Human Heart Blood Flow

Study Notes

Oxygen-rich blood from the lungs enters the thin-walled left atrium, the upper chamber of the heart.
The left atrium relaxes while collecting blood, then contracts to transfer blood to the left ventricle.
The left ventricle relaxes to receive blood from the left atrium, then contracts to pump oxygen-rich blood out to the body.

Deoxygenated blood from the body enters the right atrium, the upper chamber on the right, as it relaxes.
The right atrium contracts to transfer blood to the right ventricle, which dilates to receive it.
The right ventricle then pumps the deoxygenated blood to the lungs for oxygenation.

Ventricles have thicker muscular walls than atria because they need to pump blood into various organs.
Valves in the heart ensure that blood does not flow backwards when atria or ventricles contract.

The heart is a muscular organ, approximately the size of a fist.
It has different chambers to prevent the mixing of oxygen-rich and carbon dioxide-rich blood.

Oxygen-rich blood from the lungs enters the left atrium, a thin-walled upper chamber.
The left atrium relaxes while collecting blood, then contracts to transfer blood to the left ventricle.
The left ventricle contracts to pump blood to the rest of the body.
De-oxygenated blood from the body enters the right atrium, another thin-walled upper chamber.
The right atrium contracts to transfer blood to the right ventricle.
The right ventricle pumps blood to the lungs for oxygenation.

In animals with high energy needs, such as birds and mammals, the heart has a four-chambered structure to keep oxygenated and de-oxygenated blood separate.
This separation allows for a highly efficient supply of oxygen to the body.
In animals that do not use energy to maintain body temperature, such as amphibians and reptiles, the heart has a three-chambered structure, allowing for some mixing of blood streams.
Fishes have a two-chambered heart, with blood being pumped to the gills for oxygenation before passing to the rest of the body.

In fish, blood goes through the heart only once during each cycle of passage through the body.
In other vertebrates, blood goes through the heart twice during each cycle, known as double circulation.