Difference Between Organs And Systems
Just as cells are grouped to make tissues, tissues are combined to form still larger and more complex units of the body—the organs, including the stomach, the intestines, the trachea, the kidneys, the spleen, the liver, and the heart.
Consider the stomach: It breaks down food into simpler compounds that the body can use. The mechanical action of the stomach is made possible by its strong muscular walls. As the walls contract, anything within the stomach is churned and broken up. Three layers of smooth muscle make up most of the thickness of the stomach wall. One layer runs circularly around the organ, another layer is arranged lengthwise, and the third layer runs at irregular angles within the stomach. Thus, each layer of muscle produces its effect in a different area, but the total pattern of movement is highly effective for churning and mixing food particles.
The epithelial cells that line the digestive tract have already been mentioned. The inner surface of the stomach is arranged in folds, and over these the epithelial covering is fitted like a glove over the fingers. In the deep grooves that lie between the folds, different glands are developed from the epithelium. These glands secrete the various digestive juices that assist in the breakdown of food. Nerve fibers and blood vessels also penetrate the stomach along its outer surface and branch into a rich network within its wall.
In some organs—the stomach, for example—muscular tissue predominates. In others, such as the kidneys or liver, special epithelial tissues make up the bulk of the organ. In the brain, nervous tissue is naturally the most important element. The trachea is an example of an organ where connective tissue is predominant.
The systems of the human body include the skeletal, muscular, circulatory, digestive, respiratory, excretory, lymphatic, endocrine, nervous, urogenital, and integumentary.
In the skeletal system, the bones serve as a framework for the body and protect internal organs. They manufacture red blood cells in the marrow of long bones. They store calcium, which is supplied at intervals to the blood.
The muscular system accounts for all motion in the body, as the muscles that make up the system contract and relax.
The circulatory system carries blood to every part of the body. It consists of a pump—the heart—and a system of “pipes” or blood vessels.
The digestive system subjects food to a series of physical and chemical changes. These make the nutrients contained in the food available to the tissues.
In the respiratory system, oxygen passes to the lungs through the process of inhalation and is absorbed by the blood. Waste products—carbon dioxide and water—are carried by the blood to the lungs, from which they are expelled by exhaling.
In the excretory system, wastes are extracted from the blood and are then passed out of the body as solids or liquids.
The lymphatic system is made up of a series of ducts. Through these flows a liquid called lymph, which ultimately passes into two large veins. Lymph circulates through the body to carry food from the blood to the cells, and wastes from the body to the blood.
The endocrine system consists of a group of glands that empty their body-regulating secretions, called hormones, into the bloodstream.
In the nervous system, impulses are carried to and from the central nervous system (brain and spinal cord).
The urogenital system includes the urinary organs as well as the organs of reproduction.
The integumentary system is made up of the skin, hair, and nails (derived from the skin) and the structures associated with the skin. This system protects the underlying tissues, acts as a sensory organ, and plays an important part in regulating body temperature and in excretion.