The digestive system begins with the mouth. The mouth does both mechanical and chemical digestion. Teeth rip, shred, crush, grind, and tear food into smaller and smaller pieces which serves to increase the surface area of the food being eaten. The saliva in the mouth contains amylase which is an enzyme that breaks down sugars.
After the mouth is the esophagus. This is the tube that connects the mouth to the stomach, and it moves food down into the stomach through wave-like contractions called peristalsis.
Once in the stomach, the food is engulfed in chemicals like hydrochloric acid and pepsin. The pepsin helps chemically digest proteins. Food will stay in the stomach for around 3–4 hours, and it will then move into the small intestine. The small intestine does additional chemical digestion, and the pancreas is responsible for producing many of those chemicals. The small intestine is also where the food will be absorbed into the bloodstream. This occurs at the tiny folds in the small intestine called villi.
Next is the large intestine. A main function of the large intestine is water absorption. Any solid material left over at this point is removed from the body via the rectum and anus.
The cardiovascular system is typically divided into three parts: the blood, the blood vessels, and the heart. The heart is the pump that moves the blood through the blood vessels. A human heart is a four chambered heart, and it has two distinct sides. The right side of the heart carries oxygen poor blood and pumps that blood to the lungs where the blood will deposit carbon dioxide and pick up oxygen. The blood then returns to the left side of the heart, and it is then pumped to the "body" where it will exchange oxygen for carbon dioxide at the cells. Blood being pumped away from the heart is carried in arteries while blood returning to the heart is in veins. The capillaries are the thin walled vessels that allow the gas exchanges.
The respiratory systems is composed of the nose, nasal cavity, pharynx, larynx, trachea, bronchi, lungs, and alveoli. The nose's main function is to filter air that is being inhaled. The filtration is done by hairs found within the nose. The nasal cavity warms and humidifies the air. The pharynx is the throat, and it is technically part of the digestive system as well. The pharynx will branch off in two directions. One way is the esophagus, and the other leads into the larynx (voice box) and trachea (windpipe). The bronchi lead to the left and right lungs, and the lungs contain the alveoli. The bronchi will continually branch into smaller and smaller branches that lead to the tiny alveoli air sacs which are wrapped in a bed of capillaries. Carbon dioxide will diffuse into the alveoli for exhalation, and oxygen will diffuse out of the alveoli and into the blood for delivery to the body's cells.
The urinary system begins with the kidneys which filter blood and remove waste products. They also help balance the body's fluid levels and produce hormones that regulate blood pressure. The removed waste is called urea and combines with water to form urine. The urine is carried to the bladder via ureters, and the bladder stores it until it is time to rid the body of the urine. The urine will flow out of the body through a tube called the urethra.
The endocrine system is made up of many different glands that secrete hormones. The major glands are the hypothalamus, the pituitary, the thyroid, the parathyroids, the adrenals, the pineal body, and the ovaries/testes. Hormones are chemicals that regulate the activity of cells and/or organs. Things like sexual development, growth, and metabolism are just some of the things that the hormones of the endocrine system help to regulate.