The Human Body Everyone Needs Healthy Systems There are several systems that make up the human body. Although their functions differ, they all work together to keep your body running smoothly. Some of these systems include the circulatory, respiratory, digestive, and urinary systems. Blood Vessels The circulatory system of the human body consists of the heart, the blood, and the tubes known as blood vessels. This system constantly works to supply your body with oxygen and nutrients. There are three kinds of blood vessels that transport blood throughout your body. They are arteries, capillaries, and veins. Arteries are blood vessels that carry blood away from the heart to other parts of the body. It s their job to carry blood that is rich in oxygen and deliver it to all parts of your body. The walls of arteries are thick and muscular. They can stretch out as the heart pushes blood through them. The smallest arteries branch out into capillaries. A capillary is the smallest kind of blood vessel. The oxygen in your blood moves from the capillaries into your cells. The wastes and carbon dioxide move out of the cells and back into the capillaries. Unlike arteries, capillary walls are very thin. In fact, they are only one cell thick. Gases can pass through these thin walls. Capillaries join together to make small veins. These small veins join together to make larger veins. Veins are blood vessels that transport blood toward the heart. They help the return trip of blood to the heart. They have valves that are like doors. These valves make sure the blood only flows in one direction. They open to let blood flow to the heart and close if blood begins to flow away from the heart.
The Human Heart The heart is a vital muscle. In an adult human, it is about the size of a fist. The heart beats twenty-four hours a day when you re awake and when you re asleep in order to pump blood to all parts of the body. There are four chambers in the human heart. These chambers are made of a special kind of muscle. They work together in a rhythm that s based on how active you are being at the time. Two chambers make up each side of the heart. The right atrium and right ventricle make up one side, and the left atrium and left ventricle make up the other side. The atrium is the upper part and the ventricle is the lower part. The ventricle is stronger than the atrium. The left and right sides of the heart work separately to pump blood on different paths. The heart contracts in order to pump blood. The hearts of animals and humans are not the same. Many reptiles have three chambers that make up their heart. Fish only have two chambers.
The Job of the Respiratory System The respiratory system moves oxygen and carbon dioxide between the air and your blood. Respiratory Parts You inhale, or breathe air in, through your mouth and nose. This is how respiration starts. Your sinuses work to warm and moisten the air you breathe. Inside your nose are small hairs and a layer of mucus. These work to trap dirt and germs from getting into your lungs. From the nose, the air moves to the back of the throat and into the larynx. This is where the vocal cords are. The trachea is a tube that carries the air from the larynx toward the lungs. It branches into two tubes a little below where your neck joins your body. These two smaller tubes are the left and right bronchi. They deliver air into your left and right lungs. Inside the lungs, the bronchi branch out into smaller and smaller tubes. These are called bronchioles. The bronchioles finally end in tiny air sacs in the lungs. The air sacs are where oxygen enters the blood and carbon dioxide leaves it. At the bottom of your chest is the diaphragm. This dome-shaped muscle expands and contracts. When it expands, it forces air out of your lungs and you exhale. When it contracts, you inhale, or breathe air in.
Systems Work Together Living things require oxygen. Oxygen is important because it helps cells release the energy that they need from sugar. Insects have a respiratory system. They breathe in oxygen through openings on their sides. Worms have a circulatory system. This helps them get oxygen. Their blood absorbs oxygen that is carried through their body. Humans are more complex living things. Their respiratory and circulatory systems work together to get oxygen to cells. It all begins when you take a breath. Breathing inhaling and exhaling is the job of the respiratory system. Oxygen comes in from the air that you breathe. Oxygen fills the lungs. The lungs make oxygen available to the circulatory system so that it can be transported by your blood. Carbon dioxide is removed from your blood and exhaled back out to the air outside your body. Two things are happening at the same time in the air sacs. Oxygen leaves the lungs and enters the blood. Carbon dioxide leaves the blood and enters the lungs. If you hold your breath, carbon dioxide builds up in your blood. The muscles in your diaphragm contract, forcing you to breathe. This is one way your systems work together to supply you with oxygen.
The Digestive and Urinary Systems Digestive System Turning the food you eat into fuel that your body can use is the job of the digestive system. Digestion is the process that changes food into forms that the body can use. Many organs work together to break down food into very small bits of materials. Once the food is broken down it can be carried by the blood to your cells. The Mouth and Esophagus Where does the digestive system begin? With the mouth, of course! In the mouth you chew pieces of food so that they become small enough to swallow. The taste buds on your tongue bring out the flavors of the food. The tongue also helps move food to the back of your mouth. From there the food passes into a tube called the esophagus. This organ is made up of rings of muscle. The muscles move in a pattern to squeeze the food as it travels down the tube to the stomach. Muscles behind the food get tight as the food passes through each ring. This entire process doesn t take very long. Food reaches the stomach in about two or three seconds.
The Stomach and Intestines Where is your stomach? It s on the left side of your body under your ribs. The walls of your stomach are muscles. They can stretch enough to hold all the food you eat in a meal. When you swallow, a muscle at the bottom of your esophagus opens up. This allows food to reach your stomach. The muscle then closes to keep food there. The stomach muscles squeeze together at the same time the stomach releases acids to break down food. This soupy mixture is then ready for the next step in digestion. The partly digested food leaves the stomach and enters the small intestine. This is a narrow, winding tube. Over several hours, the small intestine breaks down food even further. This is done with the help of the liver and the pancreas. These organs send chemicals to aid in digestion. When the process of digestion is finished, the digested food particles move into the blood vessels in the small intestine. Some food, however, cannot be digested in the small intestine. This undigested food moves to the large intestine, often called the colon. Here some water is recovered from the food. The waste will be stored in another part of the colon, until it is removed from the body.
The Urinary System Wastes from your body cells enter your blood. If they stayed there for a long time, they would poison you. Therefore, this waste must be removed. In humans, waste is removed from the blood mostly by the urinary system. Humans have a pair of organs called the kidneys. Kidneys have the same dark red color and shape as kidney beans. Located below the lowest ribs, there is one kidney on either side of your backbone. Kidneys filter wastes out of the blood. This mix of water and wastes is called urine. There s a tube from each kidney that carries urine to the urinary bladder. The bladder is a sac that collects and stores urine. At the bottom of the bladder is a tight muscle. This holds the urine in the bladder until the body removes it through urination. As the kidneys filter waste, other materials leave the blood too. These materials include salt, calcium, and other nutrients your body needs. The kidneys put the right amount of these materials back into your blood. They help to maintain a balance. Other organs besides kidneys remove wastes. When you sweat, your glands release a little bit of waste too.
Glossary air sacs artery bladder bronchiole capillary digestion esophagus large intestine small intestine trachea vein the thin-walled sacs in the lungs where oxygen enters the blood and carbon dioxide leaves the blood a blood vessel that carries blood away from the heart to other parts of the body the sac that collects and stores urine formed by the kidneys a small tube in the lungs that carries air to air sacs the smallest kind of blood vessels the process that changes food into a form that the body can use the tube that carries food from the mouth to the stomach a wider tube connected to the small intestine that removes water from undigested food and stores this waste until it is removed from the body a narrow, winding tube where food is changed chemically and broken down into small particles that can be absorbed into the blood the tube that carries air from the larynx to the lungs a blood vessel that carries blood toward the heart