How Hemoglobin Picks Up and Delivers Oxygen

All of the cells in our bodies require oxygen (02) for survival and must release carbon dioxide (CO2) as a waste product. The respiratory and circulatory systems work together as delivery systems for these gases. The lungs exchange these gases between the environment and the bloodstream. The bloodstream then transports the gases to the metabolizing tissues and back to the lungs. O2 and CO2 are both transported in the circulatory system, but they are transported in different fractions of the blood. The liquid portion of the blood, called plasma, carries dissolved CO2, but can carry only a small amount of dissolved O2.A large amount of O2 loads instead onto hemoglobin molecules, which are packed inside red blood cells. This illustration represents the circulation that runs from the lungs, to metabolizing tissues, and back to the lungs again. Hemoglobin molecules are shown individually here, but in the bloodstream they are actually packed inside red blood cells. After passing through the lungs, hemoglobin is oxygenated, and as it passes through metabolizing tissues, it becomes deoxygenated. Within the lungs, oxygen diffuses into the plasma of the blood and then into red blood cells, where it is grabbed by hemoglobin molecules. Carried in red blood cells, hemoglobin delivers oxygen throughout the body. Where oxygen concentrations are low, such as in metabolizing tissues, hemoglobin releases oxygen. The oxygen diffuses into the cells and is used in cellular respiration. As cells consume oxygen, they generate carbon dioxide as a waste product during the citric acid cycle of cellular respiration. The carbon dioxide diffuses into the bloodstream, where it dissolves in the plasma and travels to the lungs. The carbon dioxide then diffuses into the lungs and, with an exhalation, the lungs release it into the atmosphere. With an inhalation, the cycle repeats.

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