible to being ruptured. And

when an aneurysm ruptures, there may be a large amount of internal bleed-

ing. Persons suﬀering a stroke, or cerebrovascular (seh-REE-broh-VAS-kyoo-

lar) accident, for instance, may well have experienced a ruptured aneurysm of

their cerebral blood vessels covering the brain. Whatever particular body

functions the oxygen-and-blood-deprived brain area carried out, are then

partially or totally lost.

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ARTERIOSCLEROSIS

Closely related to hypertension is the condition called arteriosclerosis (ar-

teer-ee-oh-sklair-OH-sis). Arteriosclerosis is ‘‘an abnormal condition of ’’

(-osis) ‘‘hardening’’ (scler) of the walls of an ‘‘artery’’ (arteri).

There is a positive feedback relationship between arteriosclerosis and

hypertension. You may remember (Chapter 14) that a negative feedback

system removes or corrects a particular change. It thus tends to re-establish

or maintain a state of Biological Order.

Positive feedback, however, is a system that magniﬁes or accelerates

change in some particular direction. Whatever is happening already, positive

feedback makes it even more extreme, or worse. Thus, positive feedback

is sometimes referred to as a vicious cycle. It magniﬁes or accelerates changes

that often veer out of control, thereby promoting Biological Disorder.

Consider hypertension and arteriosclerosis (Figure 16.6). When the BP is

consistently elevated above its normal range, then it tends to damage the

elastic ﬁbers within the arterial walls. [Study suggestion: Picture a garden hose

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Fig. 16.5 The three possible states of blood pressure (BP). (A) Normotension. (B) Hypo-

tension. (C) Hypertension.

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with an adjustable nozzle at the end. It is attached to a faucet that is left

turned on all summer, with the nozzle closed. What will eventually happen to

the garden hose?] When the elastic ﬁbers are damaged, the arterial wall gets

stiﬀer, and when the arterial wall gets stiﬀer, the BP goes up even more. This

makes the hypertension worse. And as the hypertension worsens, the elevated

pressure damages the elastic ﬁbers to an even greater extent.

Hence, there is a positive feedback relationship (vicious cycle) between

hypertension and arteriosclerosis. The worse the hypertension gets, the

more the arterial wall hardens, and the more the arterial wall hardens, the

worse the hypertension gets. Adding to the damage, calcium ions are pushed

out of the bloodstream by the high BP, becoming embedded in the arterial

wall. So the once soft, pliable, garden hose-like normal artery progressively

transforms into a rock-hard, stiﬀ, calciﬁed (KAL-sih-feyed) tube that more

resembles a heavily limed-up lead water pipe!

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Fig. 16.6 Hypertension and arteriosclerosis: A positive feedback relationship. (NOTE: Blood

pressure is abbreviated as BP.)

The Blood Connective Tissue

Having discussed the heart as a pump and the blood vessels as tubes for

carrying ﬂowing blood, it is now time to consider the blood itself. Blood is

a red, sticky, connective tissue that occupies about 5–6 liters (LEE-ters) of

volume in an average-sized adult.

PLASMA

At ﬁrst, it may seem odd to classify blood as a connective tissue. After all, it

contains no connective tissue ﬁbers (such as collagen or elastic ﬁbers) that

directly strap body parts together. Nevertheless, blood is considered a special

connective tissue, because it contains plasma (PLAZ-muh). Plasma is the

clear, watery, intercellular substance found ‘‘between’’ (inter-) the ‘‘cells’’

(cellul) of the bloodstream. Because it circulates throughout the body within

the bloodstream, plasma acts like a functional connective tissue rather than a

structural one. Recall that hormones (Chapter 15), for instance, are First

Messengers secreted by the endocrine glands and carried throughout the

body within the bloodstream. It is the plasma, not the blood cells per se,

that does this functional connecting or carrying of the hormones towards

their various target cells.

The plasma is basically a saline (SAY-leen) or ‘‘salt’’-containing solution

consisting of 0.9% NaCl (sodium chloride) solute (Chapter 4) dissolved in

water solvent. In addition to carrying hormones, nutrients such as glucose

and O

, and waste products like CO

, the blood also contains a number of

important plasma proteins. Some of these plasma proteins are critical for

blood clotting. Still others play a role in such processes as body defense

from foreign invaders.

FORMED ELEMENTS IN THE BLOOD

Besides the plasma, there are also many formed elements within the blood-

stream. Formed elements are cells and cell fragments within the blood that

have a deﬁnite shape or form.

