The Heartworm's Cardiovascular System
The heartworm is very common amongst animals, especially dogs, and in very rare cases can enter humans. The heartworm tends to enter the heart and pulmonary arteries and clogs them, leading to a large decrease in blood pressure which can exhaust, and even make ill, the host. While the heartworm rests in the heart and pulmonary arteries, the worm has the blood of the host carrying nutrients and oxygen diffuse into its long and narrow body, letting it survive. The danger of the heartworm is that its numbers can multiply as it lays more and more eggs within the host, which can lead to fatal causes.
Here is an illustration of what the inside of the heart would like look like when infected with heartworms. It is evident that the worms would eventually overcrowd the heart, and enlarge the arteries going out of the heart.