The human body has many systems of major organs that are necessary for it to perform its various functions. Some of the most vital systems of the body include the nervous system, which controls and coordinates the body and includes the brain, spinal cord, and nerves; the circulatory system, which pumps blood throughout the body and includes the heart, blood vessels, and blood itself; and the respiratory system, which regulates oxygen exchange and includes the lungs, diaphragm, bronchi, larynx, and trachea. The musculoskeletal system includes both the bones and muscles and provides support and structure to the human body, as well as the ability to move. Food nutrients are processed and absorbed by the digestive system, while waste products are excreted by the digestive system and the urinary system. Other important systems of the body include the endocrine system, immune system, lymphatic system, and the reproductive system.
Without the nervous system, the other systems of the body would shut down. Using nerve cells called neurons, the nervous system sends signals to different parts of the body, triggering both voluntary and involuntary responses, such as the contraction of a voluntary muscle or the beating of the heart. In humans and most other vertebrates, the central nervous system contains the brain and spinal cord, which transmit signals to the network of nerves in the rest of the body, known as the peripheral nervous system.
Another vital system is the circulatory system, or more specifically, the cardiovascular system, which is a major mechanism of transport that carries gases, nutrients, hormones, and blood cells throughout the body. One of the gases transported by the circulatory system is oxygen, which is obtained when blood is carried through the lungs in a process called pulmonary circulation. The oxygenation of the blood is one of the primary functions of the respiratory system. Air inhalation is begun by contractions of the diaphragm muscle, allowing the rib cage to expand and causing suction in the chest that pulls air down into the lungs. Oxygen enters the bloodstream by diffusing through hollow anatomical structures in the lungs called alveoli, which also take carbon dioxide from the bloodstream so it may be exhaled.
Humans obtain energy by digesting food nutrients with another one of the systems of the body, the digestive system. Digestion starts in the mouth, with food being broken down by the action of chewing and the chemical action of the salivary glands. Chewed food called bolus is then swallowed, traveling through the esophagus on its way to the stomach, where gastric juices continue breaking down the food molecules. Solid waste products from the stomach travel through the small and large intestines and are excreted in the form of feces. The urinary system is responsible for the excretion of liquid waste products, with the kidneys filtering the waste and excreting it into the bladder, where it is stored until it is expelled from the body in the form of urine.
The endocrine system is a system primarily made up of hormone-secreting glands. The hormones secreted by the endocrine system help regulate the body's function. Both the immune and lymphatic systems contribute to the defense of the human body by fighting off disease and disposing of foreign substances and waste. The reproductive system includes the sex organs of both males and females, and its primary function is to produce offspring.