Kenny Hunt, age 11, of Hurricane, W. Virginia, for his question:
Does a fish have a heart?
Yes, a fish has a heart. It is much simpler than a human heart, though it does the same sort of work. It pumps his blood through his arteries and veins, distributing oxygen and dissolved food to the busy cells throughout his body. A fish's heart co operates with his body's respiratory system and digestive system, just as the human heart does, though on a more simple scale.
The elaborate human heart has four compartments, or chambers, to pump the blood stream to and from the lungs, then around and around the body. A fish's heart has only two main chambers called the atrium and the ventricle. It is linked by major blood vessels to his gills and also to the veins and arteries of his circulatory system. Like the human heart, it pump pump pumps to circulate oxygen and nourishment ¬day and night.
A fish's gills absorb dissolved oxygen from the water. His gills are delicate fringes under flappable covers, situated where you would expect to find his ears. As he gulps, water flows back through his mouth, streams among the gills and emerges through the gill covers. Tiny blood vessels in the delicate gills extract oxygen and return waste carbon dioxide to the water.
Larger blood vessels carry the purified, enriched blood back from the gills to the two chambers inside the heart. It fills the atrium, then the ventricle. With the next heartbeat, powerful muscles in the ventricle send the fresh blood through a system of arteries to deliver its oxygen to the busy cells. As the cells take the oxygen they need, they return their waste carbon dioxide to the blood stream.
As the pulsing blood streams through the digestive organs, it gathers dissolved nutrients and these too are distributed to the cells.
Hence, food and oxygen are distributed constantly by the arteries. Used blood, carrying waste products from the cells is carried by the veins.
Waste food products are carried to the fish's kidneys, to be excreted. The veins carry waste carbon dioxide all the way back to the heart. There the used blood is pumped on a circuit to the gills, where it gives up its carbon dioxide and absorbs a fresh supply of oxygen. The rejuvenated blood returns again to the heart, all ready to be pumped on another trip through the fish's body.
Animals without backbones, such as worms, have much simpler hearts. But the fish has the simplest heart of the vertebrates. The mammals have four chambered hearts. However, a two chambered heart performs the same duties and works fine for a fish.