The Cardiovascular System
From the moment it begins beating until the moment it stops, the human heart works tirelessly. In an average lifetime, the heart beats more than two and a half billion times, without ever pausing to rest. Like a pumping machine, the heart provides the power needed for life. This life-sustaining power has, throughout time, caused an air of mystery to surround the heart. Modern technology has removed much of the mystery, but there is still an air of fascination and curiosity.
Development
A human being's heart is about the size of that human being's fist. As the body develops, the heart grows at the same rate as the fist. So an infant's heart and fist are about the same size at birth. In the womb, however, that similarity was not always true. During the first few weeks after conception, the fetal heart occupies most of the fetus' mid-section. The heart size to body size ratio is nine times greater in the fetus than in the infant. During those first few weeks, the fetal heart lies high in the chest. Soon, it moves down to occupy its position in the chest cavity.
There are several phases of the fetal heart's development. At first, the
heart is just a tube. It grows so fast that it needs more space, so it bends and
twists back, forming the familiar shape. During the next phase, the two atria
are partly separate but there is just one big ventricle. The next phase begins
when the two atria are completely separate and the ventricles are just beginning
to separate. Finally, the ventricles separate completely and the heart is
developed.
During the fetal heart's developmental stages, the heart actually takes on
several distinct appearances. These heart structures resemble other animal
hearts. During phase one, the tube-like heart is much like a fish heart. The
second phase, with two chambers, resembles a frog heart. The three-chambered
phase is similar to a snake or turtle heart. The final four-chambered heart
structure distinguishes the human heart.
The heart, like other body parts, needs oxygen in order to grow and develop
properly. During childhood, the body's years of rapid growth, the need for
oxygen is greatest. The heart's rate of pumping oxygen-rich blood is fastest in
infancy, about 120 beats per minute. As the child grows, the heart rate slows. A
seven year old child's heart beats about 90 times per minute. By the age of 18,
the heart rate has stabilized to about 70 beats per minute. By adulthood, the
heart is fully developed. Throughout life, the heart needs only to be maintained
and kept healthy in order to function. If you take care of your heart, by
following a prescription for a healthy heart, your heart should take care of you
for the rest of your life.
Structure
The heart you see drawn on the average Valentine is only a rough
representation of the actual structure of the heart. Your heart is actually
shaped more like an upside-down pear. The human heart is primarily a shell.
There are four cavities, or open spaces, inside the heart that fill with blood.
Two of these cavities are called atria. The other two are called ventricles. The
two atria form the curved top of the heart. The ventricles meet at the bottom of
the heart to form a pointed base which points toward the left side of your
chest. The left ventricle contracts most forcefully, so you can best feel your
heart pumping on the left side of your chest.
The left side of the heart houses one atrium and one ventricle. The right side
of the heart houses the others. A wall, called the septum, separates the right
and left sides of the heart. A valve connects each atrium to the ventricle below
it. The mitral valve connects the left atrium with the left ventricle. The
tricuspid valve connects the right atrium with the right ventricle.
The top of the heart connects to a few large blood vessels. The largest of these
is the aorta, or main artery, which carries nutrient-rich blood away from the
heart. Another important vessel is the pulmonary artery which connects the heart
with the lungs as part of the pulmonary circulation system. The two largest
veins that carry blood into the heart are the superior vena cava and the
inferior vena cava. They are called "vena cava" because they are the "heart's
veins." The superior is located near the top of the heart. The inferior is
located beneath the superior.
The heart's structure makes it an efficient, never-ceasing pump. From the moment
of development through the moment of death, the heart pumps. The heart,
therefore, has to be strong. The average heart's muscle, called cardiac muscle,
contracts and relaxes about 70 to 80 times per minute without you ever having to
think about it. As the cardiac muscle contracts it pushes blood through the
chambers and into the vessels. Nerves connected to the heart regulate the speed
with which the muscle contracts. When you run, your heart pumps more quickly.
When you sleep, your heart pumps more slowly.
