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human anatomy
1. How does skin work?
Even at its thickest point, our skin is only a few millimeters thick. But it is still our heaviest
and largest organ, making up about one seventh of our body weight: Depending on your
height and body mass, it weighs between 3.5 and 10 kilograms (7.5 and 22 pounds) and
has a surface area of 1.5 to 2 square meters. This goes to show how important skin is for
your body and metabolism.
What does skin do?
Skin has a lot of different functions. It is a stable but flexible outer covering that acts as
barrier, protecting your body from harmful things in the outside world such as moisture, the
cold and sun rays, as well as germs and toxic substances.
Just looking at someone’s skin can already tell you a lot – for instance, about their age and
health. Changes in skin color or structure can be a sign of a medical condition. For
example, people with too few red blood cells in their blood may look pale, and people who
have hepatitis have yellowish skin.
Skin also plays an important role in regulating your body temperature. It helps prevent
dehydration and protects you from the negative effects of too much heat or cold. And it
allows your body to feel sensations such as warmth, cold, pressure, itching and pain. Some
of these sensations trigger a reflex, like automatically pulling your hand back if you
accidentally touch a hot stove.
Skin also functions as a large storeroom for the body: The deepest layer of skin can store
water, fat and metabolic products. And it produces hormones that are important for the
whole body.
If skin is injured, the blood supply to the skin increases in order to deliver various
substances to the wound so it is better protected from infections and can heal faster. Later
on, new cells are produced to form new skin and blood vessels. Depending on how deep
the wound is, it heals with or without a scar.
To be able to do all of these things, skin consists of three different layers: the outer layer
(epidermis), the middle layer (dermis) and the deepest layer (subcutis). Depending on
where it is on your body and the demands made on it, your skin varies in thickness. The
thickness of your skin depends on your age and sex too: Older people generally have
thinner skin than younger people do, and men generally have thicker skin than women do.
The outer layer (epidermis)
The outermost layer of skin which you can see is called the epidermis. It is mostly made up
of cells that produce keratin (keratinocytes). These cells are gradually pushed to the
surface of the skin by newer cells, where they harden and then eventually die off. The
hardened keratinocytes (corneocytes) are packed closely together and seal the skin off
from the outside environment.
2. The epidermis constantly renews itself: New cells are made in the lower layers of the
epidermis. These move to the surface within four weeks. This constant renewal serves to
replace the cells that are lost and fall to the ground as tiny flakes of skin when the skin is
rubbed. The cells in the epidermis grow faster in response to pressure or rubbing. The
amount of skin flakes that are shed remains the same, though. As a result, the layer of
hardened skin on the surface gradually becomes thicker and a callus develops. The skin
does this to protect itself – to better withstand pressure and rubbing.
Only rarely is the balance of new cell production and old cell shedding affected by illness.
Examples include infections, autoimmune disorders or genetic diseases that cause
increased growth of rough, scaly skin on the entire body. Hardened skin in only one place
may be a sign of non-melanoma skin cancer or skin changes that may develop into cancer.
Depending on where it is on the body, the epidermis varies in thickness. For instance, it is
only 0.3 millimeters thick on your elbows and the back of your knees, and up to 4
millimeters thick on other parts of your body such as the soles of your feet and palms of
your hands.
The epidermis also contains other types of cells with special functions:
Melanocytes produce and store a black pigment called melanin. They produce more
melanin when your skin is exposed to sunlight, which is why it becomes darker. This
protects the skin from the sun’s harmful UV rays.
Lymphocytes and Langerhans cells play an important role in fighting germs. They
“grab” the germs and take them to the nearest lymph node.
Merkel cells are special nerve cells in the skin that enable you to sense pressure.
3. The middle layer (dermis)
Under the epidermis, firmly stuck to it, lies the middle layer of skin (the dermis). It is made
up of a dense network of tough, elastic collagen fibers. These make the skin strong and
robust, while at the same time stretchy. If skin is stretched a lot – for instance the skin
covering a pregnant woman’s belly – the dermis might tear. The torn dermis can be seen
as light lines (stretch marks).
In places, the dermis bulges into the connective tissue that surrounds our muscles and
bones and connects them with the skin.
The dermis contains a network of nerve fibers and very small blood vessels called
capillaries. Nutrients and oxygen in the blood pass from the capillaries into cells. The other
main function of the capillaries is to help your body cool down if it gets too hot. The dermis
is also the skin layer that contains the most sensory (feeling) cells and sweat glands.
