Circulatory system How does the blood circulatory system work?

The Circulation System

Part I: The Role of the Lungs


1. Location and Structure of
the Lungs

2. Breathing and Respiration

3. Gas Exchange

* picture of the heart and its

* picture of the body and some of
its organs

Location and Structure of
the Lungs

The lungs are a pair of
elastic, spongy organs used in breathing. In humans the lungs
take up a lot of the chest cavity. They are located just behind,
and to either side of, the heart. They extend down from the
collarbone to the diaphragm (the muscular wall between the chest
cavity and the abdominal cavity). In adult humans each lung is 25
to 30 cm. long (10 to 12 in.) and roughly cone shaped. The right
lung is somewhat larger than the left lung because it has three lobes,
or sections, whereas the left lung has only two.

When we breathe, the air
travels to the lungs through a series of tubes and passages. The
air enters the body through the nostrils or the mouth. It travels
down the throat to the windpipe. Inside the chest cavity the
windpipe divides into two branches, called the right and left
bronchial tubes
that enter the lungs. The large bronchial
tubes branch into ever smaller tubes, called bronchioles.
These in turn divide into even narrower tubes. Each small tube
ends in clusters of thin-walled air sacs, called alveoli.
It is the alveoli that receive the oxygen and pass it on to the

The alveoli are surrounded
by tiny blood vessels, called capillaries. The alveoli and
capillaries both have very thin walls, which allow the oxygen to
pass from the alveoli to the blood. The capillaries then connect
to larger blood vessels, called veins, which bring the oxygenated
blood from the lungs to the heart. The largest veins that do this
work are called the pulmonary veins, and they connect
directly to the heart.

Breathing and Respiration

Sometimes we use the terms
breathing and respiration to mean the same thing, but they
actually are distinct processes. Breathing is the process
of moving oxygen-rich air into and out of the lungs. Respiration
refers to how the cells of the body use oxygen to create energy
and how they exhale carbon dioxide that is a waste product of
this process.

The lungs have to work
continuously because the body cells are constantly using up
oxygen and producing carbon dioxide. Unlike the heart, the lungs
have no muscle tissue. Instead, muscles in the rib cage and the
diaphragm do all the work of lifting the ribs upward and outward
to let the air in, and then relaxing to force the air out.

Gas Exchange

Why are oxygen and carbon
dioxide such important gasses? All cells of the body need energy
to do their work. They get energy by combining sugars or other
food materials with oxygen. This chemical reaction is something
like burning. Inside the body cells the chemical reaction gives
off heat and other forms of energy. This energy provides the
power we need to talk and move and think.

When a fire burns, carbon
dioxide is formed. When a body cell combines sugar with oxygen to
get energy, carbon dioxide is formed there, too. But too much
carbon dioxide could poison a cell. They need some way to get rid
of carbon dioxide. The blood brings oxygen to the body cells and
takes away their carbon dioxide. The blood that travels back to
the heart and lungs is dark red. It has picked up carbon dioxide
from the body cells, and it has left most of its oxygen with the
cells. We can think of the dark colored, carbon dioxide-rich
blood as "used” blood. This is the blood that the heart
pumps into the lungs.

The carbon dioxide in the
blood is exchanged for oxygen in the alveoli. These tiny air sacs
in the lungs are only one cell thick and they are surrounded by
capillaries that are also only one cell thick. Blood from the
heart flows through these capillaries and collects oxygen from
the alveoli. At the same time, carbon dioxide passes out of the
capillaries and into the alveoli. When you breathe out, you get
rid of this carbon dioxide. The bright red, oxygen-rich blood is
returned to the heart and pumped out to the body. We can think of
it as "fresh” blood — it is as good as new!

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For Parents
/ Heart and Circulatory System

Heart and Circulatory System

Reviewed by: Larissa Hirsch, MD

en españolEl aparato circulatorio y el corazón

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
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.

Illustration: Healthy Heart

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.

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. 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.

How Can I Help Keep My Child’s Heart Healthy?

To help keep your child’s heart healthy:

  • Encourage plenty of exercise.
  • Offer a nutritious diet.
  • Help your child reach and keep a healthy weight .
  • Go for regular medical checkups.
  • Tell the doctor about any family history of heart problems.

Let the doctor know if your child has any chest pain, trouble breathing, or dizzy or fainting spells; or if your child feels like the heart sometimes goes really fast or skips a beat.

Reviewed by: Larissa Hirsch, MD
Date reviewed: September 2018

More on this topic for:

  • Heart Murmurs
  • Mitral Valve Prolapse
  • Atrial Septal Defect (ASD)
  • Congenital Heart Defects
  • Words to Know (Heart Glossary)
  • Arrhythmias
  • Cardiomyopathy
  • Cardiac Catheterization
  • Interrupted Aortic Arch (IAA)
  • Patent Foramen Ovale (PFO)
  • Patent Ductus Arteriosus (PDA)
  • Supraventricular Tachycardia (SVT)
  • Body Basics: The Heart (Slideshow)
  • The Heart
  • Heart Disease
  • Your Heart & Circulatory System
  • Heart Murmurs
  • Arrhythmias
  • Atrial Septal Defect
  • Ventricular Septal Defect
  • Heart and Circulatory System

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