The right atrium collects blood
from two large veins, the
superior vena cava (from the
upper body) and the inferior
vena cava (from the lower
body).
The oxygenated blood travels
back from the lungs through the
pulmonary veins (of which there
are four) and enters the left
atrium of the heart.
From the left atrium, the blood
is pumped through the mitral
valve into the left ventricle.
The blood that collects in the
right atrium is pumped through
the tricuspid valve into the right
ventricle.
The pulmonary artery carries
the blood to the lungs, where it
enters the pulmonary
circulation, gives up carbon
dioxide, and picks up oxygen.
After passing through
capillaries and exchanging
substances with cells, the blood
returns to the right atrium via
the superior vena cava and
inferior vena cava, and the
process begins anew.
From the right ventricle, the
blood is pumped through the
pulmonary valve into the
pulmonary artery.
From the left ventricle, the
blood is pumped through the
aortic valve into the aorta,
which subsequently branches
into smaller arteries that carry
the blood throughout the rest of
the body.