Digestive, Excretory, Respiratory, and Circulatory Systems

The Digestive System

No matter what you eat, everything that goes into your mouth is processed by your body’s digestive system. Three main processes occur within the digestive system: digestion, absorption, and elimination.

  • Digestion is the breakdown of food into nutrients, which are molecules that your body’s cells can use.
  • Absorption is the movement of nutrients from the digestive system into the bloodstream, where they can be carried to all parts of the body.
  • Elimination is the removal of undigested material from the body.

These processes occur in a series of organs called the digestive tract. The organs of the digestive system help the process by moving food around or by producing chemicals used in digestion.

The Esophagus and Stomach

Digestion starts as soon as you put food in your mouth. Chewing breaks food into smaller pieces and enzymes in your saliva help to chemically break down food. Smooth muscle in the digestive tract moves food through the digestive system. In this way, the digestive system interacts with the muscular system. Food moves from your mouth to your stomach through a muscular tube called the esophagus. Your stomach is the body’s main organ of digestion. Food is broken down by the stomach’s digestive juices.

The Small and Large Intestines

Partially digested food moves from the stomach to the small intestine, where absorption occurs. Digested nutrients and water pass through the walls of the small intestine and into the bloodstream. Waste materials continue through the small intestine and move into the large intestine. In the large intestine, waste materials are prepared for elimination, which is the passage of undigested material out of the body through a:n opening called the anus.


The Liver, Gallbladder, and Pancreas

Even though food does not move through them, the liver, gallbladder, and pancreas are important parts of the digestive system. These three organs aid digestion in the small intestine. The liver is the largest internal organ of the body and has many functions. Its job in digestion is to make bile, which breaks down fat. Excess bile made by the liver is stored in a small organ called the gallbladder. The gallbladder releases bile into the small intestine as needed. The pancreas is a leaf-shaped organ that produces digestive enzymes. Enzymes from the pancreas are proteins that speed up biological reactions. The enzymes the pancreas makes help break down proteins, carbohydrates, and fats in the small intestine.

The Excretory System

Processes within your cells result in waste products. These waste products must be removed from your body to keep your body systems in balance. The excretory system is the system that removes liquid, solid, and gas wastes from the body.

Liquid Waste

Your skin is one of the organs of the excretory system, and sweating is one process of excretion. Your skin sweats in conditions such as warm weather and during exercise. The sweat helps your body with temperature regulation. As sweat evaporates from your skin, this helps your body cool down. When your skin sweats, this also allows your body to get rid of excess water and salts. The lungs, which are part of the respiratory system, are also important structures in the excretory system. Carbon dioxide is a gas produced as a waste product from cell processes. Most of it is removed from your body through the lungs when you exhale.

Urine is produced by the kidneys, which are organs that are separate from the digestive tract. Kidneys are the main organs of the excretory system. People usually have two kidneys. The kidneys use millions of tiny filters to separate waste products in the blood from the water, glucose, and minerals the body needs.


Liquid waste produced by the kidneys is carried in urine, which passes through a tube called the ureter and is stored in the bladder. When the bladder is full, it contracts and pushes urine out of the body through the urethra. In most instances, the emptying of the bladder is under the control of voluntary muscles.

Blood enters each kidney through a large artery. Inside the kidney, the artery divides into many networks of capillaries that surround the filtering units of the kidney, which are called nephrons. Each kidney has about one million nephrons. Each nephron looks like a long coiled tube with a cup at one end. The cups of the nephrons are found in the outer rim of the kidney. Fluid from the blood is pushed through the walls of the capillaries and into the nephron. Some of the material that is moved into the nephron is waste material, and some is material the body needs. The material the body needs is returned to the blood through a process called re-absorption. The material that is waste, along with water, leaves the body as urine.


Solid Waste

Solid waste from the digestive system is prepared in the large intestine for elimination. Water is removed from the waste material, which eventually leaves the body through the anus as feces.

The Respiratory System

Did you know that your digestive system is closely connected to your respiratory system? Oxygen, which enters your body through the respiratory system, is required for the process your cells use to release energy from food molecules. Specifically, oxygen is the gas that cells use for cellular respiration. Through cellular respiration, organic molecules from the food you eat are broken down, and energy and carbon-dioxide gas are released. The released energy is used by body cells for all the cells’ activities.

Gas Exchange Within the Lungs

When you breathe, you pull air into a pair of organs inside the chest called lungs. The inside of a lung is divided into many small air sacs, called alveoli, which are surrounded by tiny blood vessels called pulmonary capillaries. Oxygen in the lungs enters the body by diffusing across the alveoli and into the blood vessels. Diffusion is the movement of molecules from an area of higher concentration to an area of lower concentration. Inside the alveoli, oxygen moves into the bloodstream because there is more oxygen in the alveoli than there is in the blood. Meanwhile, carbon dioxide leaves the bloodstream because there is more carbon dioxide in the blood than there is in the alveoli. The oxygen travels within the bloodstream to other parts of the body, and most of the carbon dioxide is exhaled.


The Circulatory System

The circulatory system transports blood through the human body. Blood delivers water·and nutrients from the digestive system, and oxygen from the respiratory system, to all cells in the body. Blood also carries wastes from body cells to the organs that remove wastes.

The Heart

The center of the circulatory system is the heart, which pumps blood. The heart is a fist-sized muscle divided into two upper chambers called atria and two lower chambers called ventricles.


Oxygen-poor blood from the body enters the right side of the heart. The right atrium and right ventricle pump oxygen-poor blood to the lungs. While in the lungs, the blood takes up oxygen gas and releases waste carbon-dioxide gas it has carried away from body cells. Oxygen-rich blood returns to the left side of the heart from the lungs. The left atrium and left ventricle pump oxygen rich blood to the rest of the body. When it reaches body cells, the oxygen-rich blood gives up its oxygen and nutrients and picks up carbon-dioxide gas. The blood returns back to the right atrium, and the cycle continues.

Blood Vessels

There are three kinds of blood vessels inside the body: arteries, capillaries, and veins. In general, arteries carry oxygen-rich blood away from the heart. As blood moves away from the heart, the arteries get smaller and narrower. Capillaries are microscopic vessels that connect arteries and veins. Capillaries are only one cell thick. Veins carry blood back to the heart from the capillaries.