4. The deepest layer (subcutis)
The subcutis (also known as the subcutaneous layer or hypodermis) is mostly made up of
fat and connective tissue. In the subcutis, between the folds of dermis that bulge into it,
there are tiny cavities. These cavities are filled with storage tissue made out of fat and
water. The fat acts as a shock absorber, protecting bones and joints from blows or bumps.
It serves as insulation too. What’s more, many hormones are produced in the fat cells of
the subcutis. One example is vitamin D, which is an essential vitamin and is made when
the skin is exposed to sunlight.
The subcutis and dermis both contain blood vessels and lymph vessels too, as well as other
things like nerves, sweat glands, sebaceous (oil) glands, scent glands and hair
rootBoundless Anatomy and Physiology
Functions of the IntegumentarySystem
Protection
The skin provides an overlaying protective barrier from the environment and pathogens
while contributing to the adaptive immune system.
Key Points
The skin provides a protective barrier from the external environment and prevents
dehydration.
Langerhans cells in the skin also contribute to protection as they are part of the adaptive
immune system.
The integumentary system protects the body’s internal living tissues and organs, protects
against invasion by infectious organism, and protects the body from dehydration.
Key Terms
vitamin D: An important vitamin synthesized thanks to the skin.
melanocytes: Cells that help protect our body from radiological damage.
Langerhans cells: Langerhans cells are dendritic cells (antigen-presenting
immune cells) of the skin and mucosa that contain large granules.
The skin helps protect our body’s internal structures from physical, chemical, biological,
radiological, and thermal damage as well as damage from starvation and malnutrition.
5. Physical and Chemical Damage
Human skin: A diagram of human skin.
The skin is composed of tough skin cells as well as a tough protein called keratin that
guard tissues, organs, and structures underneath the skin against physical damage from
minor cuts, scratches, and abrasions. Because our skin is tough and largely waterproof, it
helps protect internal structures from chemical irritants such as man-made detergents or
even natural irritants like poison ivy.
Otherwise, these dangerous chemicals would seep into our sensitive internal environment.
The waterproof nature of our skin also ensures that important molecules stay within our
body.
Biological Damage
The skin also contains important cells called Langerhans cells. These cells help our
immune system fight off infectious biological agents, like bacteria that try to get further
into our body through skin that may have been compromised by physical damage.
6. Sebaceous glands associated with the skin secrete substances that help fight off
potentially dangerous microorganisms as well. These glands also secrete substances that
help keep our skin hydrated, and thus more resistant to bacterial invasion.
Radiological Damage
Our skin also contains melanocytes that produce a pigment called melanin. This protects
the body from radiological damage via the sun’s UV radiation (or that from tanning beds).
Other Protective Roles
Part of our skin is made up of fat. This fat serves three large purposes:
1. It helps cushion internal structures against any physical blows.
2. It acts as a food source, protectingour body fromthe effects of starvation.
3. It helps insulate us against cold temperatures.
Our skin is also closely associated with sweat glands that help protect us from high
temperatures by cooling us off through the process of evaporation. These glands also help
to excrete potentially dangerous substances, like urea, out of the body.
All sorts of sensory receptors are found within the skin as well. These help move our body
parts away from potential sources of damage, like hot stoves, when they sense danger,
thereby protecting our body from great harm.
Finally, the skin is also important for the synthesis of vitamin D, which is an important
vitamin for the building of strong and healthy bones. Ergo, the skin protects the body from
fractures if we do not otherwise get enough of this vitamin from food-based sources.
Thermoregulation
The integumentary system keeps body temperature within limits even when environmental
temperature varies; this is called thermoregulation.
Explain the skin’s role in thermoregulation
Key Points
The skin’s immense blood supply helps regulate temperature: dilated vesselsallow for heat
loss, while constricted vesselsretain heat.
The skin regulates body temperature with its blood supply.
The skin assists in homeostasis.
Humidity affects thermoregulation by limiting sweat evaporation and thus heat loss.
7. Key Terms
Evaporation: What happens when water crosses the skinvia sweat glandsand then
dissipates into the air; thisprocesscools body temperature to within the body’s tolerance
range.
homeostasis: The ability of a systemor living organismto adjust its internal environment to
maintain a stable equilibrium; such as the ability of warm-blooded animals to maintain a
constant temperature.
vasoconstriction: The constriction (narrowing) of a blood vessel.
arrector pili: Any of the small muscles attached to hair follicles in mammals; when the
muscles contract they cause the hairs to stand on end.
