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Week 6 · Chapter

Chapter 6 — The Integumentary System

Human Anatomy and Physiology I Free sample edition

The module's primary reading · ~35–45 minutes · Objective 3 (identify the four primary tissue types and their subtypes from their structural features, name an epithelium by its layers and cell shape, and describe the layers, accessory structures, and homeostatic functions of the integumentary system, including thermoregulation as a negative-feedback loop), Objective 1 (use the language of the body correctly — the anatomical position, the directional-term pairs, the three body planes, and the body cavities and abdominal regions — distinguish anatomy from physiology, climb the levels of structural organization, list the functions and requirements of human life, and trace a homeostatic feedback loop through its receptor, control center, and effector, telling negative feedback from positive feedback) · Read it first this week; the lecture, the slides, the tutorial, the quiz, and the lab all build on it.

In plain language

Your skin is not a plastic wrap; it is a living organ, and the largest organ system you have. The integumentary system (the skin together with its hair, nails, and glands) guards you from germs and from drying out, feels touch and heat, and helps keep your temperature steady. This week you stack the skin's three layers, from the thin surface layer that has no blood vessels to the fatty layer that sits beneath the skin proper. You put the surface layer's own sublayers in order, from the bottom, where new cells are born, to the top, where dead cells flake away. And you run Week 1's temperature loop again, this time naming the exact skin structure that does each job.

Why you already care about this

Step out of a warm car into a cold parking lot, and within seconds the hair on your arms stands up in goosebumps. Run up three flights of stairs on a warm afternoon, and your face flushes and your shirt dampens with sweat. Scrape your knee on a sidewalk, and sometimes it only turns white and stings, while other times it beads with blood. You have lived all three, and each one is a structure in your skin doing a job.

The goosebumps come from tiny muscles on your hair follicles. The flush and the sweat are your skin shedding heat. Whether a scrape bleeds tells you how deep it went, because the top layer of your skin has no blood vessels at all. This chapter gives you the map behind those moments, layer by layer.

By the end of this chapter you will be able to:

name the skin's three layers from the surface down and say which one carries blood vessels;
put the five epidermal strata in order, deep to superficial, and count four in thin skin and five in thick;
match keratinocytes to keratin and melanocytes to melanin, and say what each product does;
match each accessory structure (hair and the arrector pili, nails, sebaceous glands, eccrine and apocrine sweat glands) to its job;
run a temperature reading through the skin's feedback loop, naming each effector by its structure;
read the depth of a scrape from whether it bleeds.

Carry this line with you: born at the bottom, dead armor at the top — not the reverse.

Where we are

Already covered: Week 1, Body Organization, Homeostasis & Anatomical Terminology, gave you the feedback loop and the superficial/deep pair. Week 2, The Chemistry of Life, gave you proteins and lipids. Week 3, The Cell & Membrane Transport, gave you diffusion. Week 4, Cellular Energy & Protein Synthesis, showed how cells build proteins. Week 5, Tissues, gave you the four primary tissue types and stratified squamous epithelium.
This week: the skin as an organ, its three layers, the epidermal strata in order, keratin and melanin, the accessory structures, the skin's functions, thermoregulation as a loop, and the depth of a scrape.
How they connect: last week's four fabrics are assembled here into your first whole organ, and Week 1's temperature loop finally gets its effectors named by structure.
Where we're going next: Week 7, The Skeletal System: Bone Tissue & Structure, picks up the vitamin D your skin starts making, because bones need the calcium it helps you absorb.

Here is what you lean on this week. In Week 1 you learned that a feedback loop has a receptor that senses, a control center that compares with the set point, and an effector that reverses the change. You also learned that superficial means nearer the surface and deep means farther from it. In Week 3 you saw that diffusion moves particles from where they are crowded to where they are sparse. In Week 5 you met epithelium, which is avascular, and stratified squamous epithelium, stacked layers with flat cells on top. You also met connective tissue, cells scattered in a matrix of fibers. The skin uses every one of those ideas.

