Gas Exchange HCS 1070 SLO: 1.8-1.9

External Gas Exchange The movement of gases between the alveoli and the capillary blood in the lungs. Gas must move through both the alveolar wall and the capillary wall which are both very thin. These membranes are also moist because the oxygen and carbon dioxide must go into solution before they can diffuse across the membranes.

Diffusion Diffusion is the movement of molecules from an area of high concentration to an area of low concentration. There is a higher concentration of oxygen in the alveoli than there is in the capillary, so oxygen will move from the alveoli to the capillary There is a higher concentration of carbon dioxide in the capillary than in the alveoli, so carbon dioxide will move from the capillary to the alveoli.

Diffusion of Oxygen and Carbon Dioxide

Internal Gas Exchange The movement of gases between the blood (capillaries) and the tissues (cells). Cells are constantly using up oxygen during cellular respiration and producing carbon dioxide. As a result, the concentration of oxygen in the cells will always be low and the concentration of carbon dioxide will be high. This concentration gradient allows for diffusion of oxygen into the cells where it is needed and for diffusion of carbon dioxide (waste gas) to leave the cells and be carried by the blood to the lungs where it can be eliminated.

Transport of Oxygen Blood plasma can only carry about 1.5% of the oxygen Almost all of the oxygen that diffuses into the capillary blood in the lungs is bound to hemoglobin found in red blood cells. Hemoglobin is a large protein molecule with four small iron containing “heme” regions. Each heme portion can bind with one molecule of oxygen

Red Blood Cells and Hemoglobin

Interesting Fact Even though blood returning to the heart from body tissues is said to be deoxygenated, the red blood cells still hold some oxygen. The blood is never completely depleted of oxygen.

Transport of Carbon Dioxide Carbon dioxide is produced continuously in the cells as a by-product of cellular respiration. Glucose + oxygen  carbon dioxide + water + ATP energy It diffuses from the cells into capillaries and into the blood where it is transported to the lungs to be breathed out.

Most of the carbon dioxide is transported as an ion (bicarbonate HCO3) which is formed when carbon dioxide undergoes a chemical change after it dissolves in blood fluids. It first combines with water to form carbonic acid (H2CO3) This acid then separates (ionizes) into hydrogen and bicarbonate ions.

This happens much more quickly inside the red blood cells where an enzyme called carbonic anhydrase increases the rate of the reaction. The bicarbonate formed in the blood cells moves to the plasma where it is carried to the lungs. In the lungs, the process is reversed. The bicarbonate moves back into the red blood cells and releases carbon dioxide for diffusion into the alveoli and breathed out of the lungs.

Transport of Carbon Dioxide

Questions Gases move between the alveoli and the blood by the process of diffusion. What is the definition of diffusion? What substance in red blood cells carries almost all of the oxygen in the blood? What is the main form in which carbon dioxide is carried in the blood?

Answers Diffusion refers to the movement of molecules from an area in which they are in higher concentration to an area in which they are in lower concentration. Hemoglobin molecules in the red blood cells carry almost all of the oxygen that diffuses into the capillary blood. Carbon dioxide is carried in the blood in the form of a bicarbonate ion which is formed when carbon dioxide undergoes a chemical change after it dissolves in blood fluids.