The integumentary system functions in thermoregulation—the ability of an organism to
keep its body temperature within certain boundaries—even when the surrounding
temperature is very different. This process is one aspect of homeostasis: a dynamic state
of stability between an animal’s internal and external environment.
Human skin: This image details the parts of the integumentary system.
The skin assists in homeostasis (keeping different aspects of the body constant, e.g.,
temperature). It does this by reacting differently to hot and cold conditions so that the
inner body temperature remains more or less constant.
8. The Skin’s Role in Cooling the Body
The skin is an incredibly large organ. It is about 2 meters squared (depending on the size of
the individual). Owing to its location at the barrier of the environment and our internal
selves, and its relatively very large surface area, it is plays an incredibly important role in
thermoregulation.
This is because in a healthy individual, when all else is held equal, their body is constantly
generating heat as a result of its various metabolic and physical processes. At rest, such an
individual is expected to increase their body temperature by 1 C every 5 minutes as a
result of these processes. Left unregulated, this would kill a person quite quickly.
The process of skin-based thermoregulation occurs through several means. The first way
involves the abundance of blood vessels found in the dermis, the middle layer of the
skin. If the body must cool down, the body vasodilates these blood vessels.
The Skin’s Role in Keeping Us Warm
Anatomy of the skin: The skin is the largest organ of the integumentary system, made up of multiple layers of
ectodermal tissue, and guards the underlying muscles, bones, ligaments, and internal organs.
On the other hand, if the body needs to prevent the loss of excess heat, such as on a cool
day, it will end up constricting the blood vessels of our skin. This process is known as
vasoconstriction.
Since the blood vessels are narrower than they were before, less blood flows through the
skin and thus less heat can escape into the environment via radiation, convection, and
conduction. The body will also limit or stop the process of sweating to minimize any
evaporative heat loss.
In addition, our body thermoregulates using our hair. The arrector pili muscles contract
(piloerection) and lift the hair follicles upright. This makes the hairs stand on end, which
acts as an insulating layer, trapping heat. This is also how goose bumps are caused, since
humans don’t have very much hair and the contracted muscles can easily be seen.
While this hair-based method of thermoregulation is effective in many mammals and birds
owing to their large and thick amounts of fur and feathers (respectively), the relative
effectiveness of this method of thermoregulation in humans is in question since we have
little to no body hair in comparison.
Finally, while technically not a thermoregulatory mechanism, the fat associated with our
skin does help insulate our body and therefore increases body temperature as a result.
9. Cutaneous Sensation
The somatosensory system is composed of the receptors and processing centers to
produce the sensory modalities, such as touch and pain.
Key Points
The somatosensory is the systemof nerve cells that responds to changesto the external or
internal state of the body.
Receptors are spread throughout the body, with large numbers found in the skin.
Several distinctreceptor types formthe somatosensory systemincluding thermoreceptors
(heat), nociceptors (pain), and mechanoreceptors (pressure).
There are four types of mechanoreceptors that respond to different pressure stimui and
provide a wide range of mechanical sensitivity—they are the keys for fine motor control.
Key Terms
somatosensory system: A diverse sensory systemcomposed of the receptors and processing
centers to produce the sensory modalities such as touch, temperature, proprioception (body
position), and nociception(pain).
sensory receptor: A sensory nerveending that recognizesa stimulus in the internal or
external environment of an organism.
The Somatosensory System
The somatosensory is the system of nerve cells that responds to changes to the external or
internal state of the body, predominately through the sense of touch, but also by the
senses of body position and movement.
Spread through all major parts of the body, it consists of sensory receptors and sensory
neurons in the periphery (for example, skin, muscle, and organs), along with deeper
neurons within the central nervous system.
While touch is considered one of the five traditional senses, the impression of touch is
actually formed from several diverse stimuli using different receptors:
Thermoreceptors (temperature)
Nociceptors (pain)
Mechanoreceptors (pressure)
Transmission of information from the receptors passes via sensory nerves through tracts in
the spinal cord and into the brain. Processing primarily occurs in the primary
somatosensory area in the parietal lobe of the cerebral cortex.
10. Thermo receptors
Mammals have at least two types of sensors: those that detect heat and those that detect
cold.
Upon deviation from the norm ,sensory receptors trigger an action potential that can
provide feedback or lead to alterations in behavior in order to maintain homoeostasis. Two
receptors that exhibit the ability to detect changes in temperature include Krause end
bulbs (cold) and Ruffini endings (heat).