1. Assemble an organ: the skin and its four tissues

Think about a winter coat. It has a tough outer shell, a thick quilted lining, wires for a heated version, and pockets. No single fabric makes it a coat; the layers working together do. Your skin is built the same way, except that every layer is alive.

The integumentary system is the skin together with its accessory structures, the hair, nails, and glands; it is the body's largest organ system. In an adult, the skin is roughly 16 percent of body weight and covers about 1.5 to 2 square meters. Treat those as estimates you know in words, not figures to memorize; they are taught in words because they vary from one person to the next.

The skin is a true organ, not a single tissue, because it is built from all four tissue types you met in Week 5:

epithelial tissue forms the outer layer;
connective tissue forms the thick layer beneath it;
muscle tissue forms the tiny muscles on its hair follicles;
nervous tissue runs through it as nerve endings that sense touch, temperature, and pain.

Misconception check. Students often think of skin as a thin wrapper, one tissue thick, like the peel on an apple. It is tempting because the surface you see looks like one smooth sheet. The cure is to count the tissues: epithelium on top, connective tissue below, muscle on the hair follicles, and nerves throughout. Four tissue types working together make an organ, and the skin is the organ system that holds the rest of you in.

2. Stack the layers: epidermis, dermis, and hypodermis

Picture a slice of skin cut straight down, the kind of cross-section a lab atlas shows. Read it from the top, the superficial side, toward the deep side.

At the top is a thin band of tightly packed cells. That is the epidermis, the thin outer layer of keratinized stratified squamous epithelium. You already know each word: stratified means stacked layers, squamous means flat cells at the surface, and keratinized means the cells fill with a tough protein. The epidermis is avascular, so no blood vessel runs into it. Its living cells are fed by diffusion from the layer below.

Under the epidermis is a much thicker band, full of fibers. That is the dermis, the thick, vascular connective-tissue layer of the skin. It holds the skin's blood vessels, nerves, glands, and hair follicles.

Below the dermis lies a soft band of loose tissue and fat. That is the hypodermis, a layer of loose areolar connective tissue and fat that insulates and cushions. It is usually not counted as part of the skin itself.

Worked example — read the cross-section. You read the three bands from superficial to deep and say what each one is and whether it carries blood vessels.

Top band: packed flat cells with no vessels. It is the epidermis, epithelial tissue, and it is avascular.
Middle band: fibers, vessels, glands, and follicles. It is the dermis, connective tissue, and it is vascular.
Bottom band: loose tissue and fat. It is the hypodermis, below the skin proper, insulating and cushioning.
In one line: epidermis → dermis → hypodermis, superficial to deep.

Misconception check. Two slips travel together here. First, students often say the epidermis is full of blood vessels, because skin is warm and a deep cut bleeds. The vessels live in the dermis; the epidermis itself is avascular. Second, students often call the hypodermis "the deepest layer of the skin." It is deep to the skin, not part of it. The skin proper is the epidermis and the dermis.

Honesty note: some sources draw the hypodermis inside the skin and some draw it outside. This course follows the more cautious wording: it is usually not counted as part of the skin itself.

3. Meet the epidermal cells: keratinocytes and melanocytes

Look at your fingernail and then at a freckle. One is hard and protective; the other is colored. Two different cells in your epidermis make those two kinds of products.

A keratinocyte is the main cell of the epidermis, which makes and stores keratin.
Keratin is a tough protein made and stored by keratinocytes that gives the epidermis, hair, and nails their toughness and water resistance.
A melanocyte is a cell in the deepest layer of the epidermis that makes the pigment melanin.
Melanin is the pigment that colors skin and hair and absorbs ultraviolet light, shielding the living epidermal cells beneath from damage.

So the pairing is one cell, one product, one job. The keratinocyte makes keratin, and keratin means toughness and water resistance. The melanocyte makes melanin, and melanin means pigment and a UV shield. UV is the ultraviolet light in sunlight, and melanin shields the living cells of the epidermis from its damage.

Worked example — match the product to the job. You read three everyday observations and name the product behind each.

