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16.3: How Bodies Work

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    The organ level of organization in the body may be the most familiar to us from our everyday experiences. Many of the common ailments we hear about—an upset stomach, a broken bone, lung disease, skin cancer—are named for the organs they affect.

    An organ is made up of tissues that work together to perform a specific function for the body as a whole. Groups of organs that perform related functions are organized into organ systems, which perform more general functions. Table 1 describes the structures and functions of some common organs.

    Table 1. Structure and Function of Organs
    Organ Primary function(s) Tissues it contains Organ system(s) it is a part of
    brain control of body systems and behavior; cognition nervous, connective, epithelial nervous system; endocrine system
    skin protection; support and containment; temperature and fluid regulation epithelial, nervous, connective, muscular integumentary system
    stomach chemical and mechanical digestion of food epithelial, connective, muscular, nervous digestive system
    sternum (breastbone) support; protection; blood cell production epithelial, connective, nervous skeletal system; immune system; cardiovascular system
    kidney waste removal; fluid regulation epithelial, connective, nervous urinary system

    Organ Systems, The Whole Body, and Populations

    Organ systems are made up of organs that work together to perform a specific function for the body as a whole. Table 2 describes the organ systems and their primary organs and physiological functions that we will cover in subsequent pages.

    Note that we have opted to organize the rest of this module into three basic groups: systems involved in “control,” systems of “cell maintenance,” and systems of “support.” It is important to remember just as organs and systems work together that these categories are not mutually exclusive. For example, we have placed the reproductive system in the control category since it is involved in controlling the process and events of reproduction. However, the reproductive system is also a cell maintenance system, as it produces and maintains the actual cells used in reproduction. Just keep in mind these are groupings to help you mentally organize your learning more than they hard rules of anatomy and physiology.

    Table 2. Organ Systems
    Organ system Key Organ(s) Primary function(s)
    nervous brain, spinal cord control of behavior and body systems; cognition
    endocrine glands control of the body systems and development
    reproductive penis, testes, prostate (male); uterus, ovaries, vagina (female) reproduction
    sensory nerves and receptors associated with tongue, ears, skin, eyes, nose detect external stimuli and chemicals
    cardiovascular heart, blood vessels transport of materials through the body; regulation of temperature
    respiratory trachea, lungs gas exchange; regulation of temperature
    immune thymus, tonsils, spleen defense agains infection
    digestive tongue, esophagus, stomach, small intestine, large intestine, gallbladder, rectum digestion of food; waste removal
    muscular muscles, tendons support; movement
    skeletal bones, cartilage support; protection; movement; blood cell production
    integumentary skin support; protection; regulation of fluid levels

    The Whole Body

    The organ systems of the body all work together to maintain proper physiological functions. Many times in the arena of anatomy and physiology, including in this course, we closely examine the molecules, cells, tissues and organs of the body to learn their forms and functions. However, it is important to consider that every molecule works as part of the entire system. Endocrine disorders such as diabetes affect glucose levels in the body. Altered blood glucose levels can affect many organ systems. For example, the immune system may not heal as well, the urinary system may experience kidney damage, and the cardiovascular system can experience vascular damage, even to the point of causing blindness. In the body, everything is interconnected.

    Assigning organs to organ systems can be imprecise since organs that “belong” to one system can also have functions integral to another system. In fact, most organs contribute to more than one system.

    This illustration shows eight silhouettes of a human female, each showing the components of a different organ system. The integumentary system encloses internal body structures and is the site of many sensory receptors. The integumentary system includes the hair, skin, and nails. The skeletal system supports the body and, along with the muscular system, enables movement. The skeletal system includes cartilage, such as that at the tip of the nose, as well as the bones and joints. The muscular system enables movement, along with the skeletal system, but also helps to maintain body temperature. The muscular system includes skeletal muscles, as well as tendons that connect skeletal muscles to bones. The nervous system detects and processes sensory information and activates bodily responses. The nervous system includes the brain, spinal cord, and peripheral nerves, such as those located in the limbs. The endocrine system secretes hormones and regulates bodily processes. The endocrine system includes the pituitary gland in the brain, the thyroid gland in the throat, the pancreas in the abdomen, the adrenal glands on top of the kidneys, and the testes in the scrotum of males as well as the ovaries in the pelvic region of females. The cardiovascular system delivers oxygen and nutrients to the tissues as well as equalizes temperature in the body. The cardiovascular system includes the heart and blood vessels. The lymphatic system returns fluid to the blood and defends against pathogens. The lymphatic system includes the thymus in the chest, the spleen in the abdomen, the lymphatic vessels that spread throughout the body, and the lymph nodes distributed along the lymphatic vessels. The respiratory system removes carbon dioxide from the body and delivers oxygen to the blood. The respiratory system includes the nasal passages, the trachea, and the lungs. The digestive system processes food for use by the body and removes wastes from undigested food. The digestive system includes the stomach, the liver, the gall bladder (connected to the liver), the large intestine, and the small intestine. The urinary system controls water balance in the body and removes and excretes waste from the blood. The urinary system includes the kidneys and the urinary bladder. The reproductive system of males and females produce sex hormones and gametes. The male reproductive system is specialized to deliver gametes to the female while the female reproductive system is specialized to support the embryo and fetus until birth and produce milk for the infant after birth. The male reproductive system includes the two testes within the scrotum as well as the epididymis which wraps around each testis. The female reproductive system includes the mammary glands within the breasts and the ovaries and uterus within the pelvic cavity.
    Figure 1. Click for a larger image. Organs that work together are grouped into organ systems.

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