Structure and Movement
You might recall that our bodies require both the skeletal system and muscular system to
work together to provide structure and movement. When you walk your bones provide
structure while your muscles do the work to move your skeleton forward.

Skeletal System

The skeletal system is the body system that contains bones as well as other structures

that connect and protect the bones and that support other functions in the body. The

skeletal system’s main function is to provide protection for internal organs and support

for movement. It is also responsible for the storage and production of cells and nutrients. 

Protection

Flat bones of the skeleton protect soft organs and tissues. For example, the cranium is

like a helmet for your brain, protecting it from impact. Feel your stomach, then feel your

chest. You will notice your chest is protected by flat bones called the rib cage and

sternum while your stomach is not protected by any bones. Flat bones are ideal for

protection as organs fit nicely inside their flat walls. 

Support and Movement

The skeletal system works together with the muscular system to support the body and

provide movement. Move your head back and forth, then side to side. The human body
has many places where bones meet, which are called joints. Joints allow flexibility and
movement like you observed with the climber at the beginning of the chapter. There are
three types of joints in the human body as observed in Table 1. These different types of
Joints offer different types of movement. As you learn about each type, find a joint on
your body to explore how it moves.

How are joints connected? Your skeletal system uses connective tissues to connect parts

of the skeleton to the muscular system. Ligaments connect bones to bones while

tendons connect bones to muscles. Cartilage offers a cushion where two bones meet,

like at your knee. 

Storage and Production
Did you know that your bones are alive? They are made up of different tissues which
work together to stay healthy. You may have heard of bone marrow, a very important
substance for the health of a human. Bone marrow is a nutrient-dense spongy tissue
located in the cavities of bones. It is often used for patients with a bone cancer
diagnosis. It contains stem cells, which make other types of cells. There are two types of
marrow including red marrow and yellow marrow. Examine Table 2 to learn about the
function and location of each type.

In 2019, Tel Aviv University researchers in Spain discovered that prehistoric humans would store bones for consumption. It was originally thought that bone marrow was consumed right after a hunt, but new evidence shows differently. You can think of animal bones and the nutritious bone marrow as the "canned soup” of its time. Humans could break open the bones and consume the contents after weeks of storage. 

The Muscular System
The muscular system is the body system that contains tissues that are responsible for
providing movement and temperature regulation. Without muscle tissues, you would not
be able to move or function. Muscle tissues make up the muscular system, working
together to help the body move.

Skeletal muscles are called voluntary muscles because you can control them with

conscious thought. Cardiac and smooth muscles are involuntary muscles, which you 

cannot consciously control, like your heart.

Movement and Stability

Recall the robotic hand model from the Quick Launch activity. The string represented the

action of the muscle on the bone, while the straws represented the bone. Muscles attach

to the bones with connective tissues called tendons. When a muscle contracts, it gets

shorter. When it relaxes, it gets longer. These changes result in movement. For example,

when a bicep contracts, it brings the arm towards the body. When the tricep contracts,

the bicep relaxes, and the arm extends.

Skeletal muscles also aid in providing stability. With information from the brain, the
muscles respond by contracting and relaxing to ensure you maintain your posture and
positioning. For example, i you are riding a bike, the nervous and muscular systems will
work together to alter movement so you do not fall over while riding down a path.

Temperature Regulation

Muscles are heat producers. Muscle fibers work together as long bundies contracting

and relaxing. During this process, cells use food energy, releasing heat as a byproduct.

With thousands of fibers working together, muscle movement generates heat, which is

circulated around the body.

Control and Coordination

You might recall that the climber at the beginning of the chapter was balancing on the
rock, coordinating the movements of the body to continue upwards. A combination of
body systems, including the nervous system and endocrine system, work together to
receive input from the outside world, process it, and coordinate a reaction or response.

The Nervous System

The part of an organism that gathers, processes, and responds to information is called the nervous system. The basic units of the nervous system are called nerve cells, or neurons. The nervous system’s main function is to receive information (called stimuli) from the five senses, process it, and respond by sending an electrical signal to other body systems. 

The nervous system is made up of two different parts called the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS is made up of the brain and the spinal cord. It receives, processes, stores, and transfers information. The spinal cord is a tubelike structure of neurons that extends to areas of the body. This allows information to be sent out and received by the brain. The PNS has sensory and motor neurons that transmit information from the CNS to the rest of the body. 