Nociceptors
A nociceptor is a sensory nerve cell that responds to damaging or potentially damaging
stimuli by sending signals to the spinal cord and brain. Nociceptors can respond to
excessive thermal, mechanical, or chemical stimulation and often result the generation of
an involuntary motor respons—for example, pulling a hand away from a hot surface.
Mechanoreceptors
Mechanoreceptors are sensory receptors that respond to pressure and vibration. Four key
types of mechanoreceptor have been described based on their response to stimulation and
receptive field.
Receptors can either induce a slow response to stimulation, whereby a constant activation
is initiated, or a fast response, whereby activation is only initiated at the beginning and end
of stimulation. The receptive field—the region in which a receptor can sense an effect—can
vary from small to large.
The Merkel receptor is a disk-shaped receptor located near the border between the
epidermis and dermis. It demonstrates a slow response and has a small receptive field; it is
useful for detecting steady pressure fromsmall objects, such as when grippingsomething
with the hand.
The Meissner corpuscle is a stack of flattened cells located in the dermis, near the
epidermis. It demonstrates a rapid response and has a small receptive field; it is useful for
detecting textureor movement of objects againstthe skin.
The Ruffini cylinder is located in the dermis and has many branched fibers inside a
cylindrical capsule. It demonstratesa slow response and has a large receptivefield; it is good
for detecting steady pressure or stretching, such as during the movement of a joint.
The Pacinian corpuscle is a layered, onion-likecapsule surroundinga nerve fiber. It is located
deep in the dermis, in the subcutaneous fat. It demonstrates a fast response and has a large
receptive field; it is useful for detectinglarge changes in the environment, such as vibrations.
Together they provide a wide range of mechanical sensitivity that enables fine motor
control.
11. Metabolic Functions
One of the metabolic functions of the skin is the production of vitamin D3 when ultraviolet
light reacts with 7-dehydrocholesterol.
Key Points
Vitamin D refers to a group of fat-soluble steroids responsiblefor increasing intestinal
absorption of calcium, iron, magnesium, phosphate, and zinc.
Foods rich in vitamin D are relatively scarce and so the body synthesisesthe majority of
vitamin D itself, in the skin.
Vitamin D deficiency is associated with poor development of bones in children and a
softening of bones in adults.
Vitamin D3 is made in the skin when 7-dehydrocholesterol reacts with ultraviolet light.
Vitamin D is produced in the two innermost strata of the epidermis. Cholecalciferol (D3) is
produced photochemically in the skin from7-dehydrocholesterol.
Vitamin D is produced in the two innermost strata of the epidermis, the stratumbasale and
stratumspinosum.
Key Term
7-dehydrocholesterol: 7-dehydrocholesterol is a cholesterol precursorthat is converted to
vitamin D3 in the skin, therefore functioningas provitamin-D3.
The integumentary system is the largest of the body’s organ systems, made up of the skin
and its associated appendages. The integumentary system distinguishes, separates, and
protects the organism from its surroundings, but also plays a key metabolic function, as
the major region for vitamin D production.
What is Vitamin D?
Vitamin D refers to a group of fat-soluble steroids responsible for increasing intestinal
absorption of calcium, iron, magnesium, phosphate, and zinc. In humans, the most
important compounds in this group are vitamin D3 (also known as cholecalciferol) and
vitamin D2 (ergocalciferol). Cholecalciferol and ergocalciferol can be ingested from the diet
and from supplements, however very few foods are rich in vitamin D; and so synthesis
within the skin is a key source.
Vitamin D deficiency is associated with impaired bone development in children, which
leads to the development of rickets and a softening of bones in adults. Deficiency in
vitamin D has been termed a modern disorder associated with both a poorer diet and
reduced time spent outside.
12. Vitamin D Synthesis
Vitamin D: The chemical structure of vitamin D.
The human skin consists of three major layers: the epidermis, dermis, and hypodermis. The
epidermis forms the outermost layer, providing the initial barrier to the external
environment. Beneath this, the dermis comprises two sections, the papillary and reticular
layers, and contains connective tissues, vessels, glands, follicles, hair roots, sensory nerve
endings, and muscular tissue. The deepest layer is the hypodermis, which is primarily
made up of adipose tissue.
Vitamin D is produced in the two innermost strata of the epidermis, the stratum basale and
stratum spinosum.
Vitamin D3 is made in the skin when the 7-dehydrocholesterol reacts with ultraviolet light
of UVB type at wavelengths between 280 and 315 nm, with peak synthesis occurring
between 295 and 297 nm.
