Final answer:
The blood carries carbon dioxide as it moves from the heart to the lungs, where it is expelled and oxygen is taken up.
Step-by-step explanation:
The gas that the blood carries as it moves from the heart to the lungs is carbon dioxide. In the process known as external respiration, deoxygenated blood travels via the pulmonary artery from the heart to the lungs, where it releases carbon dioxide into the alveoli and takes up oxygen. Carbon dioxide in the blood can be transported in three ways: most directly dissolves in the plasma, a smaller amount binds to hemoglobin forming carbaminohemoglobin, and the rest is carried as a bicarbonate ion. Oxygen is then transported back to the heart through the pulmonary veins in the form of oxygenated hemoglobin within red blood cells.
In the lungs, during gas exchange, carbon dioxide is expelled and oxygen is loaded into the blood. This is made possible by the large surface area of red blood cells, which facilitates rapid diffusion of these gases. Hemoglobin plays a crucial role in transporting both oxygen and carbon dioxide, with oxygen binding to hemoglobin for transport around the body, and carbon dioxide being released from it to be exhaled.
The blood carries oxygen as it moves from the heart to the lungs.