Arterial blood
The blood goes straight from the heart to the lungs and there it is changed into arterial blood. The change consists in taking oxygen from the air and giving off carbon dioxid to the air. From the lungs the blood goes through the capillaries again into the veins, the small capillaries in the lungs uniting to form the pulmonary veins. These veins finally unite into one vein that empties into the upper half of the left side of the heart, and from there the blood goes to the lower half of the same side. The blood has now reached its starting point and is ready to begin its journey again. The journey of the blood is as follows:
Entire circulation
From the right side of the heart to the arteries; from arteries into arterioles; from arterioles to capillaries; from capillaries into veins; from veins into the heart; from the heart to the lungs; and from the lungs back to the heart again.
THE RESPIRATORY SYSTEM
Use of the respiratory system
That portion of the body by which we breathe is called the respiratory system. This system is composed of the nasal passage, the pharynx, the larynx, the trachea, the bronchi, and the lungs. The mouth is not a part of the respiratory system; we should never breathe through our mouths.
The use of the nose in breathing
As the air passes through the nasal passage it is warmed and moistened, and a great deal of dust and dirt is taken out of it. Thus the nasal passage serves to warm, to moisten, and to clean the air we breathe, and is a very important part of the respiratory system, since either cold or dry air is very irritating to the lungs.