Muzio 1
Mammalian Circulation: A Critical Understanding of How the
Heart Pumps Blood
Anna Muzio
The heart, composed of cardiac muscle, works under unconscious control. Like respiration, the body
automatically sends signals for the heart to beat on demand without thought. The heart acts as a
mechanism for the body to deliver nutrient oxygen from the lungs to the body.
BASIC GROSS ANATOMY
The heart, comprised of a dual circuit, pumps blood through the simultaneous squeezing and ventilating of
the four chambers. The atria, or collecting ducts, consist of the upper chambers of the heart, while the
ventricles make up the lower chambers. The hearts’ atria and ventricles are described by their anatomical
position; for example, the term “right ventricle” refers to a cadavers’ right ventricle.
The opening and closing of valves catalyzes blood flow. The heart contains four valves: the mitral valve,
bicuspid valve, pulmonary semilunar valve (pulmonic valve), and atrioventricular valve (aortic valve).
Valves operate like control mechanisms to flow; their subsequent opening and closing relate to heartbeat.
Valves prevent deleterious backflow and mixing of oxygenated blood with deoxygenated blood. Valves are
analogous to checkpoints; in order to enter the next phase of the cycle, the valve must open to liberate
blood.
Property of Dr. John Waters; Penn State Biology Lecture “Blood and Circuitry”
Muzio 2
FUNCTIONING AND CIRCUITRY
The main function of the heart consists of moving blood for oxygen transport. Travelling through two
circuited systems, blood returns to heart deoxygenated from the body and, conversely, oxygenated from
the lungs. The systemic circuit refers to bloods flow from the heart to the body. The pulmonary circuit
entails blood flow from the lungs back to the body. The systemic circuit dictates delivery of oxygenated
blood, while the pulmonary circuit controls the re-oxygenation of blood. Blood travels along the tracks of
arteries and veins. Arteries transport blood away from the heart, while veins carry blood towards the heart.
Capillaries, or small vesicles, promote gas exchange between tissues. In the systemic circuit, oxygenated
blood moves from the heart, into the body, and through the capillaries, where the body emits oxygen to the
muscles, and thus becoming deoxygenated. This blood progresses back to the heart and travels to the
lungs, where it can aptly re-oxygenate. Keep in mind, the heart is a cycle!
Property of Dr. John Waters; Penn State Biology Lecture “Blood and Circuitry”
Muzio 3
PROGRESSION OF BLOOD FLOW
Again, the cycle of blood flow is circular without a finite entry point or ending point. Analogous to a toll road
on a turnpike, blood can enter the cycle of blood flow at any given point, just as a car can enter the turnpike
at several locations. For purposes of this discussion, I will reference the beginning of blood flow from the
right ventricle onward. The progression follows:
1.
2.
3.
4.
5.
6.
Right Ventricle
o Blood begins its journey by moving out of the right ventricle (1) and into the pulmonary artery
(2). This flow, categorized by the closing of the pulmonary semilunar valve, prepares blood to
flood into the pulmonary circuit to become re-oxygenated.
Pulmonary Artery
o The pulmonary artery (2) continues moving blood away from the heart and into the capillaries
of the lungs for gas exchange (3).
Gas Exchange in Lungs
o Blood begins travelling through
the one-cell thick, thin walled
capillaries to participate in gas
exchange (3).
The carbon
dioxide that the body produces
from inhalation diffuses out of
the capillaries, while oxygen
diffuses in. This oxygenated
blood continues flowing towards
the body.
Pulmonary Veins to Left Atrium
o Blood starts returning back to
the heart through transport in
the pulmonary vein (4). This
richly oxygenated blood begins
entering the left atrium (5).
Left Ventricle
o Catalyzed by the contraction of
the Atrioventricular Valve, or AV
valve,
blood
continues
progressing into the left ventricle
(5) to prepare for transport out
of the body.
Aorta
o The aorta (6) functions by
pushing blood out of the heart
and into the systemic circuit,
initiating
deliverance
of
oxygenated blood to nourish the
extremities (7 & 8).
Property of Dr. John Waters; Penn State Biology Lecture “Blood and Circuitry”
Muzio 4
7. Gas Exchange in Extremities of Upper Body
o Blood continues flowing to the upper extremities, consisting of the head, arms, and upper trunk
(7). Blood, squeezing through the capillaries, then deoxygenates.
8. Gas Exchange in Extremities of Lower Body
o Comprised of the lower trunk, legs, and feet, the lower body extremities receive nourishment
by deoxygenating in the capillaries. (8)
9. Superior Vena Cava
o The superior vena cava (9) functions by moving blood from the upper extremities and back into
the heart to prepare for entrance into the pulmonary circuit.
10. Inferior Vena Cava
o The inferior vena cava (10) works by transporting blood from the lower extremities to return to
the heart. Remember, blood is now deoxygenated!
11. Right Atrium
o The right atrium (11), acting like a collecting pool for the deoxygenated blood returning from
the heart, starts distributing blood back into the right ventricle (1). After contraction of the
Bicuspid valve, the cycle starts again.
PROGRESSION OF BLOOD FLOW
Characterized by the continuous flow between the systemic and pulmonary circuit, blood flow delivers
oxygen to the body. Blood travels through the upper atria and lower ventricles of the heart; arteries
transport blood away from the heart, while veins return blood. Blood flow can be traced in the heart
beginning at any point, as it is a circuit. Starting with the right ventricle, blood moves through the
pulmonary artery and into the pulmonary circuit. Gas exchange occurs, and the newly oxygenated blood
returns back to the heart through the pulmonary vein, and into the left atrium. Blood now progresses into
the left ventricle and aorta to distribute oxygen rich blood to the extremities. Gas exchange occurs in the
extremities’ tissues, the deoxygenated blood returns to the heart by the inferior and superior vena cava,
and into the right atrium.