A fingernail resists a scratch. That toughness is keratin, made by keratinocytes.
A long swim does not leave your body soaked through your skin. That water resistance is keratin's job, with help from an oily product you meet in Section 6.
Skin and hair carry color, and the living cells under the surface are shielded from UV. That is melanin, made by melanocytes.

Misconception check. Students often swap melanin and keratin, saying "keratin colors the skin" or "melanin makes nails hard." The words sound alike, and both products come from the epidermis. Keep them apart by their jobs: melanin is the color and the UV shield; keratin is the armor. If a sentence gives keratin a color, two words have traded places.

4. Order the strata: born at the bottom, dead armor at the top

Think of a stack of bricks being laid from the ground up, where each new row pushes the older rows higher. Your epidermis grows in the same direction. New cells are made at the bottom and pushed upward. As they rise, they fill with keratin and die, and by the time they reach the top they are flat, dead armor that flakes away.

The epidermal strata are the layers of the epidermis, named in order from deep to superficial: basale, spinosum, granulosum, lucidum in thick skin only, and corneum.

The stratum basale is the deepest epidermal layer, the stem-cell layer where new keratinocytes are born and where melanocytes sit.
The stratum spinosum is the epidermal layer that lies directly above the stratum basale.
The stratum granulosum is the epidermal layer above the stratum spinosum and below the stratum corneum, or below the stratum lucidum in thick skin.
The stratum lucidum is an extra epidermal layer, found only in the thick skin of the palms and soles, between the stratum granulosum and the stratum corneum.
The stratum corneum is the most superficial epidermal layer, made of flat, dead, keratin-filled cells that flake off.

Two names describe where the lucidum appears. Thin skin is skin whose epidermis has four layers: basale, spinosum, granulosum, and corneum. Thick skin is the skin of the palms and soles, whose epidermis has five layers because it adds the stratum lucidum.

Worked example — Dataset M, the epidermal strata (canonical). You order the strata and then count them in each kind of skin.

Order them, deep to superficial: stratum basale → stratum spinosum → stratum granulosum → stratum lucidum (thick skin only) → stratum corneum.
Count thin skin: basale, spinosum, granulosum, corneum, so 1 + 1 + 1 + 1 = 4 layers.
Count thick skin (palms and soles): the same four plus the stratum lucidum, so 4 + 1 = 5 layers.
Find the difference: 5 − 4 = 1 extra layer, the lucidum, which sits between the granulosum and the corneum.

In words: the palm of your hand has one more epidermal layer than the back of your hand, and that layer is the stratum lucidum.

Misconception check. Students often write the strata out of order, putting the corneum at the bottom or the basale on top, because a list read top to bottom on a page feels like it should start at the surface. The cure is the life story of one cell. It is born in the basale, rises through the spinosum and the granulosum, crosses the lucidum only in thick skin, and ends as dead armor in the corneum. Say it as a sentence: born at the bottom, dead armor at the top.

5. Read the dermis: a papillary layer over a reticular layer

Picture the underside of an egg carton, a sheet of small rounded bumps. The top of your dermis looks like that where it meets the epidermis, with small bumps pushing up into the layer above.

The dermis has two layers, both a mesh of collagen and elastic fibers.

The papillary layer is the upper layer of the dermis, made of loose connective tissue that rises into the dermal papillae under the epidermis.
Dermal papillae are small bumps of the papillary layer that push up into the underside of the epidermis.
The reticular layer is the deeper layer of the dermis, made of dense irregular connective tissue.

Read it as structure to function. The papillary layer is loose and sits right under the epidermis, and its bumps reach up toward the avascular cells above. The reticular layer below is dense, packed with fibers. Together they carry the vessels, nerves, glands, and follicles the epidermis lacks.

Misconception check. Students often flip the two dermal layers, calling the top one dense and the deep one loose. It is tempting because "deeper" can sound "softer." The cure is to read them in Week 5's tissue language: papillary is loose connective tissue on top, with bumps; reticular is dense irregular connective tissue below. A second slip is calling the dermis an epithelium. It is connective tissue: cells scattered in a matrix of fibers.

