The Digestive System
Molecules of food are too large to enter the cells of the body. To make use of the nutrients in food, the body must break down the food molecules. This process occurs in the digestive system. When food is chewed, the teeth grind and tear the food into small bits. Saliva not only keeps the food moist, it also begins to digest, or break down, the largest molecules.
When you swallow, the food goes down the esophagus and into the stomach. There, smooth muscles repeatedly contract to cause a churning motion that stirs the food. Strong acids begin to break the molecules apart. The food is like a thin soup by the time it leaves the stomach.
Food passes from the stomach into the small intestine. Tiny food particles move across the membrane of the small intestine into blood vessels. The blood carries the particles to cells throughout the body. Undigested food passes into the large intestine, where much of the water in the waste is absorbed into the body. The remaining solids, called feces, leave the body through the rectum and anus.
The Excretory System
Human body cells produce wastes. Most are removed as they pass through the kidneys. Inside the kidneys are millions of tiny filters that remove waste and other materials from the blood. Materials that are not waste are usually returned to the blood. Waste is carried in a liquid called urine, which passes into the bladder. The bladder pushes urine out of the body.
The liver also filters harmful substances, such as extra sugar, dead cells, chemicals, or drugs from the bloodstream.
The Circulatory System
The circulatory system transports nutrients, oxygen, antibodies, and wastes throughout the body.
Blood is a tissue. About half of the blood is plasma, a light-colored, watery liquid. Plasma carries nutrients, vitamins, minerals, and oxygen throughout the body.
The red and white blood cells and platelets are suspended in the plasma. Red blood cells carry oxygen from the lungs to the cells. White blood cells are much larger but they are able to squeeze through body tissue to reach an infection and destroy the invading cells. Platelets are tiny particles that form blood clots. Without platelets, a person could bleed to death.
The heart is a fist-sized muscle divided into four chambers. The two upper chambers, called atria, collect the blood returning to the heart through the veins. The two lower chambers, called ventricles, pump blood away from the heart through the arteries.
The two sides of the heart work like separate pumps. The right atrium receives blood from the body. This blood has little oxygen in it. The right atrium pumps the blood down into the right ventricle. From there the blood is pumped through an artery to the lungs, where the blood releases carbon dioxide, a waste product, and picks up oxygen.
Oxygen-rich blood leaves the lungs through veins and enters the left atrium of the heart. The left atrium pumps the blood down into the left ventricle. From there the oxygen-rich blood is pumped through arteries to all parts of the body, except the lungs. The arteries branch off into smaller and smaller arteries called arterioles and capillaries. In this way oxygen reaches every cell in the body. The cells release carbon dioxide into the blood and absorb oxygen.
The Respiratory System
The body can survive only a short time without oxygen. Cells use oxygen to burn food and release the energy in the nutrients. The waste product of this activity is carbon dioxide. The function of the respiratory system is to supply oxygen to the cells and remove carbon dioxide.
The hairs and the mucous lining of the nose warm, moisten, and clean the air of dust and dirt. The air enters at the rear of the mouth and then goes down the windpipe, called the trachea, through the bronchial tubes, and into the lungs. At the top of the windpipe is the larynx, or voice box, which allows us to make the sounds that include our speech.
In the lungs are millions of tiny air sacs. Each is surrounded by a network of capillaries. Here, oxygen passes into and carbon dioxide passes out of the blood vessels. The oxygen-rich blood heads straight for the heart, where it is pumped throughout the body. The carbon dioxide leaves the body when you exhale.