This section is from the book "The Human Body: An Elementary Text-Book Of Anatomy, Physiology, And Hygiene", by H. Newell Martin. Also available from Amazon: The Human Body.
The blood when flowing in the pulmonary capillaries gives up carbon dioxide (a waste product which it has gathered in its flow through the other organs) to the air, and receives oxygen from it; since its coloring matter (hæmoglobin) forms a scarlet compound with oxygen, the blood which flows to the left auricle through the pulmonary veins is of a bright red color. This color it maintains until it reaches the systemic capillaries, but in these it loses much oxygen to the surrounding tissues, and gains much carbon dioxide from them. But the blood-coloring matter which has lost its oxygen has a dark purple-black color, and since this unoxidized or " reduced " hæmoglobin is now in excess, the blood returns to the right auricle of the heart by the venæ cavæ of a dark purple-red color. This color it keeps until it reaches the lungs, where the reduced hæmoglobin becomes again oxidized. The bright red blood, rich in oxygen and poor in carbon dioxide, is known as "arterial blood," and the dark red as "venous blood;" and it must be borne in mind that the terms have this peculiar technical meaning, and that the pulmonary veins contain arterial blood, and the pulmonary arteries contain venous blood. The change from arterial to venous takes place in the systemic capillaries, and from venous to arterial in the pulmonary capillaries.
Why are we justified in diagrammatically representing the heart as made of two separated parts? Starting from the left auricle, describe the course of the blood until it returns there.
What does the blood give up in the pulmonary capillaries? What does it receive? Why is it bright red when it enters the left auricle? How far in its course does it keep this color? What gases does the blood gain and lose in the systemic capillaries?
What color is the blood when returned to the right auricle? Why? What is meant by arterial blood? By venous? What veins contain arterial blood? What arteries venous? Where does the change from arterial to venous occur? Where that from venous to arterial?