HUMAN BIOLOGY

78 Chapter 4

dermis The skin layer under

the epidermis.

epidermis The top layer

of skin.

integument An organ

system that consists of

skin and structures derived

from it.

keratinocyte Cell that

makes the protein keratin.

melanocyte Cell that

makes the skin pigment

melanin.

the skin: an example of an organ system

Human skin also makes cholecalciferol, a precursor of

vitamin D—a catchall name for compounds that help the

body absorb calcium from food. When skin is exposed

to sunlight, some cells release vitamin D into the blood-

stream, just as specialized endocrine cells release hor-

mones into the bloodstream. In this way your skin acts like

an endocrine gland.

epidermis and dermis are the skin’s

two layers

An outer epidermis and underlying dermis make up the

skin (Figure 4.11A). Sweat glands, oil glands, hair follicles,

and nails develop from the epidermis. The dermis is mostly

dense connective tissue, so it contains elastin fibers that

make skin resilient and collagen fibers that make it strong.

The epidermis and dermis (Figure 4.11B) form the cutane-

ous membrane you read about in Section 4.7. Under the

dermis is a sub cutaneous (“under the skin”) layer called

the hypodermis. This loose connective tissue anchors the

skin while allowing it to move a bit. It also contains fat that

helps insulate the body and cushions some of its parts.

n    Skin and structures that develop

from it make up the integument—

the body’s covering.

Of all your organ systems, you

know your integument the best. The

integument (from Latin integere, “to

cover”) consists of your skin, oil and

sweat glands, hair, and nails. The skin

has the largest surface area of any

organ. It weighs about 9 pounds in

an average-sized adult, and as cover-

ings go, it is pretty amazing. It holds

its shape through years of washing and being stretched,

blocks harmful solar radiation, bars many microbes, holds in

moisture, and fixes small cuts and burns. The skin also helps

regulate body temperature, and signals from its sensory

receptors help the brain assess what’s going on in the outside

world. Yet except for places subjected to regular abrasion

(such as your palms and the soles of your feet), your skin

is generally not much thicker than a sheet of construction

paper. It is even thinner in some places, such as the eyelids.

Figure 4.11 Animated! Skin is the main component

of the integumentary system. A The structure of human

skin. The dark spots in the epidermis are cells that contain

pigment. B A section through human skin. C Close-up of

a hair. Dead, flattened hair cells form a tubelike cuticle

around the hair shaft. (A: © Cengage Learning; B: Garry DeLong/Science

Source; C: Mauro Fermariello/Science Source)

cells being

flattened

dermis

outer flattened

epidermal cells

dividing cells

sweat gland

A

B

4.9

hair
follicle

duct of
sweat
gland

smooth
muscle

pressure-
sensitive
sensory
receptor

sweat
gland

blood
vessel

hair

sebaceous
gland

epidermis

stratified

squamous

epithelium

dermis

mainly

dense

connective

tissue

hypodermis mainly adipose

tissue and loose connective

tissue

dead, flattened cells

C of a shaft of hair

Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).