6. Tour the accessory structures: hair, nails, and glands

Picture a single hair on your forearm in that cross-section. It grows out of a pocket in the dermis, and a thin muscle is attached to that pocket. Nearby sit two kinds of small glands, one making oil and one making sweat. Each part has one job.

A hair follicle is the structure in the skin from which a hair grows. A hair is a filament of keratin that grows from a follicle.
The arrector pili is the small muscle attached to a hair follicle that contracts on a nerve signal to stand the hair up, raising goosebumps.
A nail is a plate of tightly packed dead keratinocytes.
A sebaceous gland is a gland in the dermis that secretes sebum, which lubricates and waterproofs skin and hair.
Sebum is an oily lipid mixture, made by sebaceous glands, that lubricates and waterproofs skin and hair.
An eccrine sweat gland is a sweat gland that releases watery, hypotonic sweat with a little salt, which cools the body as it evaporates.
An apocrine sweat gland is a sweat gland of the armpits and groin that releases a thicker secretion which skin bacteria break down into odor.

Notice the Week 3 word in the eccrine line. Hypotonic means the sweat carries less solute than your cells; it is mostly water with a little salt. Notice the Week 2 word in the sebum line: a lipid, which is why it repels water.

Worked example — match the structure to the moment. You name the structure behind each everyday moment.

A cold gust raises bumps on your arm: the arrector pili muscles contract on their hair follicles.
Your forehead runs with watery sweat on a hot run: the eccrine sweat glands.
Your hair feels oily a day after washing: the sebaceous glands, making sebum.
Your armpits develop an odor by the end of a long, hot day: the apocrine sweat glands, whose thicker secretion skin bacteria break down.

Misconception check. Students often say sebaceous glands make sweat, because both kinds of gland sit in the dermis and both release something onto the skin. The cure is the product: sebaceous glands make sebum, an oily lipid mix; eccrine and apocrine glands make sweat. If a sentence says "oil glands cool you," the job and the gland have been mismatched.

7. List the jobs: what the skin does for you

Think about a day outside: you brush past a thorny bush, you feel the sun warm your arms, you sweat a little, and you never once dry out like a leaf in the sun. Your skin was doing five jobs at once.

The skin's five functions in this course are:

Protection. It guards against microbes, chemicals, and other hazards, and it limits water loss.
Sensation. It senses touch, surface temperature, and pain through nerve endings in the dermis.
Thermoregulation. It helps regulate body temperature, which Section 8 works through step by step.
Vitamin D synthesis. Under UV light, the epidermis begins the synthesis of vitamin D, a vitamin the body needs to absorb calcium and phosphorus.
Excretion in small amounts. It releases small amounts of waste in sweat.

Each job traces back to a structure. Protection comes from the keratin armor of the corneum and the melanin shield. Sensation comes from the nerve endings. Thermoregulation comes from the sweat glands and the dermal vessels. Vitamin D starts in the epidermis. Excretion rides out in sweat.

Misconception check. On a list of choices, students sometimes pick "digests food" or "makes the body's ATP" as a skin function, because the skin is so busy that every job sounds possible. Neither is on the skin's job list. Digestion belongs to the digestive system, an Anatomy and Physiology II system, and making ATP is a job every cell does for itself in its mitochondria, not a skin function. Check every candidate job against the five: protection, sensation, thermoregulation, vitamin D synthesis, and small amounts of excretion.

Caveat: the skin's job of making vitamin D only begins in the epidermis; this course stops at that first step and the bridge it builds to bone next week.

8. Run the loop on the skin: thermoregulation with named effectors

In Week 1 you read a body-temperature loop and named its effectors in general terms. This week you can point to each effector in the skin. Thermoregulation is the negative-feedback control of body temperature near its set point of about 37 °C, with sweat glands, dermal arterioles, and other structures as effectors.

Two more terms name what the skin's vessels do. Dermal arterioles are small blood vessels in the dermis that widen to shed heat and narrow to conserve it.

