Download Chapter 36 Skeletal, Muscular, and Integumentary

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Chapter
Chapter
36# Skeletal,
Title
Muscular, and Integumentary Systems
Summary
36–1 The Skeletal System
The skeletal system has several jobs. It supports the body, protects internal organs, allows movement, stores mineral reserves,
and provides a site for blood cell formation. The skeleton is
divided into two parts.
• The axial skeleton is made up of the skull, ribs, and spine.
• The appendicular skeleton is made up of all of the bones
related to the arms and legs. This includes the bones of the
shoulders, hips, hands, and feet.
Bones are living tissue. Bones are a solid network of cells
and protein fibers surrounded by deposits of calcium salts. The
periosteum is a layer of connective tissue that surrounds a typical
bone. Under the periosteum is a thick layer of compact bone. A
network of tubes called Haversian canals runs through compact
bone. These canals hold blood vessels and nerves. Cavities that
hold bone marrow are inside the bone. Yellow bone marrow is
made up of fat cells. Red bone marrow makes blood cells.
The skeleton of an embryo is made mostly of cartilage.
Cartilage is a tough but flexible connective tissue. Cartilage is
replaced by bone in a process called ossification. Ossification
starts before birth and continues until adulthood.
A joint is a place where one bone attaches to another. Joints
allow bones to move without damaging each other. Depending on
its type of movement, a joint is classified as immovable, slightly
movable, or freely movable.
• Immovable joints allow no movement. Immovable joints
are found in the skull.
• Slightly movable joints, such those in the spine, allow a
small amount of movement.
• Freely movable joints permit movement in one or more
directions. Four common freely movable joints are: ball-andsocket joints, hinge joints, pivot joints, and saddle joints.
Ligaments are strips of tough connective tissue that hold bones
together in a joint. The bony surfaces of the joint are covered with
cartilage. A substance called synovial fluid forms a thin film on the
cartilage, making the joint surfaces slippery.
Excessive strain or disease can damage bones and joints.
• Arthritis is a disorder that involves inflammation of
the joints.
• Osteoporosis is a condition in which bones weaken.
Weak bones are prone to fracture, or breaking.
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36–2 The Muscular System
Working together with the skeletal system, the muscular system
produces movement. There are three different types of muscle
tissue.
• Skeletal muscles most often attach to bones. Skeletal
muscles allow voluntary movements such as dancing.
• Smooth muscles line blood vessels and the digestive tract.
They are not under conscious control. Smooth muscles move
food through the digestive tract and control the flow of
blood through the circulatory system.
• Cardiac muscle is found only in the heart. Cardiac muscle is
not under voluntary control.
Skeletal muscle cells are called muscle fibers. Muscle fibers are
made up of smaller structures called myofibrils. Each myofibril is
made up of even smaller structures called filaments. Filaments can
be thick or thin. Thick filaments are made of the protein myosin.
Thin filaments are made of the protein actin. Thick and thin filaments are arranged in units called sarcomeres. When the muscle is
relaxed, there are no thin filaments in the middle of the sarcomere.
A musle contracts when the thin filaments in the muscle
fiber slide over the thick filaments. During muscle contraction,
knoblike heads of myosin filaments attach to binding sites on actin
molecules. Each attachment is called a cross-bridge. ATP then
causes the myosin cross-bridge to change shape. This change in
shape pulls the actin filament toward the center of the sarcomere.
The myosin head detaches and the cycle repeats.
Tendons join muscles to bones. Tendons pull on bones, making
them work like levers. Muscles supply the force that moves bones.
Most skeletal muscles work in opposing pairs. When one
muscle contracts, the other muscle relaxes. For example, in order
to bend the arm at the elbow, the biceps contract and the triceps
relax. This creates the force necessary to bend the elbow joint.
To extend the arm, the triceps contract and the biceps relax.
Regular exercise is important in maintaining muscle strength
and flexibility. Regular exercise also strengthens bones. Strong
bones and muscles are less likely to be injured.
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36–3 The Integumentary System
The integumentary system has several jobs. It covers the body
and protects against infection, injury, and ultraviolet radiation.
It helps regulate body temperature and removes wastes from
the body.
The skin is made up of two main layers.
• The epidermis is the outer layer of the skin. The epidermis is
composed of two layers. The inner layer of the epidermis is
made up of living cells. These cells quickly divide and push
older cells up toward the surface of the skin. Eventually, the
older cells die. The outside layer of the epidermis is made up
of dead cells. It provides a tough, flexible, waterproof outer
covering for the body. The epidermis also contains cells that
produce melanin. Melanin is a dark brown pigment. It helps
protect the skin from ultraviolet radiation in sunlight.
• The dermis is the inner layer of skin. The dermis contains
many different types of structures such as blood vessels,
nerve endings, hair follicles, and glands. There are two main
types of glands in the dermis: sweat glands and sebaceous
glands. Sweat glands produce sweat. Sweat evaporates off of
the surface of skin and helps keep the body cool. Sebaceous
glands produce sebum. This oily substance helps keep the
epidermis flexible and waterproof.
Hair and nails are also parts of the integumentary system. Hair
and nails are made mainly of keratin. Hair is made by structures
called hair follicles. Nails grow from an area called the nail root.
Nails protect the tips of the fingers and toes.
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