In thehuman digestive system, the process of digestion has many stages, the first of which starts in themouth(oral cavity). Digestion involves the breakdown of food into smaller and smaller components which can be absorbed and assimilated into the body. The secretion ofsalivahelps to produce aboluswhich can be swallowed to pass down theoesophagusand into thestomach.Saliva also contains acatalyticenzymecalledamylasewhich starts to act on food in the mouth. Digestion is helped by themasticationof food by theteethand also by themuscular contractionsofperistalsis.Gastric juicein the stomach is essential for the continuation of digestion as is the production ofmucusin the stomach.

Therespiratory system(called alsorespiratory apparatus,ventilatory system) is abiological systemconsisting of specificorgansand structures used for the process ofrespirationin anorganism. The respiratory system is involved in the intake andexchangeofoxygenandcarbon dioxidebetween an organism and the environment.

Thenervous systemis the part of ananimal's body that coordinates its voluntary and involuntary actions and transmits signals between different parts of its body.Nervous tissuefirst arose inwormlike organismsabout 550 to 600 million years ago. In most animal species it consists of two main parts, thecentral nervous system(CNS) and theperipheral nervous system(PNS). The CNS contains thebrainandspinal cord. The PNS consists mainly ofnerves, which are enclosed bundles of the long fibers oraxons, that connect the CNS to every other part of the body. The PNS includesmotor neurons, mediating voluntary movement; theautonomic nervous system, comprising thesympathetic nervous systemand theparasympathetic nervous system, which regulate involuntary functions, and theenteric nervous system, which functions to control thegastrointestinalsystem.

Theendocrine systemrefers to the collection ofglandsof an organism thatsecretehormonesdirectly into thecirculatory systemto be carried towards distant target organs. The majorendocrine glandsinclude thepineal gland,pituitary gland,pancreas,ovaries, testes,thyroid gland,parathyroid gland,hypothalamus,gastrointestinal tractandadrenal glands. The endocrine system is in contrast to theexocrine system, which secretes its hormones usingducts. The endocrine system is an information signal system like the nervous system, yet its effects and mechanism are classifiably different. The endocrine system's effects are slow to initiate, and prolonged in their response, lasting from a few hours up to weeks. The nervous system sends information very quickly, and responses are generally short lived. Invertebrates, thehypothalamusis the neural control center for all endocrine systems. The field of study dealing with the endocrine system and its disorders isendocrinology, a branch ofinternal medicine.

Thelymphatic systemis a series of vessels throughout the body that drain fluid from tissues. Bacteria and other microbes are picked up in the lymphaticfluid and trapped insidelymphnodes, where they can be attacked and destroyed by white blood cells.

The male reproductive system consists of a number of sex organs that form a part of the human reproductive process. In this type of reproductive system, these sex organs are located outside the body, around the pelvic region.

Thefemale reproductive systemis designed to carry out several functions. It produces the female egg cells necessary for reproduction, called the ova or oocytes. The system is designed to transport the ova to the site of fertilization.Conception, the fertilization of an egg by asperm, normally occurs in the fallopian tubes. The next step for the fertilized egg is to implant into the walls of the uterus, beginning the initialstages of pregnancy. If fertilization and/or implantation does not take place, the system is designed to menstruate (the monthly shedding of the uterine lining). In addition, the female reproductive system produces femalesexhormones that maintain the reproductive cycle.