Vasodilation is the widening of blood vessels; when the dermal arterioles dilate, more warm blood flows near the surface and heat is shed.
Vasoconstriction is the narrowing of blood vessels; when the dermal arterioles constrict, less warm blood flows near the surface and heat is conserved.

The loop runs in the same three steps as before: receptors in the skin sense temperature, the control center compares the report with the set point of about 37 °C, and the effectors reverse the change.

Worked example — Dataset A, the body-temperature loop (illustrative readings constructed for this course, not real readings from a real person). The set point is 37.0 °C, and the model band runs from 36.5 to 37.5 °C. You read three readings on a clinic chart and name each effector by its structure.

Low reading: 36.0 °C. 37.0 − 36.0 = 1.0 °C below the set point, and 36.0 < 36.5, so it is below the model band of 36.5 to 37.5 °C. The effectors by structure: the dermal arterioles constrict, the eccrine sweat glands stop, the arrector pili muscles contract and raise goosebumps, and the skeletal muscles shiver. The response raises the temperature, opposing the drop.
Middle reading: 37.0 °C. 37.0 − 37.0 = 0.0 °C, so the reading is at the set point. Nothing needs correcting.
High reading: 38.5 °C. 38.5 − 37.0 = 1.5 °C above the set point, and 38.5 > 37.5, so it is above the band. The effectors by structure: the eccrine sweat glands release sweat that cools the skin as it evaporates, and the dermal arterioles dilate to shed heat. The response lowers the temperature, opposing the rise.
Feedback type: in every case the response opposes the change, so skin thermoregulation is negative feedback.

Say the high reading in words: the chart sits 1.5 °C above the set point, so the skin turns on its sweat and widens its vessels until the reading falls. A fever breaking in a sweat is the same effector at work as the temperature heads back down.

Misconception check. Students often say "sweating warms you up," because sweat shows up when you feel hot and flushed, as if it were part of the heat. The cure is the direction of the loop. Sweat is an effector against overheating, and it cools you as it evaporates from the skin. Evaporation carries heat away; sweating never warms you.

Pause and think, then choose one. (a) describe a moment from your own week when your skin shed heat, and name the two effectors you think were working. (b) describe a moment when your skin conserved heat, and name the effectors you think were working.

9. Read the depth: does the scrape bleed?

Two friends fall off their bikes on a gravel path. One knee is scuffed white and stings but does not bleed. The other knee beads with small dots of blood. You can say how deep each scrape went without a microscope, because of one structural fact you already know.

Worked example — read the depth of a scrape. You use the vascular rule from Section 2.

The first scrape does not bleed. The epidermis is avascular, so an injury that stays in the epidermis has no vessels to open. The first scrape stayed in the epidermis.
The second scrape bleeds. Blood vessels live in the dermis, so bleeding means the injury reached the vascular dermis.
In one line: no bleeding means the avascular epidermis only; bleeding means the dermis was reached.

That is the week's clinical connection: the depth of a skin injury read from structure. You name the layer and you stop there. How an injury heals, and any care for it, are not part of this course.

Misconception check. Students often guess that a scrape which bleeds stayed on the surface, because a bleeding scrape can look small. The cure is the structure, not the size. Size tells you how wide a scrape is; bleeding tells you how deep. Only the dermis has vessels, so any bleeding means the dermis was reached.

