Circulatory System Info:
Blood Vessels
After blood leaves the heart, it is pumped through a network of blood vessels to
different parts of the body. As it is seen in the picture left, there are three types of
blood vessels that form this network. These are
arteries
capillaries
veins
With the exception of capillaries and tiny veins, blood vessels have walls
made up of three (3) layers of tissue. The inner layer is epithelial tissue. The
middle layer is smooth muscle tissue and elastic fibers. The outer layer is connective
tissue.
Arteries
Arteries carry blood from the heart to all the tissues of the body.
**Except for the pulmonary arteries, all arteries carry oxygen-rich blood.
The artery that carries oxygen-rich blood from the left ventricle to all parts of the
body is the aorta. The aorta, with a diameter of 2.5 centimeters, is the largest artery
in the body. As the aorta travels away from the heart, it branches into smaller and
smaller arteries so that all parts of the body are supplied with blood. The smallest
arteries are called arterioles.
The walls of arteries are thicker than those of veins. The smooth muscle
cells and elastic fibers that make up these walls make arteries tough and
flexible. These characteristics enable arteries to withstand the high
pressure of blood as it is pumped from the heart.
Capillaries
Arterioles branch into networks of very small blood vessels called capillaries. The
real work of the circulatory system is done in these thin-walled capillaries. They
provide oxygen and nutrients for the cells and then they
collect carbon dioxide and waste products from the cells.
The walls of the capillaries consist of only one (1) layer of cells . They are
extremely narrow blood vessels, so narrow that blood cells moving through
them must pass in single file. That makes it easy for oxygen and nutrients to
diffuse from the blood into the tissues. The forces of diffusion drive carbon dioxide
and waste products from the tissues into the capillaries.
Veins
The flow of blood moves from capillaries into veins. The smallest veins are called
venules. Veins
collect oxygen-poor blood from every part of the body
carry oxygen-poor blood back to the heart.
The walls of veins are thinner and less elastic than those of arteries.
Although the walls are less elastic, they are more flexible and are able to stretch out
readily. This is important, because it reduces the resistance the flow of blood on its
way back to the heart.
Large veins contain valves that keep blood from flowing backward.
These valves play an important role, because blood must frequently flow against the
force of gravity.
Blood flowing through veins gets quite a push from the contractions of skeletal
muscles, especially those in the arms and the legs. When these muscles contract, they
squeeze against veins and help force blood toward the heart. When these muscles
are not used for long periods of time, this extra push is lost and blood accumulates
in different parts of the body.
Pathways of Circulation
Blood moves through the body in a continuous pathways. In this pathways, there
are two major circulation stated below.
Pulmonary cirulation: it carries blood between the heart and the lungs. This
circulation begins at the right ventricle and ends at the left atrium.
Systemic Circulation: it starts at the left ventricle and ends at the right atrium,
carries oxygen-rich to the rest of the body.
Pulmonary Circulation
As you see in the picture, pulmonary circulation starts at the right ventricle, and
ends at the left atrium of the heart. Oxygen-poor blood is pumped out of the right
ventricle of the heart into the lungs through the pulmonary arteries. These are the
only arteries in the body that carry oxygen-poor blood.
After blood gets oxgen from the lungs, it returns to the heart through the
pulmonary veins, which are the only veins in the body that carry oxygen-rich blood.
The lungs are the only organs directly connected to both chambers of the heart.
Systemic Circulation
Systemic circulation starts at the left ventricle, and ends at the right atrium of the
heart.
Oxygen-rich blood leaving the heart passes through the aorta and into a number of
arteries that supply blood to every part of the body. Systemic circulation supplies
each major organ with blood, including the heart.
A pair of coronary arteries leading from the aorta carry blood through the tissues of
the heart. These arteries branch into arterioles and then into capillaries, forming a
network throughout the heart. After capillaries, oxgen-poor blood passes into veins
and returns to the right atrium.
In general, blood travels through only one set of capillaries before it returns to the
heart. However, there is a special circulation known as the hepatic portal system
that is an exception to this rule.
Blood
Blood is referred to as the river of life. It is a fluid medium that transport material
throughout the body in the circulatory system. Blood is made of plasma and blood
cells . (See chart bottom of page 88 in text).
Functions of the blood
transports nutrients
dissolved gases (oxygen and carbon dioxide), enzymes, hormones, and waste
products
regulates body temperature, pH, and electrolytes (ions in solution that conduct
electric current)
protects the body from invaders
restricts the loss of fluid.
The human body contains 4 to 6 liters of blood, only 8 % of the total mass of the
body. Approximately 55% of blood is made of fluid portion called plasma. The
remaning 45% consists of blood cells.
The Aorta and the Arterial System
The aorta leaves the heart and heads toward, what else, the head. We have to keep
our brains well nourished so we can make good grades in school. The arteries that
take the blood to the head are located on something called the *aortic arch.* After
the blood passes through the aortic arch it is then distributed to the rest of the body.
The *descending aorta* goes behind the heart and down the center of the body.
Sometimes, if you are lying flat on your back, you can look down toward your feet
and actually see your abdomen pulsate with each heart beat. This pulsation is really
the aorta throbbing with each heart beat. Do not be alarmed, this is normal.
From the aorta, blood is sent off to many other arteries and arterioles (very small
arteries) where it gives oxygen and nutrition to *every* cell in the body. At the end
of the arterioles are, guess what, capillaries. The blood gives up its cargo as it passes
through the capillaries and enters the venous system.
The Venous System
The venous system carries the blood back to the heart. The blood flows from the
capillaries, to venules (very small veins), to veins. The two largest veins in the body
are the *superior* and *inferior* vena cavas. The superior vena cava carries the
blood from the upper part of the body to the heart. The inferior vena cava carries
the blood from the lower body to the heart. In medical terms, *superior* means
above and *inferior* means under. Many people believe that the blood in the veins
is *blue*; it is not. Venous blood is really dark red or maroon in color. Veins do
have a bluish appearance and this may be why people think venous blood is blue.
Both the superior and inferior vena cava end in the right atrium. The superior vena
cava enters from the top and the inferior vena cava enters from the bottom.
This completes our little journey through the circulatory system. I hope the blood
has continued to flow to your brain as you read this and you managed to stay
awake. If you dozed off, it's o.k., I doze off myself from time to time when I read
really boring stuff. There are lots of things that I did not talk about, such as how the
cooling system works, but I thought that you might like to look some of this stuff up
by yourself. As usual, I know you will have questions for me. I can't wait to hear
from you.