Introduction

Organisms with more than one cell require different cells to work together to carry out the necessary tasks to keep the organism functioning. The division of labour goes from the simple components of cells to the complex systems of organs that keep an organism alive.

Organ Systems

Organelles are structures in the cell that perform certain functions for the cell; e.g., mitochondria provide energy for the cells.
Tissues are groups of cells of the same type performing a specific function; e.g., muscle tissue has the ability to stretch and contract allowing movement.
Organs are structures composed of different tissues specialized to carry out a specific function; e.g., the heart creates the flow of blood through the blood vessels.
Organ systems are groups of organs that have related functions; e.g., the circulatory system contains the heart, blood, and blood vessels that enable oxygen to travel throughout the body.
Organisms are made up of all the steps above working together to create a living being.

Human Organ Systems

Organ System	Major Organs	Major Function
digestive	esophagus, stomach, intestines, liver, pancreas	physical breakdown of food
circulatory	heart, blood vessels (arteries, veins, capillaries)	transportation of nutrients, gases, and wastes; defence against infection
respiratory	lungs, trachea, blood vessels	gas exchange
reproductive	testes, vas deferens, ovaries, uterus, fallopian tubes, glands	sexual reproduction
excretory	kidneys, bladder, ureter, urethra, liver	removal of wastes
locomotive (skeletal and muscular)	bones, muscles	movement of body and body parts
endocrine	pancreas, pituitary gland, adrenal glands	coordination and chemical regulation of body activities
nervous	brain, spinal cord, eyes, ears, nose, tongue, peripheral nerves	response to the environment; control of body activities

Organ Systems in Animals

Circulatory System:

The circulatory system in the human circulates blood around the body so that it can be oxygenated (receive oxygen), and so wastes can be disposed of. This circulation then serves to bring newly oxygenated blood to the tissues of the body and allows other chemicals to diffuse out of the blood cells and into the fluid surrounding the cells of the body's tissues. Waste products diffuse into the blood cells to be carried away. The blood then circulates through organs, such as the liver and kidneys, where wastes are removed and then moves back to the lungs where carbon dioxide is exchanged for a fresh dose of oxygen. The process then repeats itself. This process of circulation is necessary for the continued life of cells, tissues, and even of the entire organism.

Watch this system to learn more about the circulatory system:

1. Blood rich in carbon dioxide is pumped from the heart into the lungs through the pulmonary arteries. Arteries are blood vessels carrying blood away from the heart; veins are blood vessels carrying blood to the heart.

2. In the lungs, CO₂ in the blood is exchanged for O₂.

3. The oxygen-rich blood is carried back to the heart through the pulmonary veins.

4. This oxygen-rich blood is then pumped from the heart to the many tissues and organs of the body, through the systemic arteries. In the tissues, the arteries narrow to tiny capillaries. Here, O₂ in the blood is exchanged for CO₂.

5. The capillaries widen into the systemic veins, which carry the carbon dioxide-rich blood back to the heart.

Humans have multiple layers of cells. Gases, nutrients, and waste products cannot simply diffuse out and into the cells by simple osmosis or diffusion. Because of this, the body needs a way to move things around to all the cells. The human heart pumps blood throughout the body using its four chambers to ensure efficient and rapid movement of highly oxygenated blood to the organs of the body. This helps thermal regulation and muscle movements by delivering oxygen to the cells. The frog heart is three-chambered, consisting of two atria and a single ventricle. It mixes oxygenated and deoxygenated blood in the single ventricle so the organs do not get blood saturated with oxygen. The human and all other mammals' hearts, as well as birds, are four-chambered with two atria and two ventricles. Deoxygenated and oxygenated blood are not mixed.

Digestive System:

The digestive system in humans involves the chemical breakdown of food and its absorption from the gut. Digestion begins in the mouth where food is chewed and swallowed. During this mechanical breakdown of food, enzymes are released from the salivary glands that speed up the chemical breakdown of starch and other sugars into simpler components.

Watch this video to learn more about how the digestive system works:

After food is swallowed it travels down the esophagus to the stomach by peristalsis. The stomach has three functions; it stores the swallowed food/liquid, it mixes up the food and digestive juices for further breakdown, and finally, it empties the contents into the small intestine. In the stomach, the acidic juices kill bacteria in the food and further breakdown of the food occurs. Damage to the stomach wall by acidic gastric juice is prevented by mucus lining the stomach walls.

The liver produces bile. This bile juice dissolves fat into the watery contents of the intestine, similar to the breakdown of grease in a frying pan with soapy water. After the fat is dissolved, it is digested by enzymes from the pancreas and the lining of the intestine. Once inside the intestines, organs like the pancreas produce juice that contains a wide array of enzymes to break down the carbohydrates, fats, and proteins in food. Other enzymes that are active in the process come from glands in the wall of the intestine.

Finally, the digested nutrients are absorbed through the intestinal walls by specialized cells, called villi, that allow absorbed materials to cross into the blood, where they are carried off in the bloodstream and transported by capillaries throughout the body. This is where the two organ systems connect. The digestive system breakdowns the food we eat and the circulatory system transports these tiny nutrient packages produced by the digestive system to trillions of hungry cells in the human body. The waste products of this process include undigested parts of the food, known as fibre, and older cells that have been shed from the mucosa. These materials are pushed into the colon, where they remain until the feces are expelled by a bowel movement.

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Now that you have learned about circulatory systems, consider your favorite sport. Consider how a healthy circulatory system can help an athlete in that sport. For example, if your favorite sport is swimming, how can a healthy circulatory system help a swimmer?