In this topic you will learn how organ systems work together to keep animals alive.

Blood is a liquid tissue made up mostly of red blood cells, white blood cells, and a liquid called plasma. Blood carries food, oxygen, and water to the body's cells and removes waste from the body's cells. The circulatory system is the organ system that moves blood through the body. The heart is part of the circulatory system.

Cells need the oxygen carried by blood to get energy from food. The respiratory system is the organ system that brings oxygen to blood and removes the waste gas carbon dioxide. Large, more complex animals with waterproof skin need a respiratory system.

Animals create many types of waste. The waste gas carbon dioxide is removed from the body by the lungs. Liquid waste, created when cells break down chemicals, are removed by the excretory system. In vertebrates, the main waste removal organs are the two kidneys. Each kidney filters waste from the blood. The waste is concentrated into a liquid called urine.

Before body cells can use food for fuel, food must be broken down. The digestive system is the organ system that breaks down food for fuel. In the human digestive system, chewed food passes through the esophagus to the stomach where strong acids break down the food. From the stomach, food passes into the small intestine. Nutrients from the food are absorbed into the blood.

The skeletal system is made up of bones. Bone is living tissue. The muscular system is made up of muscles that move the bones. Vertebrates use bones and muscles together to move.

The nervous system controls all other body systems. The nervous system is made up of nerve cells and nerves. More complex animals also have a brain. Most animals have specialized sense organs that collect information about their surroundings.

Everything you do depends on organ systems. If you understand the parts that make up these systems and how they work, you can take good care of them.