Human Physiology, 14th edition (2016)

20 Chapter 1

the stem cell population gives rise to all of the different blood
cells—red blood cells, white blood cells, and platelets (chap-
ter 13). Similarly, there are stem cells in the brain (chapter 8),
skeletal muscles (chapter 12), and intestine (chapter 18).
Scientists have recently discovered that there are also stem
cells in the bulge region of the hair follicle ( fig. 1.23 ). These
stem cells form keratinocytes, which migrate down to the
matrix of the hair follicle and divide to form the hair shaft and
root sheath. Other stem cells in the region of the hair follicle
just above the bulge form new sebaceous gland cells, which
have a high turnover. Skin wounds stimulate the migration of
stem cells from the hair follicles into the skin between follicles
to promote healing of the wounded skin.
The bulge region also contains melanocyte stem cells,
which migrate to the matrix of the follicle and give the hair its
color. Scientists have now shown that graying of the hair with
age is caused by loss of the melanocyte stem cells in the bulge
of the hair follicles. The melanocyte stem cells appeared to be
present in most of the hair follicles of people aged 20 to 30 and
absent from most hair follicles of people aged 70 to 90.
As demonstrated by the stem cells in the bulge of the hair fol-
licle, adult stem cells can form a variety of related cell types; the
adult stem cells are therefore described as multipotent. The topics
of embryonic and adult stem cells are discussed in more detail in
the context of embryonic development (chapter 20, section 20.6).

Systems

Organs that are located in different regions of the body and
that perform related functions are grouped into systems. These
include the integumentary system, nervous system, endocrine
system, skeletal system, muscular system, circulatory system,

immune system, respiratory system, urinary system, diges-

tive system, and reproductive system ( table 1.4 ). By means of

numerous regulatory mechanisms, these systems work together

to maintain the life and health of the entire organism.

Body-Fluid Compartments

Tissues, organs, and systems can all be divided into two major

parts, or compartments. The intracellular compartment is

that part inside the cells; the extracellular compartment is

that part outside the cells. Both compartments consist primar-

ily of water—they are said to be aqueous. About 65% of the

total body water is in the intracellular compartment, while

about 35% is in the extracellular compartment. The two com-

partments are separated by the cell membrane surrounding

each cell (chapter 3, section 3.1).

The extracellular compartment is subdivided into two parts.

One part is the blood plasma, the fluid portion of the blood. The

other is the fluid that bathes the cells within the organs of the

body. This is called tissue fluid, or interstitial fluid. In most parts

Figure 1.23 The bulge region of the hair follicle with
stem cells. Stem cells in this region migrate to form the differentiated
cells of the hair follicle, sebaceous gland, and epidermis.

Basal
layer

Outer root sheath

Inner root sheath

Matrix

Dermal papilla

Epidermis

Sebaceous gland

Bulge region

with stem cells

Hair

System Major Organs

Primary

Functions

Integumentary Skin, hair, nails Protection,

thermoregulation

Nervous Brain, spinal cord,

nerves

Regulation of other

body systems

Endocrine Hormone-secreting

glands, such as

the pituitary, thyroid,

and adrenals

Secretion of

regulatory

molecules called

hormones

Skeletal Bones, cartilages Movement and

support

Muscular Skeletal muscles Movements of the

skeleton

Circulatory Heart, blood vessels,

lymphatic vessels

Movement of blood

and lymph

Immune Bone marrow,

lymphoid organs

Defense of the body

against invading

pathogens

Respiratory Lungs, airways Gas exchange

Urinary Kidneys, ureters,

urethra

Regulation of blood

volume and

composition

Digestive Mouth, stomach,

intestine, liver, gall-

bladder, pancreas

Breakdown of food

into molecules

that enter the

body

Reproductive Gonads, external

genitalia, associated

glands and ducts

Continuation of the

human species

Table 1.4 | Organ Systems of the Body