Download Overview of the Circulatory Pathways

Overview of the Circulatory Pathways
The two pumps of the heart are in series, meaning that the blood flows
in one direction from one pump to the other. Starting from the left
heart, the blood flows through the peripheral circulatory system, which
provides the body with the nutrients it needs, such as oxygen, and
removes metabolic waste, such as carbon dioxide. As blood moves
through this circulation, it follows a path of blood vessels that change in
their anatomy and function. Upon leaving the heart blood enters the
major arteries. Arteries are the vessels in the circulatory system that
carries blood away from the heart. These arteries branch and divide
into arterioles that are smaller in diameter. These arterioles continue
to branch out to create even smaller capillaries. These capillaries,
which are found within each organ, are the site of gas and nutrient
exchange. Once blood has perfused the organ, the capillaries begin to
coalesce into larger venules that combine to create larger veins,
ultimately returning blood to the heart through either the superior or
inferior vena cava.
Circulatory Pathways
Blood vessels are organized into circulatory routes that deliver blood
to different organs and tissues of the body. The right ventricle pumps
blood to the lungs via the pulmonary circuit. The left ventricle
pumps blood to organs and tissues via the systemic circuit. The
output of the systemic circuit becomes the input of the pulmonary
circuit, and vice versa. The pump for the systemic circuit is the left side
of the heart, which also collects the oxygen-rich blood returning from
the lungs. The pump for the pulmonary circuit is the right side of the
heart, which receives oxygen-poor- blood from the systemic circulation.
There are two important differences between the systemic circulation
and pulmonary circulation. The first is that blood in the systemic
circulation must be pumped farther than blood in the pulmonary
circulation. The second is that, in pulmonary arteries, the diameter is
larger, the walls are thinner, and the tissue is more elastic than in
systemic arteries. Because of the characteristics of pulmonary arteries,
there is very low resistance to blood flow in the pulmonary circuit, and
thus pressures do not need to be very high to move blood through the
lungs. In the right ventricle, the peak systolic pressure is typically onefifth that in the left ventricle.
The Pulmonary Circuit
The sole function of the pulmonary circuit is to carry blood to the alveoli
of the lungs so that gases can be exchanged. Flow is matched to
ventilation so that oxygen can move from the alveoli into the
bloodstream and carbon dioxide can move from the bloodstream into
the alveoli. The pulmonary circulation begins at the right ventricle,
which pumps oxygen-poor, carbon dioxide-rich blood into the large
pulmonary trunk. From its origin at the right ventricle, the pulmonary
trunk takes an upward diagonal course for approximately 8 cm (3.1
inches (in.)). It then divides into the right pulmonary artery (to the
right lung) and the left pulmonary artery (to the left lung). With the
exception of the fetal circulation, the pulmonary arteries are the only
arteries that transport oxygen-poor blood. After entering the lungs, the
pulmonary arteries subdivide into three lobar arteries in the right
lung and two lobar arteries in the left lung. Each lobar artery supplies
one lung lobe. Further subdivisions result in the formation of capillaries
around the alveoli. Carbon dioxide from the blood is transferred into the
alveoli and exhaled. Inhaled oxygen is transferred from air in the lungs
into the blood.
Pulmonary capillaries merge, forming venules that unite to
become pulmonary veins. Two of these veins leave each lung and
transport oxygenated blood to the left side of the heart. As the left
ventricle contracts, it pumps out the oxygenated blood into the systemic
circulation. Again, except for fetal circulation, pulmonary veins are the
only veins that transport oxygen-rich blood. It is easy to remember
which vessels belong to the pulmonary circulation, because they all have
either the word pulmonary or lobarin their names.
The Systemic Circuit
The systemic circuit delivers oxygenated blood and nutrients to the
entire body except the lungs, and removes the waste products of
metabolism from the same tissues. We will review the circuit from
previous sections: After blood is pumped from the left ventricle into the
aorta, it flows into smaller and smaller systemic arteries to perfuse all of
the body's organs and tissues (except for the alveoli of the lungs).
Systemic arteries branch off into smaller arterioles that lead into
extensive networks of systemic capillaries. Nutrients and gases are
exchanged between blood and systemic tissues through the thin
capillary walls. After the blood gets rid of oxygen and picks up carbon
dioxide, it usually travels through a single capillary before entering a
systemic venule. The venules then transport oxygen-poor
(deoxygenated) blood from systemic tissues before merging to create the
larger systemic veins. All blood in the systemic circulation eventually
empties into the right atrium.
The arteries and arterioles of the systemic circulation receive
oxygenated blood from the left ventricle and transport it to the systemic
capillaries. The veins and venules of the systemic circulation carry
deoxygenated blood back to the right atrium. Blood in systemic arteries
is bright red in color. Blood in capillaries is dark red, because it has lost
some oxygen and gained carbon dioxide.
The arteries and veins of the systemic circuit share a number of
similarities, most notably, the routes and names that many of these
vessels share. The blood from the heart is pumped into a single systemic
artery, the aorta, which carries oxygenated blood away from the left
ventricle. Oxygen-poor blood returns to the right atrium via three
systemic veins: the superior vena cava, inferior vena cava, and coronary
sinus. The majority of blood that returns to the heart flows through the
superior and inferior venae cavae. The names of these veins are derived
from the location of their tributaries. The superior vena cava collects
blood from veins located superior to (above) the diaphragm, with the
exception of the wall of the heart and the alveoli of the lungs. The
superior vena cava, which delivers blood to the right atrium, is formed
by the merger of the right and left brachiocephalic veins. Each of
these veins, in turn, is formed by the union of the internal jugular
vein and subclavian vein on the right or left side of the body.
The inferior vena cava, which is the largest-diameter blood vessel in
the body, collects blood from veins located inferior to (below) the
diaphragm and delivers it to the right atrium. This vein is located
directly to the right of the abdominal aorta. The merger of the paired
common iliac veins creates its distal end. Blood that drains from the
myocardium of the heart empties into cardiac veins and returns to the
right atrium via the coronary sinus.