Key terms

Integumentary system — The skin together with its accessory structures, the hair, nails, and glands; it is the body's largest organ system. (first used: 1. Assemble an organ: the skin and its four tissues)
Epidermis — The thin outer layer of the skin, a keratinized stratified squamous epithelium with no blood vessels of its own, fed by diffusion from the dermis below. (first used: 2. Stack the layers: epidermis, dermis, and hypodermis)
Dermis — The thick, vascular connective-tissue layer of the skin beneath the epidermis, holding the skin's blood vessels, nerves, glands, and hair follicles. (first used: 2. Stack the layers: epidermis, dermis, and hypodermis)
Hypodermis — The layer of loose areolar connective tissue and fat beneath the dermis that insulates and cushions; it is usually not counted as part of the skin itself. (first used: 2. Stack the layers: epidermis, dermis, and hypodermis)
Keratinocyte — The main cell of the epidermis, which makes and stores keratin. (first used: 3. Meet the epidermal cells: keratinocytes and melanocytes)
Keratin — A tough protein made and stored by keratinocytes that gives the epidermis, hair, and nails their toughness and water resistance. (first used: 3. Meet the epidermal cells: keratinocytes and melanocytes)
Melanocyte — A cell in the deepest layer of the epidermis that makes the pigment melanin. (first used: 3. Meet the epidermal cells: keratinocytes and melanocytes)
Melanin — The pigment that colors skin and hair and absorbs ultraviolet light, shielding the living epidermal cells beneath from damage. (first used: 3. Meet the epidermal cells: keratinocytes and melanocytes)
Epidermal strata — The layers of the epidermis, named in order from deep to superficial: basale, spinosum, granulosum, lucidum in thick skin only, and corneum. (first used: 4. Order the strata: born at the bottom, dead armor at the top)
Stratum basale — The deepest epidermal layer, the stem-cell layer where new keratinocytes are born and where melanocytes sit. (first used: 4. Order the strata: born at the bottom, dead armor at the top)
Stratum spinosum — The epidermal layer that lies directly above the stratum basale. (first used: 4. Order the strata: born at the bottom, dead armor at the top)
Stratum granulosum — The epidermal layer above the stratum spinosum and below the stratum corneum, or below the stratum lucidum in thick skin. (first used: 4. Order the strata: born at the bottom, dead armor at the top)
Stratum lucidum — An extra epidermal layer, found only in the thick skin of the palms and soles, between the stratum granulosum and the stratum corneum. (first used: 4. Order the strata: born at the bottom, dead armor at the top)
Stratum corneum — The most superficial epidermal layer, made of flat, dead, keratin-filled cells that flake off. (first used: 4. Order the strata: born at the bottom, dead armor at the top)
Thin skin — Skin whose epidermis has four layers: basale, spinosum, granulosum, and corneum. (first used: 4. Order the strata: born at the bottom, dead armor at the top)
Thick skin — The skin of the palms and soles, whose epidermis has five layers because it adds the stratum lucidum. (first used: 4. Order the strata: born at the bottom, dead armor at the top)
Papillary layer — The upper layer of the dermis, made of loose connective tissue that rises into the dermal papillae under the epidermis. (first used: 5. Read the dermis: a papillary layer over a reticular layer)
Dermal papillae — Small bumps of the papillary layer that push up into the underside of the epidermis. (first used: 5. Read the dermis: a papillary layer over a reticular layer)
Reticular layer — The deeper layer of the dermis, made of dense irregular connective tissue. (first used: 5. Read the dermis: a papillary layer over a reticular layer)
Hair follicle — The structure in the skin from which a hair grows. (first used: 6. Tour the accessory structures: hair, nails, and glands)
Arrector pili — The small muscle attached to a hair follicle that contracts on a nerve signal to stand the hair up, raising goosebumps. (first used: 6. Tour the accessory structures: hair, nails, and glands)
Nail — A plate of tightly packed dead keratinocytes. (first used: 6. Tour the accessory structures: hair, nails, and glands)
Sebaceous gland — A gland in the dermis that secretes sebum, which lubricates and waterproofs skin and hair. (first used: 6. Tour the accessory structures: hair, nails, and glands)
Sebum — An oily lipid mixture, made by sebaceous glands, that lubricates and waterproofs skin and hair. (first used: 6. Tour the accessory structures: hair, nails, and glands)
Eccrine sweat gland — A sweat gland that releases watery, hypotonic sweat with a little salt, which cools the body as it evaporates. (first used: 6. Tour the accessory structures: hair, nails, and glands)
Apocrine sweat gland — A sweat gland of the armpits and groin that releases a thicker secretion which skin bacteria break down into odor. (first used: 6. Tour the accessory structures: hair, nails, and glands)
Vitamin D — A vitamin whose synthesis begins in the epidermis under ultraviolet light and which the body needs to absorb calcium and phosphorus. (first used: 7. List the jobs: what the skin does for you)
Thermoregulation — The negative-feedback control of body temperature near its set point of about 37 °C, with sweat glands, dermal arterioles, and other structures as effectors. (first used: 8. Run the loop on the skin: thermoregulation with named effectors)
Dermal arterioles — Small blood vessels in the dermis that widen to shed heat and narrow to conserve it. (first used: 8. Run the loop on the skin: thermoregulation with named effectors)
Vasodilation — The widening of blood vessels; when the dermal arterioles dilate, more warm blood flows near the surface and heat is shed. (first used: 8. Run the loop on the skin: thermoregulation with named effectors)
Vasoconstriction — The narrowing of blood vessels; when the dermal arterioles constrict, less warm blood flows near the surface and heat is conserved. (first used: 8. Run the loop on the skin: thermoregulation with named effectors)

