Human Body Systems

The human body is made up of cells, tissues, organs, and organ systems that work together to maintain homeostasis.
The levels of organization in a multicellular organism include:
- Cells: the basic unit of structure and function
- Tissues: groups of cells that perform a specific function
- Organs: groups of tissues that work together to perform a specific function
- Organ Systems: groups of organs that work together to maintain homeostasis

Function: recognizes and coordinates the body's response to internal and external environments
Structures:
- Brain
- Spinal cord
- Peripheral nerves
The nervous system is divided into:
- Central Nervous System (CNS): brain and spinal cord
- Peripheral Nervous System (PNS): nerves that connect the CNS to the rest of the body

The brain is the control center of the body
The cerebrum is the largest region of the brain, responsible for voluntary activities such as movement, sensation, and cognition
The cerebellum coordinates muscle movements and maintains posture and balance
The brain stem connects the brain to the spinal cord and regulates basic functions such as heart rate and breathing

The spinal cord is a long, thin, tube-like structure that extends from the base of the brain down to the lower back
It is the main communications link between the brain and the rest of the body
The spinal cord is protected by the vertebrae and meninges

The PNS is divided into:
- Sensory Division: transmits impulses from sense organs to the CNS
- Motor Division: transmits impulses from the CNS to muscles and glands
The autonomic nervous system is a subdivision of the PNS that regulates involuntary functions such as heart rate and digestion

Function: brings oxygen, nutrients, and hormones to cells; removes waste products
Structures:
- Heart
- Blood vessels (arteries, capillaries, and veins)
- Blood
The heart pumps blood throughout the body, supplying oxygen and nutrients to cells and removing waste products

The heart is a muscular organ that pumps blood throughout the body
The heart has four chambers: right and left atria, and right and left ventricles
The heartbeat is regulated by the sinoatrial node, a group of cardiac muscle cells in the right atrium

Function: protects the body from external damage, regulates body temperature, and aids in the senses of touch and feel
Structures:
- Skin
- Hair
- Nails
- Sweat and oil glands
The skin is the largest organ of the body, and it has many different functions, including protecting the body from infection and injury.### Skin Structure and Function
Cells in the inner layer of the epidermis undergo rapid cell division, producing new cells that push older cells to the surface of the skin.
As older cells move upward, they become flattened and their organelles disintegrate, and they start producing keratin, a tough, fibrous protein.
Melanocytes in the epidermis produce melanin, a dark pigment that helps protect the skin from damage by absorbing ultraviolet rays from the sun.
Differences in skin color are caused by the amount of melanin produced and the distribution of melanocytes.

The dermis lies beneath the epidermis and contains collagen fibers, blood vessels, nerve endings, glands, sensory receptors, smooth muscles, and hair follicles.
The skin interacts with other body systems to maintain homeostasis, regulating body temperature by constricting or widening blood vessels.
The dermis contains two major types of glands: sweat glands, which produce perspiration to help regulate body temperature, and sebaceous glands, which produce sebum to keep the epidermis flexible and waterproof.

Functions: supports the body, protects internal organs, allows movement, stores mineral reserves, and provides a site for blood cell formation.
Structures: bones, cartilage, ligaments, and tendons.

Functions: works with the skeletal system to produce voluntary movement, helps to circulate blood, and moves through the digestive system.
Structures: skeletal muscle, smooth muscle, and cardiac muscle.

Function: provides oxygen needed for cellular respiration and removes excess carbon dioxide from the body.
Structures: nose, pharynx, larynx, trachea, bronchi, bronchioles, and lungs.

Function: converts food into simpler molecules that can be used by the body and absorbs food, eliminating wastes.
Structures: mouth, pharynx, esophagus, stomach, small and large intestine, and rectum.

Function: controls growth, development, and metabolism, maintaining homeostasis.
Structures: hypothalamus, pituitary, thyroid, parathyroid, adrenals, pancreas, ovaries, and testes.

Function: produces reproductive cells; in females, nurtures and protects the developing embryo.
Structures: testes, epididymis, vas deferens, urethra, and penis (in males); ovaries, fallopian tubes, uterus, and vagina (in females).

Function: helps protect the body from diseases, collects fluid lost from blood vessels, and returns the fluid to the circulatory system.
Structures: white blood cells, thymus, spleen, lymph nodes, and lymph vessels.