As the name suggests, the circulatory system involves the organs and other body parts necessary for the circulation of blood throughout the body. Blood is made in the bone marrow and moves continuously through the body. Oxygenated blood, blood which has been supplied with oxygen from the lungs, is pumped out of the heart and moves through the arteries, arterioles and capillaries. Some organs like the liver, bone marrow and spleen do not have veins but vessels that are a little larger than veins, which serve the same purpose.
The immune system's function is to protect the body from illness. It performs this function by the use of various complex mechanisms. The largest immune system organ is the skin. Without it, the body would be susceptible to all sort of infections on a regular basis. Within the body there are other organs which help prevent and fight disease. Many cells used by the immune system to fend of disease, like white blood cells, are made in the bone marrow. The lymph nodes are another important immune organ as they are filled with fluid which filters bodily fluids. Similarly, the spleen is used to filter the blood by removing foreign material from it. This organ also destroys old red blood cells. The thymus makes T cells, another important immune system cell which can have different functions, like stimulating immune responses.
The skeletal system is composed of the bones that make up the human body. About 209 bones are present in the human body, giving it shape, making blood vessels, storing minerals like calcium and phosphorus and protecting organs, among other functions. The skeletal system is divided into two sections. The axial system includes the vertebral column, skull, sternum and ribs, and the appendicular skeleton includes the bones of the lower and upper extremities, the pelvic girdle and the shoulder girdle. Additionally, the bones themselves may be categorized according to size. There are long, short, irregular and flat bones. An example of a long bone is the femur in the upper part of the leg. Example of irregular bones are some found in the skull and vertebrae.
The main function of the respiratory system is to deliver oxygen to the blood via the process of respiration, or breathing. In respiration, oxygen is breathed in while carbon dioxide is breathed out. Oxygen goes through the mouth and enters the larynx and the trachea, a tube that connects to the chest cavity. The trachea then splits into bronchi. The bronchi then split into bronchioles which then connect to the alveoli (small air-filled sacs) in the lungs. When inhaling, the lungs fill with oxygen-rich blood. Deoxygenated blood, which has has carbon dioxide, enters the alveoli from the capillaries and leaves the body using the same organs the air took to get into the body.
The endocrine system is the means by which hormones, or chemical signals, are released in the body. This system is composed of glands which give off hormones which regulate metabolism, sexual development and body growth. The system works via a feedback mechanism. For instance, the hormones regulated by the pituitary gland (a gland important for physical growth), is sent from the hypothalmus to the pituitary gland. This "releasing" hormone then triggers the pituitary gland to secrete a "stimulating" hormone to a target gland. This target gland then secretes its own hormone and when the level of this hormone reaches a certain point in the body, the stimulating and releasing hormone stop being secreted. The target gland then stops secreting its hormone, providing for a stable means of maintaining the right concentrations of hormone circulation.