Depending on the intensity of UVB rays and the minutes of exposure, an equilibrium can
develop in the skin, and vitamin D degrades as fast as it is generated.
Vitamin D from the diet or that is synthesized by the body is biologically inactive; activation
requires enzymatic conversion in the liver and kidney.
Metabolism and pathway map for vitamin D: Vitamin D synthesis pathway
Blood Supply to the Epidermis
The blood vessels in the dermis provide nourishment and remove waste from its own cells
and from the stratum basale of the epidermis.
Key Points
The epidermis contains no blood vessels, and cellsin the deepestlayers are nourished by
diffusion fromblood capillaries present in the upper layers of the dermis.
The papillary region of the dermis is composed of loose areolar connective tissue. This is
named for its fingerlike projections called papillae, that extend toward the epidermis and
contain terminal networks of blood capillaries.
The control of blood vessels within the dermis forms a key part of the body’s
thermoregulatory capacity.
13. Key Terms
papillary region: The uppermost region of the dermis that is adjacentto the epidermis.
reticular region: The lower region of the dermis.
The epidermis does not contain blood vessels; instead, cells in the deepest layers are
nourished by diffusion from blood capillaries that are present in the upper layers of the
dermis. Diffusion provides nourishment and waste removal from the cells of the dermis, as
well as for the cells of the epidermis.
The dermis:The distributionof the blood vessels in the skin of the sole of the foot. Corium—labeled atupper right—
is an alternate term for dermis. Blood vessels that supply the capillaries of the papillary region are seen running
through the reticular layer.
The dermis is the layer of skin beneath the epidermis that consists of connective tissue
and cushions the body from stress and strain. The dermis is tightly connected to the
epidermis by a basement membrane.
14. The dermis is structurally divided into two areas: a superficial area adjacent to the
epidermis, called the papillary region, and a deep, thicker area known as the reticular
region.
The papillary region is composed of loose areolar connective tissue. This is named for its
fingerlike projections called papillae, that extend toward the epidermis and contain
terminal networks of blood capillaries.
The reticular region lies under the papillary region and is usually much thicker. It is
composed of dense, irregular connective tissue. The reticular region receives its name from
the dense concentration of collagenous, elastic, and reticular fibers that weave throughout
it.
These protein fibers give the dermis its typical properties of strength, extensibility, and
elasticity. Blood vessels that supply the capillaries of the papillary region run through the
reticular region.
Control of the blood supply to the dermis forms part of the body’s thermoregulatory
capacity. Increasing blood flow, which makes the skin appear redder, will increase the loss
of radiant heat through the skin, whereas constricting blood flow, making the skin appear
paler, reduces heat loss.
Excretion and Absorption
The integumentary system functions in absorption (oxygen and some medications) and
excretion (e.g., perspiration via the eccrine glands).
LEARNING OBJECTIVE
Describe the role of glands in excretion and absorption
KEY TAKEAWAYS
Key Points
Eccrine glands, the major sweat glands of the human body, produce a clear, odorless
substance, consisting primarily of water and NaCl. NaCl is reabsorbedin the duct to reduce
salt loss.
Apocrine sweat glands are found only in certain locations of the body: the axillae (armpits),
areola and nipples of the breast, ear canal, perianal region, and some parts of the external
genitalia.
The sebaceous glands secrete an oily/waxy matter called sebumto lubricate and waterproof
the skin and hair of mammals. In humans, they are found in greatest abundance on the face
and scalp, though they are distributed throughout all skin sites except the palms and soles.
15. A major function of the integumentary system is absorption and excretion.
Excretion
There are numerous secretory glands present in the skin which secrete a large range of
distinct fluids.
Perspiration, or sweating, is the production of fluids secreted by the sweat glands in the
skin of mammals. Two types of sweat glands can be found in humans: eccrine glands and
apocrine glands.
Eccrine glands are the major sweat glands of the human body, found in virtually all skin.
They produce a clear, odorless substance consisting primarily of water and NaCl (note that
the odor from sweat is due to bacterial activity on the secretions of the apocrine glands).
NaCl is reabsorbed in the duct to reduce salt loss. Eccrine glands are active in
thermoregulation and are stimulated by the sympathetic nervous system.
Sweat gland: A sectional view of the skin (magnified), with the eccrine glands highlighted.
