To trace and describe the flow of blood with oxygen and carbon dioxide we may start at any point in the circulatory system since blood flows in a closed system of vessels, I will start with oxygen that has already entered the lungs through the nose then the brochial tree, oxygen in the lungs specifically in the air sacs will pass through the walls of the air sacs so it can enter the capillaries surrounding the air sacs, while the carbon dioxide from different parts of the body that has reached the membrane of the alveoli will pass through the walls of the capillaries to enter the air sacs, in short there is exchange of gaseous materials, carbon dioxide the goes out the body when we exhale, while oxygen which is already in the capillaries will go out from the lungs through the pulmonary veins to enter the left atrium of the heart then it goes to the left ventricle to be pumped out of the heart through the aorta so blood can bring oxygen to different parts of the body, and when oxygen is taken by the cells of our tissues the cells will release carbon dioxide again there is exchange of gases, this time it is between cells and the blood, the blood with carbon dioxide will then go back to the heart through the vena cavae and enters the right atrium of the heart, then to the right ventricle and goes out he heart to enter the lungs passing through the pulmonary arteries to collect oxygen and to give off carbon dioxide, then the circulation continues.
