Oxygen and carbon dioxide in the lungs

Key Points

1. Dark blue, oxygen-poor blood travels to the lungs, where the oxygen is exchanged for carbon dioxide received from the capillaries.
   
   
2. The capillaries return the bright red oxygen-rich blood to the heart, which pumps the blood out to the rest of the body.

Blood was considered the "lifeblood" in the 16th century. Medical experts of the day thought it was the life-giving force or the body's "seat of vitality."¹ Today, we understand that blood and all the body's organs work together to maintain life.

The human body contains about five quarts of blood. About one-half of this is a straw-colored solution called plasma. It carries substances, such as proteins, fats, sugars, minerals, and other nutrients, throughout the body. The other half of blood is made up of cells: red blood cells, white blood cells, and platelets. Red blood cells are responsible for carrying oxygen to the body's cells; they also play an important role in transporting carbon dioxide away from the body's cells. These blood cells are red because of the hemoglobin they contain. Hemoglobin binds the oxygen and carries it throughout the body.

White blood cells help the body fight infection caused by bacteria, viruses, fungi, and parasites. They leave the blood and pass through the capillary walls to reach areas of the body where these invaders may be hiding.

Platelets are "sticky" cells that help the blood to clot. They also help repair damage to blood vessels. Without enough platelets, a cut to the skin may take a long time to stop bleeding. Low numbers of platelets—sometimes caused by certain medications and illnesses—also may cause bruises from broken blood vessels under the skin that continue to bleed. Finally, platelets can contribute to the build-up of fatty deposits called plaques that can clog arteries, a sign of coronary artery disease. Learn more in Coronary Artery Disease.

Oxygen and carbon dioxide in the lungs

How does oxygen get to the blood in the first place? We breathe air containing oxygen through the mouth and nose. The air travels down through a breathing tube (the trachea or windpipe) into the lungs. The body's two lungs lie on either side of the heart.

Oxygen travels into the smallest air sacs of the lungs, called alveoli. Capillaries around these air sacs allow blood to exchange oxygen for unwanted carbon dioxide. The oxygen-rich blood then returns to the heart to be pumped to the rest of the body. The carbon dioxide in the lungs is passed out of the body through the mouth and nose when we exhale.

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Source

1. "Lifeblood." Merriam-Webster's Online Collegiate Dictionary. 2004. Accessed June 2, 2004. http://www.m-w.com