Human Anatomy- Major Organ Systems
The human body is the most complicated organism on the planet. The
organs in a human body are tissues that together perform at least one
purpose in the body. Many serve several functions. The largest of the
human organs is also the easiest to see. The skin, along with the hair,
fingernails, and toenails, covers most of the body. The skin is actually
made up of three different layers. The epidermis is the outermost layer
which contains the pigments which determine skin color. It is also
covered with tiny holes called pores from which sweat can escape the
The middle layer of skin is the dermis. It is so tightly connected to
the epidermis that it is hard to tell when one starts and the other
stops. The dermis contains the connective tissue, hair follicles, and
sweat glands. The final layer is the hypodermis. This is technically not
part of the skin, but is there to connect the skin to the musculature
underneath. This layer also supplies blood to the skin and contains most
of the body’s fat reserve.
The skin is more than just a covering for the bloody stuff
underneath. The skin provides a waterproof layer of protection against
invaders to the body, like viruses and bacteria. The skin also has an
amazing capability to turn sunlight into Vitamin D as well as regulating
body temperature. It also excretes waste through sweat.
Part of the reason the human body is so complicated is that all the
parts are interrelated. When studying the different systems of the body,
remember that organs in one system often perform functions in another.
Keep this in mind while reading about the ten major organ systems in the
The skeletal system is composed primarily of the 206 bones of an
adult human body, but also the cartilage, tendons, and ligaments which
connect the bones together. Without the connective tissue, the bones
would float around in the body without much function. With them, the
bones both support the human body and give it its shape. It also helps
allow for body movement, mainly with the joints.
The bones also provide an important protective function. The skull
protects the brain. The ribs protect the heart, lungs, and other
internal organs. As mentioned before, the bone marrow plays a vital role
in creating red blood cells. The bones also store calcium for future
use and play a role in regulating blood-sugar and fat deposits thanks to
a hormone they release.
The muscular system is made up of three types of muscles: smooth,
skeletal, and cardiac. These muscles work together to cause the body to
move in conjunction with the skeletal system. They also provide the
strength and balance the body needs to perform any basic function, like
eating, breathing, or blinking. The energy the muscles burn while moving
also provides much of the heat the body needs.
Skeletal muscles are the muscles which are attached to bones via
tendons. These are the muscles a person can control, as in the leg
muscles needed to walk from room to room. This type of muscle is also
called striated because of the fibrous look to them. Smooth muscles are
the ones controlled by the autonomic nervous system. In other words,
they are the muscles used in involuntary movements, like the intestines
as they digest food or the iris of the eyes.
Cardiac muscles are also controlled as part of the involuntary
nervous system. They are distinct in the way they are attached to each
other. They are attached to each other rather than to bones as skeletal
muscles are. Cardiac muscles are found solely in the heart.
There are many substances that are circulated throughout the human
body, like hormones and chemicals. When scientists and doctors speak of
the human circulatory system, however, they are almost always referring
to the system which moves blood around the body. The circulatory system
is made up of the heart, arteries, capillaries, veins, and to some
extent the lungs, trachea, and the ear, nose, and throat area. Some even
include the lymphatic system as part of the circulatory system.
The main purpose of the circulatory system is to get oxygen and other
needed nutrients to all parts of the body and to pick up waste material
from the cells for elimination. Think of it as the food delivery and
trash pick-up in one efficient system. The heart is the engine for the
system, as it pumps the blood through its journey through the body.
First, though, the blood must pick up the oxygen.
After the blood is pumped through the first two chambers of the
heart, it travels through the pulmonary artery to the lungs. There it
deposits the waste picked up from cells in the body, primarily carbon
dioxide, and absorbs oxygen from the lungs. The blood then re-enters the
heart via the pulmonary vein and is pumped through the remaining two
chambers of the heart. Then most of it exits the heart through the
aorta. The aorta, the body’s main artery, then begins to branch into
progressively smaller arteries, bringing nutrient rich blood to the
A portion of the oxygen-rich blood is pumped into a special artery
that brings blood to the heart itself. The heart is a strong muscle and
needs a constant supply of blood just as the rest of the body. The heart
is not designed to absorb the blood being pumped through the chambers.
It needs its own supply that can feed the heart muscle cells.
