What Does the Heart Do?
The
heart is a pump, usually beating about 60 to 100 times per minute. With each
heartbeat, the heart sends blood throughout
our bodies, carrying oxygen to every cell. After delivering the oxygen, the
blood returns to the heart. The heart then sends the blood to the lungs to pick up more oxygen. This
cycle repeats over and over again.
What Does the Circulatory System Do?
The
circulatory system is made up of blood vessels that carry blood away from and
towards the heart. Arteries carry
blood away from the heart and veins carry blood back to the heart.
The
circulatory system carries oxygen, nutrients, and hormones to cells, and removes waste products,
like carbon dioxide. These roadways travel in one direction only, to keep
things going where they should.
What Are the Parts of the Heart?
The heart has four chambers — two on
top and two on bottom:
·
The two bottom chambers are the right
ventricle and the left ventricle.
These pump blood out of the heart. A wall called the interventricular septum is
between the two ventricles.
·
The two top chambers are the right atrium and the left atrium. They receive the blood
entering the heart. A wall called the interatrial septum is between the atria.
The atria are separated from the
ventricles by the atrioventricular
valves:
·
The tricuspid valve separates the right atrium
from the right ventricle.
·
The mitral valve separates the left atrium from
the left ventricle.
Two valves also separate the ventricles
from the large blood vessels that carry blood leaving the heart:
·
The pulmonic valve is between the right ventricle
and the pulmonary artery, which carries blood to the lungs.
·
The aortic valve is between the left ventricle
and the aorta, which carries blood to the body.
What Are the Parts of the Circulatory System?
Two pathways come from the heart:
·
The pulmonary circulation is a short loop from
the heart to the lungs and back again.
·
The systemic circulation carries blood from the
heart to all the other parts of the body and back again.
In pulmonary circulation:
·
The pulmonary artery is a big
artery that comes from the heart. It splits into two main branches, and brings
blood from the heart to the lungs. At the lungs, the blood picks up oxygen and
drops off carbon dioxide. The blood then returns to the heart through the
pulmonary veins.
In systemic circulation:
·
Next, blood that returns to the heart
has picked up lots of oxygen from the lungs. So it can now go out to the body.
The aorta is a big artery that leaves the
heart carrying this oxygenated blood. Branches off of the aorta send blood to
the muscles of the heart itself, as well as all other parts of the body. Like a
tree, the branches gets smaller and smaller as they get farther from the aorta.
At each body part, a network of tiny blood vessels called capillaries connects the very small artery branches to very small veins. The capillaries have very thin walls, and through them, nutrients and oxygen are delivered to the cells. Waste products are brought into the capillaries.
Capillaries then lead into small veins. Small veins lead to larger and larger veins as the blood approaches the heart. Valves in the veins keep blood flowing in the correct direction. Two large veins that lead into the heart are the superior vena cava and inferior vena cava. (The terms superior and inferior don't mean that one vein is better than the other, but that they're located above and below the heart.)
Once the blood is back in the heart, it needs to re-enter the pulmonary circulation and go back to the lungs to drop off the carbon dioxide and pick up more oxygen.
At each body part, a network of tiny blood vessels called capillaries connects the very small artery branches to very small veins. The capillaries have very thin walls, and through them, nutrients and oxygen are delivered to the cells. Waste products are brought into the capillaries.
Capillaries then lead into small veins. Small veins lead to larger and larger veins as the blood approaches the heart. Valves in the veins keep blood flowing in the correct direction. Two large veins that lead into the heart are the superior vena cava and inferior vena cava. (The terms superior and inferior don't mean that one vein is better than the other, but that they're located above and below the heart.)
Once the blood is back in the heart, it needs to re-enter the pulmonary circulation and go back to the lungs to drop off the carbon dioxide and pick up more oxygen.
How Does the Heart Beat?
The heart gets messages from the body
that tell it when to pump more or less blood depending on a person's needs. For
example, when we're sleeping, it pumps just enough to provide for the lower
amounts of oxygen needed by our bodies at rest. But when we're exercising, the
heart pumps faster so that our muscles get more oxygen and can work harder.
How
the heart beats is controlled by a system of electrical signals in the heart.
The sinus (or
sinoatrial) node is
a small area of tissue in the wall of the right atrium. It sends out an
electrical signal to start the contracting (pumping) of the heart muscle. This
node is called the pacemaker of the heart because it sets the rate of the
heartbeat and causes the rest of the heart to contract in its rhythm.
These
electrical impulses make the atria contract first. Then the impulses travel
down to the atrioventricular (or
AV) node,
which acts as a kind of relay station. From here, the electrical signal travels
through the right and left ventricles, making them contract.
One complete heartbeat is made up of two phases:
1.
The first phase is called systole (SISS-tuh-lee).
This is when the ventricles contract and pump blood into the aorta and
pulmonary artery. During systole, the atrioventricular valves close, creating
the first sound (the lub) of a heartbeat. When the atrioventricular valves
close, it keeps the blood from going back up into the atria. During this time,
the aortic and pulmonary valves are open to allow blood into the aorta and
pulmonary artery. When the ventricles finish contracting, the aortic and
pulmonary valves close to prevent blood from flowing back into the ventricles.
These valves closing is what creates the second sound (the dub) of a heartbeat.
2.
T The second phase is called diastole (die-AS-tuh-lee). This is when the
atrioventricular valves open and the ventricles relax. This allows the
ventricles to fill with blood from the atria, and get ready for the next
heartbeat.
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