Summary

The skin is an organ built from all four tissue types.
Epidermis on top, dermis below, hypodermis under the skin proper.
The epidermis is avascular; the dermis carries the vessels.
Keratin is armor; melanin is color and a UV shield.
Strata, deep to superficial: basale, spinosum, granulosum, lucidum, corneum.
Thin skin has four strata; thick skin has five.
The papillary layer is loose; the reticular layer is dense irregular.
Sebaceous glands make sebum, not sweat.
Eccrine sweat cools; apocrine glands sit in the armpits and groin.
Five jobs: protection, sensation, thermoregulation, vitamin D, small excretion.
Thermoregulation is negative feedback with named skin effectors.
No bleeding means epidermis only; bleeding means the dermis was reached.

Check your understanding

Work these before looking at the answers — they are ungraded, and the answers (with reasoning) are at the bottom of the page.

In an illustrative reading made up for this question, a thermometer on a camping trip shows 36.4 °C. Use a set point of 37.0 °C and the model band of 36.5 to 37.5 °C. Find how far below the set point the reading sits, place it against the band, and name two skin effectors that would respond.
A study card made up for this question lists thin skin as "basale, spinosum, lucidum, corneum." Say what is wrong and write the correct list and count.
In a made-up gardening mishap, a rose thorn leaves a thin white line on one wrist that never bleeds and a short line on the other wrist with a bead of blood. Which layer did each line reach, and how do you know?
A classmate says, "Goosebumps and sweat are made by the same gland." Using this week's structures, explain what is right and what is wrong about that statement.

Answers (worked reasoning — read after attempting)

Subtract the reading from the set point, so 37.0 − 36.4 = 0.6 °C below. Place it: 36.4 < 36.5, so it is below the model band. The loop opposes the drop, so the effectors work to conserve and make heat: the dermal arterioles constrict, the eccrine sweat glands stop, and the arrector pili muscles raise goosebumps (shivering may join them).
The card put the lucidum into thin skin. The lucidum appears only in the thick skin of the palms and soles. Thin skin, deep to superficial, is basale, spinosum, granulosum, corneum: 1 + 1 + 1 + 1 = 4 layers. The card also dropped the granulosum.
The line that never bled stayed in the epidermis, because the epidermis is avascular and has no vessels to open. The line with a bead of blood reached the dermis, because the dermis is where the blood vessels are. Bleeding tells you depth, not size.
What is right: both goosebumps and sweat are skin responses to temperature, and both are effectors in the thermoregulation loop. What is wrong: neither comes from "the same gland." Goosebumps come from the arrector pili, a small muscle on each hair follicle, not a gland at all. Watery cooling sweat comes from the eccrine sweat glands. And goosebumps answer cold, while sweat answers heat.

Next up in this module: the lecture, the slides, the Readings & Resources page, the tutorial, the practice set, and the quiz. Next week your skin's vitamin D leads into bone and its calcium-storage mechanism.

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