104 PHYSIOLOGY. 



through these layers into the plasma, and most of it is 

 quickly picked up by the colored corpuscles. The colored 

 corpuscles are the carriers of oxygen. 



Hemoglobin and Oxy hemoglobin. As has already 

 been stated, the hemoglobin in the colored corpuscles has 

 an affinity for oxygen. Hemoglobin is of a dark color, 

 and gives the dark color to the blood which enters the 

 lungs. When oxygen unites with the hemoglobin it forms 

 oxyhemoglobin, which is of a bright red color. Hence 

 the change in the color of the blood in the lungs from a 

 dark bluish red to a bright scarlet. This bright blood is 

 usually called " arterial," and the dark " venous " ; but it 

 must be remembered that the blood in the pulmonary 

 artery is dark, and in the pulmonary veins bright. 



Amount of Oxygen Used. We take into the blood 

 only about one fourth of the oxygen of the air that passes 

 through the lungs. In like manner the blood, passing 

 through the tissues, gives up to those tissues (in ordinary 

 circumstances) only about half the oxygen it contains (per- 

 haps holding the remainder as a reserve). 



The Gases in the Blood. If a quart of blood be placed 

 under the receiver, and the air exhausted, it will be found 

 that the blood contained about three fifths of a quart of 

 gas. This gas is a mixture of oxygen, carbon dioxid, and 

 nitrogen, and the proportions vary according to the kind 

 of blood taken. If from the left heart, or pulmonary veins, 

 there will be more oxygen and less carbon dioxid ; if from 

 the right heart, pulmonary artery, or caval veins, there 

 will be less oxygen and more carbon dioxid. Oxyhemo- 

 globin blood (" arterial blood ") contains about one fifth its 

 volume of oxygen. Hemoglobin blood ("venous blood") 

 contains about one tenth its volume of oxygen. Oxy- 