Receiving Information

Your nervous system enables your body to receive, process, and respond to information
about your environment. Receiving information (also called a stimulus), starts with the
sensory system which includes five senses.

The Five Senses

The sensory system is the part of your peripheral nervous system that detects, or senses,

the environment. A human uses senses such as vision, hearing, smell, taste, and touch to
detect the outside environment. There are also other senses, such as your sense of
direction. All parts of the sensory system have special structures called receptors that
detect stimuli with each part using different receptors. For example, the ear has a
specific shape and an internal structure to receive sound waves.

Responding to Stimuli

When there s a stimulus, the message is transported in the body using cells called
neurons. The sensory neuron is a receptor cell that connects to the spinal cord. The
sensory neuron sends a signal to the brain. Once the brain receives the signal, it sends a
response to a motor neuron connected to another body system, such as the muscular
system. 

For example, when the rock climber you observed at the beginning of the chapter wants
to move to another rock, the muscles in the body are activated to make that movement.
The sensory receptors in the eyes and fingers detect the texture using receptors in the
fingertips. This relays a signal to the brain, which responds by prompting the climber to
look for another rock or use the rock in hand. If the climber still wants to look for another
option, the cycle repeats until they find the right rock and move upwards.

The Endocrine System
The endocrine system is the system of glands throughout the body that regulate body
functions. The endocrine system’s function is to regulate all the body’s biological
processes using hormones. Hormones are chemical signals released by different organs
in the body called glands. These hormones are responsible for the organ systems
carrying out certain functions to maintain homeostasis and function properly.

While the nervous system sends signals to the body that travel very quickly by neurons,
the endocrine system sends out hormones, which are carried by the vessels in the
circulatory system. The nervous system signal can travel from your head to your toes in
less than one second. A hormone will take about 20 Seconds but it has a longer-lasting
effect.

Some of the glands that release these hormones include the thyroid, thymus, adrenal glands, testes (in males), and ovaries (in females). Each gland is responsible for producing a different type of hormone. For example, the testes and ovaries produce testosterone and estrogen, respectively. Some glands produce
many different types of hormones.

Hormonal Regulation
Many of the hormones made by the endocrine system work with other organ systems to
maintain homeostasis. For example, the parathyroid hormone, found in the thyroid gland,
controls calcium storage in the bones of the skeletal system. Thymosin is a hormone that
is released from the thymus. It stimulates the development of T cells, which are essential
for fighting disease.

Emotions and reactions are also related to hormones. Have you ever felt scared or
stressed out? Cortisol is a hormone produced in the adrenal gland. When it is released
into the bloodstream, it causes the heart rate to increase, moves sugar to the muscles,
and dilates pupils so the eyes can take in more light. This prepares the body for a
stressful situation. The nervous system signals the release of the hormone after receiving
input from the senses.

Human Reproduction
You have read about many of the organ systems that enable you to grow and respond to
changes in the environment. Organ systems such as the nervous system, the skeletal
system, the circulatory system, and the digestive system are all important for an
individual’s survival. But what organ system ensures that the human species survives?

The Reproductive System
A reproductive system is the body organs and structures that make it possible to
produce offspring. It enables the male and female reproductive cells to join and form
new offspring. Like other animals, human males produce sperm, the male reproductive
cells, and human females produce eggs, the female reproductive cells. 

During reproduction, a sperm joins with an egg. This usually happens inside a female’s
reproductive system. Once joined, part of the female’s reproductive system nourishes
the developing human. In order to understand how humans develop, we must first learn
the parts of the male and female reproductive systems.

Female Reproduction
The female reproductive system contains two ovaries where eggs grow and mature. The ovaries produce the hormones estrogen (ES truh jun) and progesterone (proh JES tuh rohn) that help eggs mature. When they are mature and ready for fertilization, they are released into the fallopian tubes which are connected to the uterus. If fertilization is successful, a baby grows in the uterus.

Male Reproduction
The male reproductive system produces sperm. Sperm is produced in the testes (TES teez; singular, testis). Sperm develop inside each testicle, then are stored in tubes called sperm ducts
where they mature. 

Testosterone is also produced in the testes. This helps the sperm change into long slender cells with a structure called a flagellum that can swim. The penis is a structure that delivers sperm in a fluid called semen.