When a sample of human blood is stained and viewed under a compound

microscope (Figure 16.7), several major types of formed elements can be

identiﬁed. Most numerous among these are the erythrocytes (air-RITH-

roh-sights) or ‘‘red’’ (erythr) blood ‘‘cells’’ (cytes). Erythrocytes (red blood

cells) are anucleate (ay-NEW-klee-aht); that is, they are ‘‘without’’ (a-) any

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nucleus. They are also quite special in that they are shaped like biconcave

(buy-KAHN-cave) discs, being ‘‘caved-in’’ on ‘‘both’’ (bi-) sides. Viewed from

the side, this makes them look like red hourglasses! The red color is mainly

due to the presence of hemoglobin (HEE-moh-glohb-in). Hemoglobin is a

reddish-colored, ‘‘globe’’-shaped (glob), ‘‘protein substance’’ (-in) found

within the cytoplasm of the red ‘‘blood’’ (hem) cells. There may be as

many as 10,000 hemoglobin molecules present within a single erythrocyte.

The main job of these thousands of hemoglobin molecules is carrying oxygen

) molecules through the bloodstream, and to the tissue cells.

Leukocytes (LEW-koh-sights) are the ‘‘white’’ (leuk) blood ‘‘cells’’ (cytes).

These cells typically have a large, purplish-staining nucleus, but they are

named for the clear, whitish appearance of their cytoplasm. (This is in

marked contrast, of course, to the red, hemoglobin-rich cytoplasm of the

erythrocytes.) The leukocytes have signiﬁcant roles in protecting the body

from various foreign invaders. (Details are given in Chapter 17.)

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Fig. 16.7 The major formed elements of the blood.

The third common formed element are the platelets (PLAY-teh-lets)or

thrombocytes (THRAHM-buh-sights). The platelet name comes from their

shape – ‘‘little plate’’-like fragments of disintegrated bone marrow cells. The

thrombocyte name derives from their function – ‘‘clotting’’ (thromb) ‘‘cells.’’

About 1/3 the size of an erythrocyte, the platelets (thrombocytes) are scat-

tered here and there in small groups, throughout the plasma. These purplish-

colored, plate-like cell fragments have very sticky surfaces. Thus, whenever a

blood vessel ruptures and hemorrhages, the platelets soon collect around the

open hole and stick to a network of thin, ‘‘ﬁber’’-like strands, composed of

the protein, ﬁbrin (FEYE-brin). As the platelets collect and stick to one

another, a ‘‘clot’’ or thrombus (THRAHM-bus) is soon formed. The hole is

blocked, hemorrhaging stops, and wound healing follows.

Serum Cholesterol, Atherosclerosis, and

‘‘Heart Attacks’’

When a person has a so-called ‘‘heart attack,’’ it usually involves a problem

with abnormal clotting or an occlusion (uh-KLEW-zhun) – ‘‘shutting up’’ – of

the blood vessels serving the myocardium. The site of the troublesome occlu-

sion (shutting up) is somewhere within the coronary (KOR-uh-nair-ee)

arteries. These vessels are named for their resemblance to a ‘‘crown’’

(coron) encircling the top of the heart, just below the auricles (OR-ih-kls)

or ‘‘little ear’’-like ﬂaps. The right and left coronary arteries look like a red-

colored crown slipped down over the ears of a real prince (Figure 16.8, A)!

CHOLESTEROL AND ATHEROSCLEROSIS

The blood serum (SEER-um) is the clear, yellowish watery portion of the blood

plasma that remains after a clot (thrombus) has been formed. The blood serum

contains cholesterol (kah-LES-ter-ahl). Cholesterol is a white, fatty substance

that tends to be laid down as atheromas (ah-ther-OH-mahs) or ‘‘fatty’’ (ather)

‘‘tumors’’ (-omas) whenever its concentration becomes too high.

Atherosclerosis (ah-ther-oh-slair-OH-sis) is a fatty hardening of the arteries

that occurs when excessive blood cholesterol is deposited as atheromas (fatty

tumors) upon the inner walls of arteries. Atherosclerosis is especially harmful

in cases of coronary artery disease, where atheromas can build up and sud-

denly occlude (stop up) a coronary artery opening (Figure 16.8, B).

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Fig. 16.8 Atherosclerosis and occlusion of the coronary arteries.