Download Blood Flow Through the Heart

Blood Flow Through the Heart
Blood circulates in a closed system. It flows from the heart, through the
lungs, back to the heart, and then to the rest of the body. The blood
always returns to the heart. The amount of blood that circulates and the
rate blood circulates is controlled by the pumping of the heart. The
system is similar to having a rubber ball filled with water and a tube
connected on both sides of it. When the ball is squeezed, the water
enters the tube on one side. When the ball is relaxed, water is drawn into
it through the other side. Squeeze again, and more water is ejected into
the tube. Relax again, and water is drawn into the ball. The valves
between the opening of the tube and the ball, which only open one way,
control the entry of water into the tube and prevent its tendency to move
backwards.
Blood continuously flows in a circle. Oxygen-poor blood enters the lungs
where carbon dioxide, a waste product of the organs’ cells, is exchanged
for oxygen. This oxygen-rich blood returns to the heart by way of the
pulmonary veins. When it arrives from the lungs, it enters the left
atrium. Some of the blood flows passively through the bicuspid (or
mitral) valve into the left ventricle. When the left atrium contracts, the
remaining blood in the left atrium is pushed through the bicuspid valve
into the powerful left ventricle.
After the left ventricle contracts, the blood exits through the aortic valve
into the aorta to be circulated throughout the body. The first section of
the aorta is the ascending aorta, which runs only a short way before
becoming the aortic arch at the apex of an inverted U, from which
vessels branch to supply the head, neck and arms. After the aortic arch
turns downward it becomes the descending aorta. The thoracic aorta
supplies blood to the thorax above the diaphragm and the abdominal
aorta supplies blood to the region below the diaphragm. Arteries that
branch off of the aorta continue to branch into smaller and smaller
arterioles and eventually form the capillaries within the organ. It is
within the capillaries that the gas exchange occurs. In the organs,
oxygen is provided and carbon dioxide is removed. This is the opposite
of gas exchange in the lungs where carbon dioxide is removed and
oxygen is provided.
Once exchange has occurred, oxygen-poor blood exits the capillaries,
flowing into venules, or small veins. These venules combine to form
veins, which eventually return the blood to the right atrium. The blood
from the head, neck and arms returns to the right atrium through
the superior vena cava, while the blood from the lower body returns
through the inferior vena cava. The right atrium contracts and blood
passes through the tricuspid valve into the right ventricle. From the
right ventricle, the blood is ejected through the pulmonary semilunar
valve into the pulmonary artery to travel to the lung arterioles and
capillaries. Oxygen-rich blood flows into the venules and back to the
heart through the pulmonary vein and is ready to enter the left atrium
again and circulate around the body again.
The Fetal Circulation
The path that blood takes through the fetal circulatory system and heart
differs somewhat from that in the postpartum state. In particular, a
majority of the blood passes through three shunts, bypassing the liver
and lungs that do not require as much blood in the fetus as they do after
birth. These shunts include the ductus venosus, which bypasses the
liver, and two shunts that bypass the lungs; the foramen ovale and the
ductus arteriosus.
As highly oxygenated blood travels along the placental vein into the
fetus, some of the blood perfuses the liver, while a majority bypasses the
liver through the ductus venosus and directly enters the inferior vena
cava. The fetal liver matures late in development, when it prepares to
take over functions such as processing chemicals and nutrients absorbed
by the GI tract. Because it is not carrying out such functions in the fetus,
there is not a need to send significant blood flow there.
The lungs are also not functional in the fetus, as the placenta carries out
the functions of gas exchange. In fact, the lungs are deflated, with the
alveoli collapsed. This condition also narrows the vessels of the
pulmonary vasculature, creating high resistance to blood flow. To keep
the workload on the right heart from becoming too high in the fetus,
most of the blood bypasses the lungs. This occurs via two shunts. The
first is an opening between the right and left atria called the foramen
ovale. Because the pressures are higher in the right heart than the left
heart in the fetus (due to the high pulmonary resistance), blood moves
from the right atrium to the left atrium through this shunt. The
remaining blood moves into the right ventricle where it is pumped into
the pulmonary artery. But even from here most blood will not go
through the remainder of the pulmonary circulation. Instead, it travels
from the pulmonary artery to the aorta through the ductus arteriosus.
Soon after birth the lungs inflate, greatly reducing the resistance
through pulmonary circulation and lowering the pressures on the right
side of the heart. This briefly reverses flow through the foramen ovale,
pushing two flaps of tissue over the opening, closing it. These flaps grow
into the atrial septum, leaving a slight depression called the fossa ovalis.
The decreased pressure in the pulmonary artery also triggers the ductus
arteriosus to collapse, converting it into the connective ligamentum
arteriosum over the next three months or so. The ductus venosus
become a connective tissue remnant, called the ligamentum venosum,
found on the inferior surface of the liver.
Aging and the Heart: Valvular Stenosis
If any of the four valves within the heart become stiff, the valves are
unlikely to open to the fullest extent. Blood flow is impeded and the
pressure in at least one of the heart chambers increases.
Mitral valve stenosis increases the stiffness of the mitral valve that
separates the left atrium from the left ventricle. This results in increased
pressure in the left atrium, which backs up into the pulmonary
circulation, causing edema there. If the atria can’t compensate enough
to maintain flow through the narrowed mitral valve, heart failure will
develop. People with untreated mitral stenosis typically develop atrial
hypertrophy in an effort to generate enough atrial pressure to maintain
this flow. Similar results and compensatory mechanisms occur in
tricuspid valve stenosis.
Aortic valve stenosis occurs when the aorta valve stiffens. This increases
the pressure in the left ventricle and increases the stress developed in
the wall of the left ventricle during ejection. The stroke volume is
reduced, as the left ventricle has to contract more forcefully to eject the
blood into the aorta. This causes left ventricular hypertrophy, although
this is not always enough to maintain flow.