16. Apocrine sweat glands are inactive until they are stimulated by hormonal changes in
puberty. Apocrine sweat glands are mainly thought to function as olfactory pheromones,
chemicals important in attracting a potential mate. The stimulus for the secretion of
apocrine sweat glands is adrenaline, which is a hormone carried in the blood.
The sebaceous glands are microscopic glands in the skin that secrete an oily/waxy matter,
called sebum, to lubricate and waterproof the skin and hair of mammals. In humans, they
are found in greatest abundance on the face and scalp, though they are distributed
throughout all skin sites except the palms and soles. In the eyelids, meibomian sebaceous
glands secrete a special type of sebum into tears.
Absorption
Due to the absorptive capabilities of skin, the cells comprising the outermost 0.25–0.40
mm of the skin can be supplied by external oxygen rather than via the underlying capillary
network. Additionally certain medications can be administered through the skin.
The most common mechanism of administration through the skin is the use of ointments
or an adhesive patch, such as the nicotine patch or iontophoresis. Iontophoresis, also
called electromotive drug administration, is a technique that uses a small electric charge
to deliver a medicine or other chemical through the skin.
Overview of Metabolic Reactions
Metabolic processes are constantly taking place in the body. Metabolism is the sum of all
of the chemical reactions that are involved in catabolism and anabolism. The reactions
governing the breakdown of food to obtain energy are called catabolic reactions.
Conversely, anabolic reactions use the energy produced by catabolic reactions to
synthesize larger molecules from smaller ones, such as when the body forms proteins by
stringing together amino acids. Both sets of reactions are critical to maintaining life.
Because catabolic reactions produce energy and anabolic reactions use energy, ideally,
energy usage would balance the energy produced. If the net energy change is positive
(catabolic reactions release more energy than the anabolic reactions use), then the body
stores the excess energy by building fat molecules for long-term storage. On the other
hand, if the net energy change is negative (catabolic reactions release less energy than
anabolic reactions use), the body uses stored energy to compensate for the deficiency of
energy released by catabolism.
Catabolic Reactions
Catabolic reactions break down large organic molecules into smaller molecules, releasing
the energy contained in the chemical bonds. These energy releases (conversions) are not
100 percent efficient. The amount of energy released is less than the total amount
17. contained in the molecule. Approximately 40 percent of energy yielded from catabolic
reactions is directly transferred to the high-energy molecule adenosine triphosphate (ATP).
ATP, the energy currency of cells, can be used immediately to power molecular machines
that support cell, tissue, and organ function. This includes building new tissue and repairing
damaged tissue. ATP can also be stored to fulfill future energy demands. The remaining 60
percent of the energy released from catabolic reactions is given off as heat, which tissues
and body fluids absorb.
Structurally, ATP molecules consist of an adenine, a ribose, and three phosphate groups
(Figure 1). The chemical bond between the second and third phosphate groups, termed a
high-energy bond, represents the greatest source of energy in a cell. It is the first bond that
catabolic enzymes break when cells require energy to do work. The products of this
reaction are a molecule of adenosine diphosphate (ADP) and a lone phosphate group (Pi).
ATP, ADP, and Pi are constantly being cycled through reactions that build ATP and store
energy, and reactions that break down ATP and release energy.
18. Figure 1. Structure of ATP Molecule. Adenosine triphosphate (ATP) is the energy molecule
of the cell. During catabolic reactions, ATP is created and energy is stored until needed
during anabolic reactions.
The energy from ATP drives all bodily functions, such as contracting muscles, maintaining
the electrical potential of nerve cells, and absorbing food in the gastrointestinal tract. The
metabolic reactions that produce ATP come from various sources (Figure 2).
19. Figure 2. Sources of ATP. During catabolic reactions, proteins are broken down into amino
acids, lipids are broken down into fatty acids, and polysaccharides are broken down into
monosaccharides. These building blocks are then used for the synthesis of molecules in
anabolic reactions.
Of the four major macromolecular groups (carbohydrates, lipids, proteins, and nucleic
acids) that are processed by digestion, carbohydrates are considered the most common
source of energy to fuel the body. They take the form of either complex carbohydrates,
polysaccharides like starch and glycogen, or simple sugars (monosaccharides) like glucose
and fructose. Sugar catabolism breaks polysaccharides down into their individual
monosaccharides. Among the monosaccharides, glucose is the most common fuel for ATP
production in cells, and as such, there are a number of endocrine control mechanisms to
20. regulate glucose concentration in the bloodstream. Excess glucose is either stored as an
energy reserve in the liver and skeletal muscles as the complex polymer glycogen, or it is
converted into fat (triglyceride) in adipose cells (adipocytes).