Considering how much work it has to do, the heart is surprisingly small. The
average adult heart is about the size of a clenched fist and weighs about 11
ounces. Located in the middle of the chest behind the breastbone,
between the lungs, the heart rests in a moistened chamber called the pericardial
cavity which is surrounded by the ribcage. The diaphragm, a tough layer of
muscle, lies below. As a result, the heart is well protected.
To monitor the heart, scientists can use x-ray or scanning technology to get a
picture. To really explore the heart, scientists have to perform surgery. Heart
surgery is very risky because the heart's pumping action is so critical for
survival. If the heart stops pumping, the body cannot survive. Before beginning
heart surgery, doctors connect the patient to a machine that pumps the blood for
the heart. Only then is it safe for the doctor to stop the heart in order to
operate.
Blood
The average adult has about five liters of blood living inside of their body,
coursing through their vessels, delivering essential elements, and removing
harmful wastes. Without blood, the human body would stop working. Blood is the
fluid of life, transporting oxygen from the lungs to body tissue and carbon
dioxide from body tissue to the lungs. Blood is the fluid of growth,
transporting nourishment from digestion and hormones from glands throughout the
body. Blood is the fluid of health, transporting disease fighting substances to
the tissue and waste to the kidneys.
Because it contains living cells, blood is alive. Red blood cells and white
blood cells are responsible for nourishing and cleansing the body. Since the
cells are alive, they too need nourishment. Vitamins and Minerals keep the blood
healthy. The blood cells have a definite life cycle, just as all living
organisms do.
Approximately 55 percent of blood is plasma, a straw-colored clear liquid. The
liquid plasma carries the solid cells and the platelets which help blood clot.
Without blood platelets, you would bleed to death.
When the human body loses a little bit of blood through a minor wound, the
platelets cause the blood to clot so that the bleeding stops. Because new blood
is always being made inside of your bones, the body can replace the lost blood.
When the human body loses a lot of blood through a major wound, that blood has
to be replaced through a blood transfusion from other people.
But everybody's blood is not the same. There are four different blood types.
Plus, your blood has Rh factors which make it even more unique. Blood received
through a transfusion must match your own. Patients who are scheduled to have
major surgery make autologous blood donations (donations of their own blood) so
that they have a perfect match.
Vessels
In a general sense, a vessel is defined as a hollow utensil for carrying
something: a cup, a bucket, a tube. Blood vessels, then, are hollow utensils for
carrying blood. Located throughout your body, your blood vessels are hollow
tubes that circulate your blood. There are three varieties of blood vessels:
arteries, veins, and capillaries. During blood circulation, the arteries carry
blood away from the heart. The capillaries connect the arteries to veins.
Finally, the veins carry the blood back to the heart.
If you took all of the blood vessels out of an average child, and laid them out
in one line, the line would be over 60,000 miles long! An adult's vessels would
be closer to 100,000 miles long!
Besides circulating blood, the blood vessels provide two important means of
measuring vital health statistics: pulse and blood pressure. We measure heart
rate, or pulse, by touching an artery. The rhythmic contraction of the artery
keeps pace with the beat of the heart. Since an artery is near the surface of
the skin, while the heart is deeply protected, we can easily touch the artery
and get an accurate measure of the heart's pulse.
When we measure blood pressure, we use the blood flowing through the arteries
because it has a higher pressure than the blood in the veins. Your blood
pressure is measured using two numbers. The first number, which is higher, is
taken when the heart beats during the systole phase. The second number is taken
when the heart relaxes during the diastole phase. Those two numbers stand for
millimeters. A column of mercury rises and falls with the beat of the heart. The
height of the column is measured in millimeters. Normal blood pressure ranges
from 110 to 150 millimeters (as the heart beats) over 60 to 80 millimeters (as
the heart relaxes). It is normal for your blood pressure to increase when you
are exercising and to decrease when you are sleeping. If your blood pressure
stays too high or too low, however, you may be at risk of heart disease.
On average, your body has about 5 liters of blood continually traveling through
it by way of the circulatory system. The heart, the lungs, and the blood vessels
work together to form the circle part of the circulatory system. The pumping of
the heart forces the blood on its journey.