The mammals (and birds) have a double circulatory system with a
complete separation of the pulmonary (to lungs) and systemic (to body)
systems to prevent mixing of oxygenated and deoxygenated blood. So
that pulmonary and systemic circulatory systems are completely
"Mammal circulatory systems are divided into two circuits: pulmonary
and systemic. The pulmonary circuit carries deoxygenated blood from
the heart to the respiratory surface in the lungs, where it is
reoxygenated, and then back to the heart. The systemic circuit carries
oxygenated blood to all the body's cells via arteries, and
deoxygenated blood back to the heart via veins. The mammalian double
circulatory system is efficient because it uses a separate pump (the
two ventricles) to power each circuit."
From "Anatomy of Animals - Mammals":
The frog heart has a 3-chambered heart with two separated atria and a
single ventricle which pump the blood to the both circulatory systems
and, as you said, does not prevent for a partial mixing of oxygenated
and deoxygenated blood.
The lizard's heart is more developed, it still has 3 chambers but it
has a muscular septum which partially divides the ventricle. In the
very moment the ventricle contracts the gap in the septum closes and
the ventricle is momentarily divided into two separate chambers
preventing the mixing of oxygenated and deoxygenated blood.
In birds and mammals the septum is complete providing them a
4-chambered heart and two separate circulatory systems. The septum
(interventricular and interatrial septum) is the muscular tissue that
keep both systems independent, that is it keeps oxygenated and
deoxygenated blood 100% separated.
For more detalided explanations and graphs see "Animal Circulatory Systems":
"In the human body, the heart ... consists of four chambers, the two
upper atria (singular: atrium) and the two lower ventricles.
A septum divides the right atrium and ventricle from the left atrium
and ventricle, preventing blood from passing between them. Valves
between the atria and ventricles (atrioventricular valves) maintain
coordinated unidirectional flow of blood from the atria to the
The function of the right side of the heart is to collect deoxygenated
blood from the body and pump it into the lungs so that carbon dioxide
can be dropped off and oxygen picked up, this happens through a
process called diffusion. The left side collects oxygenated blood from
the and pumps it out to the body."
From "Heart information on Wikimirror.com":
The above text confirms us that the human heart (and this is
applicable to mammals hearts) is a 4-chambered heart divided into two
halves (left and right). Each half belongs to a unique circulatory
system and both system are independent (no mixing between them). The
tissue that mantains both circulatory systems 100% separate is the
For further readings see the following articles:
"Interventricular septum - Wikipedia, the free encyclopedia":
"Interatrial septum - Wikipedia, the free encyclopedia":
mammal heart "oxygenated blood"
mammal heart "oxygenated blood" separate
mammal "circulatory system"
I hope this helps you. Feel free to use the clarification feature if
you find something unclear or incomplete. I will be glad to give you
further assistance on this question if you need it.