Among the lipids (fats), triglycerides are most often used for energy via a metabolic
process called β-oxidation. About one-half of excess fat is stored in adipocytes that
accumulate in the subcutaneous tissue under the skin, whereas the rest is stored in
adipocytes in other tissues and organs.
Proteins, which are polymers, can be broken down into their monomers, individual amino
acids. Amino acids can be used as building blocks of new proteins or broken down further
for the production of ATP. When one is chronically starving, this use of amino acids for
energy production can lead to a wasting away of the body, as more and more proteins are
broken down.
Nucleic acids are present in most of the foods you eat. During digestion, nucleic acids
including DNA and various RNAs are broken down into their constituent nucleotides. These
nucleotides are readily absorbed and transported throughout the body to be used by
individual cells during nucleic acid metabolism.
Anabolic Reactions
In contrast to catabolic reactions, anabolic reactions involve the joining of smaller
molecules into larger ones. Anabolic reactions combine monosaccharides to form
polysaccharides, fatty acids to form triglycerides, amino acids to form proteins, and
nucleotides to form nucleic acids. These processes require energy in the form of ATP
molecules generated by catabolic reactions. Anabolic reactions, also called biosynthesis
reactions, create new molecules that form new cells and tissues, and revitalize organs.
Hormonal Regulation of Metabolism
Catabolic and anabolic hormones in the body help regulate metabolic processes. Catabolic
hormones stimulate the breakdown of molecules and the production of energy. These
include cortisol, glucagon, adrenaline/epinephrine, and cytokines. All of these hormones
are mobilized at specific times to meet the needs of the body. Anabolic hormones are
required for the synthesis of molecules and include growth hormone, insulin-like growth
factor, insulin, testosterone, and estrogen. Table 1 summarizes the function of each of the
catabolic hormones and Table 2 summarizes the functions of the anabolic hormones.
21. Catabolic Hormones(Table 1)
Hormone Function
Cortisol
Released fromthe adrenal gland in response to stress; its main role is
to increase blood glucose levels by gluconeogenesis (breakingdown
fats and proteins)
Glucagon
Released fromalpha cells in the pancreas either when starvingor
when the body needs to generate additional energy; it stimulates the
breakdown of glycogen in the liver to increase blood glucose levels; its
effect is the opposite of insulin; glucagon and insulin are a part of a
negative-feedback systemthat stabilizesblood glucose levels
Adrenaline/epinephrine
Released in response to the activation of the sympathetic nervous
system; increases heart rate and heart contractility, constrictsblood
vessels, is a bronchodilator that opens(dilates) the bronchi of the
lungs to increase air volume in the lungs, and stimulates
gluconeogenesis
22. AnabolicHormones (Table2)
Hormone Function
Growth
hormone (GH)
Synthesized and released fromthe pituitary gland; stimulates the growth of
cells, tissues, and bones
Insulin-like
growth factor
(IGF)
Stimulates the growth of muscle and bone while also inhibiting cell death
(apoptosis)
Insulin
Produced by the beta cells of the pancreas; plays an essential rolein
carbohydrate and fat metabolism, controls blood glucose levels, and promotes
the uptake of glucose into body cells; causescellsin muscle, adiposetissue,
and liver to take up glucose fromthe blood and store it in the liver and muscle
as glucagon; its effect isthe opposite of glucagon; glucagonand insulin are a
part of a negative-feedbacksystemthat stabilizesblood glucose levels
Testosterone
Produced by the testes in males and the ovaries in females; stimulates an
increase in muscle mass and strength as well as the growth and
strengthening of bone
Estrogen
Produced primarily by the ovaries, it is also produced by the liver and adrenal
glands; its anabolic functions include increasingmetabolismand fat
deposition
23. Disorders of the…
Metabolic Processes: Cushing Syndrome and Addison’s Disease
As might be expected for a fundamental physiological process like metabolism, errors or
malfunctions in metabolic processing lead to a pathophysiology or—if uncorrected—a
disease state. Metabolic diseases are most commonly the result of malfunctioning proteins
or enzymes that are critical to one or more metabolic pathways. Protein or enzyme
malfunction can be the consequence of a genetic alteration or mutation. However, normally
functioning proteins and enzymes can also have deleterious effects if their availability is not
appropriately matched with metabolic need. For example, excessive production of the
hormone cortisol (see Table 1) gives rise to Cushing syndrome. Clinically, Cushing syndrome
is characterized by rapid weight gain, especially in the trunk and face region, depression,
and anxiety. It is worth mentioning that tumors of the pituitary that produce
adrenocorticotropic hormone (ACTH), which subsequently stimulates the adrenal cortex to
release excessive cortisol, produce similar effects. This indirect mechanism of cortisol
overproduction is referred to as Cushing disease.