As the blood from the arteries reaches its destination, the arteries
become very small and transition into capillaries. Capillaries are
special blood vessels that allow the cells surrounding them to absorb
the oxygen from the blood in them and to dump their waste. The
capillaries then transition into veins. The blood in veins is much
darker as is has little oxygen in it. The veins slowly begin to combine
into larger and larger veins until they unite into the largest vein, the
interior vena cava, which brings the blood back into the heart for the
process to begin all over.
- The Circle of Blood – The Franklin Institute has a great website about the three different parts of circulation.
- The Circulatory System – Sometimes seeing something makes it much clearer. The diagram on this page gives a clear picture of how the heart works.
- Human Circulatory System – This site gives an overview of the entire circulatory system.
With the systems already discussed, there are already many complex
operations taking place. Something needs to control all the functions.
That duty falls to the nervous system. The nervous system consists of
the brain, the spinal cord and other nerves, and the retina. The retina
is the nerve connected to the eye which controls vision.
The brain and spinal cord are made up of a special kind of cell
called a neuron. A neuron is specifically designed to allow for rapid
communication between cells. This is done either via chemical or
electric communication. The brain has billions of synapses where neurons
are interconnected and constantly communicating.
The brain isn’t just where humans think, but it also controls all
body functions, emotion, judgment, memory, the senses, and all the other
systems of the body. Different parts of the brain control different
aspects of the body or different areas of personality. The brain makes
billions of decisions or connections every minute, far more than any
computer. The signals are then sent along the spinal cord.
The spinal cord is a long collection of fibrous tissues called
nerves. The signals are passed along these cords, like signals sent
along a telephone wire, to the appropriate muscle or organ. In return,
the nerves send input signals back to the brain, like pain sensations.
The respiratory system is highly intertwined with the circulatory
system. The lungs expand pulling air in through the nose or mouth. The
diaphragm, located underneath the lungs, is the organ actually
responsible for causing inhalation. The air then rushes through the
throat into the trachea, the pipe which connects the lungs to the
throat. The trachea then splits into two, each branch leading into a
different half of the lungs.
As air fills the lungs, the oxygen from the air is absorbed into the
blood the heart has pumped there for that purpose. Once the lungs are
mostly filled, the lungs begin to recoil, pushing air back out the
trachea, throat, mouth and nose. The difference this time is that the
blood which absorbed the oxygen has dumped carbon dioxide into the
lungs. So the air from an exhalation has a much higher level of carbon
dioxide than normal air.
- Respiratory System – Find out what part each piece of the respiratory system plays in the body.
- Respiratory System Diagram – Get a better idea of the structure of the lungs and other organs in the respiratory system and where they are in the body.
- Respiratory System Game – The game quizzes students on their knowledge of the respiratory system.
The digestive system might be the most complicated system in the
body. It certainly has a large number of organs. The digestive system’s
purpose is to remove nutrients from the foods and beverages a person
consumes so that they can be used by the cells in the body as fuel. As
most humans consume a variety of foods and often many that are
unhealthy, the digestive system has a big job.
The mouth, teeth, tongue, and salivary glands are the first organs of
the digestive system. They start the digestive process by mechanically
reducing the food into a substance the stomach can digest. The food or
beverage is then swallowed down the esophagus and carried into the
stomach. Here, stomach acids begin the chemical breakdown of the food.
Once the stomach is done with the food, it passes into the small
intestines. The small intestines play a vital part in digestion. Bile
from the liver and gall bladder and digestive enzymes from the pancreas
enter the small intestines to break down the proteins from the food and
to emulsify the fat. Once the nutrients are broken down like this, the
villi, small hair-like projection which line the lower end of the small
intestines, absorb the soluble nutrients so they can enter the
Whatever substances are left, then pass into the large intestines.
Here the colon removes most of the water from the substances. The colon
also has good bacteria which live there and make Vitamin K for the body.
The waste material is then squeezed into the rectum. Finally, the waste
exits the body.
The excretory system is made up of all the organs which help the body
get rid of waste products. Waste products are sometimes created by
cells; other times it is unneeded and unused substances which have
entered the body and need to be removed. Some organs that are in other
systems are also excretory organs. For instance, the skin excretes sweat
and the lungs excrete carbon dioxide.
The urinary system is also an important piece of the puzzle. The
urinary system, the kidneys, ureter, bladder, and urethra, is there to
clean out the blood, like a filtration system. The kidneys are the
primary filter. Not only does the pair of organs pull out toxic material
from the blood, they also regulate the amount of fluid in the body and
maintain the right level of acidity of that fluid.
