The human body is composed of millions of cells, myriad tissues, and numerous organs; some have unique functions, other functions are coordinated, and some functions overlap. Therefore, a classification of organ systems is not definitive. For example, the endocrine system releases hormones into the body, and the gastrointestinal tract digests food. The pancreas does both and may be classified into either of these systems. Likewise, the brain may also release hormones but is classified as part of the nervous system, though part of the brain, called the pituitary, could be called an endocrine organ.
Nervous System
The nervous system contains the central and peripheral nervous systems. The central nervous system includes the brain and spinal cord. The brain consists of the telencephalon, which includes the cerebral hemispheres; the diencephalon, which includes the hypothalamus and thalamus; the mesencephalon, or midbrain; the cerebellum, the pons and the medulla oblongata. The spinal cord has 31 pairs of spinal nerves. Peripheral nerves may originate from the spinal cord or the brain. The may be sensory or motor and voluntary or involuntary. Involuntary nerves are also called the autonomic nervous system, which includes the parasympathetic "rest and digest" and sympathetic "fight or flight" divisions.
Cardiovascular System
The cardiovascular system includes the heart and blood vessels. The heart has four chambers. Blood travels from the main veins in the body called the superior and inferior vena cava to the right atrium, then to the right ventricle, out to the lungs, back to the heart at the left atrium, then to the left ventricle and out to the body through the aorta. The aorta leads to smaller arteries, smaller arterioles, and finally tiny blood vessels called capillaries. These lead to progressively larger venules, veins, and finally back to the vena cava.
Gastrointestinal System
The gastrointestinal system starts at the mouth, where the process of digestion begins with enzymes from salivary glands and mechanical processing of food by the teeth. It proceeds to the pharynx, or throat, then to the stomach through the esophagus. Food leaves the stomach through a valve called the pyloric sphincter and enters the small intestine, which is divided into the duodenum, jejunum and ileum. The large intestine begins at the ileocecal valve, which leads to the cecum, where the appendix is, and the right ascending colon. This is followed by the transverse colon, the left descending colon, the sigmoid colon, the rectum and anus. Other organs of the digestive or gastrointestinal system include the liver, gallbladder, bile ducts and pancreas.
Pulmonary System
The pulmonary system begins at the nose or mouth where air enters and exits the body. Air passes over the epiglottis and other structures of the larynx in the throat and down into the trachea. From here it enters the left and right bronchi to go to the lungs. The bronchi divide into smaller and smaller channels until reaching the alveoli, which are small sac-like structures that exchange gas with the blood and air.
Urinary System
The urinary system includes the kidneys, which filter the blood, controlling its pH, electrolyte balance and fluid levels. The kidneys also release hormones controlling blood pressure and the creation of red blood cells by bone marrow. The waste product of kidneys is urine, which passes through the ureter to the urinary bladder, and out of the body though the urethra.
Immune System
The immune system consists of cellular and humoral elements. The cellular component is white blood cells. The humoral includes proteins, such as complement, that can attack invading organisms and communicate with the cells of the body. Other such proteins include interleukins and tumor necrosis factors. The lymphatic system also is part of the immune system; its lymph nodes hold B cells and T cells, which originate in the bone marrow. T cells develop in the thymus.
Skeletomuscular and Connective Tissue Systems
The skeletomuscular system includes the bones, divided into the axial and appendicular skeleton. The muscles attach to bones through tendons, and bones to each other through ligaments. The connective tissues include supportive structure tissues, primarily made of collagen or cartilage, throughout the organs. It also includes blood, which is composed of cells and fluid. Cells include white cells of the immune system, red cells for carrying oxygen on hemoglobin and platelets to control bleeding. The fluid portion is called plasma.
Integumentary System
The integument is another word for skin. It is the largest organ of the body and is composed of the epidermis, dermis and subdermis. It serves to protect the body from external injuries and pathogens as well as regulate body temperature, and help create vitamin D. In females, the mammary tissues make milk for offspring.
Lymphatic System
The lymphatic system includes the lymphatic channels, lymph nodes and spleen. As blood moves through the capillaries, some of the liquid is filtered through the capillaries to the surrounding tissues. The excess fluid in the tissues is removed through the lymphatics. It drains through lymph nodes found throughout the body, which carry the T cells and B cells of the immune system. The lymphatics drain into the veins through the thoracic duct in the chest. The spleen is similar to a large lymph node in that it has B cells and T cells, but blood passes directly through it.
Special Sensory
The special sensory system could be considered part of the nervous system, but it may include more than the nerves. For example, the retinas of the eyes contain the nerves, but the cornea is the outer layer of the eye. Other sensory organs include receptors for stimuli in the skin, the ears for hearing and balance, tongue for taste and so forth.
Endocrine System
The major organs of the endocrine system include the pituitary, thyroid, parathyroid, pancreas, adrenal glands and testes or ovaries. The kidney and liver also have endocrine functions. The function of the pituitary gland is to help control many of the other endocrine organs.
Reproductive System
In the male, the testes make sperm, which travel to the epididymis at the back of the testes, then to the urethra in the prostate through the vas deferens. The seminal vesicles also meet the urethra at the prostate. From there, sperm travel out of the body through the penis. Semen is made primarily by the seminal vesicles and the prostate. The female reproductive system includes the vulva and vagina, cervix, uterus, fallopian tubes and ovaries.
