84 APPLIED PHYSIOLOGY 



Not even oxygen can pass through the sides of the arte- 

 ries, but when it reaches the capillaries it passes out very 

 easily. So in the capillaries it leaves the red blood cells 

 and goes to the cells of the body. When oxygen has left the 

 blood cells, they become of a darker color. So blood in the 

 veins is always of a darker color than blood in the arteries. 



151. Oxidation in the cells. The oxygen which goes to 

 the cells of the body burns, or oxidizes, their albumin, fat, 

 and sugar, producing carbonic acid gas and water. All 

 oxidation inside the body takes place in a living cell. 

 Oxidation is the real act of breathing. The cells of the 

 body are always getting oxygen and giving out carbonic 

 acid gas and water. The water drains off by the lymph. 

 The carbonic acid gas goes through the sides of the capil- 

 laries and so back to the blood, where it is dissolved and 

 carried to the lungs. So blood in the veins has less oxygen 

 and more carbonic acid gas than blood in the arteries. It 

 is the lack of oxygen, not the presence of carbonic acid gas, 

 that makes the venous blood dark in color. 



152. How carbonic acid gas leaves the body. All the 

 venous blood flows into the right side of the heart and is 

 pumped to the lungs. In the lungs the carbonic acid gas 

 leaves the blood and passes off through the sides of the 

 capillaries into the air sacs. It is then breathed out by the 

 next breath. At the same time, oxygen passes to the red 

 blood cells again, and the blood is ready for another round 

 of the body. So the blood carries oxygen to the cells of 

 the body and gets carbonic acid gas from them, and then 

 goes to the lungs to give up the carbonic acid gas in 

 exchange for oxygen from the air. The real breathing in 

 the body is done in the capillaries and cells. The lungs 

 are places where the red blood cells get oxygen to carry to 

 the cells. 