After waste and excess fluid are removed from the blood, the ureters
carry the urine created in the kidneys to the bladder. The bladder
stores the urine until it can be eliminated. When the body is ready to
eliminate the urine, the urine passes through the urethra to the outside
of the body. In men, the urethra also acts as part of the reproductive
The endocrine system is perhaps one of the lesser known and lesser
understood of the body systems, but it is a very important one. The
endocrine system is made up of glands which produce chemicals called
hormones which control body development, puberty, metabolism, and even
mood. The glands are different from other glands in the body, like sweat
or salivary glands which have ducts. Endocrine glands are ductless.
In the brain, the hypothalamus, the pineal, and the pituitary gland
secrete a wide array of hormones. The hypothalamus controls growth,
along with many other functions. The pineal gland produces melatonin,
which influences how well a person sleeps. The pituitary gland also
helps control growth, blood-sugar levels, egg production in females, as
well as others.
There are other glands found in the stomach, duodenum, liver,
pancreas, and kidneys. The thyroid in the neck controls the body’s
metabolism. In females, the ovarian follicle produces several hormones
which regulate the menstrual cycle and development of female sex
characteristics. Males have testes, which not only cause men to develop
male sexual organs, but also increase their muscle and bone mass.
- The Endocrine System – Discover the role hormones play in the health and development of the body.
- The Endocrine System Communicators
– Like the nervous system, the endocrine system is a communication
system. Learn how this system transmits messages throughout the body.
- Endocrine System Diseases – When something goes wrong with one of the glands in the endocrine system, the effect can be devastating to a person’s health.
The reproductive system allows humans to produce offspring. Though
there are differences in other systems between men and women, they are
minor differences, whereas the reproductive systems are quite different
as they have different roles. Interestingly, many of the sexual organs
start off exactly the same in the developing fetus. Then at a certain
point during the baby’s development, the organs develop into the
appropriate organs for the gender.
A female’s main reproductive organs are the two ovaries. These two
small organs hold all the eggs a woman has plus produces many of the
hormones related to reproduction and the menstrual cycle. These are
connected to the uterus by the fallopian tubes. The uterus serves as the
home for a developing fetus during pregnancy. The cervix connects the
uterus to the vagina, the exterior opening of the body in the genital
The male equivalent of the ovaries is the two testes, which are
housed in the scrotum. These organs produce the hormones related to male
gender and reproduction as well as the sperm, which is the father’s
contribution to a baby’s genetics. The sperm is then stored in the
epididymis until it passes into the vas deferens, picking up seminal
fluid from the accessory glands. The vas deferens then takes the sperm
mixture to the penis during ejaculation.
- The Reproductive System Development
– The organs in a developing baby’s reproductive system start off the
same regardless of gender. This article explains how the sexual organs
develop differently in males and females.
- Complete Reproductive System – Learn how the brain and certain endocrine glands work with the reproductive organs in the system.
- Anatomy Bowl
– This free Jeopardy-like game tests knowledge about anatomy. This link
connects to the questions about the reproductive system.
The lymphatic and immune system also acts as a filter for the body.
Unlike the urinary system, this system filters out invaders in the body,
such as viruses and bacteria. The lymphatic system is made up of the
tonsils, thymus, spleen, and many lymph nodes found all over the body.
This system circulates a liquid called lymph throughout the body via the
This fluid circulation is important in maintaining the health of the
body. It removes excess fluid from around tissues, transports white
blood cells to the bones, and absorbs fatty acids and fat from the
intestines. It also plays a part in the immune system as it carries
antigen-producing cells to the lymph node when an infection has been
discovered. As lymph passes through the lymph nodes, they act as
filters, catching the intruders in their tissues.
When describing the immune system, scientists describe two different
types of protections. The first is innate immunity. They are the natural
protectors the body has to keep invaders, or antigens, out. The skin
acts as a barrier for most of the body. Saliva, tears, mucus and sweat
all have properties that kill many antigens. Stomach acid destroys most
antigens that are consumed.
The adaptive immune system describes the actions the body takes when a
specific antigen is detected. This includes producing antibodies to
fight the virus that has been inhaled into the lungs, for example. There
are several tools the immune system uses, including killer T cells,
which will kill infected cells and thus kill the virus. The immune
system also has an amazing ability to remember antigens which have
invaded from the past. This is why a person who has previously had the
chicken pox, for example, won’t get sick with it again. If the virus
enters the body, the immune system already has the tools needed to fight
it on hand.