Patients with Cushing syndrome can exhibit high blood glucose levels and are at an
increased risk of becoming obese. They also show slow growth, accumulation of fat
between the shoulders, weak muscles, bone pain (because cortisol causes proteins to be
broken down to make glucose via gluconeogenesis), and fatigue. Other symptoms include
excessive sweating (hyperhidrosis), capillary dilation, and thinning of the skin, which can
lead to easy bruising. The treatments for Cushing syndrome are all focused on reducing
excessive cortisol levels. Depending on the cause of the excess, treatment may be as
simple as discontinuing the use of cortisol ointments. In cases of tumors, surgery is often
used to remove the offending tumor. Where surgery is inappropriate, radiation therapy can
be used to reduce the size of a tumor or ablate portions of the adrenal cortex. Finally,
medications are available that can help to regulate the amounts of cortisol.
Insufficient cortisol production is equally problematic. Adrenal insufficiency, or Addison’s
disease, is characterized by the reduced production of cortisol from the adrenal gland. It
can result from malfunction of the adrenal glands—they do not produce enough cortisol—or
it can be a consequence of decreased ACTH availability from the pituitary. Patients with
Addison’s disease may have low blood pressure, paleness, extreme weakness, fatigue,
slow or sluggish movements, lightheadedness, and salt cravings due to the loss of sodium
and high blood potassium levels (hyperkalemia). Victims also may suffer from loss of
appetite, chronic diarrhea, vomiting, mouth lesions, and patchy skin color. Diagnosis
typically involves blood tests and imaging tests of the adrenal and pituitary glands.
Treatment involves cortisol replacement therapy, which usually must be continued for life.
Oxidation-Reduction Reactions
The chemical reactions underlying metabolism involve the transfer of electrons from one
compound to another by processes catalyzed by enzymes. The electrons in these reactions
commonly come from hydrogen atoms, which consist of an electron and a proton. A
24. molecule gives up a hydrogen atom, in the form of a hydrogen ion (H+) and an electron,
breaking the molecule into smaller parts. The loss of an electron, or oxidation, releases a
small amount of energy; both the electron and the energy are then passed to another
molecule in the process of reduction, or the gaining of an electron. These two reactions
always happen together in an oxidation-reduction reaction (also called a redox reaction)—
when an electron is passed between molecules, the donor is oxidized and the recipient is
reduced. Oxidation-reduction reactions often happen in a series, so that a molecule that is
reduced is subsequently oxidized, passing on not only the electron it just received but also
the energy it received. As the series of reactions progresses, energy accumulates that is
used to combine Pi and ADP to form ATP, the high-energy molecule that the body uses for
fuel.
Oxidation-reduction reactions are catalyzed by enzymes that trigger the removal of
hydrogen atoms. Coenzymes work with enzymes and accept hydrogen atoms. The two
most common coenzymes of oxidation-reduction reactions are nicotinamide adenine
dinucleotide (NAD) and flavin adenine dinucleotide (FAD). Their respective reduced
coenzymes are NADH and FADH2, which are energy-containing molecules used to transfer
energy during the creation of ATP.
Chapter Review
Metabolism is the sum of all catabolic (break down) and anabolic (synthesis) reactions in
the body. The metabolic rate measures the amount of energy used to maintain life. An
organism must ingest a sufficient amount of food to maintain its metabolic rate if the
organism is to stay alive for very long.
Catabolic reactions break down larger molecules, such as carbohydrates, lipids, and
proteins from ingested food, into their constituent smaller parts. They also include the
breakdown of ATP, which releases the energy needed for metabolic processes in all cells
throughout the body.
Anabolic reactions, or biosynthetic reactions, synthesize larger molecules from smaller
constituent parts, using ATP as the energy source for these reactions. Anabolic reactions
build bone, muscle mass, and new proteins, fats, and nucleic acids. Oxidation-reduction
reactions transfer electrons across molecules by oxidizing one molecule and reducing
another, and collecting the released energy to convert Pi and ADP into ATP. Errors in
metabolism alter the processing of carbohydrates, lipids, proteins, and nucleic acids, and